951Boost

So does anyone know what the turbo outlet diameter is for a k26/8? And would putting a 3" downpipe on it hurt or improve performance (asumming a 3" or 4" exhaust)?

Mike1982

RKD in OKC, WOW, 10 more db with the 4" compared to 3" at 10' from rear at 6,000 rpms!! That is QUITE a bit considering that was the only thing changed. I wouldn't mind getting my car tested some how to see where it stands, it not close to stock but just perfect for sound level for me! 3" fabspeed cat-bypass pipe with a cat back SFR system with most of the baffles removed, so it has a very raspy sound to it. Much meaner sound at idle all the way to red line!

sweanders

I believe you are stuck in N/A thinking..

RKD in OKC

Could be, but I did do a 400 rwhp pull without a 3 inch downpipe.

RKD in OKC

Most stereo shops should have a db meter, either for setting up THX for home theatre or car shops that complete in those bass thumping loudness contests. They might get a kick out of testing your exhaust loudness if you ask. I borrowed the one from work. The Safety Kings use it for deciding where to put up those ear protect required signs.

The test we did was on two separate cars. both had MAFS, mine had a Super61 the 3 inch exhaust car the K26/8 tuned to 268 rwhp. So there were some other differences in the exhaust samples.

I did try my 4 inch with a Flowmaster muffler and it was just way to loud for me, upwards of 126 db.

TurboTommy

With this exhaust system issue, you have to ask yourself what your goal is. The most important thing you should be trying to achieve (besides altering the sound) is to reduce the pressure between the the exhaust valve and the turbine, because only that will dictate your power potential.
" How will changes to the exhaust, as discussed above, actually affect this turbine inlet pressure?", is the question one should ask.
If you don't intend to measure this TIP, you're tapping in the dark and don't really know which size and combination exhaust to go with (unless, again, it's the sound issue).
The best thing for reducing TIP is the turbine housing itself and trying to reduce back pressure after the turbine has only minor effect.
If all you care about is fast spool up, then go with a small housing, I guess, and bigger exhaust will help, but your power will not be near what it should be.

That being said, I have a large housing and tried a couple different combinations of exhaust after my turbine. The result was there was no appreciable difference in TIP measurements (therefore, no power difference). I'm ending up with 2.5" from the turbo (outlet is 2.25 in", by the way) gradually tappering up to 3" straight through muffler, and no cat.

A couple of things that seem to support this line of thinking:
Dfastest has roughly 500 hp and uses 2.5" exhaust; an experienced turbo guy, whom I know, agrees with these exhaust sizes; Corky Bell's book shows 2.5" supporting 500+ hp (and that's with a cat!). He also mentions the pipe should just be a little bigger than the housing exit, which supports RKD's post.

Take all this for what its worth.
I think exhaust modifications after the turbo are way to over-emphasized.

951Boost

Gang, sorry for the re-post as I posted this on a similar ongoing thread but figured it might need more exposure. And I do agree with TurboTommy's thoughts (Edited for relevance to this thread)

"I've spent plenty of time looking at the LR and SF websites & they don't claim that kind of gains (40hp over the 3" system). Being that both systems (LR & SFR) are essentially 4" cat backs I find a 40hp gain extremely "optimistic". If one reads the info on the Lindsey site you'll see that the real gains were made in the torque department (75 lbs by the 4”!!!) and the big plus that I see is that all improvements were made lower in the rpm band. HOWEVER....they used different mufflers!! That right there ends the apples to apples comparo since they used the (much) freer flowing muffler with the 4” system.

I realize that a true 4“ exhaust, from the downpipe back will (specially in engines with larger than stock turbos, big valve heads, headers, larger displacements or sustained high rpm use etc.) outflow a 3” one (all other things being equal) that is just simple physics. However in the books “Maximum Boost” by Corky Bell & “Forced Induction” by Graham Bell the recommendations are as follows;

Corky Bell: page 131, graph for single pipe diameter shows that a 3” exhaust should be good for close to 700hp or sizing the pipe about 10% larger than the turbo outlet to maintain optimum velocity.

Graham Bell: page 270 for engine sizes of 2.5-3 ltrs he recommends a 3.5 single pipe or two 2.5 pipes for road engines developing 120 to 150hp/ltr and adding .5-1.0 pipe diameter for competition engines developing 200-250 hp/ltr.

Now if you crunch those numbers you’ll see that there is a vast difference in those recommendations!!! My personal belief is that pipe diameter is not as big of a factor (between 3 and 4” for our cars) as is muffler selection as it causes the most backpressure.

Problem is noise, some people like LOUD (ala open pipe/motorcycle/The fast and the furious sounding)! And that obviously will produce less backpressure and = more HP.

Some of us (yeah, probably the “older” crowd) prefer something mellower/deeper/quieter and are probably willing to give up a few HP for this “peace of mind”.

I think that what we need is for the top wrenches here that have relatively easy access to a dyno (specially those that can drop their exhaust in like 3 minutes ) to run some db and power test on different mufflers even if that means that the rest of us that are interested in these results would kick some $$$ their way to offset the cost (or maybe buy the mufflers from them at a markup that would allow them to make a profit).

Perhaps the test should be something like this:

1)Open pipes. DB/HP/TQ measurements
2)Most free flowing muffler. DB/HP/TQ measurements
3)Good compromise muffler. DB/HP/TQ measurements
4)Closest to factory/quietest muffler. DB/HP/TQ measurements

Comments (substantiated, none of that “my uncle's sister's brother said” or “trust me ‘cause I said so”) ideas, positive input is welcomed and solicited. And yes, something like “I tried muffler XXX and then I switched to muffler YYY and it was louder/quieter (as long as you include PART NUMBERS) is valid in my book, I trust anyone can tell the difference between louder than and quieter than."



BTW- Anyone know what the turbo outlet diameter for a k26/8 is?

TIA

eospeed

I say even if you only get 40lbs of tourque it is worth the money.Thats what wins the race anyway.

Laust Pedersen

For turbo dimensions see post #4 here .

I agree with Turbo Tommy in his main points except that the power generated by the turbine is derived from the pressure difference before and after. So by reducing the back pressure there is some advantage to be gained. However if anybody takes the calculated results that “Willard Bridgham 3” posted and my translation into a .07 psi back pressure gain halfway seriously, then it should be clear that it only translates into a single digit HP gain. But don’t despair, there are still the sound and good looks justifications for a custom exhaust.

Laust

951Boost

Laust thanks, appreciate the link.

danny951

Does anyone know about how the exhaust "pulses" work. I'm not sure if it applies only to an N/A car since the exhaust is directly tied to the pulses from the exhaust manifold, but basically each pulse has has a higher pressure at it's front or leading edge (which is exiting the back of the car) and the rear or trailing edge of the pulse has a lower pressure. This creates a natural vacuum for each pulse, the higher pressure leading edge is pulled along by the trailing lower pressure edge of the previous pulse. From what I've ready, this natural flow is disrupted with a diameter change occurs in the exhaust. the larger diameter section gives more space to the leading edge (remember, that's the high pressure side) and slows down the exhaust pulses since the difference between the leading edge of one pulse and the trailing edge of the previous pulse is not as great. .

zerMATT951

I've heard similar things, but as you say in the beginning, I think that only applies to NA cars. By the time our exhaust gets to the turbocharger, it's all "chopped" up.

Duke

If a 2.5" system is enough for 500hp then I guess everyone with a stock 951 S can upgrade to a 1.25" exhaust system and benefit from the lighter weight and smaller space without sacrifying power or lag.

951Boost

Mmm, I guess that also means that having a” BIG PIPE” must give male owners of these cars a certain amount of emotional and psychological reassurance?

See why unsubstantiated comments are a waste of everybody’s time?

RKD in OKC

Wes at Lindsey Racing did two dyno runs with his car, one with the LR 3 inch and another with LR 4 inch. If I remember correctly there wasn't as much of a big gain in HP or Torque as much as it moved the powerband 500-700 rpm lower. I think the runs were with a Sport Turbo and not the big honkin' Super75.