The Myths of Porting and Polishing!!

[ehra]

Next picture.

 

[Dan Ruddock]

I need to learn a bit more about cams. I don't yet fully understand why you lose bottom end while gaining mid-range or top end.

The condensed version, it's about port velocity, inertia and when the intake valve closes although the other three events effect power band also but to a lessor degree. I would read the glossary on the webcam site which will help you also. Dan

 

[49er]

Renobruce
You loose bottom end performance because all race/semi race ground camshafts employ greater overlap between the inlet and exhaust valves. This is the time that both valves are open together for a short period either side of TDC. The idea is to try and scavenge inertia from the exhaust valve to increase the inertia at the inlet valve. If you increase overlap dramatically, you spoil the bottom end scavenge with exhaust gases back feeding into the inlet port.
Tuners increase the overlap, opening rate and lift at the valves to gain better cylinder filling and increase their maximum HP. The consequences are a reduction in gas flow at lower rpms, but a major improvement in flow in the mid to top end. The usual increase in port and carb sizes help to further hinder bottom end flow. It is indeed all about gas velocity and accelerating the burnoff rate during combustion.

ehra
Nice pics of that port mod. No valve guide protuding into the port either.

 

[Dan Ruddock]

Renobruce
You loose bottom end performance because all race/semi race ground camshafts employ greater overlap between the inlet and exhaust valves. This is the time that both valves are open together for a short period either side of TDC. The idea is to try and scavenge inertia from the exhaust valve to increase the inertia at the inlet valve. If you increase overlap dramatically, you spoil the bottom end scavenge with exhaust gases back feeding into the inlet port.
Tuners increase the overlap, opening rate and lift at the valves to gain better cylinder filling and increase their maximum HP. The consequences are a reduction in gas flow at lower rpms, but a major improvement in flow in the mid to top end. The usual increase in port and carb sizes help to further hinder bottom end flow. It is indeed all about gas velocity and accelerating the burnoff rate during combustion.

ehra
Nice pics of that port mod. No valve guide protuding into the port either.

Not true if you increase overlap by using a narrow lobe center cam with like duration the bottom end will improve because the intake is closing sooner. Like I said earlier when the intake closes has the biggest effect on power band. Dan

 

[skip]

Well thanks for all of the input. I do not plan to race my bike so porting is out. I can see the point if you are looking for a few more HP to shave a couple of .10th off you ET but for myself I can't justify spending that kind of money for that little bit of gain. I have done the 1166 upgrade keeping the engine stock except for the pistons. This year I plan to add Adjustable Cam sprockets, a stage 3 jet kit with pods and a new V and H header. Once that is done I am going to run it on a dyno to make sure that I have max HP. I think these improvements will complete the work on my engine. Now I just have to finish my cosmetic work and start looking for a new project.

Thanks for all of the good advice,
The Beast

good choice for a street bike, plus porting usually removes some of the midrange and bottom end somthing you want in a street bike.

 

[Guest]

The smartest thing a gung-ho garage performance engine tinkerer can do is just leave your engine ALONE. The pro engine builders all agree, If you have to ask on the internet about what performance upgrades to do, then you don't belong inside the engine anyway.
A perfect example of my stupidity and ignorance in my youth was when I built my own hi-po 350 Chevy. $2700 in reciepts, and I was putting down a pittifull 237 net RWHP. A bone stock 1981 Z28 350 would put out 224. From that point on, every engine I built thereafter I built bone stock. If I needed more HP, I simply bought a BIGGER engine!

AAhhh man, you are taking the fun out of everything dude....

Can you be a man and not try to make things bigger, faster, stronger???? I ask you.

I agree with the peace meal approach though. Engines are interconnected systems. Change one thing and effect others. Throwing a couple parts on can kill performance if it isn't looked at in a whole.
That being said, there are tried and tested upgrades that will work well and get you into lots of trouble when twistin the wick. Good trouble that is.

I am sure you can find many testimonials on here from dudes who have made a couple upgrades to their beasts and dropped a jaw the next time they pin it.

OK, I'm done...

 

[U.P.dragracer]

I totaly agree , when you are a parts swapping type of guy and you throw stuff like big carbs , all out race exhaust etc. big cams , etc. on a stock engine that has not been planned out to work with those mods . most of the time it will perform like a turd \\/

I drag race my bikes quite often so I have traded off low RPM power for a "LITTLE" I repeat "LITTLE" more top end power . Yes at the dragstrip it does give me an edge over many stock bikes .
I have not done any internal mods as of yet to the GS1100 but the 600 Bandit does have bigger cams than stock . top gear roll ons with the 600 are alot like a formula one car trying to pull a trailer LOL!!!!