exhaust

[philippeF]

Hi all
I'm on the way to rebuild my 73 RS engine, i'm going to use it on the track. by this way what should be the best exhaust configuration ? dual output ? single to keep back pressure ? empty muffler ?
all advices and pics welcome
best regards
Philippe
France

[RoninLB]

MFI needs a more specific muffler than carbs for proper MFI operation.

[Zeke]

Originally posted by RoninLB
MFI needs a more specific muffler than carbs for proper MFI operation.

Ron, that seems like a blanket statement. I guess what he wants is what muffler to use. I think a little relief is in order like a dual out sport muffler.

I agree that if you go designing your own system using a Magnaflow or equivalent, that you might miss the sweet spot. A dual out sport can't be too far off the mark.

[RoninLB]

Originally posted by Zeke
Ron, that seems like a blanket statement. I guess what he wants is what muffler to use.
.

Oh.. thanks Milt.

MFI is exhaust back-pressure sensitive.. Use stock or a custom muffler designed for MFI

PS: "stock" meaning stock to the year and engine of the MFI

[Zeke]

You're saying that the MFI RS cars can't be improved? I would be of the opinion that there was a difference between what the factory installed for production RS's (erring on the conservative/quiet side) and what they would liked to have done. 1500 cars total, right?

Let's leave factory race cars out of the discussion as they surely had modified pumps, etc. and a race 'muffler' affording some backpressure.

In no way am I disagreeing that backpressure is good. I respect your knowledge.

[RoninLB]

Originally posted by Zeke
You're saying that the MFI RS cars can't be improved?

I said "back-pressure sensitive" because I don't know enough about MFI exhaust to make an educated answer.. Probably internal muffler design would have been a better answer. I never had a MFI so my info is a kinda "just in case in happens someday".

anyway a Monty will definately clash w/MFI causing big problems.. If wanting to improve on the original MFI muffler a respected and popular 911 guy called Pressler should still be doing the internal custom design to MFI HP.

[Zeke]

Let's take this to the ultimate detail because I've been reading here for awhile and have never come across this. Is there a difference between the stock factory installed banana muffler between say the '70-71 MFI S and E motors and the T carbed one? Or the '72-73 MFI motors S, E and T MFI and the ROW carbed T? I mean, do these things have different part #s?

[RoninLB]

Originally posted by Zeke
Let's take this to the ultimate detail because I've been reading here for awhile and have never come across this.

I unable to find Kurt Pressler of Pressler Parts in Calif for hard info on MFI aftermarket muffler replacements and stock muffler mods.
I'll keep searching for hard info.. no problem.

[RoninLB]

Dual inlet mufflers.

and I found out Monty makes a muffler specifically for the MFI

[philippeF]

Hi
thanks for the feedback, it's not really clear : shall i keep (or rebuilt to the same specs) the stock exhaust line , or improve something
what opinion about the "bursch" MFI racing products ?
any other experience ?
Philippe

[Two Oh]

Originally posted by Zeke
You're saying that the MFI RS cars can't be improved? I would be of the opinion that there was a difference between what the factory installed for production RS's (erring on the conservative/quiet side) and what they would liked to have done. 1500 cars total, right?

Let's leave factory race cars out of the discussion as they surely had modified pumps, etc. and a race 'muffler' affording some backpressure.

In no way am I disagreeing that backpressure is good. I respect your knowledge.

Zeke, Enlighten me and others, what is "backpressure?"

Regards, Mark

[Two Oh]

bump

[Randy Webb]

Phillipe -- 1. you can certainly keep the original muffler. 2. You might get more power by using a factory variant called the "sport muffler" - it is much noisier than the ones on the touring cars. 3a. You can do historical research to see if your car type ever used a different muffler. 3b. You can experiment on a dyno with different types to see if you can get more hp and torque with something else. that would get expensive fast.

2-0 -- Backpressure refers to the pressure buildup in the exhaust sytem. It is not a static quantity, but changes in a cyclic fashion with exhaust impulses as the cylinders fire. Backpressure can (and since the 1940s, is) used to "scavenge" (pull) the exhaust out of adjacent cylinders, which yields more power, lower emissions, and etc. This isdifficult to measure or to calculate and is a major reason why shade-tree engineers usually futz up exhaust system design. That may change in the future as every homeowner's garage computer begins to run CAE software....

[Zeke]

An open header system has little or no back pressure. Racing exhaust is designed to extract exhaust from the cylinder, thru the valve and into the head pipe. This is accomplished by having the correct tube size and length. At a certain RPM (the target of the design,say at max HP RPM or max troque RPM), the efficiency is at optimum.

However, this does nothing to control emmisions. In fact, it sucks out raw unburned gas from the incoming charge. Race cars backfire a lot because of this.

Back pressure is simply designed flow characteristics that control exhaust flow due to the fact that there is a pressure present in the exhaust system preventing scavenging of the exhaust. This would be typical of a street car. Mufflers cause backpressure.

Many engines, racing and otherwise, do better with some backpressure. It has to do with the cams and the induction design of the particular engine. This is a complex science and I may be giving you 10% of the pertinent information on the subject. I probably understand 50% of the science involved.

Apparently, the Porsche MFI engine is one of those engines designed to use the effects of the exhaust to optimize HP. All gas moving in a tube creates a pressure, either positive or negative. The cycling of these oposite pressures is where the magic is. The only time the pressure is equal to atmospheric is when ist isn't moving. That would be only for an instant as the pressure changes from pos to neg.

Backpressure, length of pipe and shape of a chamber was the big performance factor in my experience as a kart racer for many years. We used 2 cycle engines which are fully dependent on exhaust design and the correct scavenging, pulses in the system and pressures. An open pipe reduced the HP by about half.

[Two Oh]

Milt and Randy,


Now I understand what backpressure is.

Thanks very much.

Regards, Mark

[skinnerd]

I had one of them "Pressler" dual-in dual-out sport mufflers built 'specially for MFI. I ran it on my 2.7 RS MFI clone. It was a great muffler, and the only one I run on that engine. It is stock and dyno'd at 208 at the rear wheels, so I think Pressler's research is good on these mufflers. It is a little quieter/mellower than your standard twin pipe sport exhaust, but still sounds great. I am not sure where Mr. Pressler is these days, but I was able to look him up a few years back. Good luck.

[RoninLB]

Originally posted by skinnerd
I had one of them "Pressler" dual-in dual-out sport mufflers built 'specially for MFI.

thanks for bailing me out on that one..

[philippeF]

Ok that's more accurate
I' ive been working on race engine many years ago and agree with your opinions, btw i won't use dyno testing because of the cost and difficulty, my aim was to get informations because i can built (they can build for me) ........... a custom exhaust line
i shall get the right one !

i believe i 'll go for an empty dual in / single out and factory like headers

regards
Philippe
France