Propane Compression

neptco19

Well-Known Member
Joined
Apr 5, 2005
Location
Athens, GA
I'm in the middle of building a 383 stroker and need to order some pistons, so I'm just trying to figure out what the ideal compression ratio for running 'pane is. And since I know everyone like pics, heres some of it after the rough bore was done and being drilled for 4 bolt mains...


ai74.photobucket.com_albums_i269_neptco19_383_20Engine_383engine005.jpg


ai74.photobucket.com_albums_i269_neptco19_383_20Engine_383engine010.jpg
 
'bout 11:1 is nice....
 
Rich said:
'bout 11:1 is nice....
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The kit I'm looking at getting comes with cast pistons, will they be alright for this or will I need to upgrade to forged stuff?? Not planning on using nos at all.
 
Yo,
Propane has a high octane rating of around 105. So, for a street motor I would say 11:1 would work very well as previously suggested. I have driven low compression motors that were retrofitted with propane and they did not perform well as expected. In the early 70's, when I was stationed in California, the CHP fleet ran propane and there was nothing slow about their cars. Most had Dodge cars with the big, high compression, 440's. Use synthetic motor oil and you will never have a sludge or carbon problem in that motor! Cast pistons will be fine unless you plan on running it really hard all the time (think early Z28's).

My $0.02 :beer: burp.....fart
 
I wouldn't run synthetic on a fresh motor, but that's just my $.02

You don't need forged stuff at all, but if you're in there, I've been told SS exhaust valves are a good idea.

Also, you've read up on the different ignition curves for 'pane?
 
would someone please explain the non synth during break in to me? i have always heard it but i have no earthly idea why?
 
awspence said:
would someone please explain the non synth during break in to me? i have always heard it but i have no earthly idea why?
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a lot of people do it for cost...same as running cheaper stuff when you swap gears...

you're gonna dump it after 500 miles, then again after 1000 (or whatever your "break-in theory" is)...so no need to worry about high mileage breakdown of the oil...

the reason most people say not to switch to synthetic on an old motor, is that it will "find" (not create) leaks that the non-synthetic had kinda clogged up for so many years...so maybe there's some reverse engineering of that theory in not using synthetic on the first one or two oil changes...

Greg
 
awspence said:
would someone please explain the non synth during break in to me? i have always heard it but i have no earthly idea why?
Click to expand...

I think it was on Bob the oil guy website, but Apparently Synthetic is too slick. and doesn't let the engine break in. I don't know if that's accurate or not, I'm no Chemisist and I didn't even stay at a Holiday Inn Express anytime lately.
 
i've also heard (which sounded logical) was that the synthetic doesn't allow the bearings/races to seat properly, but i'm no enigine mechanic :confused:

edit: too slow, same idea as mbalbritton
 
Synthetic is GREAT at reducing wear.. but it's that wear during break-in that actually seats the parts together... so you want SOME wear...

A few years back, Mobil did a test with a few engines of various makes.. filled 'em with Mobil 1 and ran them for what would be 150,000 miles, then tore them down. (oil changes/filters were done) The parts were still within spec for a new engine. That's good stuff...
 
I did alo of research on propane before I built my motor. I built a Dodge 360 into a 408 LP burnig beast. Don't be cheap on the pistons or the valves. Propane burns hot and can be hard on some parts. Keith Black has forged pistons for a good price. You are going to want to go with a good SS valve and a good valve job on the heads. For the comp. ratio I went with 12.5 to 1, and could have gone alittle higher. But the higher you go the harder it is to start and the harder it is on your starter(something to think about). A few things that I would make sure and do: MSD ignition, Cold plugs, thick wires, spend ALOT of time on the heads porting-FLOW, good thick exhuast(that shit is hot--WRAP IT), coat your bearings, and give it time to break in before you stomp it. As for oil I run Rotela HD30 for the first 4 oil changes and then syn.

My .02

Chris
 
I did alo of research on propane before I built my motor. I built a Dodge 360 into a 408 LP burnig beast. Don't be cheap on the pistons or the valves. Propane burns hot and can be hard on some parts. Keith Black has forged pistons for a good price. You are going to want to go with a good SS valve and a good valve job on the heads. For the comp. ratio I went with 12.5 to 1, and could have gone alittle higher. But the higher you go the harder it is to start and the harder it is on your starter(something to think about). A few things that I would make sure and do: MSD ignition, Cold plugs, thick wires, spend ALOT of time on the heads porting-FLOW, good thick exhuast(that shit is hot--WRAP IT), coat your bearings, and give it time to break in before you stomp it. As for oil I run Rotela HD30 for the first 4 oil changes and then syn.

My .02

Chris
 
Well I was gonna buy new heads to go on it, so I'm hoping they can "upgrade" the valves to SS ones. Thanks for the pointers on the break in oil. Will def. go with some single weight to break it in. Talked to the co. with the kit and they said I can get pretty much any compression piston for the same $$ and its not too much more to upgrade to forged stuff. So I'll prob end up doing that.
 
792.5cj5 said:
I did alo of research on propane before I built my motor... Propane burns hot..
Click to expand...

Actually, propane burns cooler than gasoline.;)
 
We have had discussion about all this before. Propane is cooler than gas. But. Because of the long burn, it build more heat. The exhaust valves take the majority of this heat. We recently had a Ford 429 engine that was on a propane delivery truck. The owner had taken it to another shop. The other shop removed the worn seats. Then told the guy he could not get seats to fit it. At the same time, he did a butcher job on the removal. We had to go a size bigger because of that, but, we could have put the right size in no problem. Even if we couldn't but them, we could have made them. I have seen the boss make them for obsolete heads before, out of a cam shaft! Anyway, back to what is the topic. Yes do use stainless valves, and hardened seats. The Valves from the 429 were the worse I had ever seen. Think of the days of old, and the spurs the cowboys used. That's pretty much what they looked like.
As far as break in, yes it's been said, you need conventional oil for the break in. Other wise, no,.........break in! We use 30w in all the engines we build, and let them decide on the 500 to 1000 mile change. Biggest thing is to break it in right. Too much skinny pedal, or, too little won't do the job right.
 
upnover said:
We have had discussion about all this before. Propane is cooler than gas. But. Because of the long burn, it build more heat. .
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Yes, it burns longer, just like a higher octane gas. But, BTU's are BTU's. Take a gallon of gas and make it last 1 hour or one day, it's still gonna throw off the same amount of total heat, right?

But part of the problem (as I've said before too), is that people leave the same ignition curve on their distributors, which makes the exhaust valve heat worse.

I would run a slightly hotter plug, but I didn't change mine. Do make sure the ignition system is up to spec though, and cooler coolant temp is good.

good info here: http://franzh.home.texas.net/engine.html

Chris - have you dyno'd that 408 engine? That's an impressive number.
 
Yo,

Lots of high end cars are being delivered from the factory with synthetic oil.
Personally, if the motor is right at start-up, why compromise what you just built with dyno earl? If you are worried about trash in the oil passages, then change the filter after the initial run in the driveway. Serious racers build and fill with synthetic every day. You can bet they depend on quick ring sealing and protection. While I don't recommend, you can put your hand over a carburetor at idle, choke the motor, and the raw gas will seat the rings. Done by racers for decades.

My $0.02 :beer: Miller time again..Ahaaaaaa
 
SEAIRESCUE said:
Yo,
Lots of high end cars are being delivered from the factory with synthetic oil.
Click to expand...

Yo.

The way those cars' engines are assembled and manufactured are a little different than going down to the machine shop with a bunch of parts, dontcha think?

What's done for race engines have little bearing on whether a K&N filter is any good, or what's best for break-in. They are rebuilt frequently. Durability for 100k miles isn't their concern... ;)
 
SEAIRESCUE said:
Yo,
Lots of high end cars are being delivered from the factory with synthetic oil.
Personally, if the motor is right at start-up, why compromise what you just built with dyno earl? If you are worried about trash in the oil passages, then change the filter after the initial run in the driveway. Serious racers build and fill with synthetic every day. You can bet they depend on quick ring sealing and protection. While I don't recommend, you can put your hand over a carburetor at idle, choke the motor, and the raw gas will seat the rings. Done by racers for decades.
My $0.02 :beer: Miller time again..Ahaaaaaa
Click to expand...

As Rich said, little different factory vs the machine shop. As far as flooding a cylinder with gas, all they are doing is washing the walls of the cylinders down, taking away the protective barrier. putting ring against wall, seating the rings. IMHO kinda hard to judge how much gas you have flooded into the cylinder. and how much wear you are putting on the engine.
 
So what would you guys suggest for this engines break in (not oil type but rpm range and length of time). Granted it will prob. NEVER see 100k miles unlikely even 50k. Shooting for around 450-500 hp. Its not like its going to be in a vehicle that I can hop in and drive around town for 500 miles before giving it all kinds of he**.
 
I was always advised that after the initial cam break in (what is it.. 2000 rpm for 30 seconds or a minute?) that you should constantly vary the throttle - you don't want to ride around at one throttle opening.. accelerating/decelerating often...

Personally, I'd start at a very moderate max RPM like 2500, then gradually increase it up over whatever break-in period you can afford.
 
Since the subject of propane is at hand, I finally got to take my rig out for some actual trail riding this past weekend. You may remember I have a CJ7with a 258 fueled on propane. It was just a day at Uwharrie, but what the heck, it was offroading. I haven't done any adjustments to the timing since I installed the system since it runs absolutely perfect from what I can tell. I actually couldnt' be happier with the way it ran offroad. With the way it's running now, should I try to tweak it some or just leave well enough alone?:D I did determine that I can get around 5 hours of actual run time on one 33-1/2 lb. tank of propane. Nice to know that info now.
 
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