Intentionally increasing backpressure pre-boost?

[kenamond]

So after upgrading my exhaust I encountered a lot of threads mentioning that the decreased backpressure would hurt pre-boost response when the motor is struggling as a low-compression N/A motor. I think I understand the basic idea of backpressure and scavenging on a N/A motor but not enough to think it all through sufficiently. Anyway, it doesn't seem like it'd be hard to add something to the DP to increase backpressure up to a certain boost level then open it up much like a WG as boost builds.

Anyone have any opinions/info/knowledge to contribute? I'd love a 2.3L stroker to help with low-end torque, but that is a bit more involved than some sort of variable obstruction in the DP.

 

[Burnett03]

So you are saying you want to restrict your exhaust flow until you hit boost because you think you are losing power? Come on man, seriously? What turbo are you running? Is the lag that bad?

 

[kenamond]

So you are saying you want to restrict your exhaust flow until you hit boost because you think you are losing power? Come on man, seriously? What turbo are you running? Is the lag that bad?

Normally aspirated engines benefit from exhaust backpressure. Before the turbo boosts (pre-3k rpm) it is running like a normally aspirated engine. So in those low revs, I'm wondering if a backpressure-increasing gizmo would benefit the situation. I don't see why widening the powerband is a bad idea. I'd like to accelerate from a stop in normal traffic at a better pace without having to slip the clutch at 4k. I *can* take off like a bat outta hell, but I'd rather not look like I'm trying to race everyone every time I start moving.

So back on topic...

 

[silver bullit]

Why do you want the power band in the 2k range?

Increase V.E. and displacement Will net the same result.

 

[Burnett03]

Normally aspirated engines benefit from exhaust backpressure. Before the turbo boosts (pre-3k rpm) it is running like a normally aspirated engine. So in those low revs, I'm wondering if a backpressure-increasing gizmo would benefit the situation. I don't see why widening the powerband is a bad idea. I'd like to accelerate from a stop in normal traffic at a better pace without having to slip the clutch at 4k. I *can* take off like a bat outta hell, but I'd rather not look like I'm trying to race everyone every time I start moving.

So back on topic...

I just don't get why you are trying to increase off throttle response when you have a turbo that spools at 3k. If you really want to have better off boost response bump your timing and lean it out in the 1k-3k area when you aren't boosting. Still makes no sense to me why you want to do this. I also don't believe creating back pressure would make anymore noticeable power pre-boost.

 

[Boo5t_Addict]

NA engines don't necessarily benefit from more backpressure. They benefit from the vacuum behind each exhaust pulse helping to pull the next exhaust pulse out of the engine. When you have too big an exhaust on an N/A engine, all the gases kind of fill the piping and sort of hang out, until higher rpms of course. In a turbo application you want the least amount of backpressure possible after the turbo, because when the exhaust gases get to the turbo, they have to go through the hotside, while hitting the fins on the turbine wheel, and come out all scattered. The individual pulses aren't individual pulses any more. Therefore, anything you would do to the downpipe would be of no use to you. If you're just looking for more scoot down low to get around town, slap a small turbo on. The obvious downside would be less top end. That, or just downshift. You'll only look like you're trying to race someone if you actually are.

 

[gob4sho89]

I think that sounds like a good idea,.. But wouldn't that slow your spool, causing your real power to come later?

 

[Slippi84]

I don't see what's so hard about understanding this idea. You add backpressure which hurts performance up top but helps build boost quicker in an attempt to get a stroker like feel and then as soon as boost is there take away the restriction.

 

[TunaTalon]

NA engines at lower RPM's benefit from the higher velocities of a smaller exhaust pipe. The inertia of the exhaust stream is responsible for the scavenging effect not the backpressure.

Higher exhaust backpressure is an effect of the higher velocities not the cause of scavenging. Adding a restriction in the exhaust stream of a 4G63T would not increase the exhaust stream velocity, it would not aid off boost torque. It would slow down the spool up.

To get more low end torque in my Talon:
1. More displacement of the 2.3L
2. 12mm more stroke of the 4G64 crank.
3. Higher compression ratio with the 8.8:1 pistons
4. Rod ratio decreased from 1.7 to 1.5.
5. Smallest turbo to make 320 AWHP (E316G)
6. 264/264 cams optimized for low end/ mid range
7. 4200 RPM stall torque converter to spend less time in the low end.
8. Full 3" exhaust turbo back.

 

[Slippi84]

Yeah that's true because usualy you see people with open dp or Bigger exhaust spool faster. The problem is a restiction in the exhaust of a Na car would increase velocity and restict maximum flow in a turbo car it would not increase velocity as it's after the turbo and the turbo and turbo manifold really dictate along with the flow from the engine of course velocity.

 

[crimsondragon]

I was talking to my friend about something like this the other day...supposedly a friend of his runs a Mustang that has a vacuum cutout. Under partial throttle, the cutout stays closed due to vacuum, but under WOT it doesn't open till vacuum is depleted or something like that. I suppose something like that can be implemented into boost.

 

[mavisky]

Honestly my best advice would be to look into advancing the timing off boost and then setting up a cyclone intake manifold for it.

 

[romeen]

TunaTalon is exactly right. Backpressure is NEVER a desirable trait in an exhaust system. But the smaller piping that is needed to get a certain exhaust velocity will unfortunately create backpressure.

In a turbo car, maximizing the pressure differential pre vs post turbo will create the most velocity past the turbine. Using an exhaust manifold with smaller runners (to an extent) then having a nice big diameter post turbine will aid in spool up but may end up choking off high end performance and hurting V.E.

Here is a good read:

Turbo Exhaust Theory

 

[TunaTalon]

Good link romeen, thanks.

If all of that theory is too much you can remember just two rules for intake and exhaust design.

1. Beer good, fire bad.
2. Velocity good, pressure drop bad.

 

[1992awdlaser]

My first car (97 sunfire) would pull on my friends car all the time. Then the catalytic converter broke on the inside so I had to take it off or replace it. I had a guy I know replace it with a piece of pipe. After that my friends car would pull on mine. Nothing else was changed with my car or his. I don't know why this happens but it does.

 

[kenamond]

Okay. I know all about the turbo aspects of exhaust (well maybe not *all* but plenty), and in my own defense adding a restriction that goes away at 2500rpm (probably don't want to tie off of boost/vaccuum) will not hurt my turbo performance at all. I can brake boost at 2000rpm and do dick in boost. The turbine just can't do anything at that airflow. Then the restriction goes away and I have a normal 3" turboback (or cutout or whatever). But alas, the issue is not backpressure. I also wouldn't mind a continuously variable-length-runner intake (cyclone is sort of in that class), 2.3L, AEM EMS, etc. but my idea was (hopefully) a lot cheaper. Oh well. Thanks guys.

 

[Mr Peepers]

The question isn't why he wants to do it but how. Anyway, I like where you're headed but Boo5t Addict, TunaTalon and Romeen are all dead on IMO.

Backpressure is often confused with velocity since they often go hand in hand.

I'll try to scrounge up some other good links tonight; the one from Jay Kavanaugh is pretty damn good.

p.s. Your turbine housing is already a nice source of backpressure.

 

[bvass]

This - EXTREME PSI : Your #1 Source for In Stock Performance Parts - Apexi Active Tail Silencer : 115mm Tip might give you the increased back pressure at low revs. In the ad they state that you loose a little bit of power, but I'm not sure if this is across the rpm band or just peak hp.

 

[synchronic]

The only thing to keep in mind with increasing backpressure, is that backpressure is a major factor for increasing detonation, not to mention the bad scavenging. But, if you're running low enough compression, you won't ping anyhow. Then again, if you sit in a high backpressure condition long enough, you get the mean temp of the exhaust valves (or build-up and debris) that much hotter and it may not dissipate that heat fast enough and will become a problem when you get into boost, which is where you wouldn't want detonation.

 

[crimsondragon]

Well recent events have caused me to question this thread. What is this backpressure business about? I just upped my whole exhaust to a 3 inch and now, all the T-too small wants to do is spool so badly. I went from spooling at 2.5k all the way down to 2k. Screw backpressure.

 

[TunaTalon]

That's pretty much the consensus of this thread. On turbo cars, backpressure is all to the bad.

The three inch turbo back exhaust with hi-flow cat behind my T-25 was a dramatic improvement. It started pulling up steep hills in fourth like it did in third with the stock exhaust.

 

[delta448]

Like Peepers said, your turbine is a huge source of backpressure already. I don't think you really want more.

If you add even more restriction post turbine all you're going to do is slow your spool time. You won't see any noticeable low end power increase. People complaining of losing power after freeing up their exhaust, did they actually tune their cars after they changed their exhaust?

I'm willing to bet that if you pulled the turbo and ran the exhaust straight dump out the exhaust mani you'd have all kinds of engine response, low CR or not. If anything, the reason your car is a dog in the low end is the same reason mine is, our turbine housings are small and very restrictive to overall flow. Scavenging effects in N/A engines usually occur in higher rpms, when there is some actual velocity in the exhaust pipe to pull the flow through the cylinder head.

 

[PieEyedPiper]

http://www.dsmtuners.com/forums/fre...8578-exhaust-straight-scoop-backpressure.html
I'm surprised no one has linked to this yet.

Kenamond, I do sympathize. I can certainly relate to your comment about looking like you're trying to race every time you leave a stop sign. I still run a stock exhaust and the problem is still a big deal for me. I don't think that your answer lies in an exhaust manipulation scheme, but rather in weight loss of unsprung weight.

I think that some 14lbs (rx7 FD?) wheels would go a long way to increase driveability in the low end.

So, combining light weight wheels, increased timing and a leaner tune in the low end might get you to where you're going.

 

[dsm-onster]

Applying a resistance to flow in an n/a application NEVER EVER increases torque or performance or gas velocity. Backpressure NEVER EVER increases scavenging ability or torque. It's the other way around. Backpressure is a side effect of scavenging. Good scavenging: exhaust pulses are timed to allow the vacuum at the end of each pulse to suck the next pulse out of the cylinder. An unfortunate side effect: back pressure at other rpms where the pulse speeds are out of sync with the exhaust valve.

There's some good advice in this thread. For example, use an electronic or vacuum controlled cut out. Choose an exhaust diameter that best benefits the rpm range before spool up, then have the cutout open at an rpm point or boost/vacuum level after. All of this debatably won't work because the turbine wheel that is in the way may slow the pulses down and cause them to bottleneck at any low rpms anyway causing strange resonances for scavenging and other side effects. . .

Your big t28 spools pretty fast anyway .