911T Head Flow Rates

[Rich911E]

John:
I think that the 917 engine used a DOHC design to achieve two purposes: a lighter valvetrain assembly to maintain RPM and durability at high RPM plus a better spark plug placement at the top of the combustion chamber. I have always felt that the 911 combustion chamber design was weird and that twin plugging it was just a band-aid for a bad design. Max Moritz had the wedge shaped dome pistons back several years ago and JE also offers their pistons with a wedge shaped dome. If you compare them (looking at them from the intake port) they would look something like this:



As you can see, the flame travel with the wedge piston is much better as the shrouding of the backside of the piston is much diminished. You get rid of this problem with twin plugs, but that seems like a strange way to achieve the result. Also, if you were careful in the shaping of the combustion chamber and piston dome you could ensure a quench area on the backside of the piston dome to enhance swirl and turbulence in the combustion chamber which would enhance power and efficiency.

Additionally, I talked to the Gary Dyer who used to be a factory racer for Chrysler back in the 60's. They had twin plugs on the Hemi engines as well, but what they found out was that the engine didn't make any more power with the twin plugs, but that the second spark plug hole was in a much better position in the combustion chamber. So they ended up plugging the original spark plug location and using a single plug in the second location. I think this would work for 911 engines as well, especially with a wedge-dome piston in the reverse orientation. This would be because you could place the spark plug closer to the exhaust valve which is an advantage. You could start the combustion earlier and generate higher cylinder pressures, plus you could put the quench zone right at the intake port to maximize its effect into the advancing flame front.

Scott: Making these roller tipped followers would be a piece of cake. They could be machined from aluminum, but I bet that Harland Sharp would also be able to make them in stainless. For an endurance engine, stainless would be preferable. They have found that the reciprocating mass of the rocker arm is a relatively small contributor to valvetrain losses in pushrod engines. Reducing the weight of the valve makes a much bigger difference in the tendency to float the valve at high RPM. The lightweight Manley stainless valves would work in this application. They also make titanium, but that might be a bit pricey and overkill. I would bet that the roller tip would reduce the side loading inherent in the 911 iron rockers and make them more durable and RPM-able as well. However, my original thought was the ability to make it more flexible because of being able to increase the area under the curve without the long durations which would improve all around driveability rather than maximum RPM power.

Keep this discussion going!

Rich

[jluetjen]

Valve rockers aren't necessarily evil -- I believe that some F1 and/or CART engines continue to use them, abiet under the cam (AKA: Finger followers) rather then over the cam because of the advantages of the rocker arm ratio. Given the rev range that we're talking about (<8500 RPM) it doesn't sound like they are the issue.

There was an interesting article in Excellence #86 (6/99) about a 314HP 3.2 with a 10,000 RPM redline. A couple of highlights:

* Hi-valve spring pressures: 100 lbs versus the 60 lbs recommended by most engine builders. This compares with the 200lbs spring pressures often run in SBC (Small block Chevy) motors.
* Lots of ceramic coatings on the valve springs, combustion chambers and other parts.
* Titanium retainers and valves.
* Carbon fiber fan
* Lightened crank by cutting down the cranks OD (?) and knife-edging the counterweights.
* Polished non-adjustable rocker arms with shimmed lash caps.
* Titanium rods and light weight carbon clutch
* 12.8:1 pistons
* Heads with D-shaped exhaust ports and twin plugs.
* >>> The cam had "high lift" (.500), but NOT LONG DURATION.
* Slide valve throttle bodies added 54 HP at high rev's by eliminating the butterflies.

The result: 314 HP at 7750 RPM, so the BMEP = 166 PSI. This drops to 264 HP at 9000 RPM (4394 ft/sec piston speed). The engine is making at least 300 HP from 6500 RPM to 8000 RPM. Peak Torque is 250 ft/lbs (BMEP = 195 PSI) at 6000 RPM and exceeds 210 ft/lbs from 5000 to 7750.

How does that stack up? Using the BMEP (which normalizes everything) at peak hp/torque
* Fiat Brava Rally: 199 / 205
* Porsche 917 4.5: 197 / 207
* Porsche 917 5.4: 192 / 216
* '71 McLaren CanAm Chevy: 185 / 200
* GT5 Mazda GLC per Dale Shaw: 185 / 189
* 911 RSR 2.8: 173 / 191
* 1985 Chevy NASCAR motor: 173 / 186
* Hi-Rev 3.2: 166 / 195
* BMW 323 Group A Spec: 162 / 183
* Porsche 911S 2.2: 159 / 165
* Carb'd Ferrari 308: 149 / 177
* Porsche 911SC: 144 / 155
* Carb'd 2.2E (32 mm ports): 138 / 159

Going back to the original question, I guess the question is what would happen if someone used a 2.2T (to eliminate the headache of retuning the MFI) with stock 32 mm intake ports and put something like a GE60, Early S, Webcam 104/102, GE80 or 906 cam into it?

[keitho64]

***********************************************Goi ng back to the original question, I guess the question is what would happen if someone used a 2.2T (to eliminate the headache of retuning the MFI) with stock 32 mm intake ports and put something like a GE60, Early S, Webcam 104/102, GE80 or 906 cam into it?
**************************************************


If I knew how to tell, I may have just this setup. I have a 2.2T motor which seems to run very strong. The cams had 03 stampped on the end. If these are S cams then I have just the motor. We are planning a trip to a chassis dyno in the future so I will be able to post some numbers.

I must say this thread is very informative, keep it going. In the meantime, if someone wants to provide me the cams I will be very happy to test them!

[jluetjen]

Can you confirm that you have the stock 32 mm ports in your heads?

[keitho64]

No, the intake ports were hogged out to ~36mm by the PO. The exhausts were pushed out to ~35mm. The casting number on the head confirms it is a 2.2T head but you can tell the ports have been modified.

[Rich911E]

One other point to remember here is that we have not investigated the effects of the connecting rods. Longer connecting rods generally improve mid to upper end horsepower, provide longer dwell time at TDC which leads to better combustion, higher cylinder pressures, etc. The longer rod also reduces friction because of a reduced angle between the thrust surface. These are ideal for light cars with high gears like a 911.

The ratio of rod length to stroke is indicative of side loading of the piston in the bore, affects power. It is published that rod/stroke ratio greater than 1.71 is the safe thrust limit (17 degrees). So, how do the 911 engines stack up?

The 2.0/2.2 liter motor has a pretty good rod/stroke ratio with the standard rod of 130mm from center to center (5.12") with a rod stroke ratio of 130/66 = 1.96. The rods in the 2.4/2.7 motor are shorter by half the increase in the stroke (4.4mm) or have an overall center to center length of 127.8 mm or 5.031." The rod/stroke ratio for the 2.4/2.7 = 127.8/70.4 = 1.81. This is a pretty sizeable difference and may explain why the short stroke engines are have better durability and high RPM. The 3.0 liter engine also uses the 5.031" rod as the 2.4 and 2.7 because it has the same stroke. However, the 3.2./3.3/3.6/3.8 liter motors use an even shorter rod of 5.00" (5.00 x 25.4 = 127 mm) to accomodate the 74.4mm stroke. (Obviously the pin deck height is slightly higher because the entire 2.0 mm rod length change necessitated by the 4.0mm stroke length change was not absorbed entirely in the rod. Was Porsche concerned?) Anyhow, the rod stroke ratios on these motors is much worse = 127/74.4 = 1.7 -- approaching the limit.

It is interesting to note that the 996 rod length has been lengthed substantially to 5.7" (145 mm). Obviously, some new thinking here has gone to a much longer rod.

Getting it up in the 2.1 rod/stroke ratio (a 6.25" rod in a 3.0" stroke small block is a killer combination) would be useful but would require a rod of 66 x 2.1 = 138 mm (5.46") which requires reducing in the piston deck height by 4mm. Is there enough room in the piston to permit moving the piston pin up 4mm in the casting? I think so, especially if we move the ring lands up a bit without getting them in the burn off range. This could produce a lighter piston as well.

Second, to RPM these motors we need to make the reciprocating assembly lighter. I checked out the specs on the standard 2.0 rods and they can vary from (no joke) 550-650 grams and the but the 2.4/2.7 rods from 700-770 grams. The later rods are wider by 0.050". That seemed heavy to me so I check out the Pauter rods as they sell a titanium rod that only weights 390 grams!!! WOW. Then I asked about the price $3500! Yikes. Their billet steel 2.0/2.2 rods weigh about 578 grams, so not much difference from the lighter version of the standard 2.0 rods but their 2.4/2.7 rods weigh in at 585 grams which is a significant difference from the 2.4/2.7 rods and are more reasonable in price.

So, the more I look at it is that the short stroke motores are the killer ones to have and the 2.2 and 2.5 combinations with the bigger valves than the early 2.0 heads really make the power. The have a good rod/stroke ratio, you can use the lighter 2.0 rods on the 2.2 crank, the dwell time is much improved to use higher RPM cams and increase the cylinder pressure, the side loading is improved for durability. I think I will start looking for some 2.2 cores to work on.

Rich

[snowman]

Sorry I do not know the intake volume to the 911 intake. I do know the intake volume of my 912 race engine. It is 110% of the displacement. My 912 produces about 185 to 200 HP at the crank at 7800 RPM. This was confirmed with an acutal measurement of HP at the rear wheels of 140 HP at 7800 RPM. I f you work it backward it compares to the estimate very favorably. Also dyno 2000 predicts 200 HP at the flywheel. It also predicts 95HP at at the flywheel with the stock engine, right on with the actual HP as advertized by the factory.

As to more aggressive cam profiles for a 911, I do not think you can do better than they have already done.

On the more advanced theory the engine is a pulsed system. If you look at the fourier transform of the input waveform I do not think you can do better than they have done. IF you undersand what I just said I will beleive your answer.

[Scott Clarke]

Can someone explain BMEP? I know that it stands for brake mean effective pressure, but I don't understand how it is derived and how it relates to horsepower. Forgive me if this question is too fundamental (or inappropriate for the thread).
-Scott

[jluetjen]

I was thinking about this some more this morning over breakfast and remembered some data I pulled together on another thread which I think will be meaningful to the question of "What will happen if I take a 2.2T head and used something wilder then an E cam?"

A key example: the 2.7 RS which appears to have been somewhat port constricted. The 2.7RS used the same cams as the 2.4S, but had gas speeds comparable to a 2.4TE. The result was that the car felt like an E (an average of the two?) and in fact the torque curve more closely resembled an E's then an S's. The major difference between an E and an RS's BMEP curve is were two: 1) Below 2500 RPM, the RS torque curve looked as weak as an S's. 2) The peak BMEP of the RS was comparable to an S's (about 170 psi) rather then an E's (about 160 psi)

So what does this suggest would happen if you combine a S cam with 2.2T or E heads (assuming that you have acceptable piston/valve clearences)? I've made a fairly confident estimate for the S cam below, and a slightly more SWAG'd estimate for the 906 cam. What do you think?





Note that the factory didn't even plot the 906's torque curve below 4500 RPM, so I can only assume that it will be less then the other options.

[ChrisBennet]

Originally posted by snowman
My 912 produces about 185 to 200 HP at the crank at 7800 RPM. This was confirmed with an acutal measurement of HP at the rear wheels of 140 HP at 7800 RPM. I f you work it backward it compares to the estimate very favorably. Also dyno 2000 predicts 200 HP at the flywheel. It also predicts 95HP at at the flywheel with the stock engine, right on with the actual HP as advertized by the factory.

Jack,
IFIK 911's with 915 transaxle's have about a 14-15% drivetrain loss. I'm not sure what it was for earlier transaxels. You may want to run your figures again. Great thread!
-Chris

[Rich911E]

Snowman: That was a bit of high-handed quip to make, but the Fourier transform is used to move a function from amplitude as a function of time to amplitude as a function of frequency. Looking at a function which describes wave amplitude in terms of wave frequency can reveal the strength within a given range of frequencies. My question is the function to which you are applying the Fourier transform. If you are applying this to a function descriptive of the pulsations in the intake tract (for example), this is descriptive of only part of the equation relating to the flow. There is a distinct transition between turbulent and laminar flow in the intake port of a four-cycle engine within the operating ranges of the camshafts we have been discussing. I am not sure that the wave function you describe adequately considers the multiple factors involved. I am not sure that there is a continuous function describes these parameters such that you can uniformly apply the transform to describe this across the RPM range in which these camshafts and ports are working. There is an underlying assumption of the Fourier transform is that this is a continuous function. If the events we are considering are not continuous, it is difficult to apply the Fourier transform in the first place.

So, if you are taking this approach for mathematical modeling, application of the relevant equations would surely demonstrate what is clearly apparent absent this numerical analysis, i.e. it ludicrous to believe that three or four camshaft profiles designed to provide the consuming public with acceptable performance at four different engine displacements (2.0/2.2/2.4/2.7) and multiple cylinder head, port and valve sizes and compression ratios ranging from 8.5:1 to 9.8:1 are the ULTIMATE in camshaft design. Additionally, that gases which are being ducted through the port have significantly different properties (e.g. density) when comparing a carbureted engine in which a colloidal air/fuel mixture is being transitioned through the entire intake tract compared to a pulsed mechanical fuel injection system where most of the duct deals only with dry air. If the engineers managed to find the optimal camshaft for all RPMs varying bore, stroke compression ratio, I should think that the world of camshaft design would have stopped about 40 years ago.

The pulsations in the intake tract are no doubt extremely important to the design of the intake port, but these also apply to the design of the intake runner from the mouth of the carburetor as well as the exhaust pulsations. Have you experimented with alternative runner lengths to maximize the ram effect? Have you tried alternative exhaust systems that maximize the scavenging effects of the pulses?

Scott: There is a nice discussion of BMEP here: http://www.factorypipe.com/Technical/Tech_Articles/BMEP/bmep.html

Jack: The numbers you post for BMEP are pretty poor for a high performance engine. BMEP up near 180 PSI for a really hot street engine should be there. I think we have some room for improvement here.

Rich

[jluetjen]

OK guys - you'll get a kick out of this, , as well as this and further more this!

Of course is it far easier for me to copy the links then it is to understand or even describe everything that is in those papers!

Enjoy!

[TimT]

Ouch, I get headaches recalling fourier transforms and laplace transforms. Diff Eqs fun when I was there but now, Ouch!

[snowman]

Rich,

The airflows, intake or exhaust would be analyzed at a single RPM. The process repeated for a large number of RPMs to see whats happening as the rpm changes. For an old high perf engine one would optimize for only a narrow range of rpms at max power, newer engines with variable everything are completely differen't animals but at a single rpm the same analysis would still hold. The input flow can be represented by a series of pulses at a constant frequency (RPM) You will see a sinx/x waveform with a series of diminishing oscillations. If you look at how these overlap you can visually see how the pulse before and after the one of interest affect it. For example if you want to tune the input you can look at the overlaps and adjust the length of the intake runners so that at a certain rpm you will get the full ram effect. You can also see what happens when you pick weaker harmonics. You can also look at harmonics that subtract from performance. Then you can get really sophisticated and work on methods to broaden the peak response over a wider range of rpms and even shape it in some manner that might be desireable. This gets REALLY sophisicated and I suspect is at the current state of the art.
Jack

[Rich911E]

Jack: I think the papers that you cited are particularly interesting, especially the first one which made measurements in a real condition rather than by modeling and suggesting that the variation is much greater than previously estimated. One thing I found very interesting was the figure 10 photograph in the first paper. The flow differences between the top of the port and the degree of interference of the cylinder wall really affect the flow here. This is a really interesting photograph to contemplate for minute. Would a D-shaped intake port be of value? It might make a fun project to model in plaster and see what happens.

Snowman: I do think we are getting a bit too esoteric with this discussion. As much fun as it would be to redesign this engine, there are a limited amount of modifications the average guy can get into. My only point here is that we should consider some different camshaft profiles more in keeping with what has been tried with other engines to maximize power and driveability and see if they produce similar effects in this engine.

Anyhow, this is great fun!!!

Rich

[snowman]

Rich,

The advantage of looking at an engine in the frequency domain, rather than the time domain is that it is possible to visualize how the various components interact with each other. This cannot be done in the time domain. For example in the time domain if we talk about tuning the length of the exhaust pipes you have to get into lengthy discussions of what is going on, step by step, and then you add the reversions and it becomes so fuzzy you really can't understand whats, what. In the frequency domain you get a simple picture that includes everything and its relationship to everything else, ie a graphical representation which is obtained by taking the fourier transform of the time domain.

Anyway it is very very helpful for understanding how a pulsed system (an internal combustion engine) works and is VERY practical. How?

You can obtain intake and exhaust lengths needed to tune the engine for optimum performance. You say why as there is an auto math book that gives a simple formula for finding the length. One reason is the simple formula only gives one answer and there are many many answers that would also work. Say the intake runner is already to long or the exact length is impossible to use for some practical reason. It turns out there are other lengths that will also work, although not quite as well. YOu can find the other lengths and even tell how much you are giving up.

You can shape an intake horn in such a way as to broaden the power peak, this is again practical on an older engine with removeable intake horns.

The shape of the intake pulse is determined by the cam grind. The shape is very important in how well the engine will tune and how much of an improvement the tuning can make. In fact the shape is almost EVERYTHING that is important. Unfortunately the optimum shape may be limited by mechanical limitations. A roller cam is a big plus because the shape can be made closer to ideal with it. Ideal is NOT a square shape. This is were the extreem math comes into play.

Anyway the point is that it is still useful for an old engine design. You can do almost everything to an old porsche engine that the nascar people do to their engines. An as much as I dislike cars that only turn left and go in circles, they do very very well and get spectacular results with those restricted carburator intakes.

[snowman]

Originally posted by ChrisBennet
Jack,
IFIK 911's with 915 transaxle's have about a 14-15% drivetrain loss. I'm not sure what it was for earlier transaxels. You may want to run your figures again. Great thread!
-Chris

The numbers I found are 20% to 25%. I would love the have some actuals here as I consistantly get differen't loss numbers from differen't people, all of who are supposidly in the know. What I haven't seen is any real data.

My race car still has a working generator and fan (with Schmidt power pully). I est about 10 to 15 HP are lost to them at 7800 RPM.

[ChrisBennet]

Originally posted by snowman
The numbers I found are 20% to 25%. I would love the have some actuals here as I consistantly get differen't loss numbers from differen't people, all of who are supposidly in the know. What I haven't seen is any real data.

My race car still has a working generator and fan (with Schmidt power pully). I est about 10 to 15 HP are lost to them at 7800 RPM.

Jack,
I looked in my archives and found that 18 to 30% loss for a brake type chassis dyno, not the inertia type (Dynojet). That may be the source of the confusion.
Bruce Anderson says "...11 percent for pre 87 911's.The aftermarket company's use 15 percent or more to artifically boost their claims."
Greg Fordahl: "14% loss with the 915 transaxle"
One would think it would be easy to get the loss figure empirically by just going to a Dynojet and comparing it to the factory flywheel horsepower. Unfortunately (!) the factory figures are the minimum horsepower so you really can't use them as basis for comparison to work out the drivetrain loss.

-Chris

[snowman]

Originally posted by Rich911E
I replotted the graph above and superimposed the data from the Brodix SBC heads below. Note that the CARB legal heads out of the box (unported) are roughly equivalent to the 3.3 Turbo ported and unported heads. I was amazed at the improvement in flow when these CARB legal heads were ported at the low lifts. I know that there are variations between flow benches making it hard to compare but a change from 170 to 255 CFM at .400 lift is not all due to flow bench variation.



Just some more stuff for consideration.

Rich

One little problem I have always had with Chevy anything is if it is any good it ain't Chevy anymore. Chevys are a joke, any Chevy that actually makes HP no longer has any Chevy parts in it. Its an aftermarket whatever, but not Chevy. Even the so called bowtie stuff iisn't made by Chevy. Or at least I have been told. The nice thing about a Porsche is that you actually get what you are paying for. A real performance engine/car. Right out of the box, any Porsche can beat any Real Chevy built.

[Rich911E]

Well, this thread has de-volved. No point in discussing this any further.

Rich