Higher Compression

[TK'S 78 GL1000]

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.

 

[Anonymous]

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.

Cams only allow the fuel or exhaust to flow they do not force (compress) them - cams do not increase compression I may not be as mechanical as some but this is basic physics

 

[Steve83]

Like I said, compression pressure can be changed by allowing more or less flow into the cylinder - either by valve timing, turbo or supercharger, atmospheric pressure, or simply opening the throttle. This is why you need to hold the throttle wide open when doing a compression test.

Compression ratio is determined by piston stroke length, piston shape, and combustion chamber shape. It is not variable.

Throttle closed, little flow, low compression pressure, little power (idle). Throttle open, more flow, higher compression pressure, more power. The compression ratio stays the same.

 

[dan filipi]

Like I said, compression pressure can be changed by allowing more or less flow into the cylinder - either by valve timing, turbo or supercharger, atmospheric pressure, or simply opening the throttle. This is why you need to hold the throttle wide open when doing a compression test.

Compression ratio is determined by piston stroke length, piston shape, and combustion chamber shape. It is not variable.

Throttle closed, little flow, low compression pressure, little power (idle). Throttle open, more flow, higher compression pressure, more power. The compression ratio stays the same.

Exactly.
Ratio and pressure are 2 entirely different things.

 

[Anonymous]

Like I said, compression pressure can be changed by allowing more or less flow into the cylinder - either by valve timing, turbo or supercharger, atmospheric pressure, or simply opening the throttle. This is why you need to hold the throttle wide open when doing a compression test.

Compression ratio is determined by piston stroke length, piston shape, and combustion chamber shape. It is not variable.

Throttle closed, little flow, low compression pressure, little power (idle). Throttle open, more flow, higher compression pressure, more power. The compression ratio stays the same.

Exactly.
Ratio and pressure are 2 entirely different things.

When the valve opens up the pressure equalizes and it really doesn't matter how long you leave the valve open once it equalizes that's it..!! Compression is caused by the piston not the valve any differences in compression caused by the combustion chamber pressure being slightly negative would be almost (if not certainly) negligible.
We are not talking turbochargers here people..!! These are thirty year old bikes and we should be (I am) happy just to find the compression even across the four cylinders..

I should have paid more attention to George Carlin when I was younger..!!

 

[dan filipi]

It's fun to debate this stuff :hihihi:



opcorn:

 

[ekvh]

Which compression "figure" increases power more, comp ratio or compression readings?? The need for higher octane is based on compression ratio, right? How does psi pressure affect the mix?? I really want to get to fifth grade. I also am curious as to what is going to happen to my project. I spent three hours last night trying to strip the frame paint last night. Note to self: sandblasting is worth it.

 

[scdmarx]

My understanding of compression ratio is combustion chamber size - volume, compared to cylinder volume.

Liquid is visible, so using liquid works for my imagination. Move the piston to btc and fill the cylinder volume with liquid. Lets say it took 10 parts to fill the volume. Move the piston to tdc and see how much came out. Lets say only 9 parts of the liquid came out, and left 1 part remaining.
This is a 10:1 ratio. Reducing combustion chamber size will raise the compression ratio, like 10.5:1.
Of course, higher compression ratio delivers higher pressure that needs to be dealt with.

Several things can affect combustion chamber pressure in a running engine. Valve timing, ignition timing, throttle position, air/fuel ratio, quality of fuel, air density, and probably more that I can't think of right now. These things vary under varying conditions. Some vehicles now have Variable Valve Timing, but not our old wings.

 

[scdmarx]

Which compression "figure" increases power more, comp ratio or compression readings?? The need for higher octane is based on compression ratio, right? How does psi pressure affect the mix?? I really want to get to fifth grade. I also am curious as to what is going to happen to my project. I spent three hours last night trying to strip the frame paint last night. Note to self: sandblasting is worth it.

Compression readings, static such as what you'd be reading from a compression gauge during cranking, measures the cylinders ability to seal. It has nothing to do with compression ratio.