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2.5" or 3" exhaust for N/A Omni GLH?

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4.2K views 21 replies 9 participants last post by  moparfwdsleeper  
8
#1 ·
As my User ID would suggest, I own a N/A '85 Omni GLH. The car is currently set up with a 4-in-1 collector header - 1 3/4" tubes to 3" outlet, I believe. Aft of header flange, it transitions to a 3" pipe, 3" in / out high flow cat, then it steps down to the stock exhaust pipe.

I've got a few good 304 / 409 mandrel bent SS sources I've identified and will soon be purchasing two 90' and two 45' bends, a 5' to 6' length of straight pipe a 44 series muffler and a 4" flare tip. I've got a buddy who specializes in welding SS and will be fabbing my exhaust system for me. The questions I have are as follows:
- Would I be better off in the long run spending a little extra and going for the 304 vs. the 409 SS?
- Since my GLH is carbureted / naturally aspirated, would I achieve a higher exhaust velocity and, therefore, more optimized performance going 2.5" to the tip or would I achieve the best overall performance and upgradability by going 3"?
 
M
#2 ·
85omniglh said:
As my User ID would suggest, I own a N/A '85 Omni GLH. The car is currently set up with a 4-in-1 collector header - 1 3/4" tubes to 3" outlet, I believe. Aft of header flange, it transitions to a 3" pipe, 3" in / out high flow cat, then it steps down to the stock exhaust pipe.

I've got a few good 304 / 409 mandrel bent SS sources I've identified and will soon be purchasing two 90' and two 45' bends, a 5' to 6' length of straight pipe a 44 series muffler and a 4" flare tip. I've got a buddy who specializes in welding SS and will be fabbing my exhaust system for me. The questions I have are as follows:
- Would I be better off in the long run spending a little extra and going for the 304 vs. the 409 SS?
- Since my GLH is carbureted / naturally aspirated, would I achieve a higher exhaust velocity and, therefore, more optimized performance going 2.5" to the tip or would I achieve the best overall performance and upgradability by going 3"?
Click to expand...
85,

A basically stock NA 2.2 won't see any improvement going to a 3" for sure and will actually rob performance without any backpressure. You will loose all of your low end torque. 2.5 is plenty of exhaust even for a high compression/cammed up 2.2 all motor car. On my 99ACR 2.0 All Motor high compression motor with a set of Crane 0018 cams Kirk long tube header etc, I ran a 2.5" side exit and it ran sweeeet.

If I'm not mistaken 400 austentitic series SS will corrode over time as it does not have the same amount of alloying elements of either nickle or chrome I forget which exactly. I know for sure that the 400 have some ferro magnetic properties whereas the 300 series does not. Personnally I'm a aluminized steel guy myself especially on a car that will rust out way ahead of the SS piping. Save the $$$ for some 11 or 12 to 1 compression pistons and a big bump stick.

FYI,
Marion
 
V
#4 ·
2.25'' for a stock HO with only the large header.

My GLH with the same header, a 2.25'' exhaust and a .460 lift cam ran great and pulled hard! It would give my cousins Neon R/T(Header, exhaust, ICEMAN, MP ecu) a good run!

However, Im thinking of going up to a 2.5'' exhaust with my new Big Valve setup Im doing.
 
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#5 · (Edited)
I run 2.25" mandrel bend with high flow catcon and DynoMax muffler on one of my Daytona's, the N/A Daytona with the 2.5L engine. It does just fine. I believe it gave me better response. Other than the exhaust system and the 'open' air can the engine is completely stock. Before if I was on the highway doing 70 or so and I needed to accellerate I either had to down shift or get out of the way of the traffic coming behind me. Now I simply go WOT and it picks up speed just fine.

I'm planning to go 2.5" from the swing valve to the bumper on my Turbo Z. I feel that for my goals for the car that going to the full 3" would be over kill.
 
X
#6 ·
once again, the whole backpressure thing is completely bunk.
however, the right size exhaust does help with scavenging (exactly the opposite of backpressure) due to resonance.
given that, 2.25" or 2.5" is likely gonna be best.
 
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D
#7 ·
xrattiracer said:
once again, the whole backpressure thing is completely bunk.
however, the right size exhaust does help with scavenging (exactly the opposite of backpressure) due to resonance.
given that, 2.25" or 2.5" is likely gonna be best.
Click to expand...
^ +1

2.5" will be MORE than enough ; even if you later do cams, head, etc

(I vote for 3.3 beehives, PT lifters, and a roller TBI cam as an easy, cheap, and efffective upgrade for you)

on your car, or even most (mild - moderately built) turbo cars, there's no need for the additional weight, cost, and hassle of trying to shoehorn a 3" pipe through spaces designed for 1-7/8"
 
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#8 ·
Thanks all. So, I'm going to eliminate the 3" concept altogether and look at either going up to 2.5" or staying with the 2.25" size I've got now. If the 2.5" won't degrade current performance in terms of low end torque, high velocity flow and scavenging, and allows me a good future upgrade path, then I would be inclined to lean towards the 2.5".

Certainly, the price of aluminized steel is very attractive vs. SS. I live in the Rochester, NY area and we use tons of salt in the winter and continues to be a corrosion factor well into spring. That is more of a consideration as to why I would consider stainless over aluminized. Also, the buddy that's going to fab the system for me prefers to weld SS rather than aluminized.
 
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#9 ·
detobias said:
^ +1

2.5" will be MORE than enough ; even if you later do cams, head, etc

(I vote for 3.3 beehives, PT lifters, and a roller TBI cam as an easy, cheap, and efffective upgrade for you)

on your car, or even most (mild - moderately built) turbo cars, there's no need for the additional weight, cost, and hassle of trying to shoehorn a 3" pipe through spaces designed for 1-7/8"
Click to expand...
Thanks for the upgrade recommendations. If you would, could you expound on the "3.3 beehives" and "PT lifters"?
 
D
#10 ·
search in the 'Induction' section ; there is a (long) thread 'conical springs for 8V' which has oodles of info ; there are also several threads about PT lifters

basically we can use 2000-earlier 3.3 ChryCo V6 valve springs which are conical a.k.a. beehive (along with the appropriate GM spring retainer) ; conicals have lower inertia and better harmonic characteristics than straight springs ; the GM retainer is also much lighter than our stock retianer ; these factors improve high-rpm operation (less valve float)

the 'PT lifters' are used in 2001+ 2.0 DOHC and all 2.4 ChryCo motors and have a vent hole in them allowing trapped air to escape which solves our stock lifters tendence to trap air and 'collapse' whcih really is a problem at high RPM and results in power dropping off quickly

I have these mods on my 2.2 TII and it pulls hard well past 6K ; I am using 'junkyard' = used 3.3 springs and PT lifters ; neither of these items is really a wear item so used is fine altho they are relatively cheap to buy new anyways;

for an NA motor you will need the RPMs to make the power ; combine these with your free-flow exhaust and a good cam and you should be able to pull hard to 6500+

now you can start working on a good head ! :)

I have made some posts in the Carb/TBI section relating to NA power gains ; one of my posts has a link to a great article form the 80s about a 180Hp 2.2 buildup; kind of extreme HP/$ but you might get some ideas ; its all about " cam/carb/compression " for the NA motors (and a good breathing engine)
 
#12 ·
I ran a 16.9 in my Rampage with the only thing being a resonator in place of where the muffler "used" to be (rusted off)

I wish I would have kept it n/a and played with bolt on stuff, webers, header, ect..

Not to shabby for a 300k+ stock engine..
 
#14 ·
85omniglh said:
Again, great info - thanks. I don't want to go off topic, but are there any good threads for tuning / modding the Holley carb in a GLH
Click to expand...
1. remove the holley
2. toss it in the trash
3. order some dual side draft webers
4. dont smile too much from the gain in power and reliability because you'll get smile wrinkels.. :thumb:
 
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#15 ·
I've got a dual Weber DCOE 40 and intake sitting on my shelf. At some point in the future, I will be fabbing it into the GLH, but not right at the moment. I'll see if I can find a Holley tuning thread here on the forum. If someone knows of a good thread, please let me know.

Back to the exhaust scenario, I'm going to be purchasing the mandrel bends at racing-solutions and the straight pipe and a Flowmaster Super 44 at performance-curve. Have decided to go 2.5" based on the feedback and suggestions from you all. Thanks.

I'll see if I can post some pics of the fab and post it here when I get all the materials together.
 
#17 ·
85omniglh said:
I've got a dual Weber DCOE 40 and intake sitting on my shelf. At some point in the future, I will be fabbing it into the GLH, but not right at the moment. I'll see if I can find a Holley tuning thread here on the forum. If someone knows of a good thread, please let me know.

Back to the exhaust scenario, I'm going to be purchasing the mandrel bends at racing-solutions and the straight pipe and a Flowmaster Super 44 at performance-curve. Have decided to go 2.5" based on the feedback and suggestions from you all. Thanks.

I'll see if I can post some pics of the fab and post it here when I get all the materials together.
Click to expand...

how cool, you guys know about performance-curve on this forum as well..
I'm on another Toyota forum..

heres one thing I have to ask, what are you gearing your car towards?
any type of racing? top speed?

If you already have the Webers, go strait to the upgrade.. :thumb:
You bought the carbs with the intent on using em, why not now? :confused:

I'd steer clear of the flowmaster stuff, it always makes these cars sound like a UPS truck..
Magnaflow baby ;)
 
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#19 ·
More good feedback - thanks. Got the high flow cat on her already, ordering the 2.5 mandrel bends and straight pipe tomorrow. I had planned on ordering the Flowtech Super 44 tomorrow too, but I want something that's going to make my car sound good - deep, throaty and mean - and optimize the engine's performance through 6000 rpm.

Got a vote already for Magnaflow. Any other recommendations for specific makes and models of high performance mufflers and associated links would be appreciated.
 
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#21 ·
I just ordered the full stainless steel exhaust system. Everything from the cat on back only ran a total of $274.67. Here's what I've got coming, where I ordered it from and how much it cost:
- Magnaflow straight through satin SS muffler, 2.5" in/out, 6" round, offset tubes - ordered from Magnaflow.org - cost $81.64 delivered
- 2.5" 304 SS mandrel bends, (2) 45' and (2) 90' - ordered from Racing-solutions.org - cost $110.31 delivered.
- 2.5" SS straight pipe (5') and double-walled tapered cut SS tip, 3.5" x 4.5" x 2.5" ID x 7" long - ordered from Performance-curve.com - cost $82.72 delivered.

Good score or what?! :thumb:
 
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