NZ951

30psi back pressure at 15psi is not high. When I was talking to JME, they said they try not to build turbo's with more than 2.5 times the boost level in back pressure.

The problem with products like this that LR sell is they dont put dyno results up and ust talk them up. Classic example is the thermal pipe and the maf trap and a graduated 3 - 3.5 - 4" exhaust over a 3" and over a full 4".

Porsche-O-Phile

I agree with NZ - I recall Danno or someone recently posting backpressure data and indicating that numbers in the ballpark of 50-60 psi are not uncommon.

RolexNJ

I'd like to add my two cents here with some information on back pressure in general, and as it pertains to my 3.0L car. But first, I also want to thank you RKD in OKC for posting some information on my car.

So, a simple 101 lesson on back pressure. If you are unaware, back pressure is a bad thing, even more so if it's excessive. If it's to high enough, the burnt exhaust gases cannot flow out, and can actually travel back up into the head and/or manifold. At the same time, the car will not be able to breath in new air. So it has a double negative affect. So how much back pressure is bad? Well, in a perfect world, you want to maintain a positive 1:1 ratio of boost to back pressure, especially in the higher RPMs. And it should be noted, that as the back pressure goes up it can be exponential too. Where did I get this information from? Well let's see; from two local turbo shops here in NJ, Lindsey Racing itself, and yes I called Kokeln too. I had several people at Kokeln tell me this ratio by the way. That is several resouses all saying the same thing. So, I think it's fair to say the ratio is correct.

Now, in my case, we were boosting 15 PSI and had over 30 lbs of back pressure, actually probably more!! Given this problem, Lindsey felt "one" approach was to utilize a 4 inch exhaust. We all know that back pressure can be addressed by other means too; such as cam profile, different housing types; but let's stick to the thread which is a 4 inch system. Anyway, by doing this, we were able to drop 5 lbs of back pressure and saw a noticable difference in it's performance too. To "quantify" this, we did a dyno pull before and after the installation of the 4 inch, and took a look at both datalogs. We looked at the "air in" to "air out" ratio. And although the variance was still a "negative" number, you could see the numbers were lower with the 4 inch system on it. So for my 3.0L, which is flowing a lot of air, it made a difference. We are now doing other things to fully address all the back pressure.

Also note: Porsche had 4 inch systems on the Cup Cars. See the December 2004 issue of Excellence that has a write up on a Cup Car. Does anyone really think that Porsche would go and put on these cars a 4 inch system with ZERO performance gains and just to make them sound louder? I highly doubt it. Now, for us as regular people, is the benefit to cost ratio worth it? Depends on who you are. If you are a regular Joe who just drives your car for fun, probably not. Just upgrade to a 3 inch system, and I'm sure you'll be happy. But if you are building an all out race car, and want to squeeze every HP you can out of the car, then a 4 inch makes sense.

Peace

PS. For those who havent bought and read it, read Corky Bell's book called "Maximum Boost". You will learn a lot.

evil 944t

Rolex,
glad you posted. I just started the tread to get people with data to reply. I think there is a craze for a 4" exhaust that is not needed. I just want people to realize and not waste thier money.

mark944turbo

Ideal is not a 1::1 ratio. You can sometimes achieve more boost pressure than backpressure, this is when the real torque happens in some import cars. I have never heard of it in a 951. I cant tell if that is what you mean by a positive ratio.

I have a backpressure guage, and I see up to 60 psi. Laust Pederson did too with his original water injection system and a K-26.

There is a formula that determines where excessive backpressure becomes a problem, it has to do with cam duration, displacement, ve, all kinds of variables. I would say that 2.5 number is just an approximate that sometimes works, I believe Corkey Bell likes to keep it even lower. I will be figuring out what the number on a 951 is soon, I just have not gotten to it.

A 4 inch system that comes AFTER the turbo is going to have little effect on backpressure BEFORE the turbo. I would question the methods you are using to measure backpressure if you think a 4" system made a difference.

Rob, I think you worship Porsche engineers and Lindsey, Kokely, etc too much. They are people just like us, and they make mistakes. I am not saying that they are wrong (cant really do that when they all agree on something), but just a word of warning about your attitude to defend yourself.

RolexNJ

Dr. evil 944t, if I may

I just wanted to shed some light on this subject. And hopefully I did? Especially since I have a 4 inch system on my car now.

evil 944t

bueno

Tomas L

If you just look at cross section area and compare that to the fact that Porsche thought 2,5" was sufficient for the 951 at 220-250 hp then a 3" would be appropriate for 350 hp and 4" for 500 hp.
Also keep in mind that what we normaly do when we want to increase power is that we oversize part in the engine to larger dimensions than the manufacturer thought was sufficient.

One thing to note in exhaust systems is that the gases are much hotter, and take up greater volume, directly after the turbo then when they leave the last muffler. It is therefore more important that the downpipe is as large as possible than it is with the parts downstream. And yeah, the downpipe is also the most difficult part to manufacture in larger dimensions.

Tomas

RolexNJ

mark944turbo:

I hear what you are saying and respect your views. I was just sharing mine and the inforamtion I have. And sorry, I do believe in those several resources I have.

RolexNJ

mark944turbo:

PS I dont have an attitude. I was sheding light. I resent you saying I have an "attitude". Ah... yeah right

mark944turbo

I dont mean you have a bad attitude.....attitude is different than bad attitude where i come from. Im done here..

fast951

WHAT??? 2:1 backpressure is NOT HIGH??? Where on earth did you guys get this information...

Ideal backpressure (between turbo & head) is 1.2:1 (1.2 exh. : 1 intake).. The limit should be 2:1.. Yes once you exceed 2:1 (2 exh : 1 int) you will be inducing knock and all kind of problems...

Part of the backpressure problem is the exhaust behind the turbo, the main problem is the turbo hot side..

edit: SpecialTool dynoed 405rwhp with a stock/bent/partially clogged exhaust. His backpressure was unacceptable even at 10psi boost. He swapped to a 3" exhaust and freed up the exhaust.. Now at 24psi he has almost ideal backpressure, partially due to the exhaust but the main thing is the turbo...

Ski

Here's one for ideas: A tuner(used to be in AZ, well respected, now if private service) suggested and install'd on track cars(higher sustained rpm) wastegate dump pipes only to the atmosphere. A couple of engineers who he built cars from put some sensors on the exhaust system, ran the car loaded on a Mustang dyno. They found when the wastegate dumped into the exhaust above 5000 rpm, the waves of the turbo exhaust were slowed down because of the wastegate dumping into the exhaust system - make the wastegate dump to the atmosphere, the waves of the exhaust traveled faster and with less backpressure.

John, there may be a new 951 in the Atlanta area next week.

RKD in OKC

Part of the problem on Rob's car is that the hot side is already a big as will fit in the car. That would have been the first place to "make things bigger" for more performance and reduce the backpressure. Actually the first thing they did was work with a trubo manufacturer with all kinds of impellers and housing to try to get the most out of the space available. Once the turbo was optimized they started looking into other solutions to remedy the backpressure build up. And yes they were seeing carbon from backflow in the intake manifold. The graduated 4 inch exhaust helped over the 3 inch exhaust that was already on the car. Further testing on other cars in the shop set up for 340 rwhp or more with the 3 inch exhaust were making similar backpressure under full boost (15-18psi) as Rob's 3 liter car. The graduated 4 inch exhaust opened them up enough that you could feel the difference in acceleration.

PS. It's okay to disagree as that is what racing is all about. Otherwise there would never be anything new. A former Ford Cobra racing mechanic once told me, "If you do what everone else does you will mere be competitive. If you do something different and it works you will win."

RolexNJ

Fast951:

Very nice post. Thank you very much!