5.7 TBI Y-pipe sizing question & thoughts on price estimate

[94FSblazer]

Hey guys,
question about exhaust sizing for my 94 Chevy 2DR Blazer 5.7 TBI.

I want to replace my crappy stock Y-pipe, as well as my 24 year old cat. About a year ago I bought a new Flomaster Cat
(https://www.flowmastermufflers.com/...rsal-223-series-300-in-inlet-outlet-49-state/),

and the Y collector
(https://www.flowmastermufflers.com/...ries-y-collector-250-in-primary-300-in-final/),
but never got around to getting these installed. So now it is time to get this taken care of. My cat is definitely clogged up, and I want to get rid of my stock y-pipe.

So my question is what sizing is recommended for the y-pipe? The y-collector is 2.5" in/3" out and cat is 3". I have seen so many people doing 2.5" y-pipes. Is there any reason not to do 3"? and if its better to go 3", will this y-collector not work then?

The other day I went to my usual exhaust shop and asked about doing the y-pipe and cat. They estimated it would be about $700 for them to make the y-pipe and install everything. Seems higher than I would have thought it would be? What do you guys think? Should I try to find a y-pipe to bring to them? They obviously told me that it would be more expensive for me to buy all my own stuff. I never buy that!

Lastly, I was also thinking maybe I should go ahead and replace the stock exhaust manifolds too. Pretty sure I have a small leak on the driver side (I get the annoying ticking/leak sound). So would it be worth changing these out with some shorty headers instead?

Thanks for the help! Much appreciated!

cat and y-collector pics

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[94FSblazer]

Not sure if this matters or not, but I had from the cat back replaced, and put in a magnaflow muffler. I think it was part #11259. but not 100% sure. (https://www.magnaflow.com/products?partNumber=11259)
Thanks

 

[GMTNZ]

3inch y pipe too big for a stock motor, 2 1/2inch more than big enough and the tubing will slide straight into the collector you have. It's all about gas velocity, lose velocity you lose power.
I made my own y pipe using 2 1/4 tube and the same collector as you have but the 2 1/4 in 3 out version, no cat. Certainly woke the old girl up but you need to change the heads/intake/cam etc for big changes.
I run 1 1/2 primary shorties but long tube headers would work better with these motors.
The stock engines in these trucks are idea for what they are, especially the 94 where a bit more torque is produced slightly lower down the rev range than the earlier models!

 

[94FSblazer]

3inch y pipe too big for a stock motor, 2 1/2inch more than big enough and the tubing will slide straight into the collector you have. It's all about gas velocity, lose velocity you lose power.
I made my own y pipe using 2 1/4 tube and the same collector as you have but the 2 1/4 in 3 out version, no cat. Certainly woke the old girl up but you need to change the heads/intake/cam etc for big changes.
I run 1 1/2 primary shorties but long tube headers would work better with these motors.
The stock engines in these trucks are idea for what they are, especially the 94 where a bit more torque is produced slightly lower down the rev range than the earlier models!

Thanks GMTNZ for that info!

so would you suggest doing 2 1/4 like you did? would the 2 1/2 be slightly larger than I should go if I do not do the cam/heads at the same time?

Ive been working on the engine just to get it back to great running condition, but haven't done any major upgrades. And with how many different ways people are doing their exhaust, I was never 100% sure on what to go with. I would love to do long tube headers, but every time Ive seen it done on a 4x4 like mine, it requires mods like trimming the frame. i don't really want to do that. But maybe that is what I should do anyway. I just figured doing shorties if I was going to change the manifolds anyway, and figured that I could at least do a small upgrade instead of another exhaust manifold.

 

[GMTNZ]

You'd be fine with 2 1/2 inch y pipe and those collectors work really well.
Shorty headers work well enough and are an improvement over the stock manifolds, especially being lighter. I wouldn't go any bigger than 1 5/8 primary (header) pipes though, 1 1/2 work good on the stock motor.
Another good bolt on is the salad bowl, I think it was Hypertech one I used, replaces the plastic collar between the TBI unit and the air cleaner. Also make sure your fuel filter is replaced if you haven't done it before, when I first got my truck it wouldn't rev out at all until I replaced the fuel filter!
Hope your exhaust upgrades work out well.

 

[94FSblazer]

You'd be fine with 2 1/2 inch y pipe and those collectors work really well.
Shorty headers work well enough and are an improvement over the stock manifolds, especially being lighter. I wouldn't go any bigger than 1 5/8 primary (header) pipes though, 1 1/2 work good on the stock motor.
Another good bolt on is the salad bowl, I think it was Hypertech one I used, replaces the plastic collar between the TBI unit and the air cleaner. Also make sure your fuel filter is replaced if you haven't done it before, when I first got my truck it wouldn't rev out at all until I replaced the fuel filter!
Hope your exhaust upgrades work out well.

Thanks again!
I do have a volant CAI that I put on a couple years ago. I have also done the TBI mods of shaving off the ridges and buffing. Also replaced EGR, MAP, IAC, TPS, and I change the fuel filter every year. I actually just did it about 2 weeks ago. Ill make sure to pay attention to the sizing on the headers I am looking at. Ive mainly looked at JBA and Hooker headers so far

 

[Supercharged111]

I wouldn't go smaller than 1 5/8", but would also hesitate to exceed 1 5/8" primaries because TBI. 2 1/4" is all you need per manifold, but 2.5" will not hurt you at all. My Corvette with a 5.7 LS6 uses 1 3/4" primaries on its long tube headers. I installed them at the same time as a dual 3" x pipe mid pipe and gained torque from an idle to redline. If you watch Engine Masters on Youtube you'll see that an x pipe only adds about 8 ft/lb over divorced duals on a healthy V8 which is about what an H pipe delivers and my Corvette came stock with an h pipe so take that for what it's worth.