Performance parts for gs850g.

[Moarpower]

Just curious about what performance engine parts are available for the 850g.

I bought a bike with a claimed 11-1 high compression rebuild done but I'm unable to find evidence to support that claim nor can I find many parts online for the 85og.
The are lots for the 750 and 1000/1100 but the 850g seems like the forgotten stepchild when it comes to upgrade parts.

I need to pull the head and replace gaskets etc and would like to add new pistons and rings, valves and spring upgrades.

 

[Burque73]

Search for an 1100G engine to throw in your 850's frame. That's about the only way to get more out of it, I think.

Edit: Depending on what year 850 you have that may not work.

 

[Moarpower]

Search for an 1100G engine to throw in your 850's frame. That's about the only way to get more out of it, I think.

Edit: Depending on what year 850 you have that may not work.

The current project is a 1980

 

[zed1015]

I bought a bike with a claimed 11-1 high compression rebuild done but I'm unable to find evidence to support that claim

Once you pull the head it will be obvious whether the pistons are higher comp.
Either that or do a comp test when warm and see if the readings are substantially higher than stock.

 

[Moarpower]

Once you pull the head it will be obvious whether the pistons are higher comp.
Either that or do a comp test when warm and see if the readings are substantially higher than stock.

Compression test read 155, 155, 155, 160 and the rebuild was done in 2016. Good compression but not overly high

 

[bwringer]

Compression test read 155, 155, 155, 160 and the rebuild was done in 2016. Good compression but not overly high

Those are extremely healthy numbers. No way I'd take that apart.

According to my Haynes manual:
Stock compression ratio is 8.8:1
Compression range is 128-171 psi, service limit is 100psi, and maximum difference is 28psi.

The question of high-performance 850 comes up here fairly regularly, and as far as the GSR Hive Mind can tell or remember, there really aren't any performance parts made, now or in the past, for the 850G/GL. A few have installed 4-1 exhausts and pods, then entered the usual carburetor hell for a while to make them work.

For a time, you could get 1.0mm oversize pistons from Suzuki and bore the cylinder liners.

There have been a few isolated projects here and there where folks have had custom pistons made, widened the wheels, etc. But that's all very much at the deep end.

As noted above, the best route is to slide in a 1000G or 1100G engine. There's some sorting out to be done of the airbox (if you use one), and exhaust, and driveshaft (make sure you get the driveshaft with the donor engine), but overall it's been done several times. You get a nice bit of added performance, and the larger engines are actually somewhat lighter; the 850 engine is the heaviest of all the shafties.


In the end, what is your goal? I'd argue that a lot of folks have simply never experienced an 850 that's running correctly. Getting everything up to snuff and perfect in stock form is step one, so you understand what you're starting with.

Another interesting phenomenon is that a lot of folks haven't figured out that a properly running 850 loves to spin. It's a far more entertaining engine (in my opinion) than the 1000 or 1100 because you have to keep it on the boil. That rush from 6,000 to 9,000 rpm is serious fun, and seriously addictive and involving. Grip it and rip it. Spin it to win it...

 

[Moarpower]

Thanks Bwringer.

The reason for the work is just in case I need to.ooen it up. It slipped time and I will want to remove the head and check valves and the piston tops at a minimum. If they are fine I won't need to do any more besides resetting timing.
I believe I may need a new timing chain also but hopefully it's just the failed tensioner.
Cam timing will need to be degreed in as they have slotted the cam gears.
I bought this bike already with pods and a 4 into 1 exhaust.
I put the stage 3 jet kit in from Dynojet but other issues with the bike have prevented me from really dialing it in nicely.

 

[GregT]

When you pull the head to check for damage,measure the bores. It's possible it may have 1000 pistons.

 

[rphillips]

Hope your problems aren't as bad as you're thinking. With the compression you're seeing very unlikely you have valve or piston problems. Also that EX. cam gear on the IN cam has got to be straightened out. Your cam degreeing could be forgot about if you can find a set of oem cam gears or a set of cams, with oem gears on them. Just install them per manual and all should be good. Also like someone else said, check your tensioner, they rarely give problems, other than oil leaks,. Locking them down to make a manual tensioner out of them has been done a lot, if that's happened, it will no longer keep tension adjusted automatically.... I've always thought easiest, cheapest things first, then go deeper if needed..

 

[unfocused]

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.

 

[Moarpower]

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.

What will the trade off be if I use the 750 cams?

 

[unfocused]

What will the trade off be if I use the 750 cams?

You will see the power increase through out the power range. From what I understand through research, the 750 has the better cams within the 8V engine line. The G engines are kinda lazy. It's been reported that even the 1000 will wake up with this cam swap.

Also, you'll need to look into the engine to see if it's been overbored or pop up pistons. You say high compression. This would require pop up pistons or a shaved cylinderhead. Just over boring the engine will not increase compression.

 

[pdqford]

Just over boring the engine will not increase compression.

......... :jaded:

 

[unfocused]

......... :jaded:

I'm sorry, did I say something that offended you?

 

[pdqford]

I'm sorry, did I say something that offended you?

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.

 

[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,