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Discussion Starter · #1 ·
ok my track season is over now and over the winter i plan to bump my compression among other things. question is if anyone who has been sucessful with this before, did you go for a thinner head or base gasket? any issues with valve clearance? i know the base gaskets are available in different thickness, what did you go with and what did it put your final compression to? i am doing this at stock bore superstock build.
thanks
 

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APE offers a .025" head gasket, that I'm using with the +2mm big bore 13:1 compression. Still running pump gas (93) with no problems thus far.
 

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any idea what the stock head gasket thickness is?
It's a 3 layer head gasket. Each layer is about .008" You can remove middle layer for slight bump in compression. You can have heads cut for bump in compression. There is a bit of room in there. If you want to go crazy you cut a bunch off the head but you'll have to buy adjustable cam sprockets and degree cams. For the price of head gaskets, I wouldn't bother going in there just to remove middle layer.
 

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I decked the top of the cylinder .010" (not head) and removed the middle layer of a stock head gasket. This left me about .035" of piston to head clearance and I can still run on 93 with no problems. PTV clearance was not an isssue either. I would mill cylinder before I touched head.
 

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You can get by with .025" PTV or as some old school guys say squish. Who has the best price on adjustable cam gears, and cams?
 

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You can get by with .025" PTV or as some old school guys say squish. Who has the best price on adjustable cam gears, and cams?
PTV is not squish.

PTV is piston to valve clearance. I measure during assembly/disassembly with clay.

Squish is piston to head clearance. Squish = deck height + head gasket thickness.

Decreasing squish will increase compression and make combustion more efficient.

Increasing compression by decreasing squish will allow the motor to be slighly less prone to detonation compared to if you were to only increase compression by decking the head.

You need to leave some squish clearance to account for crank flex, otherwise the piston will eat the head. Most engine guys might say in the .030" range give or take. A stock motor has around .050" of squish.
 

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PTV is not squish.

PTV is piston to valve clearance. I measure during assembly/disassembly with clay.

Squish is piston to head clearance. Squish = deck height + head gasket thickness.

Decreasing squish will increase compression and make combustion more efficient.

Increasing compression by decreasing squish will allow the motor to be slighly less prone to detonation compared to if you were to only increase compression by decking the head.

You need to leave some squish clearance to account for crank flex, otherwise the piston will eat the head. Most engine guys might say in the .030" range give or take. A stock motor has around .050" of squish.
Was my understanding the PTV was accounted into the squish. Learn something every day. Don't you also have to take into consideration the base gasket thickness as that will change the deck height?
 

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Don't you also have to take into consideration the base gasket thickness as that will change the deck height?
Yes, base gasket thickness changes deck height.

When you decrease squish, you decrease the PTV by the same amount assuming you leave head, valves, cams, and cam timing what they were. (There are athe lot of variables that can change PTV.)

You could put in an thinner base gasket and that would be the same as decking the cylinder. They make thinner base gaskets, but stock one is .010" thick so sealing can become issue with thin ones.
 

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To answer the original question: by decking cylinder 0.010", taking middle layer out of stock head gasket, using stock base gaskets and pistons, and just dusting the heads to clean the surface- my compression came out to be what the advertised compression is (11.5:1ish IIRC). The advertised stock compression is higher than actual as multiple people have confirmed. Stock is actually around 10.8:1 or so.
 

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Yes, base gasket thickness changes deck height.

When you decrease squish, you decrease the PTV by the same amount assuming you leave head, valves, cams, and cam timing what they were. (There are athe lot of variables that can change PTV.)

You could put in an thinner base gasket and that would be the same as decking the cylinder. They make thinner base gaskets, but stock one is .010" thick so sealing can become issue with thin ones.
Thanks for the good info. I'm still young and hungry for information.
 

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Discussion Starter · #12 ·
Would there be any advantage to taking .010 off the deck and removing a later of gasket vs just taking like .020 off the deck? Has anyone had sealing problems just using one layer of head gasket? Our is that too much? To those that have done this what compression did you end up with and how far off was your can timing?
Thanks for the help you guys rock
 

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Would there be any advantage to taking .010 off the deck and removing a later of gasket vs just taking like .020 off the deck? Has anyone had sealing problems just using one layer of head gasket? Our is that too much? To those that have done this what compression did you end up with and how far off was your can timing?
Thanks for the help you guys rock
I know some people have run a single head gasket. I'd pull one layer and cut .010 off a mating surface.

That is about .019 (which is what CCS323 did) so you would again be close to 11.5:1. The difference is that when you cut from the head you don't get as big of a compression gain as the cylinder. Without measuring I can't tell you the difference.
 

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Would there be any advantage to taking .010 off the deck and removing a later of gasket vs just taking like .020 off the deck? Has anyone had sealing problems just using one layer of head gasket? Our is that too much? To those that have done this what compression did you end up with and how far off was your can timing?
Thanks for the help you guys rock
There is no going back if you shave 0.020", I dont see the point when it would cost the same. Id rather leave the material there to leave me more options.

does anyone know how much would have to be removed to get to 13:1? or does anyone know what the deck height is?
thanks
Thats why engine builders make $$$!

I dont know how much you can shave the heads. Gregg Spears would know. He was extremely helpful with my engine rebuild.

13:1 is approaching a ticking time bomb with stock crank and rods IMO. I would be doing a valve job, lightweight flywheel, and degreeing cams before I went above 12:1. 2nd gen cams are a huge gain, but legality may be an issue.

My stock deck height was +/- 0.023"
Stock heads are about 19cc.
Piston dish is about 6-7cc.
You could probably calculate a ball park number.
 

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Discussion Starter · #18 ·
The light weight flywheel and degree cams are all on my list of things to do this winter before the bike is fired up again. Wera legal is a big part of my build so unfortunately things like a crank or rods are out. At .050 would I run into valve clearance issues? I'm just trying to get as much legal power as possible since monday on the straight the bike just ran out of steem long before to gear red line.
Thanks
 

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Valve clearance issues will depend on where your cams are set...but I've read that the SV's are pretty forgiving in this department unlike other engines. Would be good to check what you have just to be sure you haven't created a grenade with the pin out.

I destroyed a friends 427 Chevelle when he had assembled it without checking the PTV. The intakes were kissing the piston tops...and #2 eventually broke off which destroyed the block and head. Nice! I always check even if it shouldn't be an issue.

And 'squish' is some great stuff! You can increase compression while actually decreasing the tendency to detonate. My last RD400 had the squish so tight you could read the overbore #'s from the piston on the squish band carbon. Yes..it was a crispy one, but ran like a little raped ape.:)
 

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The light weight flywheel and degree cams are all on my list of things to do this winter before the bike is fired up again. Wera legal is a big part of my build so unfortunately things like a crank or rods are out. At .050 would I run into valve clearance issues? I'm just trying to get as much legal power as possible since monday on the straight the bike just ran out of steem long before to gear red line.
Thanks
Change your gearing.
 
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