Full exhausts: Dip in HP at low rpms

[Singletrack]

Originally Posted by mixja

Generally what causes the drop in low-end torque as the result of a bigger exhaust system is the reduction in exhaust gas velocity.

QFT

[Subw00er]

So looking at the graph, is the logic then that you can open up the exhaust a little, but only up to a point, then the gains on the top end diminish the low end performance?

[PunjabiM3]

Originally Posted by Subw00er

So looking at the graph, is the logic then that you can open up the exhaust a little, but only up to a point, then the gains on the top end diminish the low end performance?

What:
1. Exhausts that are NOT using the stock primary cats are losing some power compared to stock around 2k-2500rpm.

2. BUT these exhausts are making considerably more power everywhere else across the rpm band.

So now ask yourself this:
1. How often are you actually cruising at 2k rpm? In my opinion almost never. This is a high-reving car not a AMG or American muscle car. Even when you are cruising on the highway at 70mph in a 6MT, you are close to 3k rpms so the loss is irrelevant.

2. What if I drag race? Still irrelevant because you will be launching from around 4k rpm anyhow for the best times.

Summary:
So in reality there might be a loss of 5-10whp between 2k-2500 rpm compared to stock. But throughout the rest of the rpm range you are gaining likely around 15-20whp over stock with HFC's, midpipe, and muffler. So the gains totally outweigh the losses imho. Like someone said before, it would be very rare that you will be at WOT at 2k rpm except maybe in 1st gear. Hope that helps.

[golemite]

Yup its a trade off, but one I consider inconsequential... I'm gaining alot in the top end, and thats where the M3 shines in the first place. Who revs from 2k RPMs in our cars?

[stylinexpat]

Originally Posted by golemite

Yup its a trade off, but one I consider inconsequential... I'm gaining alot in the top end, and thats where the M3 shines in the first place. Who revs from 2k RPMs in our cars?

I see another small dip around 5700...

[stylinexpat]

Originally Posted by askiles

There is no dip in this graph.

-Andy

I believe that Encore offers different types of X-Pipes. Which X-Pipe is used in this graph??

[mixja]

Originally Posted by Subw00er

So looking at the graph, is the logic then that you can open up the exhaust a little, but only up to a point, then the gains on the top end diminish the low end performance?

Ideally you want to:

1. Minimise back pressure (this increases flow capacity and assists with exhaust scavenging)
2. Ensure the exhaust system has sufficient flow capacity (this is never an issue at low RPMs, only an issue at high RPMs)
3. Maximise exhaust gas velocity as much as possible (this will affect low end performance)

Address any one specific issue above can negatively impact the others so it is a balancing act.

Increasing flow capacity by increasing exhaust size will reduce exhaust gas velocity - if you have reached the point where the flow capacity is optimum for making your top end power, then going any bigger provides no gains and will diminish low end power.

Back on topic, I think the dip is an amplification of a dip that exists in the stock system. You can't really build an exhaust system that is optimal for all RPMs without putting in a bypass system in the headers (aka Ferrari) and I think BMW engineered the system for mid to high RPM performance. Tweaking the valve timing may help reduce the dip but I don't think the time/effort/cost to do this will be worth it...

[mixja]

Just confirming I noticed a dip in torque on Akrapovic system - torque is definitely lower from 1500-2500RPM, after which it springs into life.

I actually got measurements showing this on Dynolicious (iPhone dyno app) for a run where I did standing start in 2nd gear (DCT) - however I deleted my previous results on stock exhaust with the same calibration and conditions. Once I find them I'll post up comparisons.

[MrHarris]

Originally Posted by mixja

Just confirming I noticed a dip in torque on Akrapovic system - torque is definitely lower from 1500-2500RPM, after which it springs into life.

I actually got measurements showing this on Dynolicious (iPhone dyno app) for a run where I did standing start in 2nd gear (DCT) - however I deleted my previous results on stock exhaust with the same calibration and conditions. Once I find them I'll post up comparisons.

I just have the Riss Racing X-pipe with HFC, and from 1500-2500rpm there is a like 15-20% loss in power (torque most likely) but after that it just rockets forward and feels much stronger than before. It's like a trade off. Hopefully a tune will help smooth this out.

[aus]

Isn't this why the regular 3 series have that small valve in one of the exhaust tips that' closes at low RPM's and then opens once you get going? You can see it when you're at a light. A lot of the 335 guys unplug the valve to keep it open.

Also, is there such a dip if you just remove the secondary cats?

[BPMSport]

I think that's something that we could smooth out. I never paid any attention to that before. Maybe I'll throw mine on the dyno and see what we can do.

MrHarris: A tune should solve the issue you are describing. If the car is not tuned for the modifications, sometimes the car will drive out of whack.

[mixja]

Originally Posted by Mike Benvo

I think that's something that we could smooth out. I never paid any attention to that before. Maybe I'll throw mine on the dyno and see what we can do.

MrHarris: A tune should solve the issue you are describing. If the car is not tuned for the modifications, sometimes the car will drive out of whack.

Would be great if you could do this - an enhanced Stage II that accommodates full exhaust systems...

[BPMSport]

Originally Posted by mixja

Would be great if you could do this - an enhanced Stage II that accommodates full exhaust systems...

Yeah... Thinking about this, and my car is not the best candidate due to the stock mufflers. I'm ready to get exhaust for it but I think I'm going to sell it and get a 2009/10 model and mod that one up.

Today we dyno tuned a car with full exhaust and racing cats, but my dyno graph only starts from 2700 so it's not visible. If I knew about the issue this morning we would have attacked it today on the dyno. Anyway, the next car that goes in for a custom dyno tune will also undergo some testing to get rid of that dip.

Is this mainly with the Akropovic exhaust or it does it with others as well? I am not sure if the dyno posted by Rom3n was stock or not. Reminds me of that dreaded ZHP dip but thankfully no where nearly as bad.

[mixja]

Originally Posted by Mike Benvo

Yeah... Thinking about this, and my car is not the best candidate due to the stock mufflers. I'm ready to get exhaust for it but I think I'm going to sell it and get a 2009/10 model and mod that one up.

Today we dyno tuned a car with full exhaust and racing cats, but my dyno graph only starts from 2700 so it's not visible. If I knew about the issue this morning we would have attacked it today on the dyno. Anyway, the next car that goes in for a custom dyno tune will also undergo some testing to get rid of that dip.

Is this mainly with the Akropovic exhaust or it does it with others as well? I am not sure if the dyno posted by Rom3n was stock or not. Reminds me of that dreaded ZHP dip but thankfully no where nearly as bad.

From what I've read it seems to be any full system...for the record I'm a Powerchip customer down in New Zealand, purchased through Powerchip Australia.

I do think the car is running quite lean at low RPMs with the full exhaust systems as I noticed yesterday flooring the car @ 1500RPM generated a lot of shuddering, as if the car was running too lean.

[BPMSport]

Originally Posted by mixja

From what I've read it seems to be any full system...for the record I'm a Powerchip customer down in New Zealand, purchased through Powerchip Australia.

I do think the car is running quite lean at low RPMs with the full exhaust systems as I noticed yesterday flooring the car @ 1500RPM generated a lot of shuddering, as if the car was running too lean.

If it was shuttering, it's doesn't necessarily mean that the car is running too lean. The timing may be advanced to much down low. I would have them check that out. If the gas octane isn't up to the task, I wouldn't be surprised that you are experiencing that.

[MrHarris]

An easy way for me to confirm about my "dip" in power is, I floor the car in 6th gear at 2k rpms and it feels real sluggish compared to stock, and stock wasn't that great to begin with at that gear/rpm.