Topic: Compression ratio and milling hi-comp pistons...

[JoeCooley]

I just picked up a set of 12:1 pistons for my next engine build. Anyone know how much needs to be milled of to result in an 11:1 static compression ratio? They are the 890cc 12:1 Wiseco pistons available from Cycle X. I will be running a ported F1 head, Kimble white springs, and an old Crane 284 duration cam. I have to take the cam to the machine shop and get it spec'd out. Idk what the lift and LSA is....

This is just an idea. I have access to VP race fuels. I just don't know if I want a "race fuel only" bike....

[turboguzzi]

Quite few CR online calculators out there. Wouldnt trust too much declared CR by mfg. Do a dry build and measure actual chamber volume so you know for sure where you are.

[bwaller]

TG's point is valid. No-one knows how much has been removed from your cylinder or head decks through the years. You have to measure the chamber, ask Wiseco or CycleX what the piston dome displacement is. Assemble to find out where the piston is above or below deck, then do some math. It's not difficult, just costs some time. Anything else is a guessing game.

Once you get some numbers you can adjust with gasket thicknesses. Save removing some from the dome as a last resort.

I don't think having to use race fuel on the street would be much fun either!

[MRieck]

TG's point is valid. No-one knows how much has been removed from your cylinder or head decks through the years. You have to measure the chamber, ask Wiseco or CycleX what the piston dome displacement is. Assemble to find out where the piston is above or below deck, then do some math. It's not difficult, just costs some time. Anything else is a guessing game.

Once you get some numbers you can adjust with gasket thicknesses. Save removing some from the dome as a last resort.

I don't think having to use race fuel on the street would be much fun either!

That is the key. I think you can get away with a high 10:1 CR on the street with a CB if you have flat top pistons/large bore, a small chamber (which you would obviously need) and quality fuel. I am going to try it using the VP small engine fuel which is 94 octane and no ethanol. If you digitally control the advance and DON'T GET STUCK IN TRAFFIC it will work (at least I think so). With a 125-70 or CX-7 cam it would kick butt. You should see the squish ring with the 71.25 or 72mm flat top piston and small chamber....awesome. Run it at .038 to .040 squish with good rods.

[MRieck]

TG's point is valid. No-one knows how much has been removed from your cylinder or head decks through the years. You have to measure the chamber, ask Wiseco or CycleX what the piston dome displacement is. Assemble to find out where the piston is above or below deck, then do some math. It's not difficult, just costs some time. Anything else is a guessing game.

Once you get some numbers you can adjust with gasket thicknesses. Save removing some from the dome as a last resort.

I don't think having to use race fuel on the street would be much fun either!

Right. Though riding behind somebody using CAM2 on the street is delicious .

[bwaller]

No wonder you have those googly eyes!

[scottly]

Joe, what year head are you using? The combustion chamber volumes changed over the years; IIRC, the F2/3 had the largest volume at 28cc, then the F0/1 and K7/8 at 24cc, and K6 and earlier were 22cc(?).

[JoeCooley]

I will be using a ported F1 head. I'm still waiting on it to arrive but will measure the chambers.

[PeWe]

Maybe this will help.....
When I ordered my 71.25mm JE pistons from Dynoman to fit my 24.4cc chambers (K6 head with opened chambers for old style RC836), I got the answer from Buzz that CR number is based on 22.5cc chambers (except for the F2 pistons).
I asked for CR10.5 with 24.4cc chambers.

My pistons look like DP1000 CR12.5 that is slightly machined on top. See photo below.
Photo with piston at TDC, JMR billet block, Base gasket used is RCS .020"

Dynoman DP1000 CR12.5: http://www.dynoman.net/images/engine/piston/dp1000-71r/DP1000-71r.jpg

Dynoman DP1000 10.25: http://www.dynoman.net/images/engine/piston/dp1000r/DP-1000r.jpg

DP1000 CR 10.25 is 185g, CR12.5 version 195g, mine are 190g.

I do not know the final result of CR, bike runs good, no pinging. Head gasket is the thicker MLS gasket, .040".
I'm sure that the thinner base gasket .010" and head gasket .030" will not hurt my build.
Cam is DP315 with rather much overlap that need more CR.
I'm sure it will run OK with another cam with less overlap as a Megacycle 125-75. Higher compression might crank up the temperature as well.

[JoeCooley]

Thanks for the info.