I have a pro design cool head on my LT500... it's has a 62 cc dome, and I'm curious what the stock dome size was?

My compression was reading 120 psi, which seemed excessively low, so I yanked the head off. I had expected that the head gasket would have been an o-ring, but it wasn't. Who new?

Oh, and she's bored .100 over... from this info, can my theoretical compression ratio be estimated? Assistance is appreciated!



Second question: Does this calculation seem right to you?

Stroke = 86 mm = 8.6 cm

Bore = 86 mm = 8.6 cm + .100 inch over bore = 8.6 cm + 0.100 inches x 2.54 cm/inch = 8.854 cm

Volume = Bore x Stroke = 8.854 cm x 3.1416 / 4 x 8.6 cm = 529.5 cm^3

So I actually have an LT 529.5! [img]i/expressions/face-icon-small-smile.gif[/img]

So from this, I estimated my compression (ignoring the head gasket volume, and the dome of the pistion (perhaps I shouldn't ignore these issues)):

Compression = (Cylinder volume + dome volume) / dome volume = (529.5 cm + 62 cc) / 62 cc = 9.54:1

Is this right? At this compression ratio, what compression (in psi) should I have?

Thanks

 

Your right. You have an LT 529.5! However, it's very important not to forget the piston dome height and gasket thickness to figure out compression. Cranking compression (PSI) will be affected by the height of your exhaust port. The way I like to do mine is check head volume with the piston atdc. I run 12:1 with pump gas. Obviously 11:1 is safer but I know people who run 13:1. Whatever. The biggest problem you have is the cool head squish band is way way way too wide. The stock head is shaped very good. It only needs .015" cut from gasket surface for compression to be perfect. It also flows water better. What I did with a friend who had to run a cool head on his Banshee was buy a high compression dome and cut the squish back to normal width and get normal cc's. Now you get higher, safer, compression. With the cool head you can only go so high with compression before you have detonation problems because of the wide squish band. Just my 2 cents worth.

 

Thanks for your input!

Would you define "squish band"?

Thanks!

 

The squish band is the outer most part of the head chamber. The part that matches the piston profile. Part of what it does is when the piston comes to tdc it forces (squishes) the gas to the center of the chamber. If its too close to the piston or too wide the gas can't squish out fast enough and detonation starts. That speed is called squish velocity. Look how wide your cool head squish band is compared to your stock head. Because it's so wide the gas don't have time to get out. You have to have more clearance on the cool head so the gas that can't get out don't get compressed to much and detonate. Now you have gas in part of the head not helping you out. Wasted. I hope this helps you out.

 

Well said Halfliter.

 

What would you recommend doing for a head? Get a stock one and modify it? What are you running? Does anyone have an '88 and up LT500 cylinder and head?

 

I would use the stock head. Measure the stock squish clearance. You should have to cut about .015 off the gasket surface to get a nice safe .060 to .065 squish clearance. Thats what I did to mine. The compression came out real nice. And the squish velocity is right where it should be. Mine is stock bore and I run pump gas. As for where to get one. I'm not sure off hand. I'm sure you could get one on ebay or even someone here might have one.

 

How much can you safely cut the stock head on a 1991 lt250r? I want to be able to run on premium pump gas still. I did a comp. check last year before I tore my motor down this time and it was 150 - 155 lbs. I want to run a little under the premium/race gas mix borderline just to be safe.

chuck

 

I can't help on this one. I've never checked a 250 head before. I don't know what the shape of the cumbustion chamber is. Hopefully its as good as the 500. However, I think 150 to 155 lbs sounds a little high already. You might not be able to go much more. Lets wait to see if someone else has an idea.

 

Halfliter, sounds like you know your stuff on the 'zilla.What should the PSI reading be on the 500 to still run pump gas? Mine isn't starting easily at all. I haven't checked the cranking compression yet, but it feels by the kick started like its a little easier on the foot the usual. I'd appriciate any advice you can give me!


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1987 LT500R