Converting LPG to Unleaded (High Compression)

Hey all, I have a Holden V8 355 built to run on straight LPG so it was built with higher compression of 11.5-11.6:1

It is naturally aspirated, I moved to a different state a few years ago and servo's with LPG are few and far between here and the price is not much cheaper then Unleaded so im considering converting it back to Petrol. (98octane)

With the "high" compression ratio do you think the motor will be severely knock limited? keep in mind these are a early 1990's design pushrod engine.

I know dynamic compression comes into play but I have no idea how you work that out. It has a Crane Cam F-280-2 link to full specs is here

Duration intake = 280

Duration exhaust = 284

Lobe separation = 109

Life intake = .553

Lift Exhaust = .562

If it is advisable to lower compression ratio im guessing it would be better for me to get a new pair of heads with larger CC rather then just fitting a thicker head gasket?

Hello i run 11-1 on several 355 chevs and my own holden 304 runs 12-1 on nz 98 gas no issues

carby and msd ignition

you can use these formulas to work out dynamic compression ratio;

V theta is the volume for a given crank angle, B is bore, L is rod length, R is crank pin radius, Vcc is combustion chamber volume and Vivc is volume when the intake valve is closed. Stick them in a spreadsheet and you'll be able to play around and see the effects of changing cam timing on dynamic compression ratio.

(1.1)

OK, shade tree mechanic methos would be to check the cranking compression with an ordinary compression gauge. That will give you a good idea of what the dynamic compression is without going through all the math's.

Iron or alloy heads, and exact head design, will affect things but I would expect anything under 190 PSI would be acceptable with 98, and you may be OK with higher.

The only way to be sure, though, is when you do the tuning for the new fuel - if you're 'knock limited' it's too much with that head design, if you reach 'best torque' timing the fuel isn't a problem.

At this point, I wouldn't be concerned with changing the engine spec' until I KNEW I had a problem and, even then, a minor compromise may be seen as being acceptable compared to the cost of remedying it. Heck, even then, you may be able to run a lower temp' thermostat and/or cold air induction to drop the chamber temps a little.

Cool thanks guys