They say .030 is getting dangerously close to kissing the heads when everything comes up to temp, and as the revs go up.
They say 40 to 50 is the place to be.
I ran about .025 for a little bit, and got away with it. I am currently running .034 for 80,000 plus miles.
Some of those guys on your linked forum are successfully running over 200 psi cranking compression.Even up to 225psi.
That kindof takes us back to post #105
You have a 360, with a 11.3 Scr, and the cam is the 256/268/110
I'm gonna assume the 360 is .030 over
If I got that right then with the cam in at 106*, the ICA will be 54*, and that leads to a Dcr(sealevel) of 9.69, with cylinder pressure of 203psi.
that means you are right on the bottom edge of perfect. That also means building a decent timing curve should be easy.
Again I remind you, I know nothing about propane, But I'm learning a bunch of theory....
Another thing to think about is this;The dynamic compression ratio might be 9.69 on paper. But in a running engine,as the rpm goes up, the Dynamic increases , Eventually the dynamic approaches the static as the valves start to trap the mixture. And as the rpm goes a little higher and the volumetric efficiency increases, the engine could ultimately operate on the static. When the engine reaches it's highest volumetric efficiency it is theoretically possible that the engine could operate on an effective compression ratio greater than the static.
Furthermore, any time the engine is throttled, it cannot achieve anywhere near any of these numbers, so if you plan to use a 2bbl for instance, the engine might never see more than a fraction of any of these crs.
To see what I mean just think about the last compression test you did on any engine. You tied the throttle wide open and hit the starter. Then you let it cycle through 6 or 8 or even 10 compression cycles, for it to achieve its maximum numbers. Right? But what was the first pulse? Did it even make 50%? Probably not. So now repeat the test with the throttle closed. Every pulse will likely be considerably less,pulse for pulse, and it will take several more pulses to achieve it's maximum.This is throttling, in action.
You can also see this from the torque curve on the dyno chart. At low rpms the engine is not capable of pulling in a full air charge, so the torque is low. But the torque curve climbs with rpm and eventually peaks. This is the rpm that the engine pulls in its greatest amount of air. From this point on,there is not enough time for such an amount of air to ever again enter the chamber.So the torque curve falls off.And this is with the butterflies completely open. When the engine is throttled, the torque curve will always be less. Just imagine the same engine running on a one barrel at WOT.
-The Scr has to be low enough that the peak cylinder pressure does not exceed the ability of the fuel to resist auto ignition.After that it's just a matter of delivering the ever-changing pressure spike to the crank at the appropriate time. And this is controlled by ignition timing.
-I wonder if ,with propane, the Dcr is as important as it is with gasoline.I'm starting to think,not. Cuz some are saying propane has an equivalent octane rating of 100 or better. If that is true, then theoretically you could run Dcr equal to Scr, so long as Scr does not exceed the limit of the fuel. And some guys are running over 12.5 Scr. All in all, your conservative cam choice of 256/268/110 should be no problem.
-However, I noticed that cam,with the intake closing at 54*ABDC,has a compression cycle of 126*. That is an awful long time.
-I ran, on gas, an ICA of 61*, which give a compression cycle of 119*. I was able to very easily tune a Holley 600 to make 32mpgUS, on that number. And this cam had 53* of overlap.So I wonder if you really NEED all those 126*.
The theoretical cam I built in an earlier post, had a compression cycle of 116* , and overlap of just 40*,
-The point is, Ima thinking you could trade away a bit of compression cycle, for a bit of intake duration. This would get you a bit more power with very little loss of fuel mileage.
-With those ideas in mind, I built another cam. 260/268/114/110installed. This cam has the same 40* of overlap again. But the compression cycle is reduced to 120* and the intake decreased to 260*; thus making it about one cam-size bigger than the one you chose. I made 32 mpgUs on gas, with 13*more overlap and 1*less compression distance. You should make about 100 mpg on propane,lol.With the Scr at 11.126,the dcr comes in at 9.15/189psi. So maybe you could bump the compression higher? I see 11.6 making 9.53/199psi
-I should also say that just 40* of overlap is really small amount. I suppose you will be running iron log manifolds with that? Cuz if you are running headers, you could be leaving a lot of power on/under the table. You could easily run 10* more overlap, with little to no sacrifice in economy, yet make a bit more power and torque.
-SOoo, new theoretical cam; 264/272/110/108Icl. This guy has 48* overlap, 120* compression, a 60* Ica, and 112* of power extraction. The extra overlap will work with headers to pull in a bigger A/F charge, at WOT, to make some power. The 120* of compression will make plenty of pressure, and the 112* of power extraction will make sure every last psi possible will contribute to propelling the vehicle. It will like an Scr of 11.6, to make a Dcr of 9.53/199psi. This 11.6 is just about doable with what you have. A zero deck, the 60cc heads and if you have the 5cc eyebrows you will need a very thin gasket. Your swept is 748.3cc.(@ 30 over) To get 11.5 requires a total chamber volume of 71.27cc. adding 60+5+6.4gasket(.028) comes to 71.4, oh so close. .028 is a pretty tight quench. I ran .025 and got away with it. Did I mention that?Your results may vary.
IDK, but it sure is fun working the numbers
