Performance parts for gs850g.

[unfocused]

No, you didn't say something that offended me.
I'm sorry if I implied that by using the wrong emoji..
But increasing the bore does raise the compression ratio.

Changing the bore size, changes the volume of the cylinder - CC's. This will increase power because you're taking the additional volume and compressing it down to the set compression ratio.
This set compression ratio is based on the combustion chamber SIZE. The piston can change this size.
CC's of the engine is based on the cylinder size with the piston at bottom dead center - this does not include the combustion chamber.
CC's of the cylinder head is based on the combustion chamber designed into the cylinder head.
Compression ratio is based on shoving the engines cc's into the combustion chamber cc's... not taking into account anything else.

My engine information below is based on advertised specs available on the WWW

Our beloved 79 GS850 has a compression ratio of 8.8:1. Take that piston and install it in a 79 GS750 and its stll going to be a compression ration of 8.8:1. Yet you have to bore the cylinder out to do this. This is because of the piston and combustion chamber size. Side bar - this WILL increase the advertised compression ratio of the GS750 because it ran a compression ratio of 8.7:1... Now, if there is a difference in the CC's of the combustion chamber between the 2 cylinder heads, this will alter the compression ratio - up or down. So, if the combustion chamber on the 750 head is larger, your compression ratio will go down based on CC's of the combustion chamber. Might bring it back down to 8.7:1.

A small block Chevy from the smog years (mid 70's throughout the 80's) ran a compression ratio of about 8.9:1 with a dish piston (crappy fuels, horrible government.) You can bore that cylinder out .030" with the same style of dish piston and you run the same compression ratio. You can still buy pistons just for this... However, you take the stock dish piston and turn it into a flat top (common hipo trick) and you change the compression ratio to about 9.5:1. You install a pop-up piston (similar to the GS750) and you can bump the compression ratio up to 10.0:1 while never changing the bore size.
You could take the same 8.9:1 compression ratio small block, remove the 76cc cylinder heads, install a set of 64 cc heads and you will drastically change the compression ratio without ever touching the bore size. You can shave the head of the GS750 and start bumping up the compression ratio for every .01" shaved off. Remember to check piston to valve clearance.

Also, don't forget, when you raise the compression ratio, you better start looking at the octane rating of your fuel and the ignition timing you're running.
The above information is not taking into account any differences in crank throw, connecting rod length, piston pin location, head gasket thickness or fire ring diameter.
This is a completely different math problem and all of these can alter the engine displacement.

I hope I've shared enough information to clarify my original statement of - "Just over boring the engine will not increase compression." I should have added ratio in there so this is my bad. Over boring an engine will increase compression values (PSI) and not ratio unless other changes have been made as noted above.

Tidbit - a top fuel engine smashes the pistons shorter. This is because the timing at 50 degrees advanced ,the piston still going up and the over-driven blower... all while running a compression ratio of 6.5:1. A small block Chevy with a 6-71 blower should not be running a compression ration higher than 9:1 - from my experience.

[pdqford]

Compression ratio is based on shoving the engines cc's into the combustion chamber cc's... not taking into account anything else.

So let's take your 850:
Lets say one cylinder equals 212 cc's.
Lets say the combustion chamber is 24 cc's.
Shoving those 212 cc's into a 24 cc combustion chamber.
My old slide rule comes up with a compression ratio of 8.83:1

Now lets bore it out to. say 215 cc, and shove that into the same 24 cc combustion chamber.
My slide rule now says the compression ratio has *increased* to 8.96:1

So JUST increasing the bore size DOES increase the compression ratio.


But look, lets call a truce, and celebrate Thanksgiving.

[rphillips]

I's thinking the same thing pdq, changing the volume of the combustion or changing the volume of the cyl. should change the comp. ratio... I'm trying to see how it couldn't

[Burque73]

The GS850 is a shaft drive so the only option is to swap with another shaft drive engine or upgrade the top end. No performance options however, a stock 850 will wake up with a set of 750 cams, exhaust and upgraded air filter(s)... DO NOT order any gaskets until you seperate the jugs from the engine. I did this with my 750 only to find out it was already bored to an 850.

Would the cam sprockets or chain need to be adjusted for 750 cams to work in an 850? This really piqued my interest.

[pdqford]

Would the cam sprockets or chain need to be adjusted for 750 cams to work in an 850? This really piqued my interest.

I would think that if the distance from the cam centerline to the crank centerline is the same for both engines, and the crank and cam sprockets are the same diameter, nothing would have to be "adjusted". Weather the 750 cam lobe profile and lift would be "good enough" for the 850, I would leave that to someone who has access to the timing specs for each engine, like open and close, duration, overlap, lift, etc,

[Burque73]

I would think that if the distance from the cam centerline to the crank centerline is the same for both engines, and the crank and cam sprockets are the same diameter, nothing would have to be "adjusted". Weather the 750 cam lobe profile and lift would be "good enough" for the 850, I would leave that to someone who has access to the timing specs for each engine, like open and close, duration, overlap, lift, etc,

By adjustment I meant taking a link out of the chain if the 750 was shorter or changing anything else. Just throwing things out there cause it's unfamiliar terrritory for me. Thanks for your input. I'll leave well enough alone, I think.