Turbos and mufflers - large power gains

[rerb]

Hi, yesterday I had some free time and ran some tests.

With my exhaust (Vance and Hines muffler on 2.5" pipe) fitted, I saw 12 psi in first, 13 in second and beyond.

Afr in boost was around 11.

I did 3 runs and got the same result.

 

I took it off, and there was a massive world of difference. I was hitting 14 psi in first, 15 in second, and I saw it tickle 16 in 3rd before I browned my pants and let off mid wheelie.

Afr was around 11.5 - 12.

Same thing, did 3 runs and used a gopro to verify what the gauges were reading.

So obviously less exhaust = more power. A lot more.

I assume the backpressure is putting resistance on the turbine wheel, choking it out. This would make sense as it's a larger turbo. My question is am I stuck with a loud bike to run these numbers, or has anyone figured out a setup where a muffler doesn't affect boost as much? 

Would a smaller turbo be choked out less? I also have a thought that maybe the leaner running from less exhaust adds to the power.

Just food for thought here, anyone with a muffler please chime in and tell me what you've found, thanks. I know the Vance and Hines one is pretty quiet, maybe there's a higher flowing one that works well here? Thanks.

[Gixer1460]

I've never run a silencer just dump pipes so can't comment on their use but like most exhaust systems, back pressure is the enemy! Don't believe anyone who says you need backpressure - only if you want to hamstring your engine you do! Although a turbo will take a lot of the energy out of the exhaust through the turbine, it still has heat with a degree of pressure based on flow and putting this into a pipe (even an open one) that is smaller dia. than the turbine outlet, will create a restriction. This is primarily why, particularly on cars, the post turbine pipes are 3 - 4" diameter with similar size through pipes in whatever silencer is fitted. The silencer needs to be an absorption type not a labyrinth or baffled type - they just cause backpressure! Its a law of diminishing returns on a bike - need big dia. pipes, no space to fit them, need big silencers, no space & they look fackin' ugly! So how much power do you sacrifice to benefit from quiet riding?

[Reinhoud]

I better start making a 3" exhaust then..

 

[MeanBean49]

All this guesswork, and feels like on the road etc.

Invest in some dyno time and see what its actually doing in each confiuration.

I found with mine swapping from slash cut to an underslung full length can using 63mm pipe cut about 20bhp off.

But that I think was mostly because of the very short contorted pipe to the can

When I had a propper full 63mm system it didnt make that much difference. Had to compromise though as it was decking out badly