GS750 with 850 jugs/pistons HP/TQ

[Steve]

Can anyone confirm the 850 pistons are a lower dome ?

I can get some pictures of 850 pistons, don't have any 750 pistons for comparison.

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[Brendan W]

I have to agree with that.
Has anybody done a weight analysis to see which is heavier? A starter motor or all the lever, linkage, spring(s) and gears of a kick-start system?
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Not forgetting the extra muscle mass in your right leg after a season or two

[93Bandit]

TBH IMO your $/HP gain ratio would probably be better spent with pods/pipe/jetting than the block/piston swap. Are you willing to do a top end engine build for a 5 HP gain from that extra 95cc? Better air/fueling should get you at least 5 HP, plus some weight savings from a 4 into 1 exhaust if you're still running the stock exhaust system.

How healthy is the 750 engine now in terms of compression? Have you done a compression test/leakdown test?

I'm not familiar with the particulars of the 850 block on 750 cases swap but I would wonder if a) the 850 block has the exact same spacing/size as the 750 block for the oil passages so that the oil passage o-rings will seal properly and b) if the shape of the bottom of the block is the same between the 750/850 so you can run a 750 base gasket between your cases and block.

Also consider that money spent on suspension/brakes/tires on bikes is always money well spent - HP only makes you fast in a straight line.

I plan on doing pods/pipes/jetting regardless; the displacement increase would be an additional mod. As far as health of the 750, I'm not sure. I plan on putting all new gaskets because it's leaking everywhere, and depending on condition of the cylinder walls I may hone and put in new rings. I figured at that point I may as well put in 850 jugs and pistons while it's all apart. From what I've read on GSR, you use 850 base gasket when assembling with 850 parts but I would appreciate clarification on that.

I don't know how quickly you would run into PTV issues, but you might not have to shave anything at all.

Think of it this way: you are shoving an extra 95cc or so into the same-size combustion chamber. That is 12.5% more displacement, so the compression ratio might go up about the same amount, so it <might> be close to 9.8 as it is.

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I had considered this, however I assumed that they put a different piston in the 850 to compensate for a CR increase. Since I'd be swapping pistons as well I figured any CR increase from 850 jugs and pistons would be minimal.

I note that the 850 compression ratio is 8.8 : 1. From everything I've ever seen, the 750 and 850 heads are identical.
This suggests to me that - like other examples in the 8V range - the 850 pistons are a lower dome to reduce the compression in the bigger cylinders.
Suzuki have a history of adapting by using the cheapest method. Pistons are cheaper than a new head casting.

Can anyone confirm the 850 pistons are a lower dome ?

IMO your simple capacity increase will give useful gains in torque if not a huge amount of power.

More torque is always OK with me! I'd like confirmation to your suspicions as well!

I can get some pictures of 850 pistons, don't have any 750 pistons for comparison.

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Oh? Do you have spare jugs to go along with those pistons that you would part with???

[93Bandit]

Not forgetting the extra muscle mass in your right leg after a season or two

If the engine is properly tuned and not a radical build, kick starting shouldn't require so much effort that my leg would fall off.

[Drybear]

This is something I've considered and I'd really like the 90hp in stock form, but there's a few reasons I want to pursue the big bore 750. First reason being it is difficult to source a complete GS1000 engine without buying a complete bike. If I buy a complete bike, I may as well ride the GS1000. I found one engine locally which is still in the bike but it was wrecked. There's some minor damage to the engine and he still want's $500 and won't budge on the price. I think I could source a used 850 for much less and use the parts from it to rebuild the 750.

Also, I want to keep the kick start as I plan on doing a starter delete and downsize the battery. I want to reduce weight wherever I can.

A proper cafe build needs a kickstart..
Btw, I did the jugs swap to 850 last winter and some additional performance mods as well, so I guess I can give you some advices if needed?

[Grimly]

If the engine is properly tuned and not a radical build, kick starting shouldn't require so much effort that my leg would fall off.

My 79 had a kickstart.
It kicked and started so easily you'd wonder if you'd actually done it.
Compared to the XS650, it's a dawdle.
When I put an engine together for that bike, it will definitely incorporate the kicker bits, if for no other reason than it will be right for that year, but also as a fall-back.

[93Bandit]

A proper cafe build needs a kickstart..
Btw, I did the jugs swap to 850 last winter and some additional performance mods as well, so I guess I can give you some advices if needed?

Hey thanks for the comment. Do you have a build thread or more pictures of your bike?

[wymple]

Each cylinder is only a bout 211 cc or so. Only one at a time is under compression. Fours kick over very easily. If not, burn the man card.