What size for Down-Pipe for KA

[SingleCamSam]

I keep seeing people say that 3" is too big for a T3/T4 and that 2.5" is better? Whatcha think? People keep mounting 3" DP's on little SR turbo's, so wouldn't a 3" be better for the KA as well?

[encasemyheart]

The outlet on a t3 turbine isn't 3", it's smaller, so that's probably the reason. Maybe not though.

[encasemyheart]

Also I think the SR20 3" downpipes are 2 1/2" to start and expand to 3" after that. That's how the 3" downpipe was on my Eclipse anyways.

[SingleCamSam]

Good point. It does look like they expand from 2.5 now that i look closer.

[huguetpj]

but the question is.... is a 3" downpipe/catback really that good on a 2.5" exhaust sided turbo?

[SingleCamSam]

Exactly, it's probably no better than 2.5"

[:: orion ::]

But the less restriction, the better (with KA-T).

With a turbo car, you can almost not go too big - Especially with a 2.4 liter.

Remember, an N/A 240SX can max the flow on a 2.5" full mandrel exhaust (barely), so a turbo KA can do it at barely any boost.

So while it's not needed, it will most likely give more power...

I'd say 2.5" elbow out of the turbo (it won't help to have 3" here, the outlet is 2.5" or less as you pointed out), then 3" DP to the cat/test-pipe, then 3" cat-back.

Unless you're running an open wastegate, then you can get away with a little less...hehe.

Later - Brian

[huguetpj]

Yep, thought so. I'd think I'll go ahead and have the DP done at 3", don't really have the money for the rest of the exhaust... yet.

[KApwr]

With turbo cars, you want the biggest you can fit in there. Lower the restriction in the exhaust= more power for turbo cars, not NA. Reason being is that the turbine housing itself imposes a lot of restriction in the exhaust flow so anything that will help expedite the flow of exhaust will be beneficial.

[encasemyheart]

From Max. Boost:

200bhp = ~2.15"300bhp = ~2.25"350bhp = ~2.4"400bhp = ~2.5"700bhp = ~3"

This is the size where flow starts to become a consideration. Keep in mind that this is in bhp, which is more than whp. Still, it's kind of surprising that before 2.5" pipe size would become a hinderance to flow you would have to be making ~350whp. So depending on your goals this is what you want.

[SingleCamSam]

Quote »With turbo cars, you want the biggest you can fit in there. Lower the restriction in the exhaust= more power for turbo cars, not NA. [/quote]

This is what i thought as well. But you have to take the scavenging effect into account even with a turbo set-up. If your pipe is too big, exhaust pulses won't be pulled along by each other as efficiently, therefore they will slow down, and the turbo won't spool as quickly. Just a thought.

[encasemyheart]

This is what i thought as well. But you have to take the scavenging effect into account even with a turbo set-up. If your pipe is too big, exhaust pulses won't be pulled along by each other as efficiently, therefore they will slow down, and the turbo won't spool as quickly. Just a thought.

This is true along with the fact that the exhaust gas velocity increases as size goes down. So it's like intercooler piping, to achieve best results you should size appropiately with the amount of power you will be making.

So for most people the max you want to go is about 2.75" exhaust. I am going 3" which is a bit oversized but my future goals include 400whp on race gas, so I think it is reasonable.

[C-Kwik]

With a turbo motor, scavenging effect is much less of an issue. Typically, you will find that a larger exhaust/downpipe is more beneficial to a turbo motor because of the pressure differential before and after the turbo. This is a lot of what helps a turbo to spool more quickly and efficiently. The less pressure there is behind the turbo, the less there needs to be in the manifold to create the same boost. This means more air can be bypassed through the wastegate allowing less backpressure in the manifold. This yields more power. Most turbo headers tend to have poor scavenging properties anyways. Hardly something to spend a whole lot of time thinking about.

[Xero]

have you ever seen the turbo Honda drag cars, though? They have a HUGE collector after the turbo, but it then narrows own to a smaller pipe out the side. I don't know if it's for regulations, or if it's because the turbo still needs a little back pressure, but I don't think they'd do it to hurt themselves unless it was beneficial,

[SingleCamSam]

Quote »Hardly something to spend a whole lot of time thinking about.[/quote]

Yup, probably not something to rack your brain about lol. I was just wondering since i see alot of people say that 2.5" is plenty for a T3 turbine.

Quote »but it then narrows own to a smaller pipe out the side.[/quote]

I saw this as well, which is part of the reason why i thought of this question. Maybe it's like a velocity stack for the exhaust to speed up flow and spool.

[C-Kwik]

have you ever seen the turbo Honda drag cars, though? They have a HUGE collector after the turbo, but it then narrows own to a smaller pipe out the side. I don't know if it's for regulations, or if it's because the turbo still needs a little back pressure, but I don't think they'd do it to hurt themselves unless it was beneficial,

I hate to speculate. But for one, you can't really have a collector AFTER the turbo. Collectors are the portions where one or more exhaust pipes join into one. Since all the runners would merge before the turbo, the collector would not exist there. Secondly, Are you sure what you were looking at was the exhaust pipe and not the dumppipe? Most race turbo motors will use an open wastegate dumppipe which will be smaller than the actual exhaust pipe. All drag honda motors I've seen use the same size pipe diameter from the turbine exit to the end of the pipe. Here's a couple of pictures of Myle's Bautista's drag motor:http://www.turbomagazine.com/f...m.jpgh ... /f...m.jpg

I don't see this large collector you speak of that narrows down to a smaller pipe. In fact, as far as I can tell, it remains the same size from the turbine to the exhaust exit.

As far as needing backpressure, turbos do not need any backpressure. Neither do motors. But sticking just to the point of the turbos, keep this in mind. The amount of flow through the turbine of the turbo will be directly related to the amount of pressure in the manifold vs the amount of pressure in the exhaust. The more of a difference here, the more air will want to pass through the turbine. lets put it this way. imagine trying to pump air into a tire that has 40 psi in it already. Then try to pump air into a tire that has 5 psi in it already. The tire with 40 psi will be much harder to pump air into since there is 40 psi of resistance. Turbos are much the same way. So, the less back pressure in the exhaust after the turbo, the less pressure is needed in the manifold to have the same pressure differential. And less pressure in the manifold means less backpressure for the motor, which equates to more power.

[TurboKA37]

u lost me after "I hate to speculate"

[Xero]

first off, sorry for my incorrect termonology, I was sleepy

After finally finding the pic I was looking for, it's of Team Bergonholtz's drag/show CRX.

also, I didn't mean that backpressure is great for a turbo car, but there's gotta be areason why a drag car is running an exhaust that goes large, then small again before it dumps out the side. Looking at it from my point of view, I'd assume that it's used for harmonics of some sort, don't know why, but it's gotta work, since they were the first ones to take a unibody Honda into the 9's

in reality, your not going for every ounce of HP from your car, so get something that works, not that's THE BEST, there's no need, go cheap,

[MarkEmark]

I have a T3, and it came with a little over than 2.5" downpipe, so I used that same size the whole way through...it runs very nicely, quiet, yet burly when you want it. I'm only running 6/7 psi though...