Overbore and Compression Ratio - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 13 (permalink) Old 01-20-2018, 04:14 PM Thread Starter
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Question Overbore and Compression Ratio

So I was thinking about rebuilding my engine next year. I have a few things i want to do a few things while i have it all apart. Two things i really want to do is to have it bored out a bit, and i want to raise the compression ratio to somewhere around 11.5:1. i have a few other things i'm gonna do like port and polish and some stronger clutch springs.

Does anyone know how much i can bore it out and where to get pistons to match the new bore?
And does anyone know how i can increase the compression by that much?
And are there any other things i should do while i have the engine pulled?
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post #2 of 13 (permalink) Old 01-20-2018, 06:19 PM
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increasing compression ratio wont take much, its already 10.5 to 1. might be as simple as remove a bit from the head (I dont know how much, but probably not much more than a few thou

not sure on how much bore it will take, or where to get oversized parts

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post #3 of 13 (permalink) Old 01-21-2018, 11:09 AM
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In all my parts searching, I don't think I've ever seen an oversize piston, even the standard bore size is hard to find. Wiseco doesn't even list the VN750, but they may build a couple for you. $$$

One problem I see with more compression, you can't adjust the ignition timing (in stock form).

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post #4 of 13 (permalink) Old 01-21-2018, 11:11 AM
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Check/replace the balance dampers while the engine is removed.
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post #5 of 13 (permalink) Old 01-21-2018, 03:41 PM
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If u press out the sleeves and put bigger ones in u can go to about 1000cc,i went to 890. u'll need machine shop work to match sleeve to piston( prob.wiseco),custom head gasket,another set of brushes in starter,stock carb.isn't enough,shave heads and a machine that spits out money.Go for a good shave and a thin gasket will be happy enough with that
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post #6 of 13 (permalink) Old 01-21-2018, 07:47 PM
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Quote:
Originally Posted by vnnikolouzo View Post
If u press out the sleeves and put bigger ones in u can go to about 1000cc,i went to 890. u'll need machine shop work to match sleeve to piston( prob.wiseco),custom head gasket,another set of brushes in starter,stock carb.isn't enough,shave heads and a machine that spits out money.Go for a good shave and a thin gasket will be happy enough with that
I was waiting for you to chime in...

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post #7 of 13 (permalink) Old 01-26-2018, 08:04 PM
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How much does a full engine rebuild run? Like a close ball park

Don't worry!!, It's just a bump in the road.
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post #8 of 13 (permalink) Old 01-31-2018, 07:14 AM Thread Starter
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I would also like to know how much this would cost. I can do most of the work my self including doing the port job. But about how much would it cost to get new high compression pistons (if I can find one) or to get the head milled down?

Also how much can it be safe to mill the head down to without the valves coming in contact with the piston and wreaking the engine?
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post #9 of 13 (permalink) Old 09-17-2019, 04:45 PM
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Quote:
Originally Posted by Connor Lynden View Post
Also how much can it be safe to mill the head down to without the valves coming in contact with the piston and wreaking the engine?
I'm also waiting for an answer on this, as I'm replacing a head gasket this winter.


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post #10 of 13 (permalink) Old 09-17-2019, 06:30 PM
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Quote:
Originally Posted by Jason Pittenger View Post
I'm also waiting for an answer on this, as I'm replacing a head gasket this winter.
Probably best just to lap the surface.

If you're milling to increase compression, you need to spend time with clay checking the clearances. Or find someone that has, and I'm not aware of anyone that's milled this one.

It's 10.5:1 stock, and manages to take regular gas. Much beyond that and you need fuel and ignition tuning, along with higher octane most likely, maybe even racing gas.

Ignition tuning is something nobody has cracked on this one, and that limits getting this mod right.

This engine will bend valves too.

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Last edited by Spockster; 09-17-2019 at 06:32 PM.
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