Cylinder Lift - Vmax-4 750

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Ken

Ken

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I don’t claim to know everything about this but I will share what I do know. Most of my information will relate to the 750's because that’s what I have first hand experience with. Some may not agree with some of the information in here like gasket thicknesses, squish, or decking.

To start off with the port timing is conservative on the Vmax-4's.

The 92's have the most conservative timing, you can go up to a .059" base gasket on the 92's. The 93/94 750's have higher ports than the 92's, so they are usually only raised with a .039" base gasket. I have read some of the dimensional changes to the ports that DynoTech published but I don’t know if they are correct. If anyone has any Yamaha documentation indicating the port changes I’d be interested in reading it.
 

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There were some differing opinions on the thickness of a stock base gasket so I ordered a new OEM one and measured it. The measurement that I got was .021" uncompressed with a micrometer.

The 92 OEM head gasket is 3-layers, the 93-94 OEM head gasket is 2-layers. Each layer is approximately .008-.009" (.2mm). The gaskets come from Yamaha with a coating on them to seal them. The coating isn’t on all surfaces of each layer, it’s only on one side of some layers. You will therefore get slight variations when measuring the individual layers of the gasket, but it’s only about .001" difference. Yamaha removed a layer from the 93-94 gasket to regain some of the compression that was lost due to the port height change.
 

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The heads on the 92-94 Vmax-4's have cast combustion chambers and they are somewhat inconsistent. I measured the lip or step between the head surface and the squish band and it varied between .014" and .023". I also CC’d my combustion chambers and found they were also a little inconsistent.

When you raise your cylinders you have to regain your lost compression and get your squish height back. The 92's are in the neighbourhood of .080" squish and the 93-94's are in the neighbourhood of .070" squish (due to the thinner gasket). The best way to find out what you have is to measure it yourself. I used solder and checked above the piston pin on both sides of the piston at the same time. I removed the head, put the piston about 1/8"from TDC, put a piece of solder on each side, bolted the head back on, turned the motor over by hand, removed the head, used a micrometer to measure the solder. I repeated this for each cylinder.
 

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I had my heads milled by a reputable automotive machine shop, he charged me $55 (Canadian) per head. I cut one head .018" and the other .020", I did this to blue print them. Personally I wouldn’t cut anymore than .020" off without rechambering the head. I based cutting my heads different amounts by; measuring the combustion chamber cc’s, measuring the squish band, and measuring the step in the head. I looked at all the measurements and made the decision to cut .002" less off one head. The end result was more consistent squish height across the motor and a more consistent head volume.

Prior to installing my heads I put them on a surface plate with some 600 grit wet/dry paper and smoothed out some of the machining marks. I also sprayed the single layer of head gasket with Permatex Copper Cote.

I windowed my pistons with a bench top drill press and a step drill. The step drills work excellent for doing this. I went with .5" windows on the same center line as the 93/94 pistons. If you window your pistons it would be better if you didn't stay on the same center line and kept the distance from the bottom of the skirt the same as stock.
 

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Decking is removing material from the tops of the cylinders. If you want to run .059 base gaskets on a 92, I feel that the only way to get your squish height back to a reasonable level is to deck the cylinders. I looked into doing this but couldn’t find I shop that I trusted to deck the jugs. The machinist who did my heads wouldn’t touch the jugs because he thought the Nicasil would chip.. For this reason I ended up going to a .047" base gasket, milled the heads .018/.020" and used a single layer of head gasket. So if we take the .047" gasket that I used and subtract the thickness of a stock gasket (.021"), I ended up with a actual lift to .026". To get my squish height back to an acceptable level and regain my compression I cut .020" off my heads plus I removed two layers of head gasket .016", for a total of .036". So I ended up with a squish height that was .010" tighter than stock. I started off with around .080" and ended up with squish ended up at around .070". I’d have been happier with it around .065" but .070" was the best that I could achieve without decking. I gained about 10lbs compression.
 

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As far as squish goes some feel that the tighter the squish the higher the octane. This really isn’t the case in my opinion. It’s the volume of the combustion chamber that sets the octane that you require. A tighter squish gives you more velocity of the charge as the piston reaches TDC. A large squish give you less velocity and a greater volume of late burning (useless) charge that is trapped in the squish band. If you run your squish too tight then there is a risk of the piston hitting the cylinder head.

Don't forget to :homework: The end!
 
Wow Ken!!!! Thank you for taking the time to write out this post. You did a awesome job. Verry informative to read and the photos are great too.
 
Excellent post Ken! Very detailed and informative.Been hearing about this mod for a few years now but never quite got the jist of it. Clear as a bell now. I hear others have done this to their sleds,but i haven't read about any results. Anybody? In addition ,Ken could you explain something in the way of clarification.I have a 93 for which you recommend a .039" lift.Is that on top of the stock.021" gasket for a total of .060"?
 
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hontri said:
Excellent post Ken! Very detailed and informative.Been hearing about this mod for a few years now but never quite got the jist of it. Clear as a bell now. I hear others have done this to their sleds,but i haven't read about any results. Anybody? In addition ,Ken could you explain something in the way of clarification.I have a 93 for which you recommend a .039" lift.Is that on top of the stock.021" gasket for a total of .060"?
Click to expand...

You replace the stock gasket with a .039" gasket or you can use two stock gaskets.
 
Thanks Ken, thats what i thought but wanted to make sure.I figured two stock gaskets at .021" apiece compressed would be pretty close to .039".Have you tried your sled with the mods? Any seat of the pants feelings? Again excellent post! Thanks Norm
 
would there be much of a difference if you didn't window the pistons? i would like to do what you have done but i'm not to crazy about drilling holes in my pistons. i'd be afraid it would cost me a whole lot of money and headaches if not done properly.
 
It's probably still worth your while to do the lift even if you don't window the pistons.

I can't really say if there was that much of a performance improvement because I did the mod in the off season. The sled seems to pull hard and it ratchets the track at about 80mph.

I believe a that the cylinder lift and windowing is supposed to give 6-8hp across the powerband.
 
you don't even see that kinda of detail in a shop manual..!! great job ken..!! now how about same kinda post but on carb boring..? LOL...!
 
:urock: KEN AWESOME JOB.

CAN I ADD A LITTLE. " SQUISH "

92 750

.059 BASE GASKET
CUT .050 OFF THE TOP OF CYLINDERS. THIS WILL LEAVE YOU .005 PISTON TO TOP OF CYLINDER.
CUT HEAD SO YOU HAVE .020 ON THE STEP OR SQUISH BAND.
USE A .030 HEAD GASKET FOR TOTAL OF .055 SQUISH.
NOTE THIS WILL RUN ON 92 OCTANE FUEL BUT NO LESS.

USE THE .020 HEAD GASKET FOR A TOTAL .045 SQUISH.
BUT TAKE NOTE YOU WILL NEED 100 OCTANE FOR THIS.

I ALWAYS MIX MY FUEL 2GAL 110 CAM 2 TO 3 GAL 92 OCTANE.
GAS MAN TOLD ME THIS WOULD BY AROUND 99 OCTANE.
OCTANE RATINGS ARE "MIN. OCTANE" 92 SOULD HAVE 93 TO 94 OCTANE.
SO THE 2 TO 3 MIX SHOULD BE MORE LIKE 100 TO 101.

AND MY .02, GOOD GAS COST LESS THAN PISTONS !!

POWERSHACK
 
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