GS750 with 850 jugs/pistons HP/TQ

[93Bandit]

I'd like to add a little pep to the 750 for my cafe racer, and I was planning on pods and exhaust but now I'm considering increasing the displacement via 850 jugs and pistons in addition to the previous mods. I have searched the forum and found most of the information I need, however it was scattered through different threads and I'd like to compile what I found here, in addition to asking some questions that either weren't previously answered or I need clarification on.

Correct me if I'm wrong, but the 77-79 750/850's use the same head therefore same carbs. So, the only difference is the cams and jetting in the carbs.

The GS750 has a bore and stroke of 65mm X 56.4mm respectively, which equals a displacement of 749cc's
The GS850 has a bore and stroke of 69mm X 56.4mm respectively, which equals a displacement of 844cc's

That's a 11.25% increase in displacement.

Based off the information in Suzukicycles.org:

GS750 has 72hp and 44 ft*lbs
GS850 has 77hp and 48 ft*lbs

So, my main question is this: Could one reasonably assume that if you put 850 jugs and pistons in a 750 and properly tune the carbs, you would have a similar hp/tq output as the 850? I understand the cams are different but I have read that the 850 cams basically increase mid-range torque, but aside from that power should be close to the same, right?

If you add pod filters and exhaust coupled with the increase in displacement from 750 to 844, how much hp/tq can you expect total? Does anyone have dyno results? If the factory rating of the 850 is 77hp, could you expect 80hp out of the modified 750?

Also, would there be any benefit to an increase in compression ratio?

[Nessism]

Your analysis seems sound. Between the bigger top end and pods/pipe 80hp should be achievable. Pretty sure Suzuki rated hp at the crank so rear wheel hp would be a fair bit lower.

[80GS1000]

For the time, effort, and money involved, why not do GS1000 engine swap? 90 HP stock and respond well to tuning.

https://www.thegsresources.com/_forum/showthread.php?256742-GS1000-Motor-in-a-GS750

[93Bandit]

Your analysis seems sound. Between the bigger top end and pods/pipe 80hp should be achievable. Pretty sure Suzuki rated hp at the crank so rear wheel hp would be a fair bit lower.

Agreed.

So, I know the general guesstimate in the automotive world is a ~15% parasitic power loss through the transmission and drive-line.

Does that 15% hold true in the bike world as well? I would think it's slightly less in a chain and sprocket application since you don't have the bulk rotating mass and friction associated with a final drive unit.

If you assume 80 hp at the crank from the mentioned mods, then that would be about 68 whp.

[93Bandit]

For the time, effort, and money involved, why not do GS1000 engine swap? 90 HP stock and respond well to tuning.

https://www.thegsresources.com/_foru...tor-in-a-GS750

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.

[Nessism]

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.

I'd ditch the kick start before the electric start.

[Steve]

I'd ditch the kick start before the electric start.

I have to agree with that.

All the cool points you might accumulate by kickstarting the bike are VERY quickly lost if you happen to stall the bike in the middle of traffic, waiting for oncoming traffic to clear so you can make your left turn. A simple nudge of the thumb on a button, and most people probably would not even know that you stalled it.

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?

.

[GregT]

I have to agree with that.

All the cool points you might accumulate by kickstarting the bike are VERY quickly lost if you happen to stall the bike in the middle of traffic, waiting for oncoming traffic to clear so you can make your left turn. A simple nudge of the thumb on a button, and most people probably would not even know that you stalled it.

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?

.

A starter motor, plus starter clutch, plus solenoid and wiring, plus a probable larger battery is the heavier of the two options.
But yes, I'd pick convenience every time.

[93Bandit]

Well, nothing I'm planning on doing to my bike has to do with "looking cool." I could not care less what others think; I've never been one to seek others approval as a form of gratification or feeling like I did the "cool" thing. My intentions are to build a bike with as few bells and whistles as possible yet remain street-able. I plan to retain wiring provisions so that I can reinstall the starter if I get tired of kicking it. Also, this bike will only be a weekend warrior not a daily, so when I'm tired of kicking I'll ride my 850.

Back to the technical, does anyone have thoughts on an increase in compression? Would it, in combination with the other mods, be worth doing and add any power? I believe the factory 750 ratio is 8.7:1 and the 850 jugs/pistons shouldn't affect that, right? Would an increase to ~10:1 be beneficial or complicate things?

[80GS1000]

Well, nothing I'm planning on doing to my bike has to do with "looking cool." I could not care less what others think; I've never been one to seek others approval as a form of gratification or feeling like I did the "cool" thing. My intentions are to build a bike with as few bells and whistles as possible yet remain street-able. I plan to retain wiring provisions so that I can reinstall the starter if I get tired of kicking it. Also, this bike will only be a weekend warrior not a daily, so when I'm tired of kicking I'll ride my 850.

Back to the technical, does anyone have thoughts on an increase in compression? Would it, in combination with the other mods, be worth doing and add any power? I believe the factory 750 ratio is 8.7:1 and the 850 jugs/pistons shouldn't affect that, right? Would an increase to ~10:1 be beneficial or complicate things?

You could bore out your 750 block to 844 cc and add Wiseco 10.25:1 pistons. That way you would avoid any potential gotchas of dropping the 850 block on to the 750 cases, plus the hassle of sourcing a spare 850 engine for parts. You'd need to run premium gasoline to avoid knocking with this configuration.

http://dynoman.net/engine/pistons/suzi.html

[93Bandit]

You could bore out your 750 block to 844 cc and add Wiseco 10.25:1 pistons. That way you would avoid any potential gotchas of dropping the 850 block on to the 750 cases. You'd need to run premium gasoline to avoid knocking with this configuration.

http://dynoman.net/engine/pistons/suzi.html

What potential gotchas could happen? I'm aware of the possible need to grind on the 750 case to clear the 850 jugs, but other than that I don't see major complications based off the threads I've found on here. Seems like a pretty straight forward swap. I even have an 850 I could cannibalize if I can't find the parts or a doner in time. That would save me $530 over the cost of those Wiseco pistons.

How much would need to be shaved off a 750 head to achieve ~10:1 CR with factory jugs and pistons? Or will I run into PTV clearance issues going that route?

Running premium won't be an issue for my wallet.

[93Bandit]

After more searching the hive mind of the GSR, I read that milling the head for a CR increase will require retiming the cams and possibly notching the gears? I'm not trying to extract every ounce of power out if this thing to get a 1/100th in the 1/4 mile, so milling the head probably isn't worth the gain for what I'm trying to achieve.

On a completely different note, redline on the factory gauge for the 750 is 9k if I remember correctly. It doesn't run and I've never ridden a 750, but my 850 seems to run out of steam above 9k but it doesn't exactly fall on its face (assuming the tach is accurate). Anyways, is 9k truly the mechanical saftey threshold or can these motors rev higher safely?

[80GS1000]

What potential gotchas could happen? I'm aware of the possible need to grind on the 750 case to clear the 850 jugs, but other than that I don't see major complications based off the threads I've found on here. Seems like a pretty straight forward swap. I even have an 850 I could cannibalize if I can't find the parts or a doner in time. That would save me $530 over the cost of those Wiseco pistons.

How much would need to be shaved off a 750 head to achieve ~10:1 CR with factory jugs and pistons? Or will I run into PTV clearance issues going that route?

Running premium won't be an issue for my wallet.

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.

[Steve]

Back to the technical, does anyone have thoughts on an increase in compression? Would it, in combination with the other mods, be worth doing and add any power? I believe the factory 750 ratio is 8.7:1 and the 850 jugs/pistons shouldn't affect that, right? Would an increase to ~10:1 be beneficial or complicate things?

How much would need to be shaved off a 750 head to achieve ~10:1 CR with factory jugs and pistons? Or will I run into PTV clearance issues going that route?

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.

.

[GregT]

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.

.

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.