Exhaust Mod Part IV

[Enzo]

New tail pipe

[Baldini]

Enzo, I'm no braniac but as i understand it roadspeed is a product of engine speed x gearing. Whatever you hang off your headers has nothing to do with it.

 

That rear tyre'll go to the edge of the tread pattern easy enough without you falling off if you just find some tightish, grippy, open bend to practice on - a long, slow, constant radius bend with no bumps to unsettleyou & plenty visibility. Here, we have roundabouts. Just make sure you can see what's around you, where the road's going & that it's a grippy surface. Work over gradually, smoothly - real gentle with the throttle. You'll be surprised how far it'll go over & still grip. Racetrack is ideal.

 

KB .

[Keith Foster]

I gotta agree with Baldini. Speed is set by engine RPM, trans gearing, final drive gearing then tire diameter. To get to a certain speed, the tire has to turn at some RPM (or for the nerdy-angular velocity in rads/sec.), It's all hard gearing between that and the crank, so RPM sets the speed. Throttle position can be less for an engine makng improved power, i.e. it will take less juice to make the needed RPM to get to X speed. Not to be a smart aleck, but maybe you just found top gear after running in a lower one? Keep those wheelies coming!

[Enzo]

No no no. I'm saying that IN 6TH GEAR, my rpms have dropped at 70 mph. I have noticed with fooling around with my pipes and crossover that these changes do A LOT. Everytime I mess with it, something different happens with the power curve and engine characteristics. Now I've noticed that rpms have dropped significantly while everything else is the same. It seems easier for the engine to make power without so much huffing and puffing. So, I'm guessing the new open tail pipe set up is giving the engine just the right amount of escape and backpressure for good power.

[Guest northend]

It may pull better at stated RPM and speed (needing less throttle) but its still going to be the same RPM at the same speed unless you change gearing,tire size, etc.

[Kiwi Dave]

I'm saying that IN 6TH GEAR, my rpms have dropped at 70 mph.

 

The only way for this to happen is for your gearing or tyre size to change. No amount of engine tuning is going to alter the engine speed verses the road speed. Try taking out your spark plugs and run it down a steep hill in 6th gear @ 70 mph. You will observe your engine revs to be the same.

[Enzo]

Interesting. It must be that it is smoother and quiter - running on less throttle. I'll take your word for it that the rpms are the same, but it sure seems different. Obviously not working as hard to do it. Anyway, if I'm using less throttle, it must be putting out more power.

[jrt]

A lot of it (all of it?) is perception. You are percieving that the engine is turning slower or that you are going faster. It's not happening, but the noise and the novelty of the change induce this feeling. Short open pipes are very effective at doing this- I could have sworn I added 10HP to my old G5 when I dropped the mufflers off that bike.

A followup question for those who are interested in engine mods- is the exhaust valve more prone to burning with the lower backpressure associated with very short pipes (VSP)? On the old bikes with a distributor, the fix was to retard the timing a bit.

I'm glad to see you are having so much fun, Enzo.

Cheers,

Jason

[Enzo]

Well here is another follow up question: If you increase the efficiency of the combustion, will you get better milage? Assuming you have not turned up the gas mixture, I would think so.

 

I always test my milage on the same Twin Peaks run. Last time was 33 mpg. It will be interesting to see if milage has changed. Would a change of milage indicate anything?

[Guest Jeff Kelland]

When the piston pushes the spent charge out into the exhaust pipe, it travels as a mass moving at high speed toward the open end. It creates a low pressure area in the pipe behind it. This low pressure, in a properly designed system, is used to pull fresh mixture into the cylinder. This is in addition to the low pressure created by the downward travel of the piston. It in effect is free supercharging. Naturally, it is only effective in a small rpm range as the timing between this low pressure and the initial opening of the inlet valve is critical for this to occur. When this mass exits (think "pop") the pipe,the low pressure behind it pulls fresh air into the pipe, traveling in the wrong direction. If the pipe is short enough, this fresh air can contact the superheated valve head causing it to burn. Human perception, being what it is, would have us believe that more noise means more power. Not likely. The manufacturers spend literally millions of dollars and untold hours studying these events, we see it as computer controlled valves in the high end japanese sportbike exhausts. They throttle the exhaust to reduce the effects of bad timing and to extend the good timing of the pulses to effectively increase the power across the rpm range making these engines much more user friendly. We can only hope that Guzzi would follow suit, again, not likely. I would guess there just aren't enough of us to justify the expense.

[Guest northend]

I still like being a minority though

[Enzo]

Jeff,

 

Excellent report. I've come to learn about the basics as you lay it out here. But I don't associate noise with power. I've seen plenty of loud and slow Harleys to know better. I hope I don't burn a valve. It may be helpful that my header is ceramic coated and also wrapped in pipe tape. This would keep the inside of the pipe super heated throughout. Well, the gas would be very hot, but the walls of the pipe would probably be less hot than stock - which again would be a problem with fresh cold air coming back up the wrong way. So, maybe my coating would make a burnt valve more likely.

 

My main goal was to create a clean look in the back and not lose any performance. I think I've done it, but only a Dyno will tell now. It is a bit loud, but it was worse. I'm learning things by doing these experiments. I still can't tell where the bike is now making the most power, but it does seem strangely linear. I did run it right up to 8000 rpm this afternoon and it pulled strong all the way up there like it was going to go past it making power. That seemed to be a good sign. Also, the bike feels like it is not working as hard and will cruise at slow speed with more ease. There is also less hard engine braking. I don't know what this means. Does anyone know?

[TX REDNECK (R.I.P.)]

Cap'n please have the Dr adjust your medication

[gthyni]

The manufacturers spend literally millions of dollars and untold hours studying these events, we see it as computer controlled valves in the high end japanese sportbike exhausts. They throttle the exhaust to reduce the effects of bad timing and to extend the good timing of the pulses to effectively increase the power across the rpm range making these engines much more user friendly. We can only hope that Guzzi would follow suit, again, not likely. I would guess there just aren't enough of us to justify the expense. 

Hmm..

and every serious jap sportbike owner replace the whole thing

with an Akrapovic/Yoshimura/Hindle full system which removes the Exup

(or similar) exhaust throttle.

Gives about 10 bhp on top and loose a little in the low/midranges.

 

I don't need more midrange power in my V11 but +10bhp on top would

be welcome. Opening up the intake and exhaust is the way to

go for more peak power even if it does not please the authoritize.

[Enzo]

Opening up the intake and exhaust is the way to

 

go for more peak power even if it does not please the authoritize.

 

Yes, I have noticed bikes are rather loud at the race track. And, you are correct, I've never seen one of those highly engineered stock air/exhaust systems on any of the bikes there either.

 

I still have a question for you all: What does hard engine braking MEAN? or, conversely, what does easy engine braking and coasting MEAN? Does anyone know?