My dyno Numbers

[mTk]

I made 130 whp, 144 lb-ft torque (129.7hp actually)

5300/3100 rpm respectively, here's my best recreation of the dyno sheet, don't have a scanner

Took 3 runs to get a baseline, 2 and 3 were the exact same. AFR was a steady 12.3 from 4250 to redline. 13 from around 2700-4100. Max psi was ~9.3

http://mtk.darkwire.com/dyno.jpg


MK

[evolutionmovement]

Assuming a 25% driveline loss, that's almost exactly the stock specs for crank HP and torque. Interesting.

Steve

[mTk]

WRXs on the dyno run ~155 stock, but yeah, almost stock numbers

MK

[Flip_x]

whats an NA whp hmm guessing 90 lol

[georryan]

Interesting, so the 3" exhaust didn't push up your whp by much at all, did it?

[BAC5.2]

Ryan - Don't be scared. The 3" exhaust isn't a performance booster. It's an aide for other performance boosters. It's like a cushion. It allows you to do other things, and that's why it's a good mod. You will see a gain from your exhaust when you start other mods.

Look at that torque curve! If we had a VTEC style system or some other valve timing control, you could make a table out of the torque curve.

[LegacyT]

I heard a good turbo-back should add around 30 chp, why did this system add nothing?

Mark,

[georryan]

Ryan - Don't be scared. The 3" exhaust isn't a performance booster. It's an aide for other performance boosters. It's like a cushion. It allows you to do other things, and that's why it's a good mod. You will see a gain from your exhaust when you start other mods.

Yeah I'm not too scared, just surprised. I feel like my torque has increased at least, and It feels like I have more power as well, but maybe that is just he boost creep.

[BAC5.2]

I heard a good turbo-back should add around 30 chp, why did this system add nothing?

Mark,

It will, with proper tuning and setup. A 3" exhaust on a stock car will hurt performance more than anything.

Turbo cars follow the exhaust tendancies just like N/A cars do. To large an exhaust reduces exhaust velocity. You can get 30hp out of a good turbo-back with the proper setup. But everything else stock, and a 3" exhaust, you'll actually HURT power more than you'll help it.

You have to MAKE the exhaust in order to take advantage of the 3" exhaust's ability to evacuate it.

I could explain at length exhaust flow characteristics, but I don't think it's necessary. Exhaust is one thing that I understand pretty well.

[Yukonart]

I could explain at length exhaust flow characteristics, but I don't think it's necessary. Exhaust is one thing that I understand pretty well.

We know, Phil. . . .it's all those nights eating spicy Messican food, isn't it?

[BAC5.2]

Spicy mexican and lots of beer = blown cats (if you know what I mean)...

[Yukonart]

Spicy mexican and lots of beer = blown cats (if you know what I mean)...

Now when someone says they've blown their bloody cat, I'll be traumatized for life.

[BAC5.2]

Glad I could put that image in your head.

[vrg3]

[Matt Monson]

I want my 3 minutes back...

[rsstiboy]

I heard a good turbo-back should add around 30 chp, why did this system add nothing?

Mark,

It will, with proper tuning and setup. A 3" exhaust on a stock car will hurt performance more than anything.

Turbo cars follow the exhaust tendancies just like N/A cars do. To large an exhaust reduces exhaust velocity. You can get 30hp out of a good turbo-back with the proper setup. But everything else stock, and a 3" exhaust, you'll actually HURT power more than you'll help it.

You have to MAKE the exhaust in order to take advantage of the 3" exhaust's ability to evacuate it.

I could explain at length exhaust flow characteristics, but I don't think it's necessary. Exhaust is one thing that I understand pretty well.

this is only true with the tiny turbo's you guys have, with anything decent sized you will make alot more power just by adding a exhaust.

[scottzg]

this is only true with the tiny turbo's you guys have, with anything decent sized you will make alot more power just by adding a exhaust.

I guess he needed a "exhaust manifold back exhaust"

[BAC5.2]

I've always understood it that a larger exhaust simply allowed you to run more boost, and that exhaust alone at stock boost levels would, at the very most, keep performance roughly the same, if not degrade it slightly.

[91White-T]

An exhaust on an otherwise stock car will flow more air and therefore spool faster and make more boost(if the control allows it). I really don't see how any kind of bigger exhaust could hurt a turbo car since the more exhaust flow there is through the turbine, the better performance. That's the whole point of a turbine, difference between inlet and outlet pressure, right?

[mTk]

An exhaust on an otherwise stock car will flow more air and therefore spool faster and make more boost(if the control allows it). I really don't see how any kind of bigger exhaust could hurt a turbo car since the more exhaust flow there is through the turbine, the better performance. That's the whole point of a turbine, difference between inlet and outlet pressure, right?

My thoughts too

MK

[totech]

temperature AND speed

[BAC5.2]

An exhaust on an otherwise stock car will flow more air and therefore spool faster and make more boost(if the control allows it).

Exactly what I was saying. A larger exhaust allows more boost. 8psi is 8psi, if you have a 3" exhaust, it's still 8psi.

I really don't see how any kind of bigger exhaust could hurt a turbo car since the more exhaust flow there is through the turbine, the better performance. That's the whole point of a turbine, difference between inlet and outlet pressure, right?

There is a point where big is too big. The fine line between no back pressure, and reduced velocity.

It's exactly like on an N/A car of a larger displacement. Exhaust can be to big.

Let me put this into perspective.

Stock boost. Stock piping. Full boost arrives at around 2400 RPM's making almost 9 psi.

Now strap a 4" pipe from the turbo back. Exhaust enters the turbo at the same speed as before (same diameter up and cross pipes). Exhaust exits the turbo slower because of the larger pipe diameter. The exhaust velocity is actually reduced.

On a Turbo car, this doesn't really matter much because the business end of things is already done, and the turbo is the point of largest restriction in the exhaust. But a reduction in velocity is a reduction in velocity. Slower output velocity causes poor flow and reduces output at the rear of the car. The air basically packs in the exhaust pipe, only letting a little bit out.

Get a bigass straw, and a regular drinking straw. Blow through each equally. The bigass straw is a flow reduction. Kink that straw, and that's backpressure. The regular drinking straw is the appropriately sized exhaust pipe.

You'd still make 8psi with the 4" exhaust, but your flow would be terrible. And as we know from school (lol), low flow = low go. Espically when the wastegate opens up, goddamn, got turbulence?

[vrg3]

8 psi doesn't always mean the same power, though.

Reduced backpressure (even if it's through a turbo first) means there's a smaller pressure differential between the intake and the exhaust. That means it's easier for stuff to get through the engine. That means more power at the wheels.

It's true that reducing exhaust gas velocity is bad, but the turbo flows so much more than the stock exhaust can handle anyway.

It's generally well documented (I know it's dangerous to say stuff like this!) that freer-flowing exhausts on turbo cars make noticeable improvements in torque.

I don't think MK is producing stock power. I think he's producing more than stock power.

[91White-T]

BAC5.2, what you're saying makes perfect sense for a N/A car, but a turbo, even a small one, flows so much air that the freer flowing the exhaust is, the better. Once the air exits the turbine, the flow restriction is in the size of the pipe. The exhaust coming off the turbine is moving at a very high speed, so the bigger the pipe, the better.

[Brat4by4]

Post Turbo Exhaust
The main performance goal of a post turbo exhaust is to create the least amount of backpressure possible.

The only reason to reduce the size towards the end of the pipe is for packaging, cost, and noise reasons. Tapering the diameter does not make more power, torque, or bring on boost faster. However having smaller pipe towards the end has less effect that having smaller piping at the beginning. In other words a system that has 3" pipe for the majority, and necks down to 2.5" at the end will flow enough for more power than a complete 2.5" system. The further downstream you neck down the exhaust the better……..if you decide to neck it down.

This is straight from the Cobb web site.

And for those of you that like to run really high boost on our small turbos:

Too small of exhaust housing and you severely limit the amount of boost and top end power you can make. You can only push so much gas volume through a small housing without having negative side effects. Adding to the complication is that each pound of boost created makes a ratio of backpressure before the turbo. It is different for each turbo, the amount of boost you are running, the size of the motor, RPM, and load on the motor. Once you start trying to push too much through the exhaust section of a turbo (running too much boost for the turbo) you start making a huge ratio of backpressure, and it only gets higher the more boost you run. This not only limits the amount of power you can make, but makes EGT go up, hinders the motor's ability to get the burnt gasses out of the motor, and makes the car more prone to detonation.