ptuomov

I don't know of any evidence of that. Do you know of any evidence of that?

Even if there is evidence of that, one could provide basic manifold porting template with the cams.

Comp cams sells these cams for Chevy and they are intended for a set of different kinds of manifolds, basically for all manifolds with a single throttle body element shorter center runners. I think that with the same manifold and only casting variations, you should be very close.

GregBBRD

I can't ever recall pulling the spark plugs on a 928 engine and saying "Wow, those two cylinders are richer than the rest!" Has anyone else?

Actually I'm sure I've never seen a difference in "color" in a set of 928 spark plugs, at all.....even on my big horsepower stroker engines which use the stock manifold.....at its flow limits.

I've certainly seen both big and small block Chevy and Ford engines that have different color spark plugs when comparing the outer cylinders to the inner cylinders....and I know that small jet changes will fix this.....so I know that even slight differences in fuel mixture is certainly going to show up on the spark plugs.

Makes me wonder how big of a problem this really is, on a 928 engine....

BC

Its only a problem when you magnify it way past where even the larger displacement motors are operating. This variance that Todd saw was at over 700hp, and I believe around 24psi of boost above atmospheric. It became a safety issue for the motor, and he developed around it, and eventually ditched the intake. For 98% of everyone else, it doesn't really matter much, and would be too much hassle to change.

Todd does read spark plugs, but for detonation.

ptuomov

I certainly am not skilled or experienced enough to read anything subtle from the plugs. I would have to rely on a sensor output.

The answer is I have no idea how big or small this problem or non-problem is. That's why I started this thread, to figure it out. However, I do think that it's potentially a meaningful problem. I quickly ran two simulations with a cheap engine simulator calibrated to match the 1987 S4. One simulation is with 20cm runners and another with 30cm runners. The volumetric efficiency is as much as 5% higher in the mid range for the long runner cylinders than short runner and about 3% lower at high rpms. This proves nothing, other than this issue is worth thinking about if one is forced to use the stock manifold.

Simulation graph below:

Hilton

Just wanted to say thanks Richard for posting that pic. I've just put my 89 up in the air to remove its X-pipe and weld a bung into the merge..

It hadn't occurred to me to just weld it on the outside of the merge like that.. obvious, and much easier than trying to weld into the "crotch" of the merge !

GregBBRD

I'm sure it has to be an issue....I'm just not sure how big of an issue it is.

I've not "seen" measurable power output difference with WOT mixtures all the way from 11.5 to 13.0.

Understand that I'm still trying to solve/fight through the "gross inadequacies/urban myths" of the engines and drivetrains. Hell, there's people out there that think a Chevy rod offset is proper in a 928 engine, because that's the rod offset that Devek used.

I'm a long way from thinking about subtle things like this....

I did recently have a GTS engine that apparently had excessive combustion chamber temperatures, which resulted in a burned valve. That burned valve was in cylinder #8.

simos

The unequal lenght intake tube configuration will work on very large rpm range. This is possible as induction system tuned lenghts does also have harmonic lenghts.
Some Pipemax examples from different speeds. The Intake port lenght was actually 10,5cm = 105mm. 200mm + 105 = 305mm, 300mm + 105 = 405mm(rough values, but..)
The 305mm and 405mm are the total intake path lenghts shown also in Pipemax outputs. We can see, that S4 and up Intake hits the harmonic lenghts at various rpm speeds. Perhaps there are rpm areas, where pipe harmonics and plenum resonation effect(if any available) will all meet each other, resulting sudden VE increase and high propability of knocking.. Knocking at low rpm and high load area perhaps isn't result of pipe flow restrictions as they starts to play bigger role on high rpm area. Also, if you can find the AFR differences from spark plugs, the difference should always be there, no matter in which rpm area you are running the car...

4000rpm

- Induction System Tuned Lengths MM- ( Cylinder Head Port + Manifold Runner )
1st Harmonic= 1282,868 (usually this Length is never used)
2nd Harmonic= 728,114 (some Sprint Engines and Factory OEM's w/Injectors)
3rd Harmonic= 508,321 (ProStock or Comp SheetMetal Intake)
4th Harmonic= 400,091 (Single-plane Intakes , less Torque)
5th Harmonic= 324,619 (Torque is reduced, even though Tuned Length)
6th Harmonic= 273,101 (Torque is reduced, even though Tuned Length)
7th Harmonic= 235,701 (Torque is greatly reduced, even though Tuned Length)
8th Harmonic= 207,308 (Torque is greatly reduced, even though Tuned Length)
Note> 2nd and 3rd Harmonics typically create the most Peak Torque
4th Harmonic is used to package Induction System underneath Hood

5000rpm

- Induction System Tuned Lengths MM- ( Cylinder Head Port + Manifold Runner )
1st Harmonic= 1026,295 (usually this Length is never used)
2nd Harmonic= 582,492 (some Sprint Engines and Factory OEM's w/Injectors)
3rd Harmonic= 406,657 (ProStock or Comp SheetMetal Intake)
4th Harmonic= 320,073 (Single-plane Intakes , less Torque)
5th Harmonic= 259,695 (Torque is reduced, even though Tuned Length)
6th Harmonic= 218,481 (Torque is reduced, even though Tuned Length)
7th Harmonic= 188,561 (Torque is greatly reduced, even though Tuned Length)
8th Harmonic= 165,846 (Torque is greatly reduced, even though Tuned Length)
Note> 2nd and 3rd Harmonics typically create the most Peak Torque
4th Harmonic is used to package Induction System underneath Hood

6000rpm

- Induction System Tuned Lengths MM- ( Cylinder Head Port + Manifold Runner )
1st Harmonic= 855,246 (usually this Length is never used)
2nd Harmonic= 485,410 (some Sprint Engines and Factory OEM's w/Injectors)
3rd Harmonic= 338,881 (ProStock or Comp SheetMetal Intake)
4th Harmonic= 266,728 (Single-plane Intakes , less Torque)
5th Harmonic= 216,413 (Torque is reduced, even though Tuned Length)
6th Harmonic= 182,067 (Torque is reduced, even though Tuned Length)
7th Harmonic= 157,134 (Torque is greatly reduced, even though Tuned Length)
8th Harmonic= 138,205 (Torque is greatly reduced, even though Tuned Length)
Note> 2nd and 3rd Harmonics typically create the most Peak Torque
4th Harmonic is used to package Induction System underneath Hood


6500rpm

- Induction System Tuned Lengths MM- ( Cylinder Head Port + Manifold Runner )
1st Harmonic= 789,458 (usually this Length is never used)
2nd Harmonic= 448,070 (some Sprint Engines and Factory OEM's w/Injectors)
3rd Harmonic= 312,813 (ProStock or Comp SheetMetal Intake)
4th Harmonic= 246,210 (Single-plane Intakes , less Torque)
5th Harmonic= 199,766 (Torque is reduced, even though Tuned Length)
6th Harmonic= 168,062 (Torque is reduced, even though Tuned Length)
7th Harmonic= 145,047 (Torque is greatly reduced, even though Tuned Length)
8th Harmonic= 127,574 (Torque is greatly reduced, even though Tuned Length)
Note> 2nd and 3rd Harmonics typically create the most Peak Torque
4th Harmonic is used to package Induction System underneath Hood

7000rpm

- Induction System Tuned Lengths MM- ( Cylinder Head Port + Manifold Runner )
1st Harmonic= 733,068 (usually this Length is never used)
2nd Harmonic= 416,065 (some Sprint Engines and Factory OEM's w/Injectors)
3rd Harmonic= 290,469 (ProStock or Comp SheetMetal Intake)
4th Harmonic= 228,624 (Single-plane Intakes , less Torque)
5th Harmonic= 185,497 (Torque is reduced, even though Tuned Length)
6th Harmonic= 156,058 (Torque is reduced, even though Tuned Length)
7th Harmonic= 134,686 (Torque is greatly reduced, even though Tuned Length)
8th Harmonic= 118,462 (Torque is greatly reduced, even though Tuned Length)
Note> 2nd and 3rd Harmonics typically create the most Peak Torque
4th Harmonic is used to package Induction System underneath Hood


About Intake Plenum volume:

http://www.stealth316.com/misc/grape...ionsystems.pdf

There is not going to be a simple answer to the needed plenum volume for a given application or rpm range. The good thing about plenum volume is
that there is a pretty wide range that it can be and still be effective, so general rules work well. The following guidelines are for engine operating in the
5000-6000 rpm rage. V8's with one large plenum feeding all 8cylinders does not work all that well as far as the Helmholtz resonator goes, but if this is the case, plenum volume should be about 40-50% of total cylinder displacement. On a four cylinder engine 50-60% works well. For 3 cylinders (6 cylinder engine with two plenums), each plenum needs to be about 65-80% of the 3 cylinders it feeds. If a boost is desired in a higher rpm range, closer 7000-7500 rpm, the plenum will need to be 10-15% smaller. To get a boost in the 2500-3500 rpm range, it will need to need about 30% larger. The plenum size of a Helmholtz resonator may go against the typical plenum size rules, but the rules change when the resonator is being used. The whole Idea of a plenum is to allow the gases to slow down and gain density.

Also, The plenum volume is critical on N/A engines, and a basic rule of thumb is: The smaller the plenum, the lower the rpm range, and bigger means higher rpm. The throttle body size and flow rate also affect the plenum size: Bigger TB, smaller plenum, small TB, larger plenum.
If you choose larger TB, you can live with smaller plenum volume...

ptuomov

One reason why the AFR difference may not show up on the plugs is that all the cylinders are on average over the rpm range about equally fueled. That is, the plugs have their head in the freezer and feet in the fire and they are all just fine on average over the rpm range. But that doesn't change the fact that gains could probably be had by making the cylinder AFRs close to the optimum not only on average but at each rpm. I think that a quad pattern cam could be ground to bring them much closer to that optimum, which would allow for higher powered engines that still pass the tailpipe sniff test.

The usual plenum size logic for a single plenum v8 manifold is that a larger plenum works slightly better at higher rpms, but the difference is probably small. For a dual plenum cross-plane Helmholtz resonator intake, it's the opposite: For higher rpm tuning, you need smaller, not larger tuning.

Here's a simpler formula for the resonant rpm with a Helmholtz resonator manifold:

http://speedtalk.com/forum/viewtopic.php?f=15&t=35914

It's pretty damn close as the observed peak with two 30cm runners and two 40cm runners is at 3150 rpm.

30cm total runner length: 19100 * sqrt((116.5 * 15.9) / (1850 * 30)) = 3489 rpm
40cm total runner length: 19100 * sqrt((116.5 * 15.9) / (1850 * 40)) = 3022 rpm

One can compute some impacts from the plenum size changes using that formula as well. The simple formula gives a much larger sensitivity of the tuning point to the rpm, basically 10% increase in the plenum volume gives a -4.6% reduction in the tuned rpm point.

mark kibort

we are the only car in the world that doesnt have a intake that can bolt up and rid us of the the crap stock intake. (crap, meaning "for HP maximization".

I think someone should take the BMW M5 intake , and mount it to the intake manifold adapters that some have manufactured.

it would be WAY less restrictive, and bolt right on to the proper spaced runner system AND use the stock MAF.


mk

mark kibort

This is the solution:

https://rennlist.com/forums/8147443-post32.html

premade, all you need to do is get the intake manifold stacks to mate to it, and get a Y pipe for the intake so it uses a big throttle body and then the stock maf.. piece of cake!

This would make BIG power on the 928!

ptuomov

So what you're saying isL "Driving the hood closed is totally overrated."

victor25

I agree in thinking it would make a big difference in power / performance. But I don't think it's that easy to modify or install.

But I do think once Hans starts making his aluminum intakes, everything will change!! If the 928 motorsports units weren't so darn expensive I would have already made something with those to go on my Rotrex 928 supercharger kit. But they are, so I'm patiently waiting and nudging Hans

Nudge.... nudge.... bump... bump... You listening Hans???

GregBBRD

Trust me....the short aluminum intake stubs that bolt to the head are the easy part....it gets harder from there.

victor25

Greg... I agree completely!!!!
I would like to make some nice low end torque and then let the supercharger take over from there. getting rid of the drop in HP & torque where the flappy turns on and off would be a huge bonus also

GregBBRD

I think we are all trying to accomplish the same thing....just in different ways.

One thing nice about the supercharger solution....the intake manifold "problems" become somewhat moot, since you are "pushing" through the manifold.

A big column of air acts completely different once you quit sucking on it and start pushing....