Campeck

im not sure if its ZERO...(which i didnt really mean) but its not alot at all. theres a thread somewher eon this page that POP i think posted some cam specs, and overlap isnt a big thing on an na cam.

i think a good thing to do would be get a JM cam with some overlap, a 4-1 header with 2.5" all the way back and make a chip. I would be surprised if it didnt make 20hp.

Geo

His answer is exactly as I understand it from discussions with my automotive engineer friend (the same guy who worked on the TRD GTP engine program).

The pulse will create a small vacuum behind it. Also there are waves that move along the flow (for a simple description). Like any wave, it will reflect at both a closed and an open end. Header and exhaust tuning is about getting the length right so the wave reflects at the valve at the right time to augment the scavenging. A 4-2-1 header generally has a smaller hp gain, but a wider power band because there are two reflections (at the 4-2 junction and the 2-1 junction) as opposed the the single, but larger reflection of the 4-1.

Geo

I think this is an important point for someone trying to find hp in a 944. Because the 944 does not use a MAF air flow meter, the FI system is sensitive to changes in VE. If you improve the air flow, you must add fuel. Not doing so will negate any improvements.

Geo

Guys, I think Campeck is largely correct, but as usual, Manning makes exceptionally good points. The best one is to skip the dogma and examine how those different points interact.

Clearly there is more going on besides backpressure, but Campeck I think makes a good conclusion (if overly dogmatic) that backpressure is not a good thing. Do not forget flow and the effect diameter has.

Mongo

There are things that I would love to say right now that I can't on this board out of respect for others and Ted Wright who is a great moderator on this forum. In the end the result is something that is shown on paper tested on a car that is NOT A 944. Campeck I'm going to tell you this once and you better remember it: Porsche has been building cars for about 60 years, had a tyrannical reign on racing for more than 20 years, but you can say that the 944 can gain more power merely because of the type of muffler chosen is blatantly incorrect. I deeply apologize, you are a great person man, but this is a blanket statement over truth: a muffler will not yield the power you expect; in fact, it would cause a loss than what you had predicted.

I don't have a scanner to back my claims up so I'm sorry. But I have well more than sufficient evidence proving that a restrictive muffler can and WILL yield more power than a straight through, whether it's equivalent to the factory's claim or not on a 944 8-valve naturally aspirated car.

Geo

Andy, now you are the one making blanket statements. You may have a single comparison on a 944. However, I have Jon Milledge's ITS organizer and not only does he not say to run a restrictive muffler, he tells exactly what header and what length of exhaust to run. Yes, even the length of the exhaust affects power production. Jon has countless hours tuning 944 engines (very nearly stock I might add) on a dyno and if anybody knows what works on a NA 944 engine it's Jon. He gets 185 bhp from them with stock cams and valves, a half point bump in CR (though shaving the head only) and 0.040" over pistsons and balancing.

Further, be careful believing you can't improve over what the factory did.

Campeck

yeah. I never did believe the "factory knows best" thing. everytime i see a honda making 250hp out of half the engine i have to work with. i get scepticall. And I think everyone else should as well. Given the 944 does have WAY more TQ down low then those hondas ever will, but theres still improvemts there.

thanks for not calling me names stig. your a great guy too. *hug*

PS( and stig your right. I never did think changing a muffler on a 944 would increase hp. I would rather think that a whole system do-over would yield good gains with no backpressure.
I just dont want the guy who started this thread to actually get a MORE restrictive muffler than stock thinking it will aid low end Tq...even if it does. maybe..perhaps..in la-la land )

Oddjob

Im pretty sure that there is zero valve overlap on most or all of the 944 series engines.

Years ago (probably about 12), using the valve timing from the little 944 Tech Spec handbook, I mapped out the cam/valve opening sequences and I recall there was no overlap for the turbo, S, or S2. I was talking about it with an engine design mechanical engineer professor at Purdue at the time, and he was very surprised - had never heard of an engine that did not have valve overlap. So someone should verify this with cam lobe info from another source (in case I made a mistake or the tech spec book is in error).

I have seen many PCA Club Race 944 n/a's that are prepped with the cat removed and running a 944T muffler. The 944T muffler is a straight through muffler and less restrictive (and therefore louder) than the stock n/a, S/S2 mufflers. Since the Turbo muffler section is a 4 bolt flange, it does require a welding/fab job to install on the 3 bolt n/a, S/S2 exhaust system. This exhaust setup is loud and not for street use. But the point is: these n/a's were a lot faster than a stock/street car with stock exhaust. No dyno numbers, but when I did this same exhaust mod (cat delete and 944T muffler) to my old 944S track car, it made noticeably more power through the rev range, and it showed in lap time improvement.

TD in DC

Campeck . . . that was never a thought in my mind. If you notice how the thread title is worded, I asked about the "optimum muffler for backpressure," which could indeed be "0." In other words, which muffler is a "known" good solution for an ITS spec 944 race car (i.e., not too much backpressure and not too little). I was more worried about losing HP/torque than trying to gain any.

P.S. I can't believe you guys are so busy bickering that nobody noticed the video

Manning

I am very surprised there is no overlap, which clearly points to the cam as bottleneck. As to the S and S2 not having overlap, being as they are seperate cams for IN and EX this in theory could be adjusted.

Campeck

ide love to see an S2 with some overlap. how cool would a loping idle S2 race car sound like.
muahahha!

Litespeeds

I switched out my stock muffler on my 88 944 to a Magnaflow and really like it alot. I asked an experienced muffler shop in regards to which muffler would give me the best mid to upper end performance and he recommended this over the Flowmaster. Sounds nice and deep.

yellowline

You need some backpressure or I'd think you could burn your exhaust valves. At least that's how it is for small air-cooled engines. I'd agree- less is better, but you can't just let it exhaust out the head without a manifold- that would be 0 backpressure.

Fishey

Burns Stainless ultra light Muffler!

Yabo

BTW stig, ted wright is no longer moderator as far as I know.

And THAT is my contribution to this thread.