Compression Ratio ?

[turbotony1]

My 109 block is now in for machining. I would love to make it into the low 10's with my combo. The build will be...

Billet Main Caps (center two or maybe the front cap too)
BHJ Balancer
Full Throttle Stroker Kit
Champion Iron Heads with Cometics and Champion Ported Intake, Hemco Upper Plenum
Roller Cam with Morel Lifters and TD Roller Rockers
TA High Volume Oil Pump
One of our TB Vendor's Headers T.B.D.

I'm looking into a 66-68mm Precision Billet Turbo

I'll be running either E-85 or Race Gas

I would like to hear what you guys would recommend as far as compression ratio. I've heard lots of talk about keeping the engine close to stock (8 to 1 or 8.25 to 1). However, it seems that lately lots of people are going up to 9 and 9.25 to 1. I would like to know the thinking behind the two different theories. It seems to me that the people who are for 8 to 1 say it will allow you to safely run more boost, and higher boost levels equals more power. On the other hand, it seem like the guys recommending higher compression seem to think the advantage is more low end torque and very little difference in safe boost.

Any comments are appreciated,
Anthony

 

[bison]

10:1. Its no theory that a higher CR will help you make more power if you build around it. Your choice of fuels is a great start.

 

[turbotony1]

10:1. Its no theory that a higher CR will help you make more power if you build around it. Your choice of fuels is a great start.

Wow! 10 to 1.

What do you mean about "building around" the higher CR?

 

[bison]

Wow! 10 to 1.

What do you mean about "building around" the higher CR?

What I mean is that high fuel with a very high detonation limit will be used and the cam selection and other important parts can benselected based on running 10:1. There is quite a difference between 8:1 and 10:1. You should be able to run mid/low 10's at 20psi with a small hydraulic cam. With a solid roller you could take it even further at that pressure.

 

[KL Mallender]

Dynamic or "running" compression ratio is the term to use here,
The Buick responds very nicely with 9.5-10.5 static and a healthy dose of camshaft duration, the bump in compression helps offset the sluggishness of a higher rpm camshaft, just be reasonable with choices.
Your build sheet looks very impressive, you should do very well.
Good luck,

Kevin.

 

[turbotony1]

Thanks Bison and Kevin!

After reading your responses it seems like I will try to keep it as close to 10:1 as possible! I really think a custom ground cam would help me find even more power out of this combo. I just don't know enough about anyone that makes custom cams for Buicks.

Who makes good custom cams?

From what I understand, with solid lifters, you can make more power at high RPM because it keeps better contact between the lifter and the lobe. I really didn't think the gains would be that noticeable, but if you guys think I could gain power, or more usable RPM, then maybe I'll look into it.

If I go the solid lifter route, will I need a billet cam?

 

[Nick Micale]

We usually go with 9.5-10.0 on a race build, but have gone higher on some "extreme" engines we have built.

Bison is right on about a big difference in performance, and that you MUST have the proper fuel.

My car will run 10.0 at 16-17 PSI on a good weather day, which means 60-70 deg.

Also, like was said, you do not need to get crazy on the cam duration as the higher compression will get the jiob done!

 

[vacuum 6]

So a 212/212 roller will get it done?

Also anyone try 10:1 on 93 and alky?

 

[pajo]

)

Hey Nick - would you mind telling what is the c.r. on your engine and are using race gas or ?

 

[Nick Micale]

The compression ratio is 9.50:1, and I use Sunoco 116 maxmal race gas.

 

[KL Mallender]

So a 212/212 roller will get it done?

If I were going to a true 10-1 mechanical cr, I would look for something a bit larger to help "bleed-off" excess cylinder pressure (I realise this may sound counter productive, but it's not) you are aiming for the same cranking pressure numbers.
Typically using a longer duration cam to move power/torque to higher rpms will lower your cranking numbers, also causing the sluggish feel when used with 8-1.
Look for 135-165 psi during a crank test.

8-1 use 190-200 @ .050
9-1 use 215-225 @ .050
10-1 use 230-240 @ .050

These are my general recommendations, others factors such as lobe seperation and installed centerline will need to be considered.

Good luck,

Kevin.