compression ratio

[eraser44]

Any ideas on max static compression ratio for a 250" Buick v6 for SS? Current combo is close to 17 - 1.
Thanks for any input.

[hbps1978]

I wouldn't worry about static compression, I believe what you want to look at is your dynamic compression ratio.

[Joe Mendelis]

Static compression ratio is very important, trust me. 17:1 is very high on gasoline. If you are not induction limited that's as high as I would go. Did you try 16:1 first? How can you get that much compression in that small of an engine? Does it have very shallow valve angles? Huge dome?

[eraser44]

Joe,
34cc chamber, .060 total deck & gasket, 1 cc net dome, 4.06 bore, 3.400 stroke = 16.73 static comp by my calc's. 6.220 rod, intake closes 68 ABDC = 10.90 dynamic.
I'm thinking maybe I'm a little high, just looking for input. On freshen up I can cut the dome and lower some.

[dbusch]

17.0 is fine with a cast single 4 manifold. I would want a little less, like 16.0 with a sheetmetal single 4, and not over 15.0 with a 2 carb sheetmetal.

Ultra high ratios need to be matched with the right intake duration, centerline, and lobe sep to work right. With too far advanced centerline, dynamic compression can get out of control. This goes hand in hand with the cross section and rpm potential of the induction system. You dont want 17.0 on something that isnt going to be turned over 9000 rpm, since the cam requirement for the lower rpm deal will make the dynamic go thru the roof.

[Greenlight]

A 4.060" bore and a 3.400" stroke yield 264 cid, not 250 cid.

I get 16.77:1 static compression ratio.

What fuel do you run?
Can you run VP C-25 in SS?

I don't think this is too much with a small dome, like you have. This likely allows you to run very low ignition timing (~25-27 degrees BTDC). This combination will be significantly better when run at high altitudes (Denver, etc.) and not any better than a 15:1 CR engine at sea level/mine shaft condidtions (in my opinion).

My experience shows that high domes don't like CR too high, but small domes like it a lot.

From the people I've spoken to the PS Cars are running ~16:1 and the PS Motorcycles are at ~17.5:1. Both run VP C-25. The PS Car guys run ~27 degrees (9800 rpm) and the bikes ~31 degrees (13k rpm).

If anyone has different info. please let us know.

[dbusch]

i thought the Pro Stock car stuff was only 15.0 compression. The induction system is so efficient they dont need any more than 15.0. Maybe that is wrong???

[Darin Morgan]

i thought the Pro Stock car stuff was only 15.0 compression. The induction system is so efficient they dont need any more than 15.0. Maybe that is wrong???

Exactly. They can be as low as 14.8:1. the inefficient engines run 15:1 to 15.5:1. The pro bike heads run about 14.5:1

[dbusch]

My cast single 4 engine uses a .200" dome producing about 16.0 compression. Would i be better off with less dome and less compression? Head cc is maxed out in the chamber (cannot be milled any more). I really didnt think .200" dome would hurt anything...

[Joe Mendelis]

Joe,
34cc chamber, .060 total deck & gasket, 1 cc net dome, 4.06 bore, 3.400 stroke = 16.73 static comp by my calc's. 6.220 rod, intake closes 68 ABDC = 10.90 dynamic.
I'm thinking maybe I'm a little high, just looking for input. On freshen up I can cut the dome and lower some.

I'm not saying you are too high. I don't know. But if you haven't run the motor with lower compression, you may try it. What do other guys run?

.060 sounds like a lot of clearance between the piston and head. What is the valve angle? If it is very shallow can you move the top ring up? Reducing that volume gives you a little less area for heat to transfer and exhaust to hang out. With 17:1 I would want as little exhaust left as possible.
As far as the "dynamic compression" goes, I see how people are calculating that, but it isn't, by any means, giving you an indication of peak cylinder pressure, or rate of pressure rise after IVC for that matter. The only way to figure out "dynamic" factors is by running the engine.

If you are not increasing PVD area more with the higher spike than you lose from pumping, lowering compression will help unless the higher residual gas fraction from the lower compression hurts the combustion (in theory). The higher you get with compression, the more minimal the increase becomes. Several engineers (not engine builders though) say that 17:1 is the breakover point. But as Darin pointed out, an engine with killer heads and carburetion will have that point lower.
Winston Cup plate engines used to run extreme compression ratios before they had a rule because it helped with limited induction. You have to figure out the balance point where power starts dropping because the pumping outweighs the increase. I don't know anything about those heads, but I would guess keeping it pretty high will help.

[Adger Smith]

Ok, Not enough information on your combo. Gearing, tire, tranny, ect.
I have a very good v-6 combination & If you are going to run heavy Lbs pre cu in classes you better stay away from too high a static compression.
If you run a 3 speed it will help, but you will eat parts when you let it hang on the converter with ultra high compression.
I'm not familiar with the chamber of a Buick V-6 & what it likes for timing & compression. I can tell you alot about the Splayed Valve V-6. It doesn't like high compression, timing & a powerglide. You can make some killer power with the high compression with quick pulls on the dyno, but it migh get hurt in the car. Don't let anyone tell you all you have to do is run C-25... Your combination is critical.
Good luck,
Adger Smith
NHRA SS/CX & SS/DS National Record Holder
IHRA SS/BX

[eraser44]

Top ring is way up. I use C-25, but I have run C-12 without seeing any change. I need initial timing @ 40 deg to get it moving, then gradually pull it back to 34 deg starting at 8000 rpm. This is in a 3000 lb Regal with a 1.98 low gear powerglide, shifting at 9000 and thru the traps at 9400. Cam is .824 intake X .777 exhaust, ground on 112 C/L, in at 116. 277 deg intake @ .050, 292 exhaust. Heads flow about 325 cfm intake average from .500 to .800, 290 exhaust. Uses a sheet metal manifold with a 750 CFM Race Demon.
I haven't dyno'ed it but my software says about 605 HP based on MPH and weight.
Previous motor was 4.06 X 3.200 for 249". The current is 4.06 X 3.400 for 264".
Any input is welcome (other than telling me to build a splay valve Chevy).

Thanks

[Adger Smith]

Saw your last post. What class do you intend to run?
My intent was not to tell you to build a Splayed Valve, just tell you my experence with compression & heavy weight classes.

[eraser44]

Adger,
I've been running this combo for 3 years. I had the record in SS/DS for a couple years until you built that Camaro.
I struggle to run 10.40's and 130 on good days while you click off effortless 9.90's and 135.
If I wasn't loaded up with Buick stuff I'd build a Chevy in a heartbeat (so to speak).
Like Jerry Reed, I'm just making the best of a bad situation. I can beat the other Buicks, Fords and inline valve Chevy's, while you and Oaks make me look silly.
I'm just flogging away.
John Mason

[dbusch]

112 c/l in on 116??? i would think 113-114 c/l in on 111-112 would work better.