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Old 01-24-2011, 07:00 PM
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sbc 350 305 heads on propane

I am new and have searched but haven't found great answers yet.

I just bought a jeep with the following

sbc 350 casting - 3970014 - 67-70 350 block
305 heads - casting 354434
Mild cam
flat top pistons - std -345np
4bbl propane setup

My question is
Is this a good setup - what kind of power can I expect - I have read that it will crap out above 5000rpm but has good low end torque. I guess I am looking for expert opinions about this setup and I am curious how many people run this kind of setup

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Old 01-24-2011, 07:33 PM
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im not familiar with the rest of the setup but I know if your gonna put 305 heads on a 350 416 head casting are the best. just my pennies worth.
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Old 01-24-2011, 11:13 PM
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One reason they maybe put 305 heads on that motor might be because they maybe have smaller combustion chambers than 350 heads which would increase the compression ratio.

Propane doesn't have as much energy per gallon as gas, but it has much higher detonation resistance. That allows you to run at least a couple points higher compression ratio to make up for hte power loss with the less btu's of energy.

However, the 305 heads will also considerably restrict power in anything above mid range rpms because of smaller valves and ports.

One way around that would be to use Vortec style heads as they flow good and have small combustion chambers.

A very good aftermarket Vortec style head that costs the same assembled as stock oem Vortecs is these

They have better exhaust port flow (Vortec's are a little weak) and they
have more material to avoid exhaust valve seat cracking which can happen
with oem stock Vortecs.

They're available with the same small 64cc combustion chamber to up your
compression - and you can also go with a thin head gasket such as
.015, 016 or 020 available reasonable cost at Summit Racing.

A mild cam will generally help build more compression, too, because the intake valve will close sooner while the piston is rising on the compression stroke.

The critical thing in the cam is to have the intake valve close a little earlier
on that compression stroke, which mild cams will generally do.

You can find cams that are a little higher performance but that also close
relatively early. Comp and Lunati are 2 good brands that have cams like
that. Generally you probly want to stay with a cam that has less than
210 duration at .050 lift on the intake, and where the intake valve closes
at or not much after 33 degrees after bottom center (on the compression stroke) at .050 lift. And you'd want a cam with around 110 to 112 max degree lobe separation rather than a computer / fuel injection type cam which will have 112 minimum up to 114 degrees lobe separation angle.

Last edited by macx; 01-24-2011 at 11:22 PM.
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Old 01-25-2011, 08:03 AM
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the only thing i know of this is... a friend about 20 yrs ago put a propane sys on his truck..V8 ford motor. i recall only an injector plate under the carb and of course a solinoid for changing the fuels... it ran very well... i don't recall his making any mod's on engine..
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Old 01-25-2011, 06:34 PM
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Yes, if you're running "dual fuel" you need the engine to be able to run on gas also.

If you're running straight propane you can take advantage of the higher octane rating, it's equivalent to 103 , to use more timing advance and higher compression to make more power and torque.

Here is a link that contains articles talking about that.
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