There's a night and day difference in low-end when you go from stock to aftermarket exhaust, but what you sacrifice there is made up for in top end power.

 

ive ran a few setups... dp to 3"x4" oval... ---> 4" turbo back... ---> foot long 4" dump tube... each was quicker then the one before, everywhere... i think it was mainly because of the better spool (more/quicker boost after "x" amount of time meant more power)... if you were NA you could have too much flow, but not with your turbo...

now i have a 4.5" dp into a 3"x6" oval exhaust (volume of 4.5" round) FTW...

brian

 

The check engine light and safe mode report makes me think something might be wrong with the O2 sensor. Did you ever pull the codes from the cel?

 

Too much exhaust flow no, too large an exhaust pipe, yes. Contrary to conventional thought, a large exhaust pipe will hinder flow in that the turbos can't push the air through smoothly, resulting in more turbulence. The turbulence being small whirlpools and eddies, don't evacuate the pipe, they clog up the system. An exhaust pipe should always be matched to the turbos being used. Fwiw, I made 891rwhp on a 3"OD exhaust, 2.75" ID. The stock twins aren't outflowing a 3" exhaust anytime soon.

Eric

 

and the energy goes into creating heat and noise as oppose to increasing flow

a large exhaust pipe will hinder flow in that the turbos can't push the air through smoothly, resulting in more turbulence. The turbulence being small whirlpools and eddies, don't evacuate the pipe, they clog up the system.

 

thank you sooo much for the replies especially cowboy bebop. Thats why i figured but the car came with the 4 inch down pipe so im most likely going to order the rmm dp.

and as far as Nick i have a 93 i believe its an obd1. Idk what the deal is with that. honestly i would get rid of the Safc if i could but its my daily so i cant take the chance of not wireing it up right and my car not run at all.

 

cowboy... i know you know more about this stuff then me and i dont doubt you made 900 on 3"...

BUT...

have you ever seen a supra make less hp on a 4" then a 3"?.. i mean apples to apples... smooth mandrel to smooth mandrel... i could see a possible problem with 4" if you were using it on a 400cc turbo motor, but, on ours?..

maybe overkill, but, a hindrance?.. again, just wondering...

brian

 

I'm with you on this. Wasn't it HPF or BL that did a back to back between a bunch of 4" exhausts on a car making ~450hp? I believe they were able to notice a difference between different 4" exhausts. They didn't even test a 3" unit.

Daniel

 

You may also lose boost control precision with stock twins and a 4" exhaust. Too much flow can mean that the wastegate(s) won't work properly.

 

steve... i think youd have a slight problem running the factory 12psi but if you can live with the 16psi or so you get from the free'er flowing 4" you can accurately regulate from 16 up...

cowboy... i knew there wasnt really any hp to be gained up top from a 4" at bpu... however, anything worth doing is worth doing right (the first time)... no need to buy a 3" just to turn around and want a 4" when you go single... at around 700whp people have seen nice gains in hp from the bump to 4"... why not just go there from the start (if you can afford it)... then you wont see gains going bigger til youre pushin 1200whp... and i still think my car spooled just slightly faster with the larger pipe...

so mkivtt03 doesnt ditch a 4" unnecessarily...

a good 4"@bpu doesnt = lower power than a 3", right?..

thanks again
brian

 

Based on the findings of some others who live by me, the problem was actually controlling high boost, not stock levels of boost. Those of us who choose to run stock twins have settled on 3" exhausts as a result of those experiments. Stu Hagen is one of those who can shed some light on this if interested.

Steve T.

steve... i think youd have a slight problem running the factory 12psi but if you can live with the 16psi or so you get from the free'er flowing 4" you can accurately regulate from 16 up...

 

I'm sorry guys. There is no way I will believe a 4" exhaust will not outperform a 3" at any of the power levels we are talking (400+) w/o some proof. The difference may be very small at 400hp, but I can't swallow that it will be less. I am not a flow expert by any meens, but I have stood behind a supra on a dyno and the stock turbos did not seem to be having any trouble with the 4" exhaust. I know that isn't scientific in the least, but please enlighten me with some data. I for one would never hesitate about going 4" with the stock turbos if you were planning on going single someday.

Daniel

 

You are talking about a 25% increase in surface area that should give you a hp increase but *likely* only at higher RPMs. What cowboy refers to is gas velocity and increasing the surface area of the exhaust and creating a reduction in back pressure will affect your power below peak.

In essence, the larger pipe you use, the more you will choke your engine's ability to evacuate gasses at lower RPMs for an increase in higher RPMs. In the example using the Veilslide Ti exhaust, it's probably a terrific exhaust provided you're producing power levels to generate enough exhaust volume to take advantage of the increased diameter.

I have little understanding of the 2JZ, yet. I can only therorize what your butt dyno tells you without seeing the dyno test results back to back to understand how the same engine behaves.

If dyno results from a 3" diameter exhaust tell you anything, a 5" exhaust is good for bragging rights but not much else without the rest of the setup to take advantage of it.

 

Ask Mr.5" himself- Johnny Bahn! :tongue:

 

I am running a 4" on the stock twins with a 3"dp, and to tell you the truth I really dont like it. The car does not realy sound that great, it sorta lost that Supra exhaust tone that we all love, unless you are WOT. Either way it will be single soon at which point I am hoping a 4" dp/ mp will not hurt my performance.

 

I had a friends GTR on the dyno, bone stock car made 260 AWHP and with a 3.75" catback it made 285. A bit faster spool, and lots of midrange power. If you think a bigger exhaust isn't going to make a difference on a factory turbo car, well..........

 

Your right, the turbine does backpressure the engine. However, keep in mind that a turbine works on a pressure ratio. If your turbine takes a pressure ratio of 3 to deliver the needed compressor power (for a given condition/boost), then changing the exhause backpressure will be multiplied on the engine. Here is an example for a pressure ratio of 3:

Pressure ratio = 3 = [turbine inlet pressure + atmosphere]/[turbine outlet pressure + atmosphere]

For the best case, turbine outlet pressure = 0. So,

3 = [turbine inlet + 14.7] / 14.7,
turbine inlet pressure = 29.4psi

if exaust pressure is raised to 5psi (not unreasonable)

3 = [turbine inlet + 14.7] / [14.7 + 5]
turbine inlet pressure = 44.4psi

That shouldn't be shocking (44.4-29.4 = 15psi and 3*5=15). Get it.

A small change in exhaust pressure can kill engine delta P.

Daniel

 

I guess I am not quite sure what you are trying to get across here. It seems like you are saying that raising the backpressure will cause the engine to perform more poorly. I agree...

 

Hey guys please allow me to add my opinion, I am no expert. I agree with cowboy that is the issue on a na but for a turbo its harder to argue. I understand it this way, you have a bigger volume you have to fill, Bigger the volume slower the velocity. I'd imagine a 3" is quite practical for whatever geometrical reasons. Since the gas is compressible due to its density, I would imagine that might help to cushion the change in velocity when you have a lot of flow, but probally not to much. In my mind I imagine that the velocity has to be a certain amount in order to sustain a smooth flow. If you cannot sustain that velocity because of the volume the gas has to fill is too large you cannot sustain a smooth flow of the gas your causing a cloging effect on the system because of the turbalent flow. What bothers me is that from 3 to 4 inch is a small change in volume and on top of that you need a very high velocity to compress gas.

To answer your question I say yes its very possible, but for our examples a 3,4" is just not much of a difference in our systems good operating parameters hence the small changes in performance between the two. Try a 1" or 10" and you are guaranteed to have major problems, obviously. Not too big not too small it all depends on the application

 

I have to head back on out but I just thought I'd throw in that exhaust gas is NOT compressed at these velocities. The exhaust would have to be going a lot faster. On the order of .5 Mach or so. Compressibility does not come into play here.

Also, most of the losses come from friction at the pipe/exhaust WALLS due to viscous effects, not due to turbulent eddies.

 

On a turbo car your answer is for the most part no. A correct sized exhaust is helpful for a NA application where you can use a tuned exhaust pulse wave to "pull" the exhaust gas out of cylinders as it moves through your exhaust system, but a turbo system simply benefits more from the pressure drop and less strain on the turbo.

Cowboy is right that you don't need a massive exhaust system, but I doubt any real world system is going to make more power than a open exhaust. I will say every car is different and if you have the ability to setup a perfectly sized exhaust system for your setup it would be better, but I don't think it's realisitic for the small gains (in a small range too) you would get from it.

 

Sooo are you guys saying 4inch is 'overkill' for a stock twin car.. Or a Supra that is performing at 400-500whp?

I am asking this because I'm thinking on the lines of small single Supras.

 

My understanding of turbo exhaust theory is, your exhaust (in an ideal world) should taper from the outlet 3" to what ever size exhaust pipe you want (as long as its larger).

http://supraforums.com/forum/showthread.php?t=227843