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Old 05-28-2012, 10:36 PM
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Higher compression for my 70 Chevy c10 307

I have a 70 c10 307 chevy that has used propane since 1988. Propane has 108 octane, thus the 307 low compression ratio is not optimizing mpg. I purchased the truck 4 months ago, have a 4:10 ? gear ratio at present and no overdrive. I am presently achieving around 15 mpg combined village and hiway driving. I plan to change the rear end ratio to 3:08 and will probable be getting around 16-17 mpg on the same roads. 15" wheels, stock tire size.
It needs a valve job at 140,000 miles I am aware of the need for hardened valves for the propane combustion. I am interested in the options that I can build into this engine to increase the comp ratio. I am aware of a threshold where the propane fuel will auto ignite due to high compression. What is that point and what is practical as I consider all that can optimize the propane fuel.
I am paying $2.25 per gallon at present. That calcs my actual cost per mile in the 28-30 mpg range. I am hesitant to discuss this as big brother will certainly tax the hell out of propane if it becomes more popular.
tjjack10
Northern California

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Old 05-29-2012, 01:05 AM
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Heads from a Chevy 305 with the 416 castings should do the trick and they are wallet friendly.
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Old 05-29-2012, 11:21 AM
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propane

If I remember correctly you don,t really want to exceed 11:1 on propane. It responds well to a turbo though, along with a little water /ethanol or methanol injection.
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Old 05-29-2012, 11:43 PM
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c10 307 propane

Can I bore it to a 327 ? If so which heads and how would this affect compression?
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Old 05-30-2012, 01:24 PM
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NO,But you can end up with a 327 by using a 350 block your 307 crank and 327 pistons.
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Old 05-30-2012, 03:05 PM
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So I would not be gaining much , I think your suggestion of different heads is the most practical. I am 55, occasionally get it on, but don't need to impress anyone. Not to say I won't develop that need in time.
I'm an inactive contractor, due to the meltdown. Money isn't as easy as it was pre 07.
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Old 05-31-2012, 12:14 PM
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Find some 4416 305 heads and mill them down by about .040"
or find some 601 305 heads. Install with a thin .015" head gasket for max effect
felpro 1094

The 4416 are nominally 58cc
and many of the 601's are 53 ish cc

You want to target a finished 52cc combustion chamber volume

if you want more power beyond the compression ratio increase
port them first.

Now you can change the stock cam for a propane friendly cam.
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Old 05-31-2012, 12:51 PM
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Its not practical or wise to try and bore a 307 block to 4" inch an attempt to make in into a 327. Its really not nessessary either to get great performance.

But you can bore it .060" and get close 3.935" bore
The CID will be 316 CID when bored .060+

The compression ratio will be a bit higher assuming the replacement piston is the same pin height as stock.

For all practical purposes it will give you any all the gain of a 327 over a 307
including readily accept most 2.02 valve performance heads.

A low compression 307 makes a very good base for a 142 B&M/Weiand
supercharger motor.
Especially when large chamber high perf 350 heads are swapped on.

You will slay your fair share of stoker motors with a street supercharged 307.
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