[Bicycle Repairman]

Stock '05 SV650 race bike. Full stainless Leo Vince exhaust and PC-V. The exhaust must be pretty old because I asked Leo Vince for a new can clamp and they kind of scoffed at me saying they had not made an oval can in years. Millennials, am I right?
Anyway, last fall the aluminum can shook itself apart and blew off of it's rivets. I cut off the damaged section, put new packing in it and riveted it back together so now it's about an 1 1/2" shorter than it's original 19". That was near the end of 2021 season for me. Then over the winter, I had a highly regarded shop do some SS legal enhancements like a valve job and shaved heads to up the compression a little. Adjustable gears were fitted to the stock cams. After all that it dyno'ed about the same as before which was disappointing to say the least.
Then during July round, the noise monitor guy came to visit our pit. He said our SV's never have a problem with being too loud. I told him about my exhaust repair and that it was quite a bit louder than it used to be. Then he asked me if the bike was down on power. I replied, "Yes, I think it is." He said the exhaust was tuned for the bike and changing it would affect power and that it takes energy to make noise and that quieter motors are more efficient and make more power. I've heard before that if something is making noise, it's taking energy to do it. I'm not sure if mechanical friction noise is the same as lots of rapid fire explosions noise though.
All that to ask this: Is the shorter, noisier can costing me HPs? Should I get a new can? Will it make any difference? Which one should I get? Why?
I count five questions. Go.

 

[MOHillbilly]

One answer.. I don't think the can is your problem. I have no advise as to what the actual issue might be, but I doubt it's the muffler by itself. Have you put it back on the dyno to see what the A/F is, and if it has actually lost power? Or maybe riding the power of suggestion from the sound guy?

 

[Geo Smith]

I may get some blowback here, but my thoughts are exhaust canisters are not that big of a factor (within limits) in horsepower development. They are more cosmetic (and cool sounding). Like MO, I'd look elsewhere.

And the thought that a quieter engine is a more powerful engine seems foreign to me. Ask a top fueler if he'd be willing to put mufflers on his dragster and he'd look at you funny.

 

[thedrewski86]

If your bike was tuned with x amount of backpressure, and now it has been drastically changed, sure it may affect the hp. It doesn't sound like you've changed the can too much though... 1-1/2" cut off won't change it that much I would think. Plus you re-packed it, so that should help. I would be looking at the other engine work for a solution.
I see what the inspector means by quieter = more power. 650cc engines do not have the same demands as a nitro-burning 8 liter V8. As far as that goes, even the top-fuel guys "tune" those open headers. Backpressure affects how an engine makes power, but displacement and backpressure demands are not linear.
I will stop speculating. It sounds like you need to get back on the dyno and collect some data.

 

[1neeto]

I may get some blowback here, but my thoughts are exhaust canisters are not that big of a factor (within limits) in horsepower development. They are more cosmetic (and cool sounding). Like MO, I'd look elsewhere.

And the thought that a quieter engine is a more powerful engine seems foreign to me. Ask a top fueler if he'd be willing to put mufflers on his dragster and he'd look at you funny.

Well, there is some truth and fiction to that. Louder doesn’t automatically give you more power. An exhaust needs to have the right balance of flow and velocity at the rpm range the engine is best at. A very free-flowing and loud exhaust will breathe very well at high rpm’s, because of its high flow, but it will do poor at low rpm’s because the velocity is not there to help scavenge gases out of the cylinder. The result is loss of low end torque. Is it really a gain if you gain 2 hp at 11000 rpm’s but lose 8ftlbs of torque from 3500 to 8000? If a track bike, then maybe, you won’t be on those low-mid range rpm’s much. But on the street, the bike will feel slower because that’s the range you’ll be mostly at.

As for exhaust slip-ons not giving any significant power, I completely agree. I think most of us understand this.

 

[Bicycle Repairman]

One answer.. I don't think the can is your problem. I have no advise as to what the actual issue might be, but I doubt it's the muffler by itself. Have you put it back on the dyno to see what the A/F is, and if it has actually lost power? Or maybe riding the power of suggestion from the sound guy?

I have before and after dynos. Pretty much no difference. How is that possible.

 

[Bicycle Repairman]

If your bike was tuned with x amount of backpressure, and now it has been drastically changed, sure it may affect the hp. It doesn't sound like you've changed the can too much though... 1-1/2" cut off won't change it that much I would think. Plus you re-packed it, so that should help. I would be looking at the other engine work for a solution.
I see what the inspector means by quieter = more power. 650cc engines do not have the same demands as a nitro-burning 8 liter V8. As far as that goes, even the top-fuel guys "tune" those open headers. Backpressure affects how an engine makes power, but displacement and backpressure demands are not linear.
I will stop speculating. It sounds like you need to get back on the dyno and collect some data.

After the disappointing results, I took it back for a second dyno and no new information. I want to take it to another shop (Nels) a couple hundred miles away, but this late in the season, I don't have time (and more money).

 

[MOHillbilly]

I have before and after dynos. Pretty much no difference. How is that possible.

Muffler alone just doesn't make that big of a difference. Once you get past the stock can, all aftermarket items are pretty much just resonated straight pipes. An inch or two shorter straight pipe is still a straight pipe and won't change anything other than the sound due to a bit less of the resonator effect. 2 strokes were a different beast in that the exhaust system made quite a bit of the bikes power, 4 strokes not so much.
If the dyno says there's no difference, there isn't. Butt cheek dynos can be fooled. Chassis dynos tell it how it is, and their best game is before and after comparisons.

 

[Geo Smith]

Then over the winter, I had a highly regarded shop do some SS legal enhancements like a valve job and shaved heads to up the compression a little. Adjustable gears were fitted to the stock cams.

In addition to shortening the can, you did some other modifications over the winter. Could some be working against others? Trying to isolate the problem to one variable when you have so many variables is difficult.

 

[Bicycle Repairman]

In addition to shortening the can, you did some other modifications over the winter. Could some be working against others? Trying to isolate the problem to one variable when you have so many variables is difficult.

True but increased compression and timed cams and PC-V tuning should result in some kind of HP gain, no? The only other change was the repaired exhaust.

 

[Dutch.Sv650s]

True but increased compression and timed cams and PC-V tuning should result in some kind of HP gain, no? The only other change was the repaired exhaust.

Do you have a picture of the bike with the shortend can?

I wouldnt suspect the Muffler being the problem tho. Like stated earlier. Shortening my exhaust on my first gen sv did nothing in terms of HP output. Had it on the dyno to. Same oval Leovince.

 

[Bicycle Repairman]

Do you have a picture of the bike with the shortend can?

I wouldnt suspect the Muffler being the problem tho. Like stated earlier. Shortening my exhaust on my first gen sv did nothing in terms of HP output. Had it on the dyno to. Same oval Leovince.

Here's pics. I agree there is no reason other than cosmetics to replace this poor old beat up, rebuilt can as long as it passed tech and doesn't trip the sound meter.

 

[Dutch.Sv650s]

View attachment 62217 View attachment 62218

Thanks for the message but I actually ment a picture of the shortend version

 

[Bicycle Repairman]

Thanks for the message but I actually ment a picture of the shortend version

That is the shortened version

 

[Dutch.Sv650s]

That is the shortened version

Oh now i see. Im sorry i read it wrong. I thought you cut half of. 1.5 inch definitely wont affect noticable HP. Even with replacing the packing it should not be really noticable. Im sure something else is lurking around. Are you below stock figures? Or still at stock figures in terms of power/torque?

 

[budaful]

The noise monitor guy said quite motors are more efficient and make more power. He was not talking about the exhaust system I would imagine. He forgot to include that those horses run into each other if they don't have a smooth free flowing exhaust. Noise guy was not lying, I doubt one would consider 120 gallons per mile efficient. The noise from a modern dragster is likely closely tied to its energy produced and energy used (nitromethane). I find it humorous the noise monitor guy is using half truths in a quest for lower decibels. 1 neeto has a great point about HP vs. torque. Unless one is wide open most of the time one would benefit more from focusing on torque not HP.

 

[Bicycle Repairman]

Oh now i see. Im sorry i read it wrong. I thought you cut half of. 1.5 inch definitely wont affect noticable HP. Even with replacing the packing it should not be really noticable. Im sure something else is lurking around. Are you below stock figures? Or still at stock figures in terms of power/torque?

Latest dyno says about 75 hp at about 9200rpm. max torque 46.8 at 7300. Before mods was about 74 peak hp. The variable that might make sense is before head shave, I used thinner Spears gaskets. Perhaps they put in stock thickness gaskets resulting in little to no difference in compression? I don't know the difference of Spears gaskets compared to stock or how much they shaved off the heads.

 

[gt alex]

Hi I am far from an expert but I think the issue of noise and maybe what was inferred to as quiet= power.
Rich mixture can be louder, retarded ignition can be louder to name just 2.
You should be looking for a tuning issue maybe the cams didn't end up degreed as intended, maybe 1cam is a tooth out.
Upping compression normally gives an extra 2% power per compression point, but to much and you will be dialing it out by needing to retard ignition timing.
I think just start with double checking the cam timing.
Hope this helps

 

[RecoilRob]

On the SV1K with Yoshi 3/4...changing the exhaust baffle absolutely changes the power output. I have 4 inserts cut to different lengths from stock (about 9") to about 3" and after tinkering with pretty much anything on the motor will switch them around to see which I like the best. No baffle absolutely kills the bottom end power while the stock full length baffle is much perkier down low and in the midrange but does hurt the top end a little, so the 'Goldilocks' baffle is one in the middle which had about all the bottom end of the stock insert without taking noticeable power off of the top end.
This isn't a backpressure issue...it's an exhaust timing issue. All the baffles have the same diameter outlets and only the length changes which then alters when the exhaust reflection pulse returns to the chamber.

Do you know what cam timing was installed? 106/108 is a good place to start on many motors (Spears race cams call for 105/105 IIRC) and I'm thinking you have a couple choices about how to proceed. You can adjust the cam timing to suit your exhaust or adjust the exhaust to suit your cam timing. The commonly accepted theory on exhaust is you want the reflection pulse to arrive back at the chamber just as the exhaust valves are nearing full closed. Arriving too soon or too late and you lose power either on the bottom end or top. On overlap with an open exhaust some amount of intake charge is going to go out the exhaust and this returning pulse pushes it back in to be burned.

Changing the exhaust baffle insert on the Yoshi system really doesn't affect the A/F ratio enough to worry about. Hey...just for fun, you might play around with a pipe insert that you can stick in the end then vary the length to see if it likes longer like it was before you shortened the can. Even on a track bike it also might be worth trying an insert that you can change just to fine tune the exhaust system. Some of the better race teams have several exhaust systems to use at different tracks where midrange improvements can lower times on tight tracks and top end helps on the long ones but any one exhaust system is for sure going to be a compromise...you just need to decide where you want the power then make the changes to get it.

Slotted cam gears makes adjusting the phasing pretty simple and you don't need to remove the cams, so bumping the exhaust cams ahead a couple degrees 'should' account for the shorter exhaust...or not. Anyone building motors with adjustable cam timing should be setting it to work with the exhaust system at hand and it might be arguable that any timing they set is just a starting point and unlikely to be optimum without further tweaking.

 

[Bicycle Repairman]

See, now that's the technical jargon I was looking for!
Nio, I don't know where the cams were set but I'll see if maybe the assembler took a note so at least I know where I'm starting from.
Thanks, Danny