cbseven

iTrader: (1)

Wouldn't you say that the 535whp car is a much different animal than my unmodified IX? Whatever took that car to those power levels is likely to have a significant impact on where the powerband lies. Suspect it would be much higher than where I am hoping to have this car.

Plans right now include a drop-in AMSOIL EAO panel filter, fuel pump (possibly the Aeromotive 340 stealth), and appropriately sized injectors for an e85 conversion. Short route intercooler pipes are also in the plans. I'm still undecided on the e85 because it is pretty scarce here - but it might be worth the trouble, and if so, I'll be doing dual maps.

What would you guys say this setup would net me?

Butt Dyno

iTrader: (10)

You want to shut people up -

do back to back, same day testing with both options.

No one does this because it's expensive. But if you don't do this, or at least some back to back testing on the same dyno, you'll never know if you're right.

Gardus

Yes but I don't want to shout it as I would be accused of being partial toward my company.

03whitegsr

iTrader: (8)

The 14 degree cone is meant to be a divergent cone (smaller -> larger) and 14 degrees is about the maximum angle you can use without boundary layer separation.

The 30 degree cone is for convergent transitions (larger -> smaller). Because you are constricting the flow, boundary separation is much less of a concern and the more abrupt transition works fine in that situation.

Just something to consider.

94AWDcoupe

iTrader: (125)

I disagree because I have seen good back to back tests that showed the exact results I observed.

It better stated as "some setups" will actually work better with a 2.5 exhaust over a 3.0. A 35r setup is no where near the "some setups" that actually would benefit. 35r is a huge turbo and will always work better with a 3.0. a 2.5 will just plain choke it. a DSM 16g on the other hand is a very small turbo. the turbine outlet is only 2.2 inches. a 2.5 exhaust doesnt choke it. a 2.0 exhaust will however always choke it. to see this effect take place with a 35r you would need to be in the 600-700who range. switch from 3.0-3.5 can produce some low torque losses. maybe talk to Dave about it, he made 3.5 exhausts. like 20 of them. when they were gone he stopped selling them. testing showed for most setups the 3.0 was a better choice.

on my mirage the 16g with 2.5 in fifth gear pulled harder (3000-4500rpm) than the car did in 4th with the 3.0. the torque loss was nothing short of massive.

the honda that made 550whp through a 2.5 used a 944 turbo kkk27. not much bigger than a 20g.

in most cases a larger exhaust simple will free up horsepower. the exhaust pulses helping evacuation does exist in turbo cars. the problem is its very delicate to achieve. has a very small window to get it working . normally aspirated exhausts that work best with a 2.5 would fall flat with a 2.5. a 1/4 inch increase in tubing size is a huge change.

"forced induction performance tuning " by A. Graham Bell has some good reading on exhaust system design and how it affects the power band and throttle response. the chapter is some 40 pages. lots of good tech from someone who knows far more than likely all of us put together. not once does he say bigger is always better.

barneyb

I remember reading about an X that was being prepared for being the subject of a magazine article and Mitsubishi but on a larger diameter exhaust in front of the converter while leaving the rest of the exhaust stock. It makes sense, as the exhaust cools while traveling down the piping the pressure goes down. What you are tying to avoid in a turbo car is back pressure. Most of the gain from big exhaust is up front. Having the three or four inch pipe at the rear is less important.

As far as exhaust pulses having any effect, after going through the turbo wheel, what pulses?

JohnBradley

iTrader: (30)

JohnBradley

iTrader: (30)

The changes were going from a restrictive HKS non-Ti cat back to a 2.5" catback and 60mm dump on the testpipe.

JohnBradley

iTrader: (30)

Here is my car with the same 3.5" ETS exhaust at the same boost level. The difference is the 790 is with an aeroturbine 4" straight thru and the 739 is with an offset entry/center exit 3.5" Magnaflow muffler. Just the little additional backpressure (this was prior to me adding a dump on my car) killed 50whp. Smaller is always less, less pipe diameter is less power.

crx-si

damn aaron, now you make me tempted to fire up my welder and make a 4" alum exhaust for my car

burnzy

iTrader: (1)

how come the bigger exhaust gained spool in the top dyno and lost spool in the second one? seems to have lost a good 800rpm? dunno if i would trade 800rpm for 50 up top

94AWDcoupe

iTrader: (125)

its funny to me that this graph clearly proves your point.
but what you dont see is it also clearly proves mine. the blue line is a far more usable. "powerband"

those spool and torque losses creates a dull boy. read my signature.

Construct

iTrader: (4)

Good read for sure, albeit a bit dated. Check pages 270-271, where says that bigger is almost always better. He does mention one engine that did worse on a 3.5in exhaust than a 3in exhaust, but goes on to say that the same engine performed well again with a 4in exhaust. Then he also says that using a slightly smaller exhaust (1/4 inch) in the muffler section might result in a 'slight low rpm performance gain', but he still prefers to maintain the exhaust diameter all the way through the muffler 'if at all possible'.

Good read though, definitely recommended.

By the way, I'm not trying to argue here for the sake of argument. I'm all for back-to-back testing and putting results over theory. However, I just don't want people to read through the forums and conclude that putting a smaller exhaust on their turbocharged car is going to net them any significant performance gains when all evidence and theory points otherwise.

Construct

iTrader: (4)

Did you look at the previous graph, where both spool and power are significantly lower on the more restrictive exhaust?

My guess, given the huge horsepower numbers, is that the motor is using some camshafts with some serious overlap. In this case, exhaust gas back pressure can definitely give low-end torque gains by promoting cylinder filling at slower engine speeds. But that doesn't really help us much here, because you wouldn't use cams with massive overlap if your goal was mid-range torque. The larger exhaust is still a better fit for the system.

94AWDcoupe

iTrader: (125)

of course. but I dont see that first graph as anything useful. you cant argue a point like this using a "restrictive" exhaust of any sort.

people have gone from 3.0 exhaust to 3.0 exhaust from different vender and gained 50whp.
interpreting data is always key.

anyone who thinks I am recommending you go try a 2.5 inch exhaust doesnt know how to interpret data either.