924Board.org

[masterdave]

I have an 88 924s the muffler is falling off and will be the next thing i replace, i was considering scratching the muffler all together and simply putting a straight pipe on it, has any one done this and does it actually yield more horse power? and is there a way to do it with out welding as that is the only thing i dont have the tools to do.

[dash16]

I believe running straight pipes will yield more HP, but in the higher rpm range and will cause lower rpm range to suffer.
_________________
1987 Porsche 924S Black/Black. 5-spd.
Options: AFX C79 261 398 423 650 651 657

1987 Mercedes 190E 2.3-16 Cosworth Pearl Black/Black. 5-spd.

STUTTGART PREVAILS

[edredas]

Awhile back I did a little reading on this subject. From what I recall, these cars have tuned exhaust systems. Removing the muffler will make the engine run leaner, resulting in a loss of power.

Also, there's no way to install the muffler properly without welding. If there's any leak in the exhaust system won't work correctly. Take it to a muffler shop and have it put on since you don't have the equipment. This will also assure that it's done right.
_________________
'84 944 -White, Brown interior
'84 944 -Red, Automatic
'86 944 -Garnet, Fully loaded, Koni suspension
'87 924S -Red, 300hp 951 swap
'87 924S -Red, Project Car
'88 924S -Red, Daily Driver

[bcblase]

I have heard that the engine needs a little back pressure from the cat/muffler to achieve the maximum HP. A straight pipe won't produce the required back pressure and HP will be lost.
_________________
1987 Porsche 924S - track toy
1986 Mercedes 190E 2.3-16v - autocross
2007 F-150 5.4L Lariat Supercrew - tow beast
1994 Volvo 850 Race Wagon - 24 Hours of Lemons Car
2001 BMW 325xi - daily driver

[Timmay!]

I don't know guys.........
I've run my 924S in the 944 Spec class for a number of years now and I don't seem to be down on power compared to the other drivers?
I also save about 20-25 lbs by deleting the cat and muffler. I have stock 88 style headers, then the rest is the same 2.25" straightpipe all the way back. Sounds wonderful!
Listen for yourself.

http://www.youtube.com/watch?v=3dO72XRoG88
_________________
Current project 1981 931
944 spec # 22 924S
77 924 Martini. Rescued. Sold
88 924S SE. Rescued. Sold
PCA since 1985

[edredas]

Hey Timmay! I've probably watched that video about a million times. Nice job getting those ricers out of the way lol... are you double-clutching and rev matching? If you are I can't tell from the video.

I agree that removing the cat is a good thing on a race car. Straight piping it probably freed up an additional 5-6 horses, and the weight saved maybe gave you 1-2hp.

While your engine sounds nice, I would put it on a dyno just to see if you've lost any horses when removing the back box... on a stock four cylinder, you can generally lose up to 30 hp this way.

To clarify, it's not necessarily the back pressure that you want. All you're wanting is the exhaust pattern to remain the same so the engine doesn't push the exhaust out too fast/too slow... if the exhaust escapes too fast, it can make the engine run lean and perhaps lead to burned valves. Installing a too large of pipe may not get rid of the exhaust fast enough, which is almost as bad as installing a pipe that's too small.

All of which leads to a loss of power.

Most of the confusion about back pressure is because V8's can run just fine with straight pipes and can achieve much of the same thing by adding an X shape to their pipes... this time there is no back pressure keeping the exhaust patterns the same... however, four cylinders can't really do that for obvious reasons... and a back box is usually needed.
_________________
'84 944 -White, Brown interior
'84 944 -Red, Automatic
'86 944 -Garnet, Fully loaded, Koni suspension
'87 924S -Red, 300hp 951 swap
'87 924S -Red, Project Car
'88 924S -Red, Daily Driver

[masterdave]

do you have an suggestions on pipe size and material, i leave for college in a few days and the car is being put away till i get back, i have about 4 months to plan what i will do to it upon my return. i still have the problem i mentioned in my post mystery problem. When i get back i will be dealing with all the problems. As well as installing bucket seats. I am basically creating a race car that i will use as my weekend driver, as i have no place to race, and no experience but love to go fast.

[edredas]

I've read that the best sizes are 1.5-2.5 inches for these cars.... I've personally never tested the different the sizes to see which one would best compliment our engines powerband and gearing... most racers tend to go with the 2.5 inch. However, that doesn't mean it the best... just as an example, 1.5 might help you on a track with lots of hills/turns, but hold you back on high speed tracks... where a 2.5 might give you a higher top end but, not do well on a low speed track... so you might want to meet somewhere in the middle;(again these are just examples, as I've never actually ran tests to see the difference.) Personally, I would go with a 2 inch stainless steel pipe.

If you get to moving the powerband around too much, it will feel as if your gearing is all out of whack.
_________________
'84 944 -White, Brown interior
'84 944 -Red, Automatic
'86 944 -Garnet, Fully loaded, Koni suspension
'87 924S -Red, 300hp 951 swap
'87 924S -Red, Project Car
'88 924S -Red, Daily Driver

[masterdave]

will the ECU/O2 sensor be effected by the removal of the cat, and should i disconnect the O2 sensor if i am removing the cat. Thanks for the advice the 2 inch sounds like a good plan, no all i need to do is rustle up a mig welder and i am in business.

[edredas]

Just unplug the O2 sensor, grind and weld up the hole flush with the pipe... toss in garbage. You also may want to re-tune your engine or all that time and effort is meaningless.
_________________
'84 944 -White, Brown interior
'84 944 -Red, Automatic
'86 944 -Garnet, Fully loaded, Koni suspension
'87 924S -Red, 300hp 951 swap
'87 924S -Red, Project Car
'88 924S -Red, Daily Driver

[masterdave]

what does a retune entail and what will i need to do it.

[edredas]

Take it to a dyno shop and do a test and tune... this will assure that you're making the most power with all those new parts. I've had friends that always assumed the power they were making and were always WAY off lol.
_________________
'84 944 -White, Brown interior
'84 944 -Red, Automatic
'86 944 -Garnet, Fully loaded, Koni suspension
'87 924S -Red, 300hp 951 swap
'87 924S -Red, Project Car
'88 924S -Red, Daily Driver

[edredas]

Actually, if you're not going to replace the entire pipe, I would match the stock 2.25" size and not have a big pipe going to a smaller pipe... also, the stock size will probably yield the best results... my guess of 2 inches was just a stab in the dark.
_________________
'84 944 -White, Brown interior
'84 944 -Red, Automatic
'86 944 -Garnet, Fully loaded, Koni suspension
'87 924S -Red, 300hp 951 swap
'87 924S -Red, Project Car
'88 924S -Red, Daily Driver

[Timmay!]

Edred,
In the video, you can actually see my right leg moving (because my right foot is moving) to blip the throttle for the downshifts. Watch it again. I matched the revs very well. I'm so proud.........
We don't double clutch on modern cars. We heel and toe. So the clutch is only pushed in once.

I use the factory size of 2.25 inches for the entire length of the exhaust.
I would keep the O2 sensor just to continue giving that input signal to the DME computer. That's in the section of the exhaust that includes the cat. Just take the car to a muffler shop and have them cut out the cat and replace it with a stright pipe ("off road use only").

But here's the most impotant thing to remember regarding the entire exhaust system.
The whole system is one unit and any change here affects someting there. There is an entire science devoted to exhaust flow from an internal combustion engine. You've got to do the testing.
_________________
Current project 1981 931
944 spec # 22 924S
77 924 Martini. Rescued. Sold
88 924S SE. Rescued. Sold
PCA since 1985

[edredas]