Exhaust - power

Q&A regarding engines, turbos, and intercoolers and power upgrades

Exhaust - power

Postby Rallly » Sun Sep 13, 2015 10:18 pm

Hey guy's, recently bought a 91 all Trac, 237k, felt pretty sluggish and hesitated at idle so I just finished up doing Plugs, wires, cap, rotor, fuel filter, O2 sensor, seafoam vac/gas tank, autobahn 88 intake side silicone couplers. Everything is stock and still feels sluggish. Oem boost gauge rises all the way over after 3k rpm but doesn't feel like 10psi. It feels as if the cats are plugged up.? So I want to upgrade my exhaust without hitting fuel cut, boost cut and not boost creep. I will eventually get a proper boost gauge. I've been checking out the eBay dp's and eBay manifolds.. Should I go 2" or 3"? I know I want the HKS Nur spec R cat back but not sure which dp, manifolds or mid pipes(unless it's included with Nur spec R exhaust)
I don't want crazy power but I want to unleash the power and efficiency I can on a small budget.

Thanks!
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Re: Exhaust - power

Postby underscore » Mon Sep 14, 2015 2:15 pm

Before you touch anything get a cheap mechanical boost gauge so you know exactly what's going on. Then forget about the eBay manifolds, the stock exhaust manifold is better until you start getting to higher power levels, at which point you'd likely want a custom one for a aftermarket turbo anyways. If you want to avoid the boost rising I believe the consensus is to go with 2.5", at least for the downpipe. Some people just gut the stock cat/downpipe but with how much platinum is in it a lot of places give you enough for the cat to nearly cover the cost of a new/used aftermarket downpipe.

For the midpipe and axleback there's a few options depending on what you want, I think the Motoria setup is still available and a decent bang for the buck. As far as I know HKS makes the Hi Power and Blitz makes the NUR Spec axlebacks but either way most of them don't come with the midpipe anymore, I believe some of them used to. On a budget I'd suggest a used downpipe, Motoria midpipe and axleback and then if you don't like the Motoria muffler just pick out a new one and get it welded on.
★ 1991 GTFour RC ~ "Rebel Scum" ★
It's for sale! http://www.alltrac.net/phpBB2/viewtopic.php?f=64&t=52181
Build thread http://www.alltrac.net/phpBB2/viewtopic.php?f=44&t=44216
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Re: Exhaust - power

Postby Rallly » Fri Sep 18, 2015 10:51 am

So if eliminated the two cats and piping all the way to the OEM resonator keeping it at 2.5inches I should be good? No overboost?
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Re: Exhaust - power

Postby alltracman78 » Fri Sep 18, 2015 7:36 pm

I had a turbo back 3" and never had any overboost problems. Even in New England winter.
On my 92 and 93 both with a complete Aussie exhaust.
Both cars were bone stock. IIRC the newer cars had a slightly higher boost limit though. I know it was above 12. 14 maybe?

Also, my current 93 has an Aussie 3" downpipe and Ebay full 3" exhaust. I don't think I even hit 14 PSI. I'm not positive though; I hardly ever drive the car. I never hit overboost with my older "custom" 3" exhaust either IIRC [on an RC engine with a Supra CT26].
But this is with a 205 engine.

I guess point being, you may or you may not.
The only way to tell is to try it.


But, it doesn't sound like your problem is clogged cats. If they were clogged I would think your turbo would spool really quick.
You'd have very little power, but it would spool fast.

You might just have a worn out engine/turbo. That's a lot of miles.
Get that boost gauge that was suggested too, before you do anything else.
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Re: Exhaust - power

Postby Rallly » Fri Sep 18, 2015 7:40 pm

alltracman78 wrote:I had a turbo back 3" and never had any overboost problems. Even in New England winter.
On my 92 and 93 both with a complete Aussie exhaust.
Both cars were bone stock. IIRC the newer cars had a slightly higher boost limit though. I know it was above 12. 14 maybe?

Also, my current 93 has an Aussie 3" downpipe and Ebay full 3" exhaust. I don't think I even hit 14 PSI. I'm not positive though; I hardly ever drive the car. I never hit overboost with my older "custom" 3" exhaust either IIRC [on an RC engine with a Supra CT26].
But this is with a 205 engine.

I guess point being, you may or you may not.
The only way to tell is to try it.


But, it doesn't sound like your problem is clogged cats. If they were clogged I would think your turbo would spool really quick.
You'd have very little power, but it would spool fast.

You might just have a worn out engine/turbo. That's a lot of miles.
Get that boost gauge that was suggested too, before you do anything else.





I plan too. Thank you sir. So I take it 14psi is the threshold for 91 alltrac turbo?
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Re: Exhaust - power

Postby underscore » Fri Sep 18, 2015 8:00 pm

14.7psi is when the fuel cut hits, I think most peoples overboost problems came from poorly made downpipes that didn't let the wastegate fully open. My full 3" setup with nothing but a flexpipe in it never went over 13psi on the dyno, but it will vary from car to car depending on what kind of shape everything is in.
★ 1991 GTFour RC ~ "Rebel Scum" ★
It's for sale! http://www.alltrac.net/phpBB2/viewtopic.php?f=64&t=52181
Build thread http://www.alltrac.net/phpBB2/viewtopic.php?f=44&t=44216
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Re: Exhaust - power

Postby Rallly » Fri Sep 18, 2015 9:21 pm

Which dp won't interfere with the wastegate opening? Will the cheap eBay one suffice?
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Re: Exhaust - power

Postby underscore » Fri Sep 18, 2015 10:28 pm

It can be hit and miss, I have a no name one that works fine, others have had to grind away a small amount of material from the inside so that the wastegate flapper has room. I haven't heard of one that couldn't be made to work with a little grinding though.
★ 1991 GTFour RC ~ "Rebel Scum" ★
It's for sale! http://www.alltrac.net/phpBB2/viewtopic.php?f=64&t=52181
Build thread http://www.alltrac.net/phpBB2/viewtopic.php?f=44&t=44216
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Re: Exhaust - power

Postby Mr Alltrac » Sun Sep 20, 2015 6:58 pm

If your really concerned about it,get a manual boost controller and set it for 12 psi. Mine has never over boosted and it's 3" turbo back.
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Re: Exhaust - power

Postby Rallly » Sun Sep 20, 2015 9:39 pm

Mr Alltrac wrote:If your really concerned about it,get a manual boost controller and set it for 12 psi. Mine has never over boosted and it's 3" turbo back.



U mean to do this after the exhaust is decatted right?
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Re: Exhaust - power

Postby underscore » Sun Sep 20, 2015 10:41 pm

A boost controller can't go lower than wastegate pressure, only higher, unless I'm mistaken.
★ 1991 GTFour RC ~ "Rebel Scum" ★
It's for sale! http://www.alltrac.net/phpBB2/viewtopic.php?f=64&t=52181
Build thread http://www.alltrac.net/phpBB2/viewtopic.php?f=44&t=44216
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Re: Exhaust - power

Postby Rallly » Sun Sep 20, 2015 11:33 pm

underscore wrote:It can be hit and miss, I have a no name one that works fine, others have had to grind away a small amount of material from the inside so that the wastegate flapper has room. I haven't heard of one that couldn't be made to work with a little grinding though.



Grind the wastegate flapper or the inside of the dp?
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Re: Exhaust - power

Postby alltracman78 » Mon Sep 21, 2015 1:22 am

The downpipe. If you grind the wastegate too much it will leak or possibly crack.
It's got to be hard to grind the downpipe too, it's relatively thin metal.

underscore wrote:A boost controller can't go lower than wastegate pressure, only higher, unless I'm mistaken.

You're right.

A boost controller also can't do anything if the wastegate won't open all the way.
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Re: Exhaust - power

Postby Rallly » Thu Sep 24, 2015 4:49 pm

What is the diameter size of the st185 alltrac factory exhaust?
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Re: Exhaust - power

Postby Rallly » Wed Nov 11, 2015 3:56 pm

So I installed a good mechanical boost gauge, and I am limited to 8psi in all gears. Is this normal on a stock 91 all-trac? Should I look into a Manual boost controller next?
So far I purchased a 3" autobahn dp and a Blitz Nur Spec R muffler. What diameter size should I tell my exhaust builder to use for my setup? Goals are, I would like a quicker spool, 12-14psi max, and not annoyingly loud. Thanks!



underscore wrote:Before you touch anything get a cheap mechanical boost gauge so you know exactly what's going on. Then forget about the eBay manifolds, the stock exhaust manifold is better until you start getting to higher power levels, at which point you'd likely want a custom one for a aftermarket turbo anyways. If you want to avoid the boost rising I believe the consensus is to go with 2.5", at least for the downpipe. Some people just gut the stock cat/downpipe but with how much platinum is in it a lot of places give you enough for the cat to nearly cover the cost of a new/used aftermarket downpipe.

For the midpipe and axleback there's a few options depending on what you want, I think the Motoria setup is still available and a decent bang for the buck. As far as I know HKS makes the Hi Power and Blitz makes the NUR Spec axlebacks but either way most of them don't come with the midpipe anymore, I believe some of them used to. On a budget I'd suggest a used downpipe, Motoria midpipe and axleback and then if you don't like the Motoria muffler just pick out a new one and get it welded on.
Rallly
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