Static Compression

[Jeff Rand]

Just wanted to get some feeback on what is being run and the reasons for a certain compression ratio. I have followed Kenny's advice in the past and let the turbo do the work and stayed at the 8.0 compression ratio but now thinking of going to 8.5 or 9.0.
The setup will be a 4.1 off center stage 2 with a 76-1BB turbo,224 duration solid roller cam, GN-1 14 bolt heads, 3.54 stroke crank and 6.5 off center rod.(3.965" piston)
Thanks,
Jeff

 

[black heat]

Listen to Kenny-besides, what do you hope to gain by upping the c.r.? Up the c.r., down the manifold pressure? Its a tradeoff, but c.r. is great under certain conditions. Moreover, while it may not mean doodly squat, when you go to compare your raw data with someone else (I assume most turbo racers are not running 9:1 c.r.), there will be fewer guys to dial up. Will this extend the turbo life by running lower boost? Most likely. Hey, run some data on your program at home.

Your friend,
pat

 

[Jeff Rand]

Hey Pat, thanks for the feedback. I have some time before I order the pistons so I can continue to weigh it out. By the way Richard Lee gave me information on CP pistons in Irvine,Ca. They split off from J&E. Since Richard picks his supplies very carefully, I will probably use them as well. So far they have been very helpful. (3-4 week lead time on custom piston at a price of $83.00each)
Happy Holidays,
Jeff

 

[KendallF]

I think the higher compression is a good idea. Especially with the big singles (88s), it will spool loads better...and you know you're gonna want to go faster and end up putting one on. Cal will probably post his experiences in that regard. I know he went from 8:1 to 9.5:1 and picked up a bunch of power, plus he's still running high boost.

If you were octane limited or wanted to run 40+ psi, maybe the lower CR would pay off but if you can run 30 psi on C16 with 8:1 and the same 30 psi with 9.5:1 (which a number of people are doing), why not go with the higher CR?

My new motor is going to be 8.66:1; the heads ended up CCing larger than I thought when I ordered pistons.

 

[HighPSI]

I'm not sure what boost level you are restricted to at higher compression ratios. I am currently running 9.5:1 at well over 30 psi of boost. The tuneup has to be "dead-on" since it is less forgiving. The higher compression ratio really makes the car seem faster on the street. It takes a lot less effort when leaving a traffic light to get up to speed. The only time I consider 8.0:1 when building a stage motor is when it is 90% street driven. Otherwise, I usually build them around 8.75-9.0:1

 

[Reginald West]

Jeff

Funny you should bring this up at this time. This is one of the things that I have been thinking that may be holding my car back. As you know kenny built my motor and it is 8-1 compression.But
before I go to far with this compression thing I want to rule out a few more things like timing ,boost level and cam.

REG

 

[Jeff Rand]

I am leaning towards 9:1 compression since the car sees little street time and I don't plan on more then 30 lbs. of boost at the track with C-16. If it spools better that would be another plus.
Thanks for the replies,
Jeff

 

[6banger]

I had a Blown Big Block Chevy I was wondering why you would want a lower CR for a car that will see street time?(8:1 vs 9:1)
It was a big cube engine(572) with a 4.5 X4.5 bore and stroke. I was told to go with a 7:1 CR. The problem I encountered was during low RPM operation (street driving) the engine made little or no boost and with the size of the charge in each cylinder I was getting such a slow burn from lack of compression I was still burning fuel in my exhaust. You could BBQ on my headers!

I`m not trying to doubt anyone and realize a blown car is different from a turbo. I`m just trying to learn so I don`t make the same mistake again.