Kinda stupid exhaust question

[snotrocket71]

hey everyone, the exhaust on my Jimmys shot and was looking for some insight. it has the original Y-pipe and what looks to be the original Cat on it (the thing is HUGE) and theres nothing past the cat due to muffler and tail pipe rotting off (its really quiet too for only having a cat and only a foot of pipe, i figure its clogged up). my question is what are my options for a new exhaust. regardless of what i do it will have a Flowmaster or thrush knock-off. i was thinking about maybe cutting the cat off and coupling a 2 1/2" piece of pipe to a muffler and possiby to a 90-91 style 3" walker tailpipe, or instead of that maybe go with the 2" outlet but that seems fairly restrictive even for a 305. i even thought of a stock style dual exhaust from walker. id like to do it on my own, i have a welder amd the space and time and would like to save some cash. any input, suggestions or experiences are welcome. Id love headers and true duals but would rather hold off for a nice quality or even custom set, but i dont really think its all that neccessary on a 305. As far as emissions laws, i'm not concerned considering the new rules that are in effect and the fact that my uncle inspects my vehicles at his shop. Im probably over thinking this wayy too much but would like to hear what others have to say. Thank you all very much.

 

[4X4HIGH]

A single 3" exhaust will net you the most low end torque while keeping your pocketbook as fat as possible.

 

[snotrocket71]

not to mention it'd probably be the easiest for me to do. out of curiosity anyone ever retrofitted the flowmaster '87-'91 3" system (PN 17161) to a pre '87 K5? (kinda contradicts the cheap part of it i suppose) but i do like the idea of nice clean mandrel bends and it couldnt be that hard to adapt to the 2.5" pipe coming off the Y pipe and running pipe to delete the cat. it kinda reminds me of working with my grandpa running PVC waste lines. i really need to crawl under the truck and scope things out better... i'll have to look into price differences too, like everyone else i plan on bigger and better things with my truck and i'm sure this exhaust wont be there forever but at the same time dont want total crap.

 

[K85 Octane]

the difference in years is the tbi engines
I don't know, but the newer trucks may be missing the heat riser. Messing with one of those nets you smog problems here. Might be the only difference in the two, might be able to just get rid of the riser and connect the system.
I have no idea though.
keep us posted

 

[blazinzuk]

I think the flow master kit you speak of is for the IFS style trucks. I don't remember seeing much from them for our trucks.

If your Uncle has a shop he has got to know someone who does exhaust. Take it to them and have them build a new one for you.

If your old exhaust is that rotted away the front is soon to go too, no sense in using that old pipe and having to replace in a couple years.

I would do a single 3" too. One less muffler to buy, less pipe to run

 

[snotrocket71]

as far as i can tell from the flowmaster website, the 17161 kit is for the TBI K5's and has 3" pipe as opposed to 2.5" like pre TBI motors (anyone correct me if i'm wrong). also the exhaust is scaly before the cat and appears pretty solid, id rather not have to deal with siezed bolts and hacking things to hell to pull the Y pipe off, but if it needs to be done then so be it. I dont need to worry about smog rules here, we just get a visual/safety inspection no dyno or anyhting as of January first. my uncle recommended united exhaust, which is fairly reputable, but i still entertain the idea of doing it myself.... on a side note, who created these new inspection rules? i thought everyone was so concerned about emissions and going green? i cant tell you how many people i know of who took off emissions equipment, deleted cats and did other things that wouldnt pass a normal dyno inspection. anyways thanks for the replies so far

 

[Jessie James]

One thing to keep in mind is that the stock size pipe is probably best for performance in a variety of ways. If you go from a 2" to a 3" pipe you will lose exhaust backpressure. That means the exhaust will no longer scavenge the gases from the cylinders on the exhaust stroke. Proper backpressure acts like a vacuum and helps suck the gases out, which in turn improves performance and of course economy.

The only exception to this is forced induction engines (turbo) which work best with the LEAST amount of backpressure. If you had a turbo, you'd want a 3" exhaust or larger. This is why you see all those turbo diesel trucks with giant exhaust pipes.

With that said, I'd suggest you stick with something that is as close to stock as possible. If you go with dual exhaust, you should make the pipes SMALLER accordingly to maintain proper backpressure.

I learned all this from a great book by Corky Bell called Maximum Boost. It's an amazing book and will totally blow you away in terms of building high performance engines.

http://www.amazon.com/Maximum-Boost-Turbocharger-Engineering-Performance/dp/0837601606

 

[AJMBLAZER]

The 3" Flowmaster setup would be a great upgrade to any stock TBI small block truck. The stock setup was too small. Been there, done that, loved it.

 

[4X4HIGH]

The 3" Flowmaster setup would be a great upgrade to any stock TBI small block truck. The stock setup was too small. Been there, done that, loved it.

A stock TBI Blazer had a single 3" exhaust from the factory.

 

[AJMBLAZER]

Then why'd they step down to 2.5" in the IFS trucks? Mine all had 2.5" exhaust after the cat.

 

[4X4HIGH]

Then why'd they step down to 2.5" in the IFS trucks? Mine all had 2.5" exhaust after the cat.

I have no idea what GM did on the IFS trucks but I would be inclined to say it probably had something to do with space limitations and the torsion bars but Blazers came stock with 3" single after the Y-pipe.

 

[jekquistk5]

I have no idea what GM did on the IFS trucks but I would be inclined to say it probably had something to do with space limitations and the torsion bars but Blazers came stock with 3" single after the Y-pipe.

not sure if its factory but my 94 TBI 2500 has 3" all the way back

 

[AJMBLAZER]

Weird.

Maybe it was 2.75". Walker lists 2.75" for the after cat pieces on the IFS trucks. I know it was smaller than 3". Crimp bent too.

Sigh...that 3" Flowmaster setup has me thinking about just going that route rather than my pieces and parts setup I bought.

 

[snotrocket71]

its funny how for such a seemingly simple thing theres so many ins and outs to exhaust. as far as back pressure and the science of exhaust i'm no expert but would say i know enough to be dangerous, i understand too much can hurt performance, and certain setups will have certain drawbacks in performance and such. if i was to do duals i'd go no bigger than 2 1/4" maybe even 2" for the firebreathing monster 305. but i may take a chance and try the flowmaster setup with a 2 1/2" to 3" coupling mated to the stock pipe which seems to have some life left in it. it seems like the most painless way to go about it for a slight bump in performance and exhaust tone. when i looked at it earlier i was surprised how neat and tucked away the exhaust was up near the frame, no ground clearance issues there.