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Old 02-01-22, 07:51 PM   #1
hardhat_harry
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Default Cookstown 100 - How to make cheap SV700

Bro-in-Law is looking to to run in a second class at Cookstown in Ireland and has asked to use my old Gen 1 SV race bike. Currently its running UK minitwin rules with some extra tweeks to make it 78HP (UK ones) at the rear on our local dyno. The Ireland Twin rules are basically anything up to 700cc what is the cheap way to take the gen1 650 to 700cc? I have heard talk of Hayabusa pistons, he has already spent a fortune building a fast ZXR400 for the lightweight so money is tight. Is there something that can be done to the standard carbs besides the usual jetting to suit the engine config?
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Old 02-01-22, 08:13 PM   #2
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Default Re: Cookstown 100 - How to make cheap SV700

dont is my advice as the crank wont take it. its already running 78hp which is a fekin miracle.
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Old 02-01-22, 09:43 PM   #3
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Default Re: Cookstown 100 - How to make cheap SV700

Spend the money on the best tyres and amphetamines.
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Old 02-01-22, 09:56 PM   #4
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Default Re: Cookstown 100 - How to make cheap SV700

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Originally Posted by Bibio View Post
dont is my advice as the crank wont take it. its already running 78hp which is a fekin miracle.
Nah I'm happy up to 85hp on a Gen 1 then I worry but others running JHS pistons as a 700 say they are at 90+ on an standard crank with no issues.
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Old 02-01-22, 09:57 PM   #5
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Default Re: Cookstown 100 - How to make cheap SV700

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Originally Posted by Adam Ef View Post
Spend the money on the best tyres and amphetamines.
He has done Cookstown before on a 400 so its not new to him and he can sleep on the superlong straight
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Old 03-01-22, 11:09 AM   #6
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Default Re: Cookstown 100 - How to make cheap SV700

are you saying just changing the pistons gives you a 700cc? if so then wont the compression ratio be down?
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Old 03-01-22, 11:31 AM   #7
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Default Re: Cookstown 100 - How to make cheap SV700

[QUOTE=Bibio;3134316]are you saying just changing the pistons gives you a 700cc? if so then wont the compression ratio be down]

depends on the squish of the piston. I have read with Busa pistons you need to remove 1/3 head gasket to keep compression as std.
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Old 03-01-22, 12:38 PM   #8
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Default Re: Cookstown 100 - How to make cheap SV700

Quote:
Originally Posted by Bibio View Post
are you saying just changing the pistons gives you a 700cc? ...
Quote:
Originally Posted by hardhat_harry View Post
depends on the squish of the piston. ...
Eh? How can changing pistons in standard bore affect cc? Surely crank offset is the thing that affects swept area?

I get that changing pistons could affect compression due to different shape of crown, different distance of crown from centre of small-end etc. but the top of the piston is still going to move up and down the same distance on standard crank and rods, isn't it. Distance between TDC and BDC won't change so cc will thus be unchanged unless re-bore is done? Or am I missing something here?
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Last edited by Ruffy; 03-01-22 at 12:38 PM. Reason: typo corrected
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Old 03-01-22, 06:35 PM   #9
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Default Re: Cookstown 100 - How to make cheap SV700

Busa pistons have a slightly different crown profile.

I have found that you can get +3mm overbore in both Busa and SV pistons which will give slightly less than 700cc so all good.
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Old 03-01-22, 07:02 PM   #10
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Default Re: Cookstown 100 - How to make cheap SV700

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Originally Posted by hardhat_harry View Post
Busa pistons have a slightly different crown profile.

I have found that you can get +3mm overbore in both Busa and SV pistons which will give slightly less than 700cc so all good.
Fair enough. That now makes more sense to me.

I'd just assumed that a re-bore and new-oversize pistons/rings etc. would move the project beyond the domain of "cheap". But then again, racing budgets aren't the same as normal budgets are they?

(I understand the piston skirt shape on Busa pistons is different too (deeper?), that's supposed help to reduce piston slap, especially at higher revs. Overall, I'm led to believe that the Busa piston upgrade is a decent one for SVs, though not something I have direct experience of, either in standard bore or oversize, for road or for race).
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