What is the lowest compression ratio you can have?
 

Sounds a bit odd but I mentioned it in another thread awhile back to see if anyone would bite, they dident.

What is the lowest compression ratio you can have and still have the engine run? Ive heard of people using 76cc heads on dished piston LG4's and they run but is this true? This little project were thinking about undertaking will be a dual fuel setup, propane/gasoline.

Thanks!

I'd guess you could go down pretty low, I've an inline 6 in my '31 that is 6:1, IIRC.

No reason a V8 wouldn't run as low or lower.

I've heard that propane likes a higher CR.
Are you sure you have that right?

I some more would have to do with what cam was used to build pressure too.

One thing I didn;t notice was the reference to propane...

But...myL-head 6 in my forklift is propane powered, I'll look through my MoToRs manual to verfiy, but I think it's 7.5:1

https://www.thirdgen.org/techbb2/sho...threadid=30124
I just dug up this thread that has a discussion of the topic, in case anyone's interested.

Define 'still run'.

IIRC, theoretically, anything greater than 1:1 could sustain internal combustion... just not sure with what fuel. :lol: Maybe it was gas, can't remember.

I thought propane liked higher CRs as well since it doesn't contain as much energy per molecule as gasoline. Unless you are going to put this thing under some tremendous boost and want as low of a CR as you can get and then suppress the detonation even more with propane... I thought that propane was kind of used the same way alcohol is sometimes (to prevent detonation).

I think propane has like 120 octane or something like that.

It does (like higher comp) because of the loss of air charge and btu's among other things. It is used to help control charge in a boost app. The octane is RON 112 MON 97 and LPG (which is used a lot rather than propane) is RON 104 MON 89.

The use of propane is still up in the air, thats what this application did use as a primary fuel, gasoline was a back up emergency use fuel but that can change. I did search before I posted, guess I used the wrong keywords or somthing.

So If I were to use a set of mexico crate motor heads on an LG4 or similiar low compression (avalible) engine whats the estimated compression ratio? I was assuming 7.3:1 - 7.5:1.



Edit: I remember that thread but it doesent relate to this.:)

The cam would be a stock cam nothing special assuming its still good, if it isnt then it will have an MTC-1 since I have one collecting dust.

I checked my MoToRs manual, and the CR is 7.3:1.

I'm guessng to get the best performance and effeciancy from propane, you run a high CR...high as it can stand...just like you'd go with a gas engine.

But performance and intent for use, isn't the same thing....i/e my lift spends it's life at under 1500rpms, if I were to increase the CR in the engine to it's highest limits, run the appropriate cam to bleed off enough cyl pressure to spin it over, I'd have a misearble piece of equipment to use.

I have another engine I'm putting together right now for proane use...a 218 L head Chysler engine, bored .040, I'm going to polish the combustion chambers and grind a little out tomorrow...if I can mock up something to CC the CC's, I can figure out the exact CR...my initial guess would be in the upper 6's...for sure not in the 7's:1.

FIW i want to propanize and turbocharge my 78 Ford 300-6. It has 8.5:1 compression so propane and a turbo will work awesome. Mmmm major boost! But 2500rpm redline meow.

Lawnmower engines are regularly down around 6:1 compression. They'll have cranking compressions down around 80 PSI. I had a really worn out one once that only had about 30 PSI of cranking compression and it would still run..... barely.

You'll also find that engines with holes in the pistons are a lot closer to 1:1 (no compression). Heh heh.

Ok so were talking some where in the 6's and there shouldent be any reason it wouldent rev under load to 5K. This is actually replacing a rusted 440 in a field water pump system that was housed in a shed. Thats why propane was used and it was probably cheaper back then. There isnt a problem changing the enclosure to run gasoline assuming it would run better on 85.

Model T's were around 4:1, IIRC.

Details of 1932 Cadillac Fleetwood (140" wheelbase, 5560 lbs.)

• 45 degree overhead valve
• twelve cylinders
• Cast Iron on aluminum crankcase
• Bore and stroke: 3-1/8 x 4 in.
• Displacement: 368 cu. in.
• Compression ratio: 5.30:1 (std), 5.08:1, 4.90:1 (opt.)
• Brake horsepower: 135 @ 3400 rpm
• SAE/Taxable horsepower: 46.9
• Main bearings: four
• Valve lifters: mechanical with hydraulic silencer on rocker bushing
• Carburetor: Detroit Lubricator Type L-13, R-13/Model 51

Seems you can go pretty low if necessary.

Nice one, Damon... :)

Wow that almost has the same power-to-CI ratio as the '79 400 did, 47 years later. Amazing what the late 70s did to muscle cars...