What to build

[JMurray]

I am building a Propane motor and I was wanting some input on what to go with. I have a 350 to start with the casting on the block are 39GM3970010, and J241 I don't know if its a 4 bolt or not havent yanked the pan. I am wanting to make around 450hp, my question is should I make a stroker or spend the machining $$ to throw at more hp upgrades for a 350. Since LP is between 110-115 octane what cam, pistons, rod and so forth should I be running? Thanks for your help.

Should mention this is going to be a street strip application.

[Marv D]

No expert on the LPG motors for sure, but I ran propane years ago on a driver and I can tell you making 450 from a 350 small block burning LPG is going to be a real chore,,, if not next to impossible. You loose around 15-20% power going from gasoiline to LPG.

Like you already know, LPG is akin to a very high race gas octane. It has something to do with it already being a gas as well as the burn rate. You can run 11.5:1 with ease on the street (that's how I got into running LPG back in the late 80's,, the old LT1 motors LOVED it.

With gas prices the way they are, I hear a lot of guys talking about building methonol motors,, I think that could be some interesting builds,, run LPG on a daily basis, swap on a Alky carb and go to the track. Some of the old LPG systems were switchable on the fly. (a hat to deliver the LPG set over the carb, solenoids controled which fuel you ran)

This could get very interesting!!!

[JMurray]

I'm wondering what setups the LP racers are using? Are they attaining these large HP's by blowers, turbo's or would nitrous even work? Would a dual setup get my the fuel rate I need? I"m just in the planning stage now trying to see what it will take if its possible to build this motor. Since Ive never delt with LP before I wasn't sure what the restrictions were. I had thought about the dual fuel but was concerned with the extra weight of two tanks and limited space.

Thanks for input

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What it Is

Mothers 1970 Chevrolet Chevelle ProPane Preview – SEMA Show: To heck with hybrids, diesels, and fuel cells. Ask the folks over at the Mothers car care products company and you just may be convinced that propane is the way to go. It’s more environmentally friendly than the 87-octane juice you’ll find at your local gas station, and in the case of a certain 1970 Chevrolet Chevelle SS, is responsible for making a 1,000-horsepower small block fire on all eight cylinders. That engine is only one part of the Chevelle ProPane, a collaborative effort between multiple partners including Chip Foose, that was showcased at the 2007 SEMA Show.

[bb1979]

Build a 450hp petrol motor and drop on two impco 425 mixers and twin convertors, go to www.google.com.au type in "how to make power with lpg" and select pages from Australia and you will find a whole lot a info, I have been running it on my small block for the last 5 years (no hp figures) and it went great. I am currently researching myself on how to maximise performance from my LPG setup on my 454. All I know right now is I need a second carb to boost the rev range past 5000rpm

[JMurray]

bb1979?
Are you using both gasoline and LPG or just LPG? I had thought about a dual intake setup not much more $$ but a lot more power. do you have engine specs for a SBC running LPG I know the timing and compression are different but what about all other internals? Since LPG is 110-115 octane would a setup designed for that octane racing fuel work best?

Thanks for the input

[Super73]

This is just an assumption, but I would think building the motor similar to a motor designed for the higher octanes is what you would want. I would assume you could run a dynamic compression range of at least 9.5 with that high of octane. High static compression with a intake valve closing point optimizing your wanted power band. Pick the IVC then figure out the max DCR based on the gas you wish to run and work it backwards to figure out what pistons/head chamber size you need for that SCR.

Hope that made sense.

[Super73]

You also might want to see if you can find someone running LPG and see if the heads might need some different bowl work due to the way it burns..

[JMurray]

thanks for the input. The compression most of the info I have found were in the 11-13 range.

[Super73]

To me that seems low for an optimized set up. For example, you can run 91 octane in a motor with 8.8-8.9 dynamic compression (compression based on where the intake valve closes). Here are some examples of what I'm talking about:

11-1cr 285* duration at .006 108 intake center = 8.35 dcr
12-1cr 285* duration at .006 108 intake center = 9.09 dcr
13-1cr 285* duration at .006 108 intake center = 9.87 dcr


11-1cr 308* duration at .006 108 intake center = 7.45 dcr
12-1cr 308* duration at .006 108 intake center = 8.10 dcr
13-1cr 308* duration at .006 108 intake center = 8.80 dcr


IMO, choosing close to max DCR you can run based on fuel is a good starting point. Then from there, figure out what intake valve timing you need for your planed RPM usage. Once that is established, work it backwards and figure out what SCR you need to have X DCR. Then mill heads, buy appropriate gaskets and pistons to position your SCR where it needs to be.

[JMurray]

Thanks super73. There are several variations of LP gas in use for autos. I have to talk to the local stations and see what they are running so I can narrow down the octain. Ive contacted a LP parts distributor to see who makes lp carbs for my hp range or if I will have to use a dual setup. I will post when I get the info.

[Super73]

I would think you could get to your goals with a well planed out combo. It takes roughly 30% more ethenol based on the burn temp to produce the same power. Any idea what the BTU rating is for LPG vs Gas?

[Super73]

Here is some good info regarding BTU for different fuels. but it doesn't show the BTU rating of automobile LPG. Straight LPG is roughly 90k per gallon vs US 93 octane at 120k per gallon. I would guess looking at the chart that it wouldn't be that far off of E85, but I could be way wrong.

http://en.wikipedia.org/wiki/Gasoline

[JMurray]

Here is what I came up with on BTU ratings LP is about 34 grand less than gasoline. Looks like Natural Gas would be the way to go if it was available to fill.
Commodity Btu MMBTU PriceExample Priceper MMBtu Propane Equiv $/Gal

Propane ($/gal) 91,500 0.091500 $0.52000 5.683060 0.520000
Natural Gas ($/MMBtu) 1,000,000 1.000000 $5.00000 5.000000 0.457500
Electricity ($/kwh) 3,414 0.003414 $0.05080 12.879906 1.361511
Crude ($/bbl) 5,800,000 5.800000 $30.0000 5.172414 0.473276
Fuel Oil ($/gal) 138,690 0.138690 $0.84000 6.056652 0.554184
Gasoline ($/gal) 125,000 0.125000 $0.95000 7.600000 0.695400