hot rodding my gs 750/850

[nastyjones]

My 78-750 gs with an 82-850 top end is undergoing a mild hop up. It will get minor intake & exhaust port shaping as well as combustion chamber & exhaust port polishing. I will include rejetted 850 cv carbs & a 4-1 exhaust system. I am using the excellent book "performance tuning in theory & practice" by A.Graham Bell that has been previously mentioned on this site.
I also want to raise the compression ratio by skimming the head & deck. What are the guidelines for this? I have heard .030 thou. but don't know if that means .020 off the head & .010 off the block or a different combo.
Can anyone tell me what has worked for them?

Paul

[duaneage]

You need to do a piston to valve check with modeling clay and make sure you have enough clearance to tolerate a shave of the head. I would look for at least .100" as you revolve the engine around all four strokes.

You might be able to find a thinner head gasket that drops the head down. What are the combustion chanber volumes of the 750 versus the 850? If you bored the engine you may not need to deck it.

Planing the head must be done right. Some places just run the head across a milling wheel by hand and mic a spot until they are close enough. The proper way is with a milling machine that is set up accurately.

[Nerobro]

The GS850 carbs are smaller than what came on the 750's. The BS32SS carbs were the stock equipment on the GS550, 650, and 850. I'd hunt down larger carbs if you're goal is high horsepower.

Since you're doing everything else in metric, it's best to discuss things like "milling the head" in metric. I'd be more apt the mill the cylinder block rather than the head. Your bore and stroke are 69.0 x 56.4 mm and the stock CR is 8.8:1. Right now, your engine compresses everything into a volume of 96.59cc's. By milling one mm off the cylinder block you loose 24.88cc's. And that will put you at 11.85:1.

Hmm... .030 is .762mm. That means you loose 18.95cc's. And that will net you 10.95:1 CR. I'm not sure I'd go that high. Something about air cooled motors and high compression ratios makes me nervous.

[arveejay]

Since you're doing everything else in metric, it's best to discuss things like "milling the head" in metric. I'd be more apt the mill the cylinder block rather than the head. Your bore and stroke are 69.0 x 56.4 mm and the stock CR is 8.8:1. Right now, your engine compresses everything into a volume of 96.59cc's. By milling one mm off the cylinder block you loose 24.88cc's. And that will put you at 11.85:1.

Hmm... .030 is .762mm. That means you loose 18.95cc's. And that will net you 10.95:1 CR. I'm not sure I'd go that high. Something about air cooled motors and high compression ratios makes me nervous.

Man, how do you know all that. Do you do it for a living?

[nastyjones]

Duaneage, I don't think I'll have clearance issues since I know this mod has been done before.

Nerobro, maybe .025 would be better then. Can you tell me the metric equivalent? I'm still not sure about whether to mill the head or block.
I was hoping someone who has done it would give us their expertise.

Paul

[Nerobro]

..... there are 25.4mm per inch. Radius times radius times pi gives your the aera.

I'd recomend milling only the block, not the head. That's an ameatures opinion but you don't want to change the shape of the combustion chamber, just the depth. Milling the head will change the shape, and may cause piston/head interference.

[BadBillyB]

Since your using the top end from one motor and the bottom of another, first thing to check is the deck height. This will determine what you need to do. Measure it with a dial indicator. Check it on more than one cylinder. Also measure the thickness of the head gasket you plan to use.....What you should be looking for is .035" to .040" clearance, from the top shoulder of the piston to the cylinder head surface......BadBillyB

[BadBillyB]

Read page 158, second paragraph in A. Graham Bells book....Piston/combustion chamber shape is more critical to making good power than mechanical compression ratio.......BadBillyB

[nastyjones]

Billy, the difference in these 2 motors is in the bore isn't it? Since the cylinders, pistons, cams & head are all 850 I think everything should work. Feel free to correct me if I'm wrong. A thin head gasket from Cometic should help. I'm not sure of the exact thickness.
And what do you think I should do with combustion chamber shape?

Paul

[duaneage]

Duaneage, I don't think I'll have clearance issues since I know this mod has been done before.

It does not matter who has done it or what was done, Check the clearances with the plugs in. At high RPMs the pistons reach higher and the valves tend to go lower as well. New valves seats will also change the relationship. Good engine builders measure everything and assume nothing.

If you mill the block ( cylinder bank) will that affect the o ring seals betwen the ehad and block or does this engine not have them?

[BadBillyB]

Billy, the difference in these 2 motors is in the bore isn't it? Since the cylinders, pistons, cams & head are all 850 I think everything should work. Feel free to correct me if I'm wrong. A thin head gasket from Cometic should help. I'm not sure of the exact thickness.
And what do you think I should do with combustion chamber shape?

Paul

OK, we're talking mild hop up right?....You should do nothing to the combustion chambers other than smoothing any sharp edges. You are not going to know what thickness head gasket to use until you measure the deck height. What you want to do is maximize the set up you already have. Dont take for granted, like Duaneage said, that the deck height is correct. This will effect the "squish" area of the piston/chamber. This is critical. A thin head gasket could be disaster if your pistons are at or just above deck.....Measure it....Your main focus should be on the ports. Are the 850 intake ports much larger than the 750 ports??........BadBillyB

[crc1214]

Duaneage, Nerobro, Arveejay and BadBillyB - I have read this post top to bottom and the entire time my jaw has been open and drool has been running out. How the F*&K do you guys know this stuff? Seriously - you guys have to have extensive hands on experience or a room full of tech books to recite this info lickity-split. Either you all four should get together and open up a bad-ass race shop or all start working for NASA - many props to you guys.

[nastyjones]

Billy, What type of equipment do I need to measure deck height & how do I do it?
As for whether the 850 ports are bigger, I believe the 850/750 heads are identical except for the cv carbs on the 850. Speaking of carbs, I plan on using the cv's. The motor will only modded as explained in my earlier posts. So no radical cams or pistons.

Paul

[arveejay]

Duaneage, Nerobro, Arveejay and BadBillyB - I have read this post top to bottom and the entire time my jaw has been open and drool has been running out. How the F*&K do you guys know this stuff? Seriously - you guys have to have extensive hands on experience or a room full of tech books to recite this info lickity-split. Either you all four should get together and open up a bad-ass race shop or all start working for NASA - many props to you guys.

Thanks for the compliment but I'm as impressed as you and I didn't contribute any help to anyone at all in this thread.

[BadBillyB]

Billy, What type of equipment do I need to measure deck height & how do I do it?

Paul

This is what I use. Its just a piece of tubing with a tab welded to it for the indicator. You first zero it on the top of the block, then carefully rotate it (with the plunger lifted) and set it down on top of the piston. The measurement is taken directly above the wrist pin as the piston can rock in the bore.