Higher Compression

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TK'S 78 GL1000

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Does anyone know of a manufacturer for higher compression pistons for a GL1000? The manual lists the compression ratio of 9.2:1. I have never experienced any spark knock regardless of the fuel octane rating that i have used. I'm wondering about upping the compression ratio. Course, that Randakk Supercharger WOULD be cool........
 
One way to raise compression would be to skim some metal from the head. Just make sure that you still have valve clearance. This is an irreversible job and should NOT BE TAKEN LIGHTLY.

Just a thought - hope it helps.
 
they do but if intake opens sooner or longer it can allow more in to compress resulting in higher compression.
 
I don't see how cams can increase the compression either. I know Dan started a string about 205lb compression not sure where it went but I don't think it was ever resolved.
 
More than likely the increased compression is caused by the valves seating properly!
 
I'm not interested in arguing the point but an air compressor builds pressure by increasing volume compressed. If certain cams allow more volume into the cylinder ....... Well draw your own conclusions.
 
A compressor has no exhaust stroke as such and uses a non return valve to increase pressure.
 
Ansimp":59eef1co said:
A compressor has no exhaust stroke as such and uses a non return valve to increase pressure.
Click to expand...
And as such increases volume to increase pressure in the tank right? Or is there a compressor out there that fills the tank to 150 psi with one stroke. Think of a bicycle pump. If you push air at half strokes does it take longer to fill the tire? Longer stroke can be obtained by how long a valve is open and how far open it is. If the cam starts to open after the piston has begun the charge stroke it's creating a vacuum until the valve opens. If it opens just before it is pulling in the fuel charge right away and if the carbs and intake are spaced correctly the fuel charge pulse has pressure behind it so more gets in the cylinder. Then on compression stroke there is more mix to compress. Sound right?
 
A turbo charger works in the same way. I think that we are getting confused with compression ratios and forced air compression. The first uses a mathematical formula to calculate the volume above the piston at BDC and divides it by that at TDC using standard atmospheric air pressure at sea level ( 14.7 psi )whilst the second is the actual measured pressure using air pressurized above ATM.

I am not an expert but I think this is right - maybe someone can expand on this and settle the debate once and for all.
 
David, you are correct. Compression ratio is purely mechanical - the difference in cylinder and combustion chamber volume at both ends of the piston stroke. It can be increased by lenghtening the stroke, milling the head, or adding material to the piston top. As carbon builds up on the piston, valves and combustion chamber, the compression ratio is increased.

Compression pressure is variable, depending on the volume of air-fuel mixture allowed to enter the cylinder on the intake stroke, atmospheric pressure, and cylinder leakage (rings, valves, head gaskets...). A turbo or supercharger increases compression pressure by forcing more mixture in, but does not increase the compression ratio.

:read: :laptop: :heat:
 
Compression ratio and psi generated are strictly a mechanical ratio of what the original volume was to what it is now compressed to. The only other way to increase cylinder pressure is by forced induction, in other words the inlet air is already under pressure. The thing to remember no matter how long the cylinder stays open ( open inlet valve) you cannot exceed the total cylinder volume. You could reduce the amount of volume available to compress by leaving the valves open during the compression stroke so this may create the difference between cylinder pressures with different cams.
 
Valve timing affects compression. Cams are what time valves, so they can affect compression. The lobe angle also affects it, and the later years had better angles for more overlap. 75-77 had more lift, but less lobe angle, 78-79 had more overlap. Guys with turbo or superchargers use 78-79 cams for more filling. This also pulls the powerband to a lower rpm.

Easiest way to get more compression is shave the heads some, but you have to check clearances of valves and may need to deepen the notches in the piston face. I took 0.025 off my 77 heads which are on a 1200 block. They would collide with a stock 1200 piston, but only because the 1200 cams were at a different lobe angle than the 77 cams. I forget which is which, but i think one was 105' and the other was 112'. I think you could safely shave 0.20 off a stock 1000 for a modest boost. But the only safe way is to put some modeling clay in and turn the engine to see how much clearance there is.
 
From Trevor White article:

Cam angle 75-77 112 degrees
78-79 107.5 degrees
80-83 109 degrees
87 Swiss. 105 degrees
87 US. 107.5 degrees


I believe this is the steepness of the valves as they tilt towards the face of the piston, hence the deeper cut outs and more hemi-shaped piston head of the gl 1000's than other years. If you go to a static compression calculator they figure these numbers along with stroke and open and close times and when they open and close.

I really hope to get my 1200 project running in a month or less. I have a set of 77, 83, and 85 carbs and intakes to try on it. I am beginning to lead towards predicting the the 1200 carbs on the 1000 heads as being the best of the bunch. But that's just a wild-assed guess as this is more than my fourth grade education can handle. If I don't make fifth grade by 2015, I'm hanging it up.
 

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