Custom Exhaust...what size piping?

[GarnetRed4.3]

I'm planning to do a custom dual exhuast on my '94 which has the 4.3L, but I'm not sure what size piping to run.

I've already got a mildly used H-pipe with two high flow cats that is left over from my old Fox Body Mustang. I'll just have the cats cut off, and the crossover section cut down, so that it can be used on the truck and run down the passenger side like the factory pipe route...these parts are 2.5in diameter. So should I run 2.5in piping to match, or use some reducers on the inlets and outlets and run 2.25in piping?

[RednckChevy]

since its the V6. I would stick with the factory size pjpe

[GarnetRed4.3]

since its the V6. I would stick with the factory size pjpe

So run 2.25in piping and then use reducers on the inlets and outles of the cats and the mid pipe?

[nick613]

2 1/4" or 2" would be good for a dual exhaust setup on your 4.3... As for the reducers, I have never heard of it being done but I don't see why it would give you that much trouble.

[Redz Z]

2.5" is a no. I tried 2.5" pipe on my old 96 4.3 Vortec GMC and it lost torque and started throwing codes. I support what these guys are saying.

[Quyonmob]

My brothers 94 C1500 stock was 2.25" to the Y, 2.5" to the cat, 3" to the muffler and tailpipe. These were OE or OE replacement parts.

[GarnetRed4.3]

2 1/4" or 2" would be good for a dual exhaust setup on your 4.3... As for the reducers, I have never heard of it being done but I don't see why it would give you that much trouble.

Here are the reducers that I was talking about...
http://www.summitracing.com/search/Department/Exhaust/Part-Type/Exhaust-Pipe-Adapters-and-Reducers/Exhaust-Pipe-Reducer-Inlet-Size-in/2-500-in/?Ns=Price|Asc

They can go from 2.5" down to 2.25" or 2"...seems like that would be the solution if I use smaller diameter piping than the diameter of the cats and crossover pipe. Just slip them on the inlets and outlets and weld up the pipes.


2.5" is a no. I tried 2.5" pipe on my old 96 4.3 Vortec GMC and it lost torque and started throwing codes. I support what these guys are saying.

I thought it may be a bit much on this truck...I ran full 2.5" on my Fox Body, which was probably putting out somewhere around 300hp, which I think would have been fine unless I had gone up to around 400hp or so. Considering this truck has around half that, I didn't figure 2.5" was needed, but I don't want any adverse effects.


My brothers 94 C1500 stock was 2.25" to the Y, 2.5" to the cat, 3" to the muffler and tailpipe. These were OE or OE replacement parts.

Yeah, I looked under the truck recently...appears to be a 2.25" Y-pipe, 2.5" at the merge of the Y, a 3" cat, and then back down to 2.25" from the cat to the muffler and tailpipe. Looks like the cat-back was replaced at some point with just stock replacement stuff.

So I guess if 2.5" is too much, then the question becomes 2" or 2.25" piping?

[RednckChevy]

I can't remember what show I saw it on but they were talking about pipe size compaired to horsepiwer. for 100-150 hrse it was 2.25. 150-250hrse 2.50 and so on all the up to 5 inches for big numbers

[GarnetRed4.3]

I can't remember what show I saw it on but they were talking about pipe size compaired to horsepiwer. for 100-150 hrse it was 2.25. 150-250hrse 2.50 and so on all the up to 5 inches for big numbers

I did a Google search yesterday, and found quite a bit of information. However, it seems that there are a TON of variables, so it's extremely hard to try and calculate the exact size piping needed. But for my engine (262ci, making about 180hp...I think that's the factory rating), some sources say 2", some say 2.25", and others say as much as 2.5" when running a dual exhaust set up.

I also read somewhere that exhaust piping for a street application should be about 50% larger than the exhaust valve diameter. Not sure how accurate that statement is. But I figured, "what the hell?" and I did the calculation...exhaust valve on these trucks is a 1.50" diameter, so that multiplied by 50% comes out to 2.25". So maybe there's something to that theory...who knows.

Long story short, 2.25" piping seems to be that middle road between too much and too little, so I think I'll go with that and run some reducers.

[chucksrt]

I wouldn't go any larger than 2.25". I think that is too much! How is this truck driven? Are you looking for light to light performance, highway performance or just loud sounding? 300+hp cars run dual 2.5" exhaust and I think that it still oversized for that HP. Your probably better off (if you are looking for light to light performance) with 1 7/8" duals or 2" duals for some more top end. You can always put some sort of dressy tips on to man it up! how about a pair of 2" pipes y'd into a single 2.5" pipe? I would hate to see you lose power just to man up the size of the pipe!

[GarnetRed4.3]

I wouldn't go any larger than 2.25". I think that is too much! How is this truck driven? Are you looking for light to light performance, highway performance or just loud sounding? 300+hp cars run dual 2.5" exhaust and I think that it still oversized for that HP. Your probably better off (if you are looking for light to light performance) with 1 7/8" duals or 2" duals for some more top end. You can always put some sort of dressy tips on to man it up! how about a pair of 2" pipes y'd into a single 2.5" pipe? I would hate to see you lose power just to man up the size of the pipe!

Thanks for the input.

I'm starting to rethink this true dual set-up because the truck only has a single O2 sensor, and I've got mixed feelings about just running it on the passenger side pipe, and not having any readings from the driver side of the engine going to the computer.

So I may just be sticking with a single exhaust. Probably 2.25" into a 3" merge Y-pipe; 3" high flow cat, and then a SI/DO muffler (3" inlet, 2.25" outlets).

[chucksrt]

I think 3' is over kill for 190hp! That will decrease bottom end power, so around town mileage will drop! If you really man the "Man" size pipe look then I would use a 1 7/8"-2" pipe (whatever stock off of the manifolds is) and y them into a single 2.5" pipe back to a muffler that has a 3" outlet or a dual 2.5" outlet tips.