Propane compression ratio

Moderator: Mod Squad

Rusty_Old_F250
Registered User
Posts: 59
Joined: Tue Oct 23, 2012 2:12 pm

Propane compression ratio

Post #1 by Rusty_Old_F250 » Sun Dec 03, 2017 10:00 pm

I've been researching this topic again, still haven't been able to get a solid answer. What is a safe compression ratio for propane? Assume propane at 105 octane for simplicity. My build will be for a HEAVY truck, so lower is better for me. As it sits the truck weighs about 9,000lbs and I've been know to run it heavier (I've had it up to 11,000), plan to run up to 18,000 towing a trailer. I'm not concerned much about power, I'm very happy with it as is. Before cyl #6 gave up the ghost, that is. :mrgreen:

Basically what I need here is a motor that will be happy with sustained full throttle in all kinds of weather.

The number I have picked out in my head is 10.5

Here's my priorities, in order of importance:
1) Durability/reliability i.e. NO DETONATION!
2) Fuel mileage, current setup has been proven to get 13.5 on gasoline, I'd like to keep this if possible.
3) Power. This will be what it will be, like I've said I've been very happy with my current 1bbl and HD exhaust on a very tired engine. I have slow standards. :rolflmao:

Sam

wallen7
Registered User
Posts: 112
Joined: Thu Jun 09, 2016 8:38 am

Re: Propane compression ratio

Post #2 by wallen7 » Mon Dec 04, 2017 2:33 pm

I would build it back stock on the bottom end and maybe advance the cam 2 - 4 degrees and put chevy 1.75 rocker arms on it. Clean up the ports in the head and maybe a light cut on the head. But I would stay away from higher compression. In a heavy vehicle the engine is going to run under loaded conditions a lot, so I would pay attention to ignition advance to avoid detonation.

pmuller9
Registered User
Posts: 1441
Joined: Tue Aug 06, 2013 12:33 am
Location: Columbus, Indiana

Re: Propane compression ratio

Post #3 by pmuller9 » Mon Dec 04, 2017 2:59 pm

If you were to port inject liquid propane then a 10.5 compression ratio would be OK because the cylinder temps drop as the propane vaporizes.
Otherwise stay with a 9.5 compressio ratio to be safe.
Normally HD 300 truck engines are 8:1 or less.

You will need hard exhaust valve seats designed for propane and Stellite exhaust valves.

I'm assuming you will be using forged pistons.

What RPM do you work the engine at?

Rusty_Old_F250
Registered User
Posts: 59
Joined: Tue Oct 23, 2012 2:12 pm

Re: Propane compression ratio

Post #4 by Rusty_Old_F250 » Tue Dec 05, 2017 7:49 pm

9.5, that's a good deal lower than I thought, here I thought I was being conservative. :mrgreen: Makes sense though, heat is the enemy here.

I have been told a WILD range of octane ratings for propane, anywhere from 105 to 130, turns out most "propane" is really propane with a percentage of butane, the more butane, the lower the octane. I could probably go higher on the CR, but a "bad" tank of gas would put me into risky territory.

Were the pistons in the factory HD motors forged? If they were, maybe I could get my CR number using factory slugs? :hmmm:

Planned RPM range is 1500-4000, shooting for a cruising rpm of 2600. My driving style is with a load is rolling into the throttle below 1800, floored by 2100.

Sam

wallen7
Registered User
Posts: 112
Joined: Thu Jun 09, 2016 8:38 am

Re: Propane compression ratio

Post #5 by wallen7 » Tue Dec 05, 2017 8:46 pm

None of the engines had forged pistons from the factory. The 96 efi motors had hypereutectic pistons but they are not forged. To get forged pistons they will have to be custom made, try Autotec or Racetec.

pmuller9
Registered User
Posts: 1441
Joined: Tue Aug 06, 2013 12:33 am
Location: Columbus, Indiana

Re: Propane compression ratio

Post #6 by pmuller9 » Tue Dec 05, 2017 11:04 pm

There is more than just the octane rating to consider when trying to figure the correct compression ratio.
A liquid fuel changes to a gaseous state and absorbs thermal energy which lowers cylinder temps.
This is known as Heat of Vaporization. The amount of fuel being used compounds this effect as in Alcohol versus Gasoline.

If propane is introduced into the intake manifold in its gaseous form then there is no temperature drop due to evaporation.
This affects the max compression ratio.
Also when propane is being used as a gas or vapor it displaces much of the air in the intake manifold and reduces the engines ability to make power.

If you multi port inject the propane as a liquid you get the temperature drop during evaporation and can use a higher compression ratio.
Since liquid propane is 270 times more dense than its gaseous form there is very little air displacement and the engine can make power at its normal potential.

The smallest dish hypereutectic pistons at zero deck with an EFI head will have about a 9.4 compression ratio.
Otherwise you are looking at Autotec custom forged pistons for any compression ratio you need.

Powdered metal exhaust valve seat inserts for propane (LPG) and natural gas, Dura-Bond 70000 / 90000 Series Valve Seat Inserts.
Stellite exhaust valves. Last time I checked SI valves had them for the 4.9

Wesman07
Registered User
Posts: 107
Joined: Tue Aug 23, 2016 10:00 am

Re: Propane compression ratio

Post #7 by Wesman07 » Wed Dec 06, 2017 5:16 am

That sounds pretty cool. What are the benefits of propane?
86 f150 300 efi with advanced stock cam. Np435, Dana 60/ 10.25, 35" BFG's, four link front suspension with 12" travel fox coil overs, custom deaver leaf pack in the rear.

wallen7
Registered User
Posts: 112
Joined: Thu Jun 09, 2016 8:38 am

Re: Propane compression ratio

Post #8 by wallen7 » Wed Dec 06, 2017 9:33 am

Unless you have a gas well or live in Canada or are already operating on propane conversion I don't really see the benefit from changing to propane. where I live the current cost of propane is $2.86 a gallon vs gas at $2.30 a gallon.

Wesman07
Registered User
Posts: 107
Joined: Tue Aug 23, 2016 10:00 am

Re: Propane compression ratio

Post #9 by Wesman07 » Wed Dec 06, 2017 10:01 am

Probably not from a finance point of view, but there are other benefits.

I know the off road guys like it because they can flop their rigs upside down and not spill any fuel... and you can run the rpm's so low you think you stalled out, but nope your still running lol.

I just figured there has to be more benefits than that.
86 f150 300 efi with advanced stock cam. Np435, Dana 60/ 10.25, 35" BFG's, four link front suspension with 12" travel fox coil overs, custom deaver leaf pack in the rear.

Lazy JW
FSP Moderator
Posts: 5390
Joined: Fri May 16, 2003 10:25 pm
Location: Careywood, Idaho

Re: Propane compression ratio

Post #10 by Lazy JW » Wed Dec 06, 2017 11:23 am

Here is a link to a thread I posted quite some time ago about propane vs gasoline.

http://fordsix.com/forum/viewtopic.php?t=46638

I personally would not want to chance it with a heavy vehicle running more than about 9.5:1 compression.
Joe
74 F-350 300-6, EFI manifolds into single exhaust, Carter YF, T-18B, Dana70 w/4.11, Flatbed dually w/dump bed. Great farm truck!
Image

wallen7
Registered User
Posts: 112
Joined: Thu Jun 09, 2016 8:38 am

Re: Propane compression ratio

Post #11 by wallen7 » Wed Dec 06, 2017 1:48 pm

Yep that goes back to what I said , You better own a gas well or live in Canada. Propane is widely available in Canada.

Rusty_Old_F250
Registered User
Posts: 59
Joined: Tue Oct 23, 2012 2:12 pm

Re: Propane compression ratio

Post #12 by Rusty_Old_F250 » Wed Dec 06, 2017 9:32 pm

Thanks guys, very interesting stuff. Here in Seattle propane can be found for as low as $1.25 a gallon. In rural areas it can be had even cheaper. But you have to know where to look, most gas stations around here that fill bbq bottles want $4.00+ per gallon.

The main benefit is the driveability and clean running, motor runs smooth and quiet, the inside of the engine will look brand new after 200,000 miles, and the tailpipe emissions are dramatically lower. Forklifts are run off of it to keep the air cleaner in enclosed spaces. Propane never goes bad like gasoline, and since it works off bottle pressure, there's no fuel pump.

Sam

Who is online

Users browsing this forum: Google [Bot], Wesman07, Yahoo [Bot] and 13 guests