100hp/L - How to questions?
 

I am currently doing a top-end refresh on my 3.0 thanks to broken head studs.

I was playing around with Desktop Dyno the other night trying to estimate my newfound output thanks to my "while-your-in-there" syndrome. While the numbers are relative they are a healthy bump from stock internals. Then I started thinking...

What does it take to make a NA 911 motor hit a target output of 100HP/L? I know there are many builder secrets to achieving this output goal. I would love to have details. I know the whole package must work together. I think 300NA HP out of my 3.0 would be great.

A few questions for the discussion:

How high rpm is required? Can you build the motor to limit at 7500 or do we need 8500? What is it about high rpm besides charge momentum that increases VE to the point of making 100 HP/L?

Is Intake velocity or air flow volume more important? Intake velocity helps VE due to charge density. You must be able to support the volume of air based upon displacement

Lift or duration more important on cam design? Lift only gets you to the point of matching the intake volume flow. Duration must take over after lift is limiting factor.

How much overlap is really required for savaging effects that help charge density?

Tuned intakes? We know equal length runners are important. ITB setups work better than common plenum. Steve W. has mentioned ITBs under a common plenum (3.6) intake manifold.

Tuned exhaust? (How much do headers help compared to SSIs?) What about individual tracks compared with cross-over designs. I think it is understood that a cross-over tube helps with low speed torque curves by providing balance to the exhaust pulse. What about high rpm.

I am hoping to open a discussion that we can all learn from.

The magic number is usually 8000 RPM with cams that can breathe big enough. It usually requires a lot of overlap to get that much air, but on a 3 liter, the displacement tends to overcome the loss of low end torque from big overlap. Look at 906 cams as a baseline, although there are more modern cams with more lift/less overlap that can do the same thing. I think a properly built 3 liter on possbily GE80s or GE100s built to rev to 8K would do it. I'd like one of Henry's 9 bolt 66mm cranks to build a short stroke motor on a 3 liter case. His short stroke 2.8s make quite a bit over 100HP/L on RSR sprint cams.

I can say with 100% confidence that this is not the case. I have driven a 2.8 short stroke. Smooth and powerful, all the way through the powerband.

We recently built a 3.0 Twin plug, 10.5: 1, DC80 w/104 lobe center MFI engine for a european rally car that achieved a reported 310 DIN @ 7260 rpm. This dyno number was achieved by a tuner that had no vested interest in the build. Stated another way, he had no reason to fabricate this number.
The engine produced a flat torque curve from 3800 to 6400 268ft/lb @5800 . We ran a 1 5/8 conventional header and 36 mm Venti port. That's correct a 36 mm intake port. The valves were 50mm in and 42.5 mm ex.
38 mm exhaustt.

http://forums.pelicanparts.com/uploa...1218590580.jpg
http://forums.pelicanparts.com/uploa...1218590601.jpg
http://forums.pelicanparts.com/uploa...1218590617.jpg

Nice dizzy ;-)

A question for Henry... can a 3.0L be made to perform, as you state like this example, with (what would be the set up be ?? ) the least expensive induction & ignition set up and not have to use the expensive $$ MFI set up ?? Can this be done with carbs or a modified cis or ?? what is the least expensive induction & ignition set up to allow this hp/torque development ?? What are your thoughts on the Bitz Racing ignition set up just reviewed in the recent issue of Excellence magazine ??
Thanks, Bob

Jamie, back to your questions about specific output, here is some historical data from Bruce Anderson's book about the engine types over the years. This is a good starting point for the discussion about power-to-weight-- the next step is to look at the cams, intake ports, valve sizes and other modifications necessary to achieve these numbers.

http://forums.pelicanparts.com/uploa...1218636882.jpg

Henry, Thank you for posting the specs on the engine. Again, nice work as usual. John Thank you for posting the list. Let's see what we ideas we can pull from the information presented so far. Let's start with the port size and resulting velocity.

Henry posted the specs of an engine that achieves 103 hp/l. What peaks my interest is the intake dimensions. He describes smaller SC ports (36mm) with larger valves compared to stock. My calculations come up with an intake velocity of 119.3 m/s at 7300 rpm. This is a function of displacement pumping air through the intake port. I am not sure how a "Venti port" changes things or what machine work has been done to the heads. I recall a previous description of proprietary head work to achieve this number. I respect that but would really love to know how the valve pocket changes. It makes me wonder if Henry has been reading the MototuneUSA site. :)

John Luetjen posted years ago that factory 911 engines achieve optimum output with an intake gas speed of approximately 100 m/s at the peak HP. Henry's engine exceeds this value but a good margin (20%) and yet still makes good power.

So far, the key is to have a cam that has peak power at a high rpm and yet keeps the intake velocity sufficiently high. The question is: what is the rpm limit? Kenik thinks this is 8k. Henry showed it can be done 740 rpm less.

Now if we look at the cam selection we see that the DC80 cam has a lobe center of 104. Usually I see a lobe center of 98 or 100 deg for this cam. Increasing the lobe center has the effect of reducing overlap. This results in increased low-speed torque but less high-speed scavaging. I do not think this is unusually wide but an effort to lower the expected peak HP. I think this is the case since the DC80 cam would have an expected peak HP of around 7900 rpm based upon exhaust duration. (I used John's formula for calculation). This raises the question if the peak HP is lowered due to the high intake velocity and the inability to support air flow at 7900 rpm.

I think I have port diameters for some of the listed engines. Time to look at port velocity at the rated peak HP rpm values.

Moving the lobe centers from 100 to 104 will widen the usable powerband, but lose a little peak torque. Between the two, I would expect more top end from the 104 lobe center cam vs. the 100. But a slight loss in peak torque. Also, I would imagine the power curve on Henry's engine didn't fall off like a rock after the peak.
On Henry's engine I am amazed it comes on the cam at only 3800 rpm. The heads must have very good velocity numbers.


Here is a straight forward build that makes 95? hp/litre.. at the crankshaft anyways.

http://forums.pelicanparts.com/porsche-911-technical-forum/246563-another-rsr-clone-project-dyno-day.html

A little more compression, and more camshaft and maybe some head work and He would be real close to 100/litre.

This is either Sherwood's or Thom's but I think it's originally Bruce's (actually, it's originally the Factory's :))

http://forums.pelicanparts.com/uploa...1218667469.jpg

i was actually epxecting closer to 100hp/L out of my engine... but i guess cam choice (not done by myself) really limited that...

the agruement my engine builder held was engine life... how long do our f-6's live @ 8000+rpm?
the other point was stress on the case, as my unit is holding onto 100mm (3.5L from 3.3L, 100mm bore with 74mm stroke) barrels, is there potential for case sagging even with good studs etc...

given that this unit is using 46mm webers and nice big headers is it simply a nice cam away from higher hp?

edit: using a mod-s cam at the moment, made 278 at the rear wheels. I am willing to loose some mid-range for more up top gustor, especially for a few more revs... jut got a civic Si with the k20a and reving to 8200 rpm is run in itself!

Facey,

You are Cam limited and Carb limited to get 100HP/L on that big motor.

Bob

Jamie,

I would expect to see 100 hp/liter starting around 7300-7500 rpm provided that you are able to achieve 100% volumetric efficiency at the given rpm. 110 hp/liter @ 7800-8000 rpm.

Some of the controlling factors are;

camshaft selection
port size
rod ratio

Henry's engine has a minimum port size of 36mm. The length of this area is relatively short. My guess is that there is enough velocity to help lower rpm cylinder filling yet does not impede maximum flow. the Dyno result was very surprising considering we were shooting for 285 Hp.

how do u calculate rod ratio?

rod length/stroke

130/66=1.969 good high rpm stability/velocity.
127.8/70.4=1.815 good solid street ratio. higher piston velocity sooner in the rpm range
127/74.4=1.706 bottom end of good street ratio. Even higher velocities. larger ports recommended.
127/76.4=1.662 WTF....requires big ports and valves. high side loads. Stuff you see on truck motors.

i am assuming u had to have measured the rod length before u assembled the engine...or is it evident in the stroke? as in all 70.4mm strokes have a 127.8mm rod length?

of course it could be changed, but is it typically?

The numbers I gave are what Porsche used on all of their motors up untill the GT3 cup.
So a 2.7 and a 3.0 have the same rod length.
It depends on budget and application. Engine builders may not choose to do it to keep component prices and turnaround at a minimum.
Engineers at Porsche increased the rod length for their GT3 cup engine to 130mm in an attempt to reduce sideloads of their engines at 9300 rpm.

130/76.4=1.701

Their sprint rods are even longer but I do not know that dimension.

To achieve this, Porsche had to do away with the second compression ring. They were truely at the limit of their current engine design for the stroke they were useing.

I am interested in what they have done on the future engine design.

so in my engine using 100mm Mahle P&C's w/ pauter rods it is likely that i have the 127mm rod length? given that i believe non-stock rods were an option i'd guess that is the case, given that i am using a 74.4mm stroke.... what would be your guess at the rev limit of that setup? I mean for safe street use, but it does see the odd (maybe not so odd) rev up!


this is neat to finally know why the 66mm stroke is so desireable...i've heard about the importance of rod/stroke ratio but never had it explained... next build = 66mm stroke with 102mm bore...something like 3.24L? should be able to rev way way up! (with the right cam , v. spring, retainers..)...

Thanks to work travel and other things in life that get in the way I have not had a chance to repost on the subject. I did have some time to think about what others have said.

Let's start with a collection of data. The following is compiled from a listing of cars and their outputs by John Luetjen. The list has many makes and models but I have limited it to the 911s. I have also added the information for Henry's 3.0. There is more to the data and the spreadsheet calculates many engine parameters that might be of interest but fall outside this discussion.

http://forums.pelicanparts.com/uploa...1219425749.jpg

Aaron mentioned that the specific output would be rpm dependent and as such a range of 7300- 7500 rpm would generate the majic 100 hp/l. So I calculated the specific output and graphed it vs the rpm of peak power. I fit a linear curve for reference. Don't ask about fit statistics... They are not great in such scattered data. It is interesting that the 100 hp/l output falls right at 7500 rpm. This would lead us to believe that a cam that provides peak HP at about 7500 rpm would be a good choice.


http://forums.pelicanparts.com/uploa...1219425760.jpg

As I looked through the data I found something interesting. There were a few engines that had relatively high outputs compared to the fit line. When I find the individual points I notice that the intake port sizes are slightly smaller than what the factory did. This is the 2.0 and 2.2 E Production specifications using a GE60 cam. Both of these engines produce peak HP right at 100 hp/l and 7000 rpm. Of course, they have a more favorable rod ratio but I think I might have found the ideal combination.

The smaller intake port size will increase the intake port velocity at peak power. So I scaled the rpm value by a ratio of the intake port to intake valve size and came up with a new dependent parameter.

http://forums.pelicanparts.com/uploa...1219425771.jpg

What started this discussion is my building of a new 3.0. It will have a Mod_s-108 cam, EFI, 9.5:1 JEs and 78 SC heads. Using desktop dyno I get a Peak HP value of 260 at 6900 rpm. This corresponds to a specific output of 86.6 hp/l. The strange thing is that if I use the ratio of port size to valve and then scale my engine fits right on the curve with a calculated VE of 87 resulting in 261 HP. I guess large data sets create the general formulas for baseline values.

If I was to reduce the port size I might expect more power according to my graph. For example, using a 36mm intake port would give me almost exactly 100 hp/L. If only I had the funds to build another set of heads and a second engine to find out... If only I knew the build secrets that keep me on the upper size of the curve.

YMMV.

Get yourself engine analyzer pro ($$) or dynomation ($$$) and program in a stock engine or something you know the exact figures for. I prefer Engine Analyzer Pro because you can get every last nitty gritty detail in there, which is also it's downfall as if you don't, it will skew the results (usually on the low side at least). I've designed a few engines in EAP and had the results be within +/- 1.5%. EAP is especially useful for choosing a cam or choosing a compression ratio based off of knock index & dynamic C.R.