Our group of boosted 928 owners has grown steadily more knowledgable over the last two years. I am pleased to see the conversations go from absolute HP gain to better comparative methods, like HP produced per Pound of boost.

But even that method of comparison - "HP per Pound" has its flaws....

Where you measure your boost.

DANGER: we are about to enter ANOTHER area that has no one-right-answer. I'll just add it to the list of conversations that have no one-right-answer right now and get it over with:

1) supercharger vs. turbocharger
2) centrifugal vs positive discplacement
3) air-to-air vs. air-to-water
4) where to measure boost

Having said that - I'd like to open this discussion.

Where are you measuring your boost?

I have read and seen 3 main categories on this:
1) right after the boost-producing unit
2) right before the boost enters the engine, and after any intercoolers
3) within the intake manifold plenum.

I use #2.

Your thoughts?

 

Exactly Carl, directly over the port. Well shucks we agree on something.

BTW how do you do this on the S4 manifold?

Andy K

 

Within the intake, specifically after the throttle body. That is the amount of boost that the engine actually utilizes isn't it?

 

Depends on where the throttle body is!

 

#2...on Andys manifold. appx 1 1/4 inch above the intake port for #7 cyl i think?...appx 3-4 inches above the intake valves.
I assume that in this postion It measures all the losses in the system to that point and is what the engine sees...and thus measures the boost level that is responsible for "X" hp. ...in my case 402.22...so far

 

Tony is correct

You want to measure it after anything that may effect boost such as a bend in an intake runner, an intercooler etc. While the stock S4 manifold may be great for NA, with forced induction, each 90degree bend should be looked at as causing a 1% flow loss. The Stock S4 intake has around 1000 degrees of turns.(look closely and add them up) 1000/90=11.1% flow loss from the MAF to the port (with forced induction). So boost measured at an SC or Turbo that blows throught the stock manifold would differ significantly at the intake port. So to get an accurate measure with the Stock S4 manifold you really need to remove it so that you can tap into it at the end of a runner just above a port. Be careful as the magnesium is probably pretty brittle and may crack if you use an NPT fitting.

It's the number of degrees of turns after the SC or turbo that matters. It would be hard to add a turbo or SC to a stock S4 manifold without adding an additional 315 degrees of turns or another 3.15% flow loss. So with the Stock S4 manifold and forced induction, ypou are looking at 1300degres of turns after the SC and 13% flow loss minimum. You'll never see a complicated manifold like the S4 manifold on a factory engineered forced induction car.

This is one of the reasons I choose to design a new manifold. My non-IC manifolds have only only ~135 degrees of turns after the SC and the ICed manifolds only have about 295 degrees fo turns after the SC.

The only way you could get a more accurate measure than how my guys do it is to drill a fitting into the heads just above the valves.


Andy K

 

i use #3 on the s4 stock manifold. closest place to the cylinders so it will give the most realistic boost number that the cylinder actually sees.
also my boosted track car has no flappy so less turns in the manifold.

 

In the intake tract a few inches from the intake valve.

 

The absolute, most important thing to consider with a manifold design on a batch fired fuel injection system is that it distributes the same amount of air to each cylinder at all times. All other design criteria come after that. Uneven exhaust pressures can contribute to unequal air distribution at the cylinders which will also contribute to lean conditions and ultimately harmful detonation. It doesn't matter as much how many turns the air has to take as much as the pressure at each intake port MUST be the same or you WILL have problems. The stock manifolds on all 928 model years do a great job of distributing even amounts of air to each cylinder, even in the high horsepower boosted applications. If required, we have a design on the table to individually tune each intake runner but so far we haven't had to do that.

For all you SuperShark guys, please don’t take anyone else’s advice where to monitor your manifold pressure, there is a reason I tell you to tap it into the line that feeds the Super FMU, please keep it there.

 

Wastegates...pull the boost signal right off the compressor housing. Measurement for the cockpit...right off the intake manifold. You have to figure on at least a 1-2 psig drop across the system, specifically with an intercooler that is working well. Over 4 psig loss across your intercooler...then it is too small and the additional pressure you have to put in will make the intercooler basically useless.

You guys want to talk about bends...check out my plumbing! Seriously though, you can restrict the discharge of a compressor down and make a lot of bends from the compressor outlet and back to the throttle body without killing performance. Consider, you can blow over 600 HP through just a 2" pipe. My throttle body feed pipe is 2.25" in diameter and this has no issues blowing out 400+ HP.

 

...see what I mean? its all over the map. With John measuring boost within the intake manifold, and Andy and I measuring at the entrance to the intake manifold (or more simply put - before the throttle body and after thte throttle body) ... if you are not careful and knowledgable in these discussions - John would be able to truthfully say " I produce 100 HP from 2 psi" compared to somebody else's "100 HP from 6 psi" etc, because he is measuring boost after the TB rather than B4 it.

John's right, Andy's right, Tim's right, Bill Ball and Jim Nowak are right - they are just different.

Just thought I'd bring this to your collective attention.

 

I think we all need a "Boostard Bonding Session" to clear the air. I've been driving the Goldmember all weekend, what a great fun toy! Now somebody PLEASE make me an offer on this car so I can start another project!