Planet 9

i am showing my dyno charts. i have been doing some dyno comparisons with an ipd plenum and softronics snapflash. i have seen some very strange readings. the strange thing is that i have lost torque with the plenum and the software, but also i have lost torque with just the plenum instead of just the software. it seems that the software was giving more torque than just the plenum. it is very strange, because when i had the flash on, i was racing my friends boxster S (280hp) and we had 2 car lenths difference until 4th gear, but then when i installed the plenum we were going the same until 4th gear. another strange thing is when i removed the flash we were going the same again, but with just the flash he was in front of me. so, there should have been a larger difference in hp since we were exactly the same in our race, but as you can see in the charts the hp numbers are very close together. by the way, the comparisons took place the same day with same weather conditions, same guy in the dyno, and same dyno as you can see. what are your thoughts on that?

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The attachments won't open for me.

Do you have the SRP flash, or the Snapflash flash? They are VERY different.

If you are using the Snapflash, it's really not made to be used with any plenum. It's made to give you the most performance from the factory parts/plenum, so mixing it with a plenum would result in a loss.

If you are using the SRP flash, that is completely different and is made to work with the SRP plenum. There have been other tests that have shown that the IPD plenum results in a little less torque down bottom, a little more up top.

The factory plenum has a flap in it that opens and closes at different RPMs to tune the intake. This tuning results in more torque down bottom, but it's a bit of a hindrance up top. So no matter which plenum you put in, I would expect to see a small loss in the lower RPMs. That's just the way these things work. The SRP flash has made many changes to bring this loss back, and it works well with the SRP parts, but your results may vary with other parts.

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Gator Bite
PCA Cayman Register Advocate

OK, I got your attachments opened. Very intesting, but not at all unexpected.

• Plenum Alone is giving you the least power
• Tune alone is giving you the most mid range torque
• Plenum and Tune is giving you the best low and top end with slightly less mid range than the Tune alone

What we can't see are the fuel/air ratios.

Everyone thinks that the bigger they make their intake, the larger then make their throttle body and plenum, the more power they'll get. Well, guess what? That's not true. If it were, why not just put a 100mm throttle body on there and get lots more power?

As the throttle body and plenum get bigger, three things happen. (1) Intake air velocities go down, (2) intake air turbulance goes up and (3)resistance to flow at wide open throttle goes down. 1 & 2 are bad for low end torque/power. 3 is irrelivant to low end power, helpful to high RPM power.

There are always compromises. It's important to understand these things before diving in. If there were no compromises, then Porsche wouldn't have spent so much time and energy making the dual stage resonant intake manifold.

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Gator Bite
PCA Cayman Register Advocate

thank you gator bite for your intel. the more i was looking at the dynos the more iam convinced that the snapflash which i have, and not the srp, cannot work with the plenum. i think i will have to work a custom tuned to get the most from the plenum...

GB - (2) makes no sense to me. Can you elaborate, please? Also, I believe that (1) is irrelevant. Velocity, per se, has no significance either on the intake or the exhaust sides (see prior rants for details).

X cubic feet of air moving through a smaller pipe will have more velocity than that same air moving through a larger pipe. With velocity comes inertia, inertia that was considered when developing the dual stage resonant intake system.

Porsche does extensive calculations to develop the intake system on our cars. They started this back in the days of the 928/968 and continue to use these tuned resonant intake even on their latest GT-3 engine. They discovered that when an intake valve closes, the rushing air, which has inertia, compresses on the closed valve creating a high pressure area. This high pressure flows back toward any lower pressure area, causing that high pressure area at the valve to turn into a low pressure area. This can be detrimental to performance if a valve opens during a period where there is a low pressure at the valve. By reducing the air velocity in the intake pipe, the dynamics change.

Furthermore, the throttle body is a circular flap inside a round opening, sometimes referred to as a butterfly valve. The more open the throttle body is, the less angle there will be on the throttle plate, which will result in less turbulence and pressure differential after the throttle plate. A larger throttle body, which has a larger circumference on its throttle plate will flow more air with less of an opening and with more of an angle on the plate. At wide open throttle, the bigger throttle plate is insignificantly different from the smaller throttle plate. But at near closed or part throttle, the smaller throttle plate will disturb the air less than the larger throttle plate.

This is one of the reasons why high performance motorcycles will use 'Constant Velocity' carburetors. In a CV carburetor, there is a vacuum actuated piston after the throttle plate that narrows the opening after the valve. This keep the air velocity more 'constant' which is particularly important when you are drawing fuel from a metered jet that is pressurized by the atmosphere. But it's also a benefit because of what I mentioned above.

Fort, remember when we are talking about throttle bodies we are discussing gas flow, not fluid flow. Most of these principles are a product of the compressibility of gasses, something that doesn't apply when discussing fluids.

Here's an interesting read that takes it a little deeper: http://auto.howstuffworks.com/question517.htm

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Gator Bite
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I've been holding my tongue on this post for a week, but I can't stand it anymore... I'm not trying to bash you Gator, but I need to address some errors.

Porsche didn't discover this - the existence and importance of pressure waves in the intake system has been well-understood for maybe 70 years...

The velocity in the pipe has very little to do with the dynamics of the pressure waves! The velocity in the pipe is far less than the speed of sound, and therefore has very little effect on the timing of the pressure waves. This is why I keep saying, over and over, that velocity is relatively unimportant. It's all about pressure waves.

This argument only concerns part-throttle operation. But who cares? If you need more power just open the throttle a little more. Normally when we discuss engine power capability we restrict our discussion to wide open throttle (WOT).


Fluids are compressible, too. Otherwise there would be no acoustics in water. Compressibility affects the air flow in an intake system in a way that is not affected very much by the fact that air is a gas. The Mach numbers are so low that it is essentially an acoustics problem, just like it would be for fluid flow. Mike - I know you've got acoustics training. Think about the intake system as if it were a speaker, and you'll be on the right track.

The link is riddled with errors. For example, it states that the wave speed in air depends on the density, which is absolutely false. It depends on the temperature only. I could go on, but be cautious citing this kind of "Popular Science" stuff to me as a source!

I'm an engineer, and a while (ok, long time) did some work with the major car companies in fluid dynamics (my master's thesis). Specifically I did computer modeling of finite rate chemistry in cylinder combustion. Without complicating things too much I'll say that this is one of the most complex and empirical fields in all of engineering and science. There is still no accurate computer model of fluid flow at high Reynolds numbers. Solving the Navier-Stokes Equations is one of the most difficult and still unsolved problems in all of engineering and theoretical physics.

There aren't a lot of theoretical statements you can make about airflow other than very general and basic laws. For the most part, you have to setup a benchtop experiment and develop whatever gives you the best results. That's why F1 teams spend so much on testing.

Basically as Forte said, bigger is better, and you can pretty much go with your gut in terms of design (a Y should be better than a T, and smooth is better than rough, and higher pressure is good in the inlet, and lower in the outlet). The bottom line is you have to have some really good test data to figure out what works and what doesn't.

Fort,

I don't understand why you feel you need to hold your tongue. If you disagree, say so. What's the big deal...? I don't consider someone disagreeing with me as a personal bash.

I think you're splitting hairs. I don't mean to suggest that this was some discovery made by Porsche, I meant to convey that they 'learned' they could achieve better intake efficiency by using an adaptive resonant intake and have been using some variation of the design since.

EDIT: I just re-read my original statement, and I agree I could have written that paragraph differently to better to convey my point.

I'm not entirely sure I understand what you're trying to convey.

Once again, I think you may be misunderstanding my intention. I shared the link to help other spectator reading this thread understand the discussion, folks who aren't going to pay much attention to effect of density, temperature and barometric pressure on the speed of sound. I didn't share the link as some kind of definitive thesis that proves a physical law.

So I've got some questions for you Fort. If a bigger TB is all around better, then why doesn't Porsche just stop messing around and put a 100mm TB on our cars? The cost difference between a small and large TB can't be much at all. Porsche absolutely loves to share parts across cars. Why not just pick one TB that's big enough for the most powerful engine and use it on every model? Also, why do you suppose the larger TB and plenum made less torque in the lower RPMs than the smaller combo when you tested them?

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Gator Bite
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