I have a Piston Question.....

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Apr 15, 2022 | 11:07 PM

08FXSTC

Thread Starter

Intermediate

Joined: Jun 2008

Posts: 27

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From: West Oz

I have a Piston Question.....

Hi All,
Starting to look at new build but am wondering how the manufactures work out their piston compression ratio.
When they state the compresion, is this worked out with standard head gasket thickness and standerd chamber size etc?
Wiseco do 10:1 and KB do a 10.5:1, are they worked out based on the same info?
Thanks.

Apr 16, 2022 | 09:50 AM

1997bagger

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Joined: Aug 2008

Posts: 5,080

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From: Ohio

Quote:

Originally Posted by 08FXSTC

Yes on stock gasket and head cc's, when good engine builders check the head cc's they are not exact as designed plus each head can be different, they will surface the head to get them equal. 10:1 is optimal for good everyday power on a Evo plus leaves some wiggle room for head surfacing, 10:5:1 will start getting into a healthy cam with a later intake closing needed to let some "air" out of the engine and head work becomes spooky plus becomes more of exotic area of a build.

9:5 with a early intake closing cam is a good target for just want to ride but may need to get out of it's own way to pass a car, problem is there is a percentage of us in the Evo forum that just can't do this, have to go to the exotic area because men are wired that way.

Last edited by 1997bagger; Apr 16, 2022 at 09:57 AM.

Apr 16, 2022 | 09:53 AM

Uncle Larry

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From: Michigan 15 Minutes East Of Hell

Good point ... Where the "squeeze" comes in at is important

Apr 16, 2022 | 10:33 AM

Schex

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From: SE Louisiana

You can get a close calculation of the static and corrected compression ratio here.
If you get the cranking compression around 170+, you will need compression releases or you could run into hot start problems.

http://www.bigboyzheadporting.com/EVOComp.htm

Apr 16, 2022 | 11:34 AM

Dano523

HDF Community Team

Joined: Jan 2020

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From: Colorado

Lets start here, since heads make a huge difference to start with when it comes to what pistons they use.

Apr 16, 2022 | 08:55 PM

08FXSTC

Thread Starter

Intermediate

Joined: Jun 2008

Posts: 27

Likes: 4

From: West Oz

Thanks for the replies.

The build is for an 85 fxwg and I am leaning towards Wiseco 10:1 +030 (bike already at 020" but will clean up with 030"), EV46, 030" HG, port clean up on the heads but stay with standard valves.
I Already have a CV40+big sucker and also 2 exhausts, Turbo Tapers and std headers with SE tapered slip ons, not sure which one I will use yet.
Big Boyz puts my ccp at 190ish, will I have issues starting? It seems to be a fairly common build but want to avoid comp releases if possible. I have rebuilt the starter and am fitting a new solenoid.

Apr 17, 2022 | 06:14 AM

Schex

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Joined: Apr 2011

Posts: 5,803

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From: SE Louisiana

I had a ev46 cranking 180, it would buck the starter sometimes on hot restarts.

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Apr 17, 2022 | 08:44 AM

1997bagger

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Joined: Aug 2008

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From: Ohio

At a 41 intake timing a hot starter buck is possible on a smoking summer day like Shex mentioned, if your concerned use the stock head gasket. Another safety net is put beehive springs on the heads, if the EV46 has starting issues you can jump to the next level of cam with later intake timing/longer duration, starting issues are gone and power range goes longer and you'll have the compression to carry it through the low range, light bike also gives you advantages in cam choices. Personally I would go the direction of your build with beehives and take advantage of 580-590 lift cams, they are just as dependable as baby cams but this is just me.

The stroker I ran for years was 10:2:1 with .030 hg, 42 intake timing cam 590 lift, All ***** starter, cables, Big Crank battery dependably started for years but went through a couple of starter drives. This was in a big bike so kept intake timing around 42, if it was a light bike would of had a later intake timing/larger duration cam. Very dependable engine.