GS750 850 upgrade

[doctorgonzo]

Going back to my 79 GS750, I am leaning towards an 850 cylinders and pistons for simplicity and cost (ie no machining needed) and I think stock compression may be preferable as the bike was originally designed this way. Also I want to keep stock airbox and have no interest in messing with the cams as currently I really like the rideablity and amount of power as it is (anything extra is a bonus of course).


But for arguments sake and cost aside, what is the best end result, in everyday use, between a rebore of my existing 750 cylinders with an 844 wiseco kit that ups the compression, and the above? Given that in my case I will be riding 95% of the time in peak hour traffic commuting. Would the wiseco upgrade reduce the longevity of the engine and be more temperamental?

I don't think it will effect the longevity. It might require premium gas, and it's more expensive. $50 a hole for the bore job, so $200, another $90 for the rings unless they are included in your $300 (or more) set of wisecos. You're in 5 or 6 bills minimum. IMO, and others may disagree, but Pod filters and carbs properly jetted for them would give you almost as much top end increase as boring it 100 CC and keeping the stock airbox.

[Coady]

I don't think it will effect the longevity. It might require premium gas, and it's more expensive. $50 a hole for the bore job, so $200, another $90 for the rings unless they are included in your $300 (or more) set of wisecos. You're in 5 or 6 bills minimum. IMO, and others may disagree, but Pod filters and carbs properly jetted for them would give you almost as much top end increase as boring it 100 CC and keeping the stock airbox.

Thanks for the reply.

My current cylinders are scatched and burning oil, so I have to rebore or replace them, thats why I figured I may as well go up to 850cc.

Will the wiseco's give a noticeable performance advantage over stock GS850 cylinders and pistons? And are they more likely to overheat? Is premium petrol the only other modification required and I guess what I'm really wondering is why the original GS750 compression is 8.7:1 and how the wiseco manages 10.25:1 and yet a new GSX1400 compression and similar is still 9.5:1.

[doctorgonzo]

Thanks for the reply.

My current cylinders are scatched and burning oil, so I have to rebore or replace them, thats why I figured I may as well go up to 850cc.

Will the wiseco's give a noticeable performance advantage over stock GS850 cylinders and pistons? And are they more likely to overheat? Is premium petrol the only other modification required and I guess what I'm really wondering is why the original GS750 compression is 8.7:1 and how the wiseco manages 10.25:1 and yet a new GSX1400 compression and similar is still 9.5:1.

I'm pretty sure it has to do with the shape of the top of the piston (the compression of the Wiseco Pistons). The premium gas is just because of the high compression. Compression makes the gas easier to ignite, so in a high compression engine the residual heat from the last time the cylinder fired can cause the gas to ignite before the spark plug fires. That is pre-ignition and will damage the engine. The upside is that higher compression, and it's more violent explosion get's more horsepower out of the same displacement. So to answer your question I think yes, there would SOME performance advantage to Wisecos over stock 850 pistons, but also a much bigger price tag. The cheapest route would be probably an 850 engine with pretty good compression then you just hone the cylinders, maybe re-ring, and off you go. Saving the 200 for the bore of the 750 cylinders and the several hundred for the wiseco pistons.

[boingk]

Lower compression engines generally give less power but a better spread of it and from lower rpm - think of a Harley and you're on the money. Higher compression engines generally produce more power over a narrower spread - think of a supersport 600.

As far as compression ratios go, 10.25 isn't much. Modern supersport 600's are commonly around 13:1. 850's run a standard compression ratio of 8.8:1, which is really quite low. Nudging it up a tad would give you a bit more power for sure.

Cheers - boingk

[Coady]

Lower compression engines generally give less power but a better spread of it and from lower rpm - think of a Harley and you're on the money. Higher compression engines generally produce more power over a narrower spread - think of a supersport 600.

Cheers - boingk

So lower comp means more low end torque? Which I have to say is why I really like my motor.

[49er]

So lower comp means more low end torque? Which I have to say is why I really like my motor.

Not true. I run 10.5-1 on my '79 850 which torques great from 3000 - 9500 rpm. Still running 91 octane (Ron), without detonation.
Minor changes to valve pockets and have ditched the points for electronic ignition. Still running stock VM carbs with modified airbox. Don't you believe for a moment that the stock 850 setup is the best it can be.

[boingk]

While lower compression won't automatically give you more torque, it does often make for a cruisier engine. It'll still charge hard if its tuned that way though (GS's are). Increasing the compression will give you a bit more kick and a meaner exhaust note. It may make the engine run a tad (I said 'tad', not 'a lot') hotter, but thats about it. I'd actually be interested to see its effect on fuel economy.

As for stock setups from this era being the best they could be? Nope, not in my book. One look at the stock airbox had me gagging, I figured I may as well fit pods as it couldn't be worse in terms of intake turbulence and would only be better in terms of weight, airflow and ease of maintenance. Plus they look and sound cool B)

Cheers - boingk

[doctorgonzo]

While lower compression won't automatically give you more torque, it does often make for a cruisier engine. It'll still charge hard if its tuned that way though (GS's are). Increasing the compression will give you a bit more kick and a meaner exhaust note. It may make the engine run a tad (I said 'tad', not 'a lot') hotter, but thats about it. I'd actually be interested to see its effect on fuel economy.

As for stock setups from this era being the best they could be? Nope, not in my book. One look at the stock airbox had me gagging, I figured I may as well fit pods as it couldn't be worse in terms of intake turbulence and would only be better in terms of weight, airflow and ease of maintenance. Plus they look and sound cool B)

Cheers - boingk

Probably the biggest "bang for the buck" performance thing you can do (Pods) for $200 or so (the pods and the re-jetting) it wakes it up quite a bit. It does move the power curve up a little bit.

[Beaver]

The older rides with VM carbs will generally benefit in the top end from fitment of a slightly larger size than stock, often just from the next model up and the jetted to suit. As mentioned above, you will be able to find these commonly and relatively cheaply. If you can retrofit the CV carbs they are good for an extra horse or two, plus potential economy benefits.

When you say the next size up, are we talking looking for a set off an 850 or just something as simple as if I have VM26's now looking for VM28's?

Cheers

[TheCafeKid]

Pods on the VM equipped bikes isn't a bad deal. Takes some
patience and tuning but the mechanical carbs are more accepting
of the fitment than the CV carbs which rely on vacuum to operate correctly and changing airflow can cause issues which even a jet kit won't resolve completely. Provided you understand that you simply can't jus wack the throttle open (which leads to a vacuum dump and will make to
motor bog for a second)

Anyway IMO the bang for the buck issue. Using the 850 jugs and pistons will give you a little pick me up but if you can couple that with pods and a pipe and even aftermarket cams (which will require degreeing to get what you want out of them) and you can really wake those 750 motors up. The bonus here is that the 750 motors are one of if not THE most durable of the GS motors built. They're the only one that used a straight cut gear set. I've not seen but heard tales of them getting overbores and turbo kits and getting flogged to hell and back and lapping it up with vigor. Hop that baby up. You're not gonna hurt it unless you're actually trying to and even then it may laugh at your attempts.

[Beaver]

These are described as Mikuni Flatsides coming off a 86/87 GSXR, the guy adds that he has heard of them being compatable with GS750's and 1000's (but I have heard those kind of claims before) they are a 40mm fitting on the motor side.



Are these out of the question on a '77 750 or '79 1000?

Sorry if I am majorly hijacking this thread but it just seems like we are on a good roll here and I am learning tons of new stuff here

Cheers

Angus

[doctorgonzo]

I THINK those are too big for the stock boots. You can jam a 38mm in there, but I dunno about 40s.

After looking forever A member here found me a set of the 36mm slingshots for my 1000 as an upgrade over the stock 34 roundslides.

I am with you on carb confusion, it's REALLY hard for me to keep it all straight, what year gsx had this mm vs that mm, etc...

Also, someone talked back a bit about using CVs on a pre 1980 bike, but I was under the impression the port spacing was different so you'd have to mod the rail.

[Beaver]

So you mean more like these?



Mikuni 36's off a GSX 1100EF

[doctorgonzo]

So you mean more like these?



Mikuni 36's off a GSX 1100EF

I believe so yes. Some years had 38mm, some years 36mm. I'd have to look at my notes for 750 vs 1100 and which years, but those should be the carbs. BS36SS and BS38SS Mikuni. I think on an only moderately built 1000 the 38s would be to much.

[Coady]

Anyway IMO the bang for the buck issue. Using the 850 jugs and pistons will give you a little pick me up but if you can couple that with pods and a pipe and even aftermarket cams (which will require degreeing to get what you want out of them) and you can really wake those 750 motors up. The bonus here is that the 750 motors are one of if not THE most durable of the GS motors built. They're the only one that used a straight cut gear set. I've not seen but heard tales of them getting overbores and turbo kits and getting flogged to hell and back and lapping it up with vigor. Hop that baby up. You're not gonna hurt it unless you're actually trying to and even then it may laugh at your attempts.

I like what I'm reading. Already got the pipes...

One thing at a time though. Gotta sort out my scored cylinders first. Some good info here.