i want my truck to sound like a dump truck, so i thought that a single 3" pipe from the 2nd cat would make a good sound. is this pipe too big? should i settle for just a glasspack? 96 300 5spd

 

that's what i got. sounds like a dumptruck........no cat though.


go for it

i lost nothing in the bottom end either.

any questions, i'll try to help.

DT

 

well im going to keep the cats on, because its a 15,000 dollar fine if i get pulled over and the cops dont find a cat, that should provide enough backpressure to make it run like it should. do you have an o2 sensor?

 

um no, it has a duraspark conversion. No o2 or crappy computer carb.

but the cats will provide enough bp


DT

 

The Gibson single 3 inch cat back system on my 95 sounds great. I also eliminated one converter(legally) by using a 2 to 1 hi flow converter headpipe replacement. It comes with an o2 bung welded in the right place too. It worked out to be the perfect exhaust system.

 

a full three inch exhaust is to much.

 

I am running 3' pipe on my 88 F-150 I-6. I have a dynomax welded muffler on it with 3" pipe sounds real mean, plus I still got my cats.

 

I seem to be missing the whole backpressure thing. It sounds odd that you would want any resistance against the rising piston on the exhaust stroke to evacuate the cylinder before the next intake stroke. I was told by an exhaust shop not to run an engine without manifolds because of the risk of warping a hot exhaust valve but not that a system can be too big after the cats though.

 

Designing an exhaust system is an entire science in itself. If you have too small of a pipe diameter, it will be restrictive and the piston on the exhaust stroke will have to work against it. This is known as pumping loss. The engine is using power to expel the exhaust instead of transferring power to the crankshaft.

If your pipe is too large you lose exhaust gas velocity. Bad. The exhaust gas comes out of the engine in pulses. The beginning of the pulse is high pressure, while the end is low pressure. The low pressure side of the pulse will "attract" the high pressure side of the next pulse (known as scavenging), and each pulse will pull (like a vacuum) every subsequent pulse along with it. Scavenging is good. When going through a pipe that is too large for the volume, the velocity slows down and has time to cool off some. Again this is bad because cooler "air" is denser and heavier than hot "air". So again this something that the engine needs to work against.

What you want is an exhuast system large enough to reduce back pressure, but small enough to maintain the highest possible velocity(this will also help keep EGT up). This can be hard because our engines are a varible rpm engine. So the exhaust volume and hence velocity are not the same at 1500 rpm and 4500 rpm.

Smaller diameter pipes work better in lower rpm power bands due to less volume / velocity requirements, but will run out of "breath" in the higher rpms. This is because you have increased exhaust volume without increasing pipe diameter. Larger diameter pipes are better suited to higher rpm power bands (6000rpm and up), due too the increased volume of the exhaust gas.

I suppose it is possible to burn up exhaust valves without a manifold of some sort.
The exhaust is hottest coming from the exhaust port. You need something there to route (scavenge) that heat away from the engine. So yes, I can see not having a manifold will cause the exaust port to heat up substanially and burn / warp a valve.

Another thing to consider is the amount of air/fuel you are dumping into engine. What works on a stock 200hp engine is not going to be the best if you build it into a 400hp engine. This is because you have altered the amount of the air/fuel coming in, so you need a larger pipe to carry it away once it has been burned. Even if the power band is kept the same (unlikely).

Just like a 500hp 460 is not going to run well with a 1.5" diameter pipe, a 130hp Honda 1.8L is not going to run well with a 4" diameter pipe.

Also inline 6's have the worst exhaust manifold because you have 6 exhaust ports dumping into a common plenum, whereas 4 cyl and V8's only have four ports going into the same plenum (V6's have 3). I think this is why EFI 300 manifolds are such a good improvment over the carbed manfold. Well, that and the log style manifold design just plain sucks. After exiting the exhaust port, the gas has to make a sharp 90° turn only to be interupted by an exhaust pulse that may not "line" up with it(remember the high pressure/low pressure sides?). Definately there is minimal scavenging going on here, especially for the #1 cylinder.

 

Yes, I believe you are correct. But all that pertains to the design of headers right off the cylinder head. Scavenging is an important part of the exhaust stroke and header construction but all that velocity is gone by the time the exhaust gasses get passed the converters and are on their way out. I think an analysis of exhaust flow from the converters back is in order.

 

Right now I have 2.5" flexible exaust tube comming off my efi's on my 84 f150(crane truck cam, offyc, 5004v, efi exaust manifolds, mild headwork) And there going into a home built y into a 3" stack. Im in a place that has no polution controll bilaws.. so no cats or egr ect. Mine sounds like a tractor... but its getting anoying. I have not got a ticket so far but the sound is just insane. I live in the country... and thats where i do most of my driving... but when i bring it to town.... going down the streets The buildings just amplify the sound X2(can you say echo)... and every body looks to see wtf was that!

So This spring im thinking of getting the local shop to put a halfway normal exaust on it. Im thinking two 2" or 2.25" pipes into a hooker areo chamber muffler with a single 3" outlet and then just a turn down before the rear axle... or would just a walker down pipe flow good enough?

Im tired of the strait pipe stack and not hearing my cd players sound detail lol.

 

Machman, what brand of highflow converter did you use? I haven't been able to find one that will match up with the two down pipes. Thanks.

 

I used a pipe and converter from a company called Fulline exhaust. It is billed as a 2 to 1 converter replacement. They send you the 409 stainless headpipe pipe that bolts to the manifolds and a high flow catco converter. It fit like a glove. You can get just the converter too. I think you need a dual 2" inlet with a 2 1/2" outlet. I have seen them in Summit and the Catco site. I did a search on yahoo and came up with Fulline Exhaust. They are in Dothan AL.