stephenscamaro

Your dynamic compression ratio is 7.68:1

Is this ok?

stephenscamaro

Static compression ratio of 10.15:1.
Effective stroke is 2.90 inches.
Your dynamic compression ratio is 7.68:1 .
Your dynamic cranking pressure is 150.76 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of PSI is 7.68 :1.
V/P (Volume to Pressure Index) is 134

sofakingdom

iTrader: (1)

Kinda low... especially for that much static CR.

Sounds like not enough motor for whatever the cam is.

big hammer

Typical yeah. Often if you actually blueprint your static confession to bring up your dynamic compression you'll end up closer to the 11:1 range.

sofakingdom

iTrader: (1)

That's TERRIFIC!! I love it.

Sometimes these things are positively Freudian.

cosmick

DCR isn't a goal, it's a tool to help you get to the goal: cranking compression; for pump premium around 200 psi.

skinny z

They go hand in hand. Dynamic compression is what gets you your cranking compression taking into account that valve and ring seal are what they should be. One can be predicted from the other all else being equal.

skinny z

Too much...

cosmick

Not really, switching from 200cc heads on a 350 to 180s will add at least 5 psi, for one example I've personally seen myself, and the 200s weren't leaking past the valveseats. Swirl-port heads will add cranking compression over 081 / 083-style, TPI will add vs TBI, etc. I've even seen higher-ratio rockers help one engine, but hurt another.

skinny z

Intake port volume will have no effect on cranking compression. Combustion chamber volume will.
If all things are equal, that is: chamber volume, compression test cranking speed and duration, then it's physically impossible to have the shape/style of a chamber or port affect the cranking pressure. The dynamics of the test are too slow so as to allow anything that might happen due to an improved design make a difference. I would say that the differences you observed were due to changes in the fundamentals; ie. chamber volume, rather than any design change.
And yes, DCR and cranking pressure are inextricably linked.

sofakingdom

iTrader: (1)

200 vs 185 (runners) has ABSOLUTELY NOTHING WHATSOEVER to do with compression.

What REALLY happened was, the PARTICULAR SET of 200 cc runner heads that went on THAT YOU SAW, either had smaller chambers, or had been decked more, or the 185s had "tulip" valves where the 200s had "flat", or thinner head gaskets were used.

Same with the stuff about "swirlies". ABSOLUTELY NO CONNECTION WHATSOEVER to compression. Sure, they'll bump compression up if their chambers are smaller etc., but that's not a property of "swirlies" as such. Just an accident.

Note that I'm not saying you didn't see it; rather, that what you're attributing what you saw TO, is wrong. Correlation != causation.

stephenscamaro

I was trying to stay in pump gas range. Dangit.

stephenscamaro

I got it now~ but based on these specs, how should I proceed?

skinny z

A dynamic compression ratio of 7.6:1 is very workable with pump gas. Depending on the situation premium fuel may not be a requirement.
By the way, 7.6 DCR will not give you 200 psi cranking pressure.

skinny z

200 psi is starting to stretch the limits of pump gas. A proper timing curve is the issue. Keep in mind that timing has more impact on engine performance than compression ratio.
Post up what you have, iron or aluminum heads, what cam are you using, carbed or EFI?

stephenscamaro

383
9.005 Deck Hieght
.030 Over
205cc 64cc 2.02/1.60 Angle Plug Aluminum Cylinder Heads Edelbrock
COMP Cams Magnum Hydraulic Roller Camshafts 08-430-8
Carbed
12cc KB HE pistons
.041 Head gasket

skinny z

My numbers are similar to yours. My DCR result is closer to 7.9:1 however I'm not using my regular calculator to work this out.
That said those numbers, 7.6-7.9 are all workable with pump gas. The aluminum head gives you a safety margin as well.
Without actually crunching the numbers, past experience tells me your cranking compression would be close to 190 psi depending on your altitude and the overall health of the engine.

stephenscamaro

Good to know, makes me feel a little better. The engine is all new, not even the first fire yet.

skinny z

I checked on the cranking compression and based on the 280 degrees of cam timing and an altitude of 600' above sea level, you should drop in around the 185 mark. To get to a 200 psi cranking pressure, the SCR would need to be bumped up about half a point. That will bring your DCR as well. Or you could reduce cam duration by about 10 degrees for the same result.
Without debating your choice of cam, you package looks very workable.