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How to increase power


Jim Ryder's Avatar
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12-19-01, 05:28 AM   #1  
Jim Ryder
How to increase power

What is the easiest/cheapest way to increase power on 10-12hp engines like Briggs, Tech? We race riding lawnmowers and change the pulley sizes and rejet the carburators but I need more "go" to keep up with those other guys.

 
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Fisher's Avatar
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12-19-01, 07:31 AM   #2  
Fisher
Have you rejetted for alcohol? Another thing to try is remove
the head gasket and use Indian head gasket shellac instead.
Run it a while then resnug the head bolts. If you hear a chirping
then it didn't work so don't run it long like that. Remove your
flywheel key and advance your timing a hair.
Use these ideas with caution, as your engine's life is in peril,
but that is expected in that type of activity.
Fish

 
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12-19-01, 12:09 PM   #3  
Hello and Welcome Jim Ryder to my Small Engine forum.

Being that your in the racing industry, I am sure you hear the phrase below.

However, for the many others reading this topic, I'll quote it:

......"There is NO substitution for CUBIC INCHES!"

....... Engine cubic inches = HORSEPOWER!

Illegal suggestions:

BORE THE ENGINE and don't tell a sole.....

Install a 16 HP engine on the riding machine and then swap the:

.......MANUFACTURERS RATING PLATES......

Good Luck,
Tom_Bartco.... Fast...Fair...Friendly & Highly Efficient....
"Accurate Power Equipment Company."
Small Engine Repairs and Diagnostics Service Technician.
Personal Quote:
"If it ain't broke, don't fix it until it is broken!"
Bare in mind my company no longer services nor repairs lawn and garden powered equipment. Rest assured and fully confident, the help I offer you <if any> is based upon my prior years in this industry, with this type of equipment and is specific and accurate to the best of my knowledge.......maybe?........

 
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12-19-01, 05:11 PM   #4  
You can effectively shave the head with a belt sander and use the gasket, which is preferred. The gasket allows the head and block to expand and contract independently in relation to heat, minimizing the risk of leaks and warpage. I put my sander in a vise belt-side up, turn it on, and hold the head on it until the desired height is obtained.

Also... use a straight pipe for exhaust, 8 inches is a good length. Too short and you'll risk valve problems. You can port the block intake hole to match the carburetor throat diameter, allowing more and better flow.

High performance camshafts are available and not prohibitively expensive. And like Tom said, bore it if you can. 30 thousandths won't do much, but 1/4 inch will make a noticeable difference. You have to make sure the block can be bored to your liking without making the cylinder walls too thin, and get the oversized piston set in your posession before you get the block bored!!!

Like fisher said, you can advance timing by removing the flywheel key and the flywheel position. Don't go too far! And try grinding 1/16 of an inch off your intake valve stem, sometimes it makes a difference. You can get a lot more power with these things, done right. Happy racing!!!

 
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12-19-01, 05:21 PM   #5  
Fisher
Not trying to disagree, but what does grinding the valve stem do
but shorten the time and height of the valve's opening?
Fish

 
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12-20-01, 05:18 PM   #6  
I honestly don't know... I have friends that are really into go-cart racing and they swear it will help. I've never tried it myself, but I probably will one day just to test. I agree with your logic, seems like it would hurt horsepower more than help. I think the valve tends to float a little at high RPM, assuming the engine is un-governed. That's the only reason I can see to grind the valve stem a little, but I would think a heavier spring would do better.

 
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12-21-01, 03:00 AM   #7  
Briggs_Man
With the valves adjusted at the right spec's, The intake valve is open at the top of the Intake stroke, By Grinding the intake valve your in relation of changing its timming, So you have more vacume thats increasing as the piston drops, Then the Valve opens and the engine Gulp's in more air and fuel mix, Poor mans turbo lol

 
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12-21-01, 02:21 PM   #8  
cool...I was thinking of that at one time, but changed my mind and went with the floating valve theory. Guess I was right the first time. Does the engine actually get more volume of air/fuel this way? Seems like it would help torque a lot more than horsepower.

 
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12-22-01, 11:31 AM   #9  
mikejmerritt
Some More Thoughts.....

I assume you guys have long ago lost the governors. The rest of these things done alone don't make much difference but added together can put you up front. Use synthetic oil. Its thinner and the internals have less trouble getting through it and splashing it around. Shave the head some and polish the combustion area to a mirror finish. Some creative metal removal can also help flow. Widen the intake and exaust ports to the point that can almost see through them or until as wide as the carb/header. Do go to alcohol not so much for the HP gain(its actually less than gas) but for the cooling effect on the engine. The HP loss is made up by the vast amount of alcohol it will burn. Play with the jet size and a good place to start is to double the size. Advance the timing about 4 degrees or about half the width of the keyway slot. The valve thing works out better to leave the valve height and remove the easy spin from the cam by grinding off of the low point on the cam (both sides). Did you guys know that the valve travel overlaps on the compression stroke so the compression is reduced by intention. This is Briggs famous easy spin starting. Get that lap off of the cam and you'll feel the difference the next time you pull the rope if it has one. This would be about all there is without going to aftermarket parts. Not much to be gained by boring but a light racing piston and rod is helpful but mighty expensive for the gain. Better money spent in the carb and valve timing area. True, any time you add performance you give up reliability but it will run while its running.....Mike

 
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12-28-01, 07:52 PM   #10  
KUZBRO
A good start would be making sure flywheel is balanced. Maybe a modified cam if thats legal in your realm. Open up the crankcase and smooth rough surfaces on the crankshaft. Also, take a look at the bearings for engine, transmission, wheels etc. Ridng mowers aren't that precision bulit. Blueprint everything possible. Make sure pulley are aligned right. With your horse power, you can't afford to lose much to poor alignment, cheap bearings, friction lost etc. Good luck, keep us posted.

 
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