Testing exhaust back pressure

[diesel des]

Yea I suppose if you want zero backpressure then 3 inch isn't enough for 500 bhp...

[diesel des]

Update. I hurriedly made and fitted a twin 2.5 inch system with 2 straight through boxes to my car last night to this morning to be ready for mapping. I haven't the valve fitted yet to one pipe to quieten it down. Man does it drone between 2.5 and 3.2k rpm!

Anyhow backpressure is reduced from original 0.5 bar to 0.25 bar.

Original system.


New system with 3 inch downpipe splitting to twin 2.5 inch at propshaft.

[EspenW]

Sweet!
Piping size is as before, but the mufflers obviously flow better?

[diesel des]

Original pipe sizes were 3 inch from down pipe to near rear diff. Then a magnaflow 3 inch in 2 X 2.5 inch out. That went to the sebring back box made for stock urs4 UrS6 were it reduced to 55mm twin to tail pipe. All boxes were straight through but sebring looks slightly restrictive to me.

New system is the same 3 inch downpipe to the end of the gearbox. Then just after the gearbox splits to 2 X 2.5 inch with smaller straight through boxes to the tail.

Less back pressure but much more noise. Very bad drone noise now at cruse. I might try the exhaust valve idea or go back to what I had and suffer the backpressure. Performance feels similar on both exhausts. Sounds faster though!

For reference these measurements are at 450bhp 518 ft lb torque stroker.

I wonder if a 4 inch downpipe and original system would be a compromise. If the original system is causing 0.5 bar back pressure total and the new system 0.25 bar maybe the 3 inch down pipe is say 0.2 bar of the restriction. Don't know. Could waste weeks and lots of stainless trying for a few bhp and psi backpressure.

Maybe keeping the wastgate separate and running a second 2 inch pipe for it to near the rear with a small silencer on that would be good. Almost 50% of the flow is through the wastgate at max power so diverting that away from turbine flow might be a good thing and it wouldn't need a big pipe as it's at high pressure.

Anyone running a long screamer pipe with silencer?? I have heard short screamer pipes and I don't want that!

[EspenW]

yep, would be interesting to see the effect of the reduced backpressure had on power. Probably not a whole lot, as it was not that bad before.
Do you have any WOT logs in 3rd or 4th gear (on the road) with the old exhaust?

[driftin jo]

If you shut one of the 2.5 inch pipes for cruising it will be much better.

[diesel des]

If you shut one of the 2.5 inch pipes for cruising it will be much better.

That's the plan. I have a actuated valve I will fit in one pipe to shut it down on cruse. even at idle it's very booming now but if you cover 1 tail pipe it quitens up nicely

[diesel des]

yep, would be interesting to see the effect of the reduced backpressure had on power. Probably not a whole lot, as it was not that bad before.
Do you have any WOT logs in 3rd or 4th gear (on the road) with the old exhaust?

I didn't take any logs before or after exhaust mods. But Afr stayed similar and my butt dyno didn't notice a power change.

[EspenW]

I didn't take any logs before or after exhaust mods. But Afr stayed similar and my butt dyno didn't notice a power change.

Yep, we would likely be talking about small differences.
Spool about the same too? Butt dyno will notice that better than changes in high rpm power.
The butt is most sensitive to changes in peak G (torque) and not so much to the duration of that G-force through the pull (power).

[diesel des]

Spool exactly the same. Its only once fully spooled the backpressure rises.

[diesel des]

So I fitted the valve and the exhaust is much quieter. I have it actuated from a boost switch set at just over 1 bar.





It's amazing how much quieter the exhaust is. Quiet and flows well, perfect.

Very happy.

[Greg_S]

Looks like a nice solution

[diesel des]

Just to conclude the valve stuck and I got fed up with that problem so my original system is back on.

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[Error404]

Interesting timing Des I was looking at these valves just yesterday with a view to rigging one up. This is how the OEMs seem to do it.
Do you know what the problem was with it?

[diesel des]

It just stuck and needed me to wiggle it before every drive. A well put together one with some sorta electrical fast acting actuator would be brill. Maybe one from an oem exhaust cut out and welded in would be the answer.

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