Homebrew Head Porting: Attempting the dark art .....
 

I am chasing 300hp on my ’78 3.0 Sc engine on somewhat of a limited budget as per this thread and will be doing the head porting myself, initially to save money, but now more to try and test the below views on the topic from a home DIYer's point of view, and ultimately have the satisfaction of having ported my own heads.

There are various different views on this and other forums on home brew “porting", mostly that it is just a hack and does not make power through a reasonable RPM range.

My mission is to see if I, as a DIYer with limited skill, knowledge and tools, and with a lot of time for research and testing , can do a home brew head “porting job” that can make good Hp and Tq through a resonalbe RPM range

( I am not sure how to measure the performance of the heads other than to make an assessment of the before and after average flow numbers, make the target as planned above on the dyno as other engine builds with a similar build sheets came short of 300Hp, and compare flow numbers with other known ported heads although I am not holding my breath on this)

Here is how I mapped things out:

1) Do a lot of research and get as much as possible literature on the subject
2) Get (build) a flow bench and test equipment
3) Flow measure one of my stock heads to get a baseline with manifolds and carbs
4) Make mold of the port and a plaster copy of the head
5) Make changes and optimize as far as possible and compare average to stock heads
6) Transfer changes to one of the heads “port match” manifolds and do final optimization
7) Make a mold of the ported head
8) Transfer to other heads
9) Equalize flow between heads
10) Dyno test to see how close I got to 300hp.

1st update to follow.

Velocity of air is much more important than volume of air. Remember that before you start hogging out a head and end up with a powerless lump. Having seen good and bad port jobs at the same size, the difference is dramatic. Start by looking at these for what "good" looks like: http://www.early911sregistry.org/forum/showthread.php?t=22220

Thanks for the link Kenik,

Wow those look great have been looking for good visuals on ported Porsche heads to start forming some initial ideas for a while but with limited success.

You are right, from the research i have done up to now, port velocity is one of the key components to make the head work especially with big cams (reversion, bad low RPM response etc. ) There is not a lot of port velocity data available for Porsche heads but a fair amount on Honda, Harley(hemi heads) and various V8 heads which will provide some ballpark parameters to work with.

It will be an interesting exercise to get the velocities up on the big port SC heads, but thats part of the goal.

Part 1 Flow Bench (update1)
 

I have been doing research for the last month on the whole porting deal and looked at various non commercial flow bench designs and came across this forum that is into flow benches and engine dynos. I decided to build the one shown in the Plans Area as there has been a lot of development done around the design.

This is not the least expensive by any means but will provide results comparable to other commercial flow benches. The alternative is to do a simple shopvac based flow device with a simple water manometer, this will provide data on the effect of flow changes made fairly accurately but will

1) Will not be comparable to other's flow results and
2) Will not flow enough to test port flow and velocity at high depressions.

The selected bench is a simple (well relatively simple) orifice design with no moving parts other than a divider plate to change the direction of flow. It uses 8* 100 CFM vac motors of which 2 will have electronic speed control. The orifice plates are calibrated and precision machined for specific flow rates and are changed manually. I will be using a digital manometer connected to my notebook for flow and velocity measurements.

The bench's tested accuracy is within 1% of an SF600 commercial flow bench which is seen as the leading commercial flow-bench. .

Attached are some pictures of flow bench construction so far....


http://forums.pelicanparts.com/uploa...1233873680.jpg

It’s nice when you have the equipment to test the changes you want to make, but often it’s just not feasible to buy professional equipment for limited home use. Building your own Flow Bench is a great solution for the serious DIY like yourself. Some Nay Sayers might chime in about the accuracy of homebuilt equipment but really this is not an issue. There are successful examples of home build flow benches online that work very well and I’m sure yours will to. It establishes a Base Line that you can work from and this important piece of equipment will provide you with all the data you need to determine the success of your porting.

It’s pretty well a one-way street when you start removing material so go slow and take it easy when you get into the actual porting. It would be nice to have a few scrap heads to play with first. That way you can always putty up the ports and try different things without risking your good ones.

Also Check out what Porsche is doing with the new intake plenum designs, the runners taper down to increase the velocity

Have fun and enjoy what your doing

I wish you well and commend you for doing it!


While doing some research a while back, I ran across the following and saved it, hope it’s helpful. Sorry I did not think to save the link at the time.

Flow Bench Fallacies: Our era is often referred to as the Information Age, but not all of the information is necessarily useful. I am beginning to think flow benches should come with a government warning:
"Caution! Excessive reliance on flow numbers may be harmful to your engine!"
I'm kidding of course. Used wisely, a flow bench can be a useful tool in engine development, just like a timing light or a dynamometer. Unfortunately, some racers believe that a flow bench is the ultimate answer machine. When the subject is cylinder heads, the four words I dread to hear are, "What do they flow?" Novice racers and magazine writers share a fixation about airflow.
Their mistaken belief that "more is better" is often the false assumption that produces an under-performing engine. A flow bench measures air movement in a very rudimentary way - steady-state flow at a constant depression (vacuum). Obviously, the conditions that exist in a running engine are quite different.
The flow bench can't simulate the effects of pistons going up and down, the reversion pulses as the valves open and close, the sonic waves that resonate inside the runners, the inertia of fuel droplets, and all of the other phenomena that influence engine performance in the real world.
When you flow test a cylinder head, you are simply measuring how far you can move the liquid in a manometer. The bigger you make the port, the more it flows. That's hardly shocking news. Bolt a sewer pipe on to a flow bench it will generate terrific flow numbers.
So should we use ports as big as sewer pipes on our racecars? The flow bench says we should - the time slip says something completely different.
If airflow were everything, we would always use the longest duration cams we could find after all, more duration means more airflow. In fact, we know that there is a finite limit to how long the valves can be open before performance suffers. That is because the valve events have to be in harmony with the rest of the engine. That same principle applies to cylinder heads. Simple airflow capacity should never be the first consideration in evaluating cylinder heads. Characteristics that are far more important include air speed, port cross-section, port volume and shape, and the relationship between the size of the throat and the valve seat.
If these attributes are wrong, you can work forever on the flow bench and not overcome the fundamental flaws.
Here's a do-it-yourself example: Turn on a garden hose and the water dribbles out a couple of feet. Now put a nozzle on the hose and the water will spray across your backyard. The water pressure and volume haven't changed, but the velocity has increased dramatically. Now think about air and fuel going into your engine's cylinders. Which would you prefer: slow and lazy or fast and responsive?
An engineer will tell you that an engine requires a prescribed amount of air and fuel to produce's "x" amount of horsepower. In a perfect world, that may be true - but we race with imperfect engines. The shape and cross sectional area of the runners is absolutely critical to performance. For example, I have two sets of Pro Stock cylinder heads that produce nearly identical flow numbers, yet one pair produces nearly150 more horsepower at 9200 rpm than the other. The flow bench can't tell the difference between them, but the engine certainly can.
There are software programs that claim to be able to predict an engine's performance based on airflow numbers. Unfortunately, a critical shortcoming of many of these programs is that they are based on inaccurate information or false assumptions. A computer is an excellent calculator, but it is not an experienced engine builder. The software doesn't know if the short-turn radius is shaped properly, whether the flow is turbulent at critical valve lifts, or whether the flame speed is fast enough. Racers have a tendency to believe that computers are infallible, so they accept the software solutions as gospel when in fact they be badly flawed. Textbooks would have you believe that an exhaust to intake flow ratio of 80 percent is ideal - yet a typical Pro Stock head has exhaust posts that flow less than 60 percent of the intake runners, (Bruce here, YES!)
You can improve the exhaust flow tremendously with about 40 minutes of work with a hand grinder, but the supposed improvements will just about kill the engine's on-track performance. I know because I've been there. We also have learned that the low-lift flow, (meaning anything below .400 valve lift in a Pro Stock engine with a .900 lift camshaft) is relatively unimportant.
Think about the valve events in a racing engine: From the point when the valve is first moves off its seat until it reaches mid-lift, the piston is either going the wrong way, (that is, it is rising in the cylinder) or it is parked near Top Dead Center. The piston doesn't begin to move away from the combustion chamber with enough velocity to lower the pressure in the cylinder until the valve is nearly halfway open.
Consequently, it is high-lift flow that really matters in a drag race engine. The shape of the combustion chamber also has a significant impact on performance.
A conventional chamber with deep reliefs around the valve seats and a relatively flat valve seat angle can produce terrific flow at .200 - .300 valve lift. Today, a state-of-the-art chamber typically has 55-degree valve seats and steep walls that guide the air/fuel mixture into the cylinder to enhance high lift flow. This doesn't mean that every racer needs state-of-the-art Pro-Stock cylinder heads - along with the high maintenance they require.
The heads have to match the application. Conventional combustion chambers and 45-degree valve seats are just fine for a dependable, low-maintenance racing engine that will run a full season between overhauls.
The classic Hemi combustion chamber is capable of producing impressive flow numbers, but it's not going to make impressive power. Engine technology in all forms of motorsports is converging around smaller, high-efficiency combustion chamber designs. You can see the result in lower brake specific fuel consumption (BSFC) numbers, which indicate improved engine efficiency. Twenty years ago, a racing engine with a .48 BSFC was considered

Thanks for the post great info, I will be doing most of the initial work on plaster copy of the head before i touch the real heads. think this was an interview with one of the Reher-Morison head porting guru's.

Gert

I think this is great. My concerns would be:

1) You may increase the flow of the head but it may not correlate to better performance on the dyno; and

2) You may see better performance on the dyno but it may not correlate to better performance on the track.

To alleviate both concerns, I would start by COPYING the known porting solutions in the Porsche community. While the real secrets are still secret, there is enough information in the public domain, including here on the forum, to get you started. Then you can iterate from there.

You seem like a technical guy, how will you replicate the port shape from cylinder to cylinder? I wonder if there are any shops with a digital Coordinate Measuring Machine that could digitize your successful port shape, which would then allow you to replicate it across all the cylinders with a CNC milling machine? That is the modern approach.

But enough caveats which you know, I admire your DIY approach and will be watching this thread closely for ideas.

Talk about a Labor of Love, 2 weeks of Clean up work on my own 3.2 heads :)

Wish the pictures turned out better, they don't do justice to the results.

http://forums.pelicanparts.com/uploa...1233944992.jpg


http://forums.pelicanparts.com/uploa...1233945040.jpg


http://forums.pelicanparts.com/uploa...1233945081.jpg

Eh! once you get your flow bench up and running, if possible could you flow test a Motronic (barn door) AFM? It would be nice to know the restriction of this unit when calculating boost. Thanks

Buy a used camera off of Ebay with "super macro" feature. Works wonders.

I just use a cheapy because someday its going to get dropped into the wash tank!

http://i30.photobucket.com/albums/c3...1/100_7844.jpg

Craig, that looks like a Georgia O'Keefe painting.

SOLD

I take paypal:D

OK, but only if you will deliver it and hang it on the ceiling.

What was this thread about anyway? :)

subscribed

The stock Porsche inlet and exhaust ports are pretty nice right out of the mold. But I love what you guys do to them...

I wonder what the cost vs. time graph would look like for die grinding by hand next to CNC?

Has someone done this?

Mark

Can't say's I have, but.......

A CMM to digitize a stock ports (or one of your choosing) as a baseline.
A CATIA work station to model it, reshape, then program for the mill.
A 5 axis mill.

There's about 600-700K.

You may find someone that has the necessary equipment/personnel with a labor rate of under 150 per hour.....but I doubt it. Couple hundred man hours for the first attempt.

Reinventing the wheel costs money. :D

Better to find a sucessful undertaking (with proof of the pudding) and pay to have yours done.

JP,
I didn't mean to compare a start-up from scratch! That is always big bucks. But now that there are some shops out there that have made the committment, like Rennsport Systems, the per unit cost should come down. Probably never approaching the DIY cost, but who knows?

Mark

You can do a "street port" job yourself where you're just smoothing out the origonal casting lines and marks and the abrupt and unsmooth factory machine marks where it transitions to the valve seat.
If you don't overdo it and screw up the shortside radius that might give you around 5-8 horsepower up top, maybe a little more.

You can get a generous supply 80 grit sandpaper cones and flaps and arbors in a little $25 kit at harbor freight.
You use them on an air powered straight die grinder, but you better have some experince and have excellent hand~eye coordination and feel.

You want a course finish on the intake ports to create surface turbulance on the surface of the ports, and a polished smooth finish in the exhaust ports so carbon won't stick as well.

Update Flowbench
 

John, Craig interesting views :)

John, you are right these are some of my main concerns, i dont have a mega budget and time to test every change on the dyno and track and this is where i think some of the real secrets are created. ;) I have collected as much as possible visuals and plan to use that as a starting point. There are some literature on the subject and a lot of information on the net (Mostly American hot rodders and drag racers). In my situation i will have to do a lot more research before making changes and hope to uncover the secrets as i go along, the key being research both on and off the flowbench and a lot of help from knowldeable folks in this and other communities.


I am going to try and stay away from new technology and stick to purely "home brew " porting that anyone should be able to do with a dremel or die grinder and some hand tools. I am planning on making a positive mold of the final product create 1/4" or 1/2" measurement segments and transfer the measurements / shape to the other heads and finally balance them from a flow point of view.


Dynamohum: did your labor of love yield any positive results? and yes i will flow test your AFM.


Here is sone updates on the flowbench:

I have been working mostly on completing the flowbench which is taking a lot longer than anticipated. MDF sucks, this stuff gets in everywhere is as bad as sanding drywall.

I completed the trail construction and will be knocking it down to the core again to start sealing and painting the inside. By the end of the week i should be done with the construction and will start installing the vacuum motors and wiring.


http://forums.pelicanparts.com/uploa...1234275503.jpg


http://forums.pelicanparts.com/uploa...1234275578.jpg

http://forums.pelicanparts.com/uploa...1234275615.jpg

Cheers
Gert

Ps: Spell check didnt work

Hey Kenik, you mentioned the velocity #s can you share those? It would be interesting to compare them to the stock heads once i get the flowbench rigged up.

Have you already completed the engine?

Thanks
Gert

That material should be shared privately and not on the board.

Totally agree

I never got velocty numbers - those are rightfully kept confidential by the builder. I did get volumes at lift.

A lot of my thoughts on the subject are encompassed in this thread from a few years back. In fact there is lots of information collected on this BBS over the years. Here's another thread that I think you will find contains a lot of good data.

As far as getting 300 bhp out of a 3.0 SC engine, a couple of questions would help me to understand the scope of the project better...

1. Are you staying with the 3.0 liter capacity?
2. What sort of induction system are you using? Carb? CIS? To be honest I doubt that you'll get to 300 HP in a 3.0 liter engine using CIS no matter what you do to the heads.
3. What sort of cams are you going to be using?

As I mentioned on the earlier threads, I think that Porsche actually did pretty well with their head design. As long as your peak torque intake velocity doesn't go above 75 meters/second, your porting is most likely fine. The happy range seems to be from a low of 60 meters/second up to 75. Low end of the range is defined by engines such as Porsche's early 2.0S, the 906 and a few other early models that generally suffered from "peaky" engines. As time went on Porsche tended to push the intake velocities up which most likely helped the overall driveability of the motors.

Here's a chart of some head flow data that I've collected over the years.

http://forums.pelicanparts.com/uploa...1234738934.jpg

A few things to consider...
1) Valve Size
2) Port Size
3) Port shape

Valve size seems to have a much bigger impact at lower lifts. So putting bigger valves in an engine without touching the ports can make the engine act like it has a bigger cam, except it will give away some peak RPM HP compared to an engine with a bigger cam and bigger ports.

Port size has two impacts..
1) It determine the peak RPM HP by limiting the ultimate amount of air that the engine can take in.
2) If the ports are too large (in the quest for big HP numbers) you can wind up with a big loss at the low to mid-RPM performance.

Port Shape is far more subtle. Keep in mind that compared to a Detroit V8, 911 ports are almost perfect. They are not compromised by uneven intake lengths (Until the advent of CIS and later injection systems) nor the intrusion of push-rods or water jackets. So the ports that you see in a 911 are pretty much how Porsche's engineers wanted them. In general Porsche's intake tracks tend to be conical, so they have the largest diameter at the tip of the intake trumpet, and then they tend to get smaller until just before the curve in the port before the valve. The reason for this appears to be that it improves the harmonic performance of intake track, which helps to minimize reversion and flat-spots. It also insures the maximum turbulence by having the highest port velocities as the mixture goes past the valve.

To sum it up, I think that the motto of the medical profession bears consideration... "Do no harm". If you can avoid messing things by making changes, you've accomplished 90% of the battle.

Me to!

To answer your question "did your labor of love yield any positive results?"

I'm still building the engine, so nothing to report yet and the heads were not flow tested prier, so really there is no base line to go by.

The work done on the heads can be classified as a Street Port. Cleaning and removing casting flaws, cleaning up the areas around the valve guide boss for a smoother transition. There was no hogging out or enlarging anything. The majority of the work was all done by hand, with lots of sore fingers. I used a dremel where I could (with the guides out, which helped allot) but still found the best results were by hand sanding.
The intake ports will be left as a sanded finish and the exhaust ports are going to be flow coated with a slick Thermal Barrier Coating (TLHB) from Tech Line Coatings, Inc.

And Yes of course, in True Spirit of the Hard Core DIY I'm doing all the Coatings myself! :)

I Have all the equipment, and nobody has a greater vested interest in giving A 110% to do it right, so why not!

Your Flow bench is coming along nicely and thanks for the offer to test a AFM

Hi John,

Thanks for all the great information, my head wants to explode from all the information from the last months research, on this forum, Speed-talk, books (like Dalton, Vizard etc) basically everything i can get my hands on.

I will have to start formulating objectives for various parameters like inlet tract lengths, port CSA, port to valve ratios, flow and flow ratios, velocities, anti-reversion etc, based on the information that i have and put the theories and objectives out there for scrutiny, and i hope that both, here and at Speed-talk, the knowledgeable folks will confirm, reject and add what is missing before i even start doing anything.

To answer the first few questions

1) Yes, i need to keep it below 3.0l for Historics racing
2) Induction: 46 PMO (40 or 42mm) venturis on their standard lenght manifold
3) After consultation with John D, DC62 Cams with 106 lobe centers
4) Equal length tube headers 1 3/4 ( i know its a little big but that's what i have) with Burns SS merge collectors on reverse cone mega-pones or muffled 6-2-1 config for sound restricted tracks and street.

Where did you get the peak tq velocity intake numbers, are they measured dynamic on a running engine, if so how would that relate back to a static test at 28" H2O?


Cheers

Gert

OK. That sounds like the right approach without going through the details. Ask John D that the expected peak torque engine speed is for a 3 liter with your cams. You can also use the rule-of-thumb equations in my "How to pick a camshaft" thread to calculate this for yourself. You will also need to know the lift so that you can ensure that valves and ports will flow throughout the valve's entire lift. Since I don't have any SC head flow data, you'll still need to flow them. I believe that your 78 SC should have the bigger 39 mm intake ports. They should really big enough to get you to 290+ HP. I wouldn't make them much bigger then 40 or 41 mm at the largest.

An example, CStreit had a 290 HP, 3.0 911 race engine that he made which had 39 mm intake ports and generated it's peak torque at about 6000 RPM. This equated to an average intake velocity of 83.5 meters per second at 6000 RPM. Prior to some tuning he was developing his peak torque at 5600 RPM, which worked out to about 78 meter/second.

Looking back at my notes, I said something incorrect earlier. The smallest diameter in the factory air-cooled 911 intake is at the intake manifold/head mounting plane. So that is the area which is going to be gating the maximum airflow. From there they generally open up at about 1 mm for every .5 inches that you move in from that plane. This is to slow the mixture down a little prior to making the corner behind the valve. In order to calculate the peak torque intake velocity, you need to do the following:

1) Know your engines expected peak torque engine speed in RPM.
2) Know your engine's stroke
3) Know your engine's bore
4) Know the diameter of your intake port at it's smallest point.

Then use this equation: =(Cylinder Stroke * (peak torque engine speed))/30000 *(Cylinder Bore/Intake Port Dia (mm))^2

This actually is a calculated average intake velocity rather then an actual measurement. It's fine as a "rule of thumb" for modeling. In reality the actual peak intake velocity will higher since the mixture is only flowing when the intake valve is open, which is a much shorter period of time. I haven't spent the time to understand the relationship between flow data and this number. If we were talking about different engines with different intake port configuration this number might not be as meaningful, but since we're talking about comparing one air-cooled 911 engine to another, in which the ports are all pretty similar in shape (but not dimensions), it seems to work close enough.

If you really wanted to map the relationship between the flow bench and reality, you would need to look at the actual amount of time that the valve is open. I've already done this for a number of factory configuration 911 engines and noticed that in general the engines' torque peak generally coincided with the engine speed when the intake valve was open .00015 seconds. Using some calculus, you should be able to sum up the total flow available (from seat to seat) based on your cam and the engine's capacity. That will get you close, but won't take into account such dynamic factors as the crank-angle and piston speed during that time as well as the harmonic tuning aspects of the intake and (in the case of cams with overlap) exhaust. Hopefully you'll find that you can flow enough air in .00015 seconds to fill up your desired cylinder capacity.

You're welcome to figure all of that stuff out, but I suspect that you'll discover that you know less about the process then you need to in order to build a solid model. In that case you may conclude (as I did) that my empirical model gets you close enough to avoid fouling things up too badly.

" Looking back at my notes, I said something incorrect earlier. The smallest diameter in the factory air-cooled 911 intake is at the intake manifold/head mounting plane. So that is the area which is going to be gating the maximum airflow. From there they generally open up at about 1 mm for every .5 inches that you move in from that plane. This is to slow the mixture down a little prior to making the corner behind the valve."

(From SF Flowbench manual i think), this tapers down as you mentioned earlier but deeper into the port and the divergence happens closer to the valve seat creating a venturi passing the valve(tulip valve flow pattern?) The port approach to the valve in this case seems to have a higher angle maybe the reason for the later divergence. Ratios are interesting to note.


http://forums.pelicanparts.com/uploa...1234813875.jpg

good collection of data!.... in about 45days i hope to start down this road on my own heads

With a 930 head the smallest diameter in the ports is where it joins up with the injector bocks and intake manifold.
They are 32mm at that point, and some people open them up up from 36-38mm for more power during boosted rpms.

Gertvr; The intake port that you showed is significantly more of a downdraft port then a 911 has. The configuration shown is what the Super-Touring engine tuners do when they would update 1 2-liter sedan engine into a Super-Touring race engine. (Keith Duckworth did the same thing with some of the Ford engines he worked on in the 60's and 70's.) Basically they'd weld up the intake ports on a street head which generally make a 90 degree turn at the valve, and open the angle up so that they came straighter down valve. That would be about the only major change to a 911 port that I think would make a significant difference to the head's performance. But since a 911 combustion chamber is a hemi design, shrouding of the valve isn't really an issue and so I'm not even convinced that changing to a straighter port would make a huge difference.

Anyhow, the factory heads that I've looked at measured out like this...

2.4TK head: Distance from manifold face...
- 0.0" 30 mm diameter
- 0.5" 31 mm dia.
- 1.0" 32 mm dia.
- 1.5" 35 mm dia (this is almost at the leading edge of the valve)

2.2E head:
- 0.0" 32 mm
- 0.5" 33 mm
- 1.0" 34 mm
- 1.5" 35 mm

2.2S head:
- 0.0" 36 mm
- 0.5" 38 mm
- 1.0" 38 mm
- 1.5: 40 mm

BTW - Does your nickname suggest that you are a TVR owner???

Cool, you are way more adventurous than i am.

John, from what I have seen from the port designs of gurus like Xtreme cylinder heads and Dick Evelrude, they tend concentrate on two things:

Improving turbulence
Making the intake tract a true, tapered venturi

My understanding is that they build velocity by ensuring the entire port is tapered from the port opening to the valve. Every infinitely small increment behind the intake port is an aperture when you cross section a head. Performance is about port design which maximizes velocity as a function of air volume, not a static diameter reading. For example, my 2.3L heads measure at any given diameter on the intake port between 39mm and 40mm. The air flow was modeled to my specific displacement (2.3L, 85mm x 66mm), camshaft (DC44 on 102 lobe centers) and top engine speed (8000 RPM). I provide this example as a counterpoint to the notion of "small" ports: it ain't the ports as much as what is behind them (between the port and the valve).

This what you get when that kind of methodology is applied:

http://i25.photobucket.com/albums/c7...n/IMG_0502.jpg

Also notice the ridges from the CNC work - that adds additional turbulence to the mixture.

kenikh; I didn't mean to come across as saying that there is nothing to be gained. I'm sure that there is. My point is that if there is 5%-10% of a performance improvement to be gained in a SBV8 from porting the heads, there is most likely only about 1/3 of that available within a 911 head since it is already far more optimized then most other heads due to the fact that it is not compromised by push-rods, water jackets and uneven port lengths.

You bring up a good point which I missed earlier -- namely that there are really two reasons for the ports opening up as they get closer to the valve:
1) To slow down the charge prior to making the 90-degree turn behind the valve.
2) To maintain an even cross-sectional area in spite of the valve boss and guide intruding into the port.

I suspect that the second is more important then the first.

Your point is well taken - OCD guys like me tend to obsess on that last 5%. :D

John,

No I whish, I never had a TVR only saw them occasionally in Historic races, where I came from they are pretty scarce

Gertvr = Gert van Rooyen

I have taken some measurements of the 78 sc Heads and dont have a bore gauge so it might not be that accurate

0.0" 39.3mm
0.5" 39.8mm
1.0" 40.5mm
1.5" 41mm
2.0" 46mm

0.5" in from seat 47.5mm

I used your formula and came up with 81m/s or (276fps) @torque peak RPM but came up with a different number on the time available to fill the head. I came up with 0.01522s with an inlet duration of 274 @ 6Krpm which moved the decimal 2 places.

I think changing to a straighter port would minimize the SSR effect and the valve area utilization would increase for higher velocities.

Kenikh, I agree with making the whole intake tract a taper venturi as on the picture above and i am going to aim for that as far as possible and a lot of literature points to that.

What do you mean with turbulence and how is it good for flow.

My impression is that turbulence is generally bad i.e. signifies that the flow is unstable and you are giving up port area at high velocities? I can see it being potentially good for slow moving air @ low lift as it could dynamically reduce the port area and speed up the flow in the remaining area for higher velocity and better cyl filling(BTW this is one of the concepts that i want to verify once I have the flowbench rigged up)

Gert

From most of the research i did so far: a lot of effort goes to try and get the air to turn around the SSR or to do away with the SSR as you mentioned above. At high velocities it becomes a dead area and the air overshoots the turn and all tries to enter the cyl via the back of the port on the high flowing radius and this reduces the effective cyl entry area.

Some porting designs widen and take material away from the high side radius around the valve guide to try and lift the airflow higher while it makes the turn around the short side radius.
By lifting and widening the air flow at the short side radius the theory is more air can flow through that area.
I've also heard of "eyebrow cuts" or grooves cut 90 degrees to the airflow along the top side or roof of the port just before the 90 degree bend starts to try and create a low pressure area there and lift the air flow higher away from the short side radius so more total air can flow through there.

You want to straighten out the air flow as much as possible before it hits the back of the valve so it flows out all the way around it as close to equally as possible. Removing too much material from the short side radius will make it so that doesn't happen as much and you can end up with the air hitting the back of the valve at an angle which will cause most or even more of the air flow to exit through the side of the intake valve farthest from the short side radius causing weird turbulance right at the valve opening and possibly reduceing air flow.

Anotherwords porting by line of sight rather than experince and a flow bench on an intake port with a 90 degree bend in it can make it flow less because of possibly creating bad turbulance.

I've looked at 962 heads and intake ports when they were sitting on a workbench and the factory ones I saw didn't look much different than a stock early SC head.

Gert;
Keep in mind that peak air flow really only matters from the peak-torque engine speed up to the peak-HP engine speed. More specifically it matters at the peak HP engine speed. Those are the times when the engine is not getting as much mixture as it needs. How much time in a lap do you spend at the peak HP engine speed?

Turbulence on the other hand improves the combustion quality, and this matters across the rev range, and will also tend to push the peak torque value up since it allows you to harness more of the energy available in the fuel. But turbulence is not a bi-modal factor. It's not like you have it or you don't. There are different forms of turbulence which can be effective...

- Swirl
- tumble
- random turbulence
- Combinations of the above.

The important point is that you want to do two things:
1) keep the fuel suspended in the incoming air charge. If the fuel falls out due to a lack of energy (aka: movement) in the charge, it will puddle or collect on the port walls, which will be fuel that doesn't power your car forward.
2) Make sure that the fuel is well mixed within the cylinder. If the mixture in the cylinders consists of a "clump" of rich mixture surrounded by larger areas of lean mixture, the chances of getting a good burn will not be good. The result will be fuel pumped out the exhaust which does nothing but heat the exhaust and not help to power your car forward.

The "art" of combustion engineering is designing an engine which maximizes the combustion quality across the whole rev range. Some engines are just not that good at this -- for example the original Ford Cleveland V8 Trans-Am motor. Other engines (such as most modern 4-valve, pente-roof designs) are actually very good at doing this.

As far as the inlet valve duration at the peak torque engine speed, you helped me find an error in my earlier calculations. Thanks! SmileWavy Here's a restatement of what I found...

911T: .00889 seconds at peak torque, .00644 seconds at peak HP.
911E: .00919s / .00636s
911S: .00856s / .00644s
906: .00757s / .00585s
Webcam 120/104: .00763s / .00592s

I suspect that some of the variation has to do with the flow capability. If you came up with 0.01522s, something still doesn't add up. Here's the formula that I used...

Intake duration in seconds =1/(360*Engine speed in RPM)*Intake duration in crank degrees*60

Did I miss something? :confused:

John,

No problem this is the formula that i used

Inlet duration on sigle power stroke= ((1/(number power strokes/s))/360 )* duration

i.e. ((1/(6000/120))/360) * 274 = .0152s Does this make sense or did i get somethig wrong?

At peak HP RPM it would be 0.0117s

Gert