Shave engine heads

[finley]

Hello again
Is there any formular that can be use to determine the amount of comrpession increase you gain as to how much you shave your angine heads.I`am trying to increase the compression in my yamama 250 hp engine.Thanks.

[woodboat]

How about this little utility
http://www.smokemup.com/auto_math/compression_ratio.php

[Corpus Skipper]

I`am trying to increase the compression in my yamama 250 hp engine.

BAD BAD BAD BAD BAD idea. These engines are designed to very close tolerances, and everything is light weight and thin for obvious reasons. Increasing the compression ratio will cause more heat, more stress, you'll have to run higher octane fuel, re-jet your carbs, change your ignition timing and advance, and the list goes on. If you want more performance from your engine, might I suggest looking into high performance reed valves, carb tuning, and exhaust reliefs. These can add 20 HP or so without risking that expensive engine! Good luck.

[finley]

BAD BAD BAD BAD BAD idea. These engines are designed to very close tolerances, and everything is light weight and thin for obvious reasons. Increasing the compression ratio will cause more heat, more stress, you'll have to run higher octane fuel, re-jet your carbs, change your ignition timing and advance, and the list goes on. If you want more performance from your engine, might I suggest looking into high performance reed valves, carb tuning, and exhaust reliefs. These can add 20 HP or so without risking that expensive engine! Good luck.
Thank you for your reply, can you tell me a little about exhaust reliefs

[Corpus Skipper]

Just below the cavitation plate, racers will drill 3 or 4 1/2" holes through the lower unit into the exhaust passage to help increase exhaust flow and reduce back pressure, which frees up some horsepower. I'm not sure if I would do it, as I think it might tend to ventilate the prop. Talk to a technician before doing anything of course. If they don't know, they'll know where to find out.

[gonzo]

The formula is quite simple. Calculate the volume of the cylinder -diameter of cylinder and what you are shaving off- and the ratio of that volume to displacement is your compression increase.

[finley]

Thanks for the info,

[Black Swan]

I would have to side with CS on this. Primarily, the only reason to "shave" the heads should be to get a clean machined surface for good gasket sealing on rebuild. The "top" of the block should also be cleaned up for the same reason. For performance increases you would be better off installing performance reed sets, perhaps spend a little time "polishing" the insides of the intake manifolds to remove casting lines, installing forged pistons, and even altering the exhaust housing to accomodate more flow. For racing performance there are direct exhaust setups that send the exhaust straight out without sending the exhaust down through rhe leg, but the accomanying noise would make this unsuitable for anything but organized racing. (Stand next to an unmuffled dragster some time.)

Many outboard manufacturers use the same block for a number of power outputs and "de-tune" the engine with exhaust, intake, and carburation restrictions to achieve the lower power ratings. The parts breakdown for an entire lineup of engines will tell you whether there are improvements that can be made between models. but my guess would be that a 250HP probably has all the "stops" already taken out. The concepts of "boring and stroking" and "beefing up the compression ratios by shaving the heads" really only apply to old cast iron block automotive engines, unless you've got LOTS of money to burn in expirimentation. Even there, the economics lean towards "add on" systems to increase the ability of the engine to "breathe". Good Luck!

[firestorm]

If you are looking for more power out of your 2 stroke shaving the head wrong direction. 2 strokes are like a jet engine in one aspect the more air that you shove thru them the more power u make. I have built and modified many 2 stroke engines over the years mostly on motorcycles and there are several ways to get power from reducing the crankcase volumes to improve transfer pressure to altering ports size and timing to get more power changing reed cases helps a lot but exhaust flow is critical so many of the previous posts covered these my one point is that I never had to swim back when I went too far in engine mods. Simply put a 10-15% power increase could reduce engine life from years to months a 20-35% increase could go from years to weeks, Full out mods could get you in the 350-400hp range with nitros and might get you down to few seconds of life span and a long swim back to shore.

[DChrls]

New to this forum and I'm no Yamaha guru but shaving the heads on most 2 stroke is the best bang for the buck. Just don't go over 145psi on compression. Also understand you WILL have to run superunleaded with that much compression. The higher compression will give you a performance increase mostly in the low to midrange and a little on the top end.

Don't waste your time drilling out the exhaust. Plus the drilling is done just above the anti-cavitation plate, in the midsection. About all this will do is make a BUNCH more noise. It only helps if you are running over the hub exhaust props,ie choppers, cleavers, ET's, Yammy drags...

Boysen 2 stage Reeds will help with the ideling of the engine and lowend power but not much else.

If ya want more input contact Hydro-Tec, http://www.hydrotecmarine.com/

[john dezman]

New to this forum and I'm no Yamaha guru but shaving the heads on most 2 stroke is the best bang for the buck. Just don't go over 145psi on compression. Also understand you WILL have to run superunleaded with that much compression. The higher compression will give you a performance increase mostly in the low to midrange and a little on the top end.

Don't waste your time drilling out the exhaust. Plus the drilling is done just above the anti-cavitation plate, in the midsection. About all this will do is make a BUNCH more noise. It only helps if you are running over the hub exhaust props,ie choppers, cleavers, ET's, Yammy drags...

Boysen 2 stage Reeds will help with the ideling of the engine and lowend power but not much else.

If ya want more input contact Hydro-Tec, http://www.hydrotecmarine.com/

how do you predict the pressure increase? is there a formula?? lets say you start with 115 psi, how much of a cut to get to 145?

[woodboat]

I posted a link to a formula table early on. As you shave the head the combustion chamber size will decrease. Plugging in a lower CC head number will show an increase in compression.