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Whats your opinion on exhuast size?

Fancy

Fancy

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So i have a 90 with 350 tbi and i want to rework the exhaust much like k85 octane (pictured below). If not mistaken it is 2.5" exhuast tubing in the picture.

I wanted to make the same Y-pipe and go straight back from there. I was thinking of doing the same 2.5" pipe from the stock headers (until i can afford to put in edelbrocks or something) to a flowmaster colector with a 3" out put. Then run the 3" through my cat, cutout and muffler then run a 2.5" out.

I already have a 3" cat, cut and muffler so that is why the change

I know these engines are picky about there back pressure so is this to big? It is worth buying all new components so it is 2.5" all the way through?
What do you think and what would you do?

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I put 2.5 mandral bent all the way back, true duals, runs great, sounds great.
 
i checked out your build thread! pretty cool. What headers where those you put on the black ones?
 
Brians1990Blazer said:
i checked out your build thread! pretty cool. What headers where those you put on the black ones?
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Thanks!!
No Idea, a buddy gave them to me lookin like junk, and I sandblasted them. They werent very beefy feeling so I dont think they are a big name brand. But they work for now. They had a 3" collectors, but I would def go with either the shorty style headers or some long tubes as these are a pain to hook up the exhaust nicely. Im sure their is a thread on here about good headers for these, I think I will look into the Ram Horn style manifolds next time, but the 2.5" ones. Those look pretty cool.
 
The efficiency gain going from a 2" outlet manifold to a 2.5 inch pipe will be negligible unless operated at a higher RPM. You are going to be operating an output of 2" into a larger pipe, which in turn with loose velocity throughout the larger pipe, unless some sort of scavenging is taking place.

When proper velocity is maintained within the pipe, you tend to loose droning, and gain efficiency. Low end torque is maintained, and the engine tends to work less, rather "forcing" the exhaust gases out, they are essentially "pulled" out, which is scavenging.

A properly thought out single system if far more efficient than a dual system, excluding highly modified engines. A lot of people put bigger pipe on because they feel it sounds cool, but in reality it is killing their performance, and their engine is working harder than it should be. The exhaust gases exiting the tailpipe must overcome atmospheric pressure, and with an already lessened velocity rate will make the engine "force" the exhaust out the tailpipe.
 
being an engineering student i understand what you are saying and agree completely! My reasoning for the 2.5" pipe is that when i put on the edelbrock headers i beleave their, along with most other shorties headers, have a 2.5" out put. This would make it a steady flow system as the cross sectional area would not change untill the cat which would not matter by that point as it will always change at the cat.
I also would not what to go from a 2.5 to a 2" exhaust correct as i would be restricting my flow? Or would this restricting be seen as optimal back pressure? I figured that the cat and muffler would add sufficient back pressure?
The whole idea is to limit the volume change correct? The more steady state you can make it the better. That is why when combining tubes you want to keep as close to the same volume as you can correct?
 
Brians1990Blazer said:
That is why when combining tubes you want to keep as close to the same volume as you can correct?
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The 2.5" to 3" is actually where the scavenging will take place. The flowmaster "Y" that everyone uses, has what they call "D" ports. When the exhaust exits the primaries, the gas will "hug" the flat wall, once passing through the gas will create a negative pressure at the end of the tube next to it, in turn causing the next pulse to be "pulled" out of that tube, and that becomes more efficient as RPM increases, which will lead back to the cylinder where the gases are then pulled rather than forced.

They will gain velocity in that transition and the 2.5-3.0 will become more efficient than say a 2.5-4.0, unless more volume is needed.

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On the TBI blazers the stock exhaust was a 2 1/2" Y pipe into a 3" single exhaust for the remainder of the exhaust system and is the best system you can run.
 
Some notes about that exhaust.

1) I knew I already had to, but waited till I later coated the Y pipe, but I put on a remote mount oil filter on it. It's placed on the frame behind the shackle.

2) I reused the stock collectors @ the manifold. I had to open them up to fit nicer in the 2.5" pipe. So it feels like the stock manifold exits are smaller than 2.5". On my system, I'm using Edelbrocks.

3) The Edelbrocks don't fit like the factory manifolds and my guess is most aftermarket headers "direct replacements" don't. So if you are building a system around manifolds, you may have trouble with fitment once you go headers. Mine wouldn't have fit at all. Also, be careful with the heater riser, since you are in CA, though I'm not sure if you have one.
 
Before you do too much, I see your in California, so check out the emissions situation first! I'm a big fan of factory manifolds to the stock y pipe (which on my '91 is 3"), into a straight thru Magnaflow 3" cat, then into a 3" flowmaster catback exhaust setup. Nice fitment, good quality, and cheap!!! Also, everything I have is 50 state legal for emissions. No header and collector gaskets to mess with, I have had horrible experiences with headers in the past. My truck seems to like this setup, and there's been pretty good evidence that the 3" single will outperform the dual setups on these trucks. Check out TBIchips.com and CFM technologies for some good info on things you can do to these TBI setups. If done right you can actually gain performance and improve your fuel mileage, and let's face it, gas isn't getting any cheaper...

Edit: I forget to mention, only took me about 3 hours, start to finish. Remember, the sawmill is your friend, the cat was a pain in the arse to get off...
 
I just did the 2.5 inch ramhorns and 3" single on mine a couple weeks ago. Before I had dual 2.25 with stock manifolds.

I have the GMPP 350 HO. Honestly I was pretty disappointed with the engine when I put it in. This has completely woken it up. I gained improved gas mileage. Quicker response, but the most noticeable is the 300 RPM reduction at highway speed. I have the original TH350 K-case, non lock up.
 
Cornfield creations said:
I just did the 2.5 inch ramhorns and 3" single on mine a couple weeks ago. Before I had dual 2.25 with stock manifolds.

I have the GMPP 350 HO. Honestly I was pretty disappointed with the engine when I put it in. This has completely woken it up. I gained improved gas mileage. Quicker response, but the most noticeable is the 300 RPM reduction at highway speed. I have the original TH350 K-case, non lock up.
Click to expand...

Interesting, I've heard of this before, but why does it happen? You would think the rpms at highway speed, say 65 mph, would stay the same... Maybe this is why mine seems to run down the highway and not work so hard anymore? I don't have a tach, so I have no idea.
 
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Thanks for all the good replys and info!!! What would you guys recommend for shorty headers. I want shorties and they need to be CA carb legal i do not have A.I.R. but i do have a heat riser to the intake. I was looking at the edelbrocks but im not sure they would work i just see them for the A.I.R. applications
 
The Edelbrocks listed on Summit (which are the ones I got with AIR and Riser) will work for you but the heat riser was a b1tch. They don't have the mount clocked correctly, so it puts the vacuum canister right into the frame. I have to run a ~1-1.25" spacer (designed to take the space of a missing riser and still use all the exhaust) and run the heat riser attached to that. Long grade 8 bolts to hold everything together. It doesn't leak and works fine, but it's kinda a b1tch putting it all back together if you need to take the pipe off.
 
All the questions you're asking you can pretty much look at on my rig when you come for that sway bar. :deal:
 
K85 would you mind posting some pics? Are are there some in your build thread?

4x4High cool ya if i get time i plan on coming tomorrow night if not i will try sat evening either way ill call you before i leave
 
Brians1990Blazer said:
K85 would you mind posting some pics? Are are there some in your build thread?

4x4High cool ya if i get time i plan on coming tomorrow night if not i will try sat evening either way ill call you before i leave
Click to expand...

Tomorrow or sunday would be best. It's my sons b-day on the 31st but he wants to go do something on saturday so we might not be around most of the day and early evening. I know we'll be home tomorrow as my friends wife is coming over to pick up our dog we gave them.
 
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