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Noise Suppression for Bike Motors

woodsy144 - 1/2/22 at 05:47 AM

Hello All,

Have a question, what is best method for noise suppression.
I will have to pass an noise test, 3/4 of max power RPM. so it is a challenge.
I am using a 08 R1, with the exup valve in place. Test will be in neutral, so closing the exup value will help.
But no I am looking at other methods such as mufflers and silencers, or a another butterfly.

Issue when looking at mufflers, they dont really give any info on noise suppression, just seems to be tone to make your car sound tough. I just need to shut the thing up. After registration things make change to get performance back, but initially its just what I need to do to get a pass.

If anyone has some experience and willing to share it would be awesome. Thanks.

mark chandler - 1/2/22 at 09:13 AM

I got refused at Goodwood due to noise, the exhaust itself was okay but general mechanical and induction noise pushed it over the edge. I stood next to the car in front of the exhaust exit it dropped, after that moved the exit to the rear and it was no-longer a problem.

Straight through 3” with a turbo

russbost - 1/2/22 at 09:20 AM

Use an ordinary car type silencer from a production vehicle, for a 1000cc bike motor I'd go for something from a 1.6 or 2.0L car. Make sure the exhaust exit is as far from the engine as possible to reduce pickup of general engine & induction noise & have the exit pointing toward the ground to damp noise against the road

dB killers do work, particularly with some accoustafil packed behind them, just be careful not tostrangle it or you'll affect emissions, also, don't put a dB killer too near the exit of exhaust as he won't be able to get the tube in to check emissions

Just remember that to reduce noise the gases nead somewhere to expand & something in that gap to muffle the sound

woodsy144 - 1/2/22 at 10:28 AM

quote:
Originally posted by russbost
Use an ordinary car type silencer from a production vehicle, for a 1000cc bike motor I'd go for something from a 1.6 or 2.0L car. Make sure the exhaust exit is as far from the engine as possible to reduce pickup of general engine & induction noise & have the exit pointing toward the ground to damp noise against the road

dB killers do work, particularly with some accoustafil packed behind them, just be careful not tostrangle it or you'll affect emissions, also, don't put a dB killer too near the exit of exhaust as he won't be able to get the tube in to check emissions

Just remember that to reduce noise the gases nead somewhere to expand & something in that gap to muffle the sound

I have hear many mixed thing about db killers. What style do you recommend?

russbost - 1/2/22 at 10:41 AM

Some dB killers literally strangle the exhaust emission, rather than deflecting sound back to within the muffler, I used to sell these (now sold by my daughter & her partner), I still have one in my exhaust now, been in 9 years so far!

Link

Sanzomat - 1/2/22 at 02:03 PM

I was always told that bike silencers work differently to car silencers (original equipment type in both cases) due to the frequency ranges being different i.e. bike needs to work 800rpm to 12,000+ rpm whereas car engines rarely spin much over 6,500rpm. On that basis car type silencers would struggle with the high frequency. That may be b*llocks though!

My R1 4XV engine'd Locost has a 4-1 manifold then a standard Yamaha 4XV motorcycle can with an additional 90 degree outlet on the end of the can pointing down at 45 degrees to the ground. This gives 99.5dB @ 8,000 rpm based on Castle Combe's noise meter/set up. The 8000 was agreed with the noise tester based on my rev limiter being set at 11k (set in the ECU) so 3/4 of rev limit being 8,250 so he said go with 8k.

As you say (and also my understanding) the IVA noise limit is at 3/4 of "max power" revs not the red line. Not sure how the IVA tester would know that without a rolling road print out though. On the rollers mine made peak power at 10,400 so 3/4 of that would be 7,800 (so slightly less than as tested at Combe).

IIRC the stock 2008 R1 revs right up to near 14K and peak power is at 12,500 so would be tested at much higher revs than me. I guess (depending on your ECU) you could map it differently so peak power was lower down and show that print out to the tester so it is tested at lower revs?

I have a DB killer similar to the one Russbot linked to. I've not tried it in the BEC but I did use it on my GTM (Rover K series engine) when I was preparing for a Thruxton trackday. The GTM usually makes 93dB@4500 rpm at Combe so a over the 90 limit for Thruxton. Messing around at home the DB killer baffle thingy took only 2dB off. I wrapped some wire wool around the narrow section and that took an additional 1dB off so 3dB reduction total. When I went to Thruxton I tried without it first - it was 93@4500rpm but the tester said he reckoned a lot of what he was picking up was general mechanical noise (Mid engined) so he let me try at 4000rpm and it just scraped the 90dB so I never used the baffle in the end! On my measurements (uncalibrated!) they are good for 2 to 3dB on a car engine but maybe better than that on a bike engine??

adithorp - 1/2/22 at 09:42 PM

I'd say...
Fit the largest diameter and length repack able silencer you can. Diameter makes more difference than length. Don't over stuff it. Accoustafil is the best packing IMO but only use about 2/3 to 3/4 of the amount recommended by the online calculator for best results. Make sure the tip points downwards.

The other side is induction and mechanical noise. Fit an airbox if you've got room or a sausage filter if not. Line the bonnet with (fire/heat resistant) sound proofing.

Tell them your max power is at slightly lower revs but don't over do it. Oh and... CROSS YOUR FINGERS!

BenB - 1/2/22 at 10:05 PM

How do they measure rpm? At SVA my rpm dial was (it later turned out) under reading the true rpm by quite some margin. And being a v4 with twin pipes the noise was split to both sides yet the test was only from one side. Crazy low reading given the complete lack of effective silencing and the fact that when chopping down the bits to fit I had inadvertently removed most of the baffles.

russbost - 2/2/22 at 09:45 AM

quote:
Originally posted by BenB
How do they measure rpm? At SVA my rpm dial was (it later turned out) under reading the true rpm by quite some margin. And being a v4 with twin pipes the noise was split to both sides yet the test was only from one side. Crazy low reading given the complete lack of effective silencing and the fact that when chopping down the bits to fit I had inadvertently removed most of the baffles.

At all tests I've had they've used the rev counter on the car, they could of course use the one on the 4 gas machine, but if they choose to do so, you need to insist bonnet is replaced as otherwise you are releasing a load of extra mechanical & induction noise. I thought if you had 2 exhaust exits they measured from both & took the average, but could be wrong on that as I've always had single exits

I'd second the comments re insulating engine bay, particularly if exhaust exit is anywhere nearby

Re car/bike silencers, I appreciate what's been said re car/bike frequencies being very different, but I've always used car units to get low noise & to very good effect, most bike units are straight through with baffle tube packed outside between the tube & the can, whereas most car silencers use internal baffles & a path some considerable way from straight thro'

You may be able to get away with telling mild porkies about the rpm that max power is achieved at, but just bear in mind that a quick search on his phone will soon reveal the truth, to stray far from manufacturers given no.'s I think you would need a rolling road report showing max power at the rpm you are specifying. Max power is often a long way below max revs, varies hugely from engine to engine, so make sure you are looking at the correct (or perhaps most favourable if it could be one of 2 choices!) engine derivative

coyoteboy - 2/2/22 at 10:00 AM

Fit a resonator section tuned to the test RPM?
If not, fit as large a silencer as you can fit, and not a straight-through-with-perforations type, use a reflective type like an OEM car type.

woodsy144 - 9/1/23 at 10:10 PM

So I have installed the biggest and heaviest muffler I could squeeze in the gap. It is a triflow unit.

https://flic.kr/p/2oaHdUz

https://flic.kr/p/2oaEHVp

https://flic.kr/p/2oaHdWt

I have conducted some initial noise testing, while the exhaust noise appears to be low, it is the mechanical noise of the engine causing the issue at the moment - just due to the proximity and lack of covers.

While my local regs allows me to some degree to exclude this mechanical noise, it is a bit of a process. So I would like to put it out there to see if there are any other ideas to reduce this noise.

Thanks.