SC stock exhaust back pressure
03-19-07, 12:29 PM
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SC stock exhaust back pressure
Guys,
I had originally wanted to do a definitive and thoroughly and detailed documented analysis of the stock system. That didn't happen for two reasons: one, it proved unneccessary to me at least. Two, I could not monitor the pressure at the outlet of the ex manifold before the gasses enter the first cat. as that o2 sensor was impossible for me to remove without some serious special purpose tool, namely, a hydraulic six point box claw foot extension in the 19 mm persuasion.
I purchased a manifold boost gauge to do this. The one I bought had a full scale deflection of 20 psi and that covered on 90 degrees of the dial face. I could not get any closer than .5 psi as an increment. 1 psi was just barely readable with confidence. The reports of 12 psi of system back pressure must be at the manifold output and that would mean that the front cat was generating nearly 10 psi of back pressure alone and I seriously doubt that is the case.
Conclusions:
1. The system is limited to less than 300 crank hp due to the tubing at the rear half being 2 inches in diameter. And a little less at the bends due to very slight colapsing.
2. The ex manifold is the worst component of the system and that is established fact even if you only subject it to a cursory "visual" inspection.
3. The back pressure at the point between the first cat outlet and the first Y input shows 3/5 s of the back pressure is presant at that point. That was my first measurement and I got 2.5 psi at 5K rpm. Not a screaming deficiency, at that. 6K rpm would have been a better measurement but I had trouble holding that rpm and my guess was that it was only .5 psi above the 5K reading. All things being relative, measurements at 5K are good for evaluating the system for comparitive analysis. Flow resistance increases at a geometric rate, however.
4. The back pressure measured at the outlet of the middle section cat was 1 psi. The obvious conclusion is that the front Y and the cat are causing 3/5s of the back pressure. Considering that the rear Y plus the resonators and mufflers and crimped 2 inch tubing are only gererating a single psi, I conclude that the front psi are generated almost entirely by the cat and that the front Y is a very efficient item. As is the rear one.
5. Further splitting this "hair" would be akin to picking fly shxx from pepper. I stopped at that point as I felt that was far enuf to draw conclusions.
6. None of the stock system is salvagable if you are either going to nitrous or installing headers. With nitrous you need to be going higher than a 75 hp shot to bump the limit. My 93 is said to have only 225 crank hp despite Lex advertisements of 250.
The center cat is required on the 93 model ONLY in the California version. Those made for everywhere elso use a resonator in the center section. I have spoken with two people that removed the center cat and still breezed through the smog test inspection. I guess you could say that they are still actively supporting the planet. Smog rules are such that any mod to the required system is prohibited and they don't care if your version is even better than stock. That Toyota went to the trouble of installing a resonator when the cat was not required leads me to believe that a straight pipe might have some cabin noise associated. I will find that out for myself. JBRADY said a 2.5 inch tube was good for the center section. My local "high dollar" ex shop has given me an estimate of $100 to do that.
It seems that Toy engineers did a pretty good job in the design of our system. You can get some improvement but the bang for the buck is the question. Still, if you are going to invest in cams, nitrous or headers I think you can add the new exhaust to your planning budget. If not, save your money.
None of this will ever apply to those that are upgrading for exhaust "tone".
I write this with the full knowledge that any engineer can poke theoretical holes in this or any conclusion or test proceedure. Still!
I hope this is of use to some of you and I certainly invite comment, constructive or even otherwise. I can find out where you live. heh heh heh
John
I had originally wanted to do a definitive and thoroughly and detailed documented analysis of the stock system. That didn't happen for two reasons: one, it proved unneccessary to me at least. Two, I could not monitor the pressure at the outlet of the ex manifold before the gasses enter the first cat. as that o2 sensor was impossible for me to remove without some serious special purpose tool, namely, a hydraulic six point box claw foot extension in the 19 mm persuasion.
I purchased a manifold boost gauge to do this. The one I bought had a full scale deflection of 20 psi and that covered on 90 degrees of the dial face. I could not get any closer than .5 psi as an increment. 1 psi was just barely readable with confidence. The reports of 12 psi of system back pressure must be at the manifold output and that would mean that the front cat was generating nearly 10 psi of back pressure alone and I seriously doubt that is the case.
Conclusions:
1. The system is limited to less than 300 crank hp due to the tubing at the rear half being 2 inches in diameter. And a little less at the bends due to very slight colapsing.
2. The ex manifold is the worst component of the system and that is established fact even if you only subject it to a cursory "visual" inspection.
3. The back pressure at the point between the first cat outlet and the first Y input shows 3/5 s of the back pressure is presant at that point. That was my first measurement and I got 2.5 psi at 5K rpm. Not a screaming deficiency, at that. 6K rpm would have been a better measurement but I had trouble holding that rpm and my guess was that it was only .5 psi above the 5K reading. All things being relative, measurements at 5K are good for evaluating the system for comparitive analysis. Flow resistance increases at a geometric rate, however.
4. The back pressure measured at the outlet of the middle section cat was 1 psi. The obvious conclusion is that the front Y and the cat are causing 3/5s of the back pressure. Considering that the rear Y plus the resonators and mufflers and crimped 2 inch tubing are only gererating a single psi, I conclude that the front psi are generated almost entirely by the cat and that the front Y is a very efficient item. As is the rear one.
5. Further splitting this "hair" would be akin to picking fly shxx from pepper. I stopped at that point as I felt that was far enuf to draw conclusions.
6. None of the stock system is salvagable if you are either going to nitrous or installing headers. With nitrous you need to be going higher than a 75 hp shot to bump the limit. My 93 is said to have only 225 crank hp despite Lex advertisements of 250.
The center cat is required on the 93 model ONLY in the California version. Those made for everywhere elso use a resonator in the center section. I have spoken with two people that removed the center cat and still breezed through the smog test inspection. I guess you could say that they are still actively supporting the planet. Smog rules are such that any mod to the required system is prohibited and they don't care if your version is even better than stock. That Toyota went to the trouble of installing a resonator when the cat was not required leads me to believe that a straight pipe might have some cabin noise associated. I will find that out for myself. JBRADY said a 2.5 inch tube was good for the center section. My local "high dollar" ex shop has given me an estimate of $100 to do that.
It seems that Toy engineers did a pretty good job in the design of our system. You can get some improvement but the bang for the buck is the question. Still, if you are going to invest in cams, nitrous or headers I think you can add the new exhaust to your planning budget. If not, save your money.
None of this will ever apply to those that are upgrading for exhaust "tone".
I write this with the full knowledge that any engineer can poke theoretical holes in this or any conclusion or test proceedure. Still!
I hope this is of use to some of you and I certainly invite comment, constructive or even otherwise. I can find out where you live. heh heh heh
John
03-19-07, 06:41 PM
#2
Good work John. I have been waiting for these numbers. I am glad to finally see them. On behalf of everyone who gets any bit of information from this: thank you!
From experimental evidence, most have said that the early section of the exhaust, namely the header, is the most performance-restricting part of the entire system. This is certainly consistent with your results.
I would be curious to see how your results would differ after seafoaming your 14 year old cats. I know the factory units are able to flow with the best of them when they are fresh and clean.
If sound is your goal, the rear section of the exhaust is certainly the area to begin modifying. A muffler swap can change tone dramatically. On the other hand, if you intend to milk every last horsepower from the 1UZ, it starts up front. Headers are a must.
I can't wait until I am able to throw the money together to get my exhaust system upgraded.
Again, great work John. This is surefire confirmation as to where gains can be made in the stock system.
From experimental evidence, most have said that the early section of the exhaust, namely the header, is the most performance-restricting part of the entire system. This is certainly consistent with your results.
I would be curious to see how your results would differ after seafoaming your 14 year old cats. I know the factory units are able to flow with the best of them when they are fresh and clean.
If sound is your goal, the rear section of the exhaust is certainly the area to begin modifying. A muffler swap can change tone dramatically. On the other hand, if you intend to milk every last horsepower from the 1UZ, it starts up front. Headers are a must.
I can't wait until I am able to throw the money together to get my exhaust system upgraded.
Again, great work John. This is surefire confirmation as to where gains can be made in the stock system.
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