Leyland P76 Owners 2004

Technical problem

[215V8] LPG Compression Ratio


Question

I'm planning on building a 4.0L/4.6L stroker with large-valve 300 heads to run on LPG (propane).
I can improve my efficiency if I use a CR of 11:1 - 12:1.
Anyone know what CR I can safely and reliably run the engine on? Warm
Regards
Shawn

Answer

Hello Shawn,
As you know, propane has a much higher octane rating than the best available
pump gasoline -- nominally 105 octane on the R+M/2 scale. That said, there are
a number of factors that effect an engine's octane requirements. Each of the
following can have an influence: camshaft timing and duration (overlap),
cylinder head design, bore diameter, spark plug location, material of construction
(aluminum is more tolerant), engine temperature, air temperature and ignition
timing.
I have zero experience with the 4.0L/4.6L stroker or the Buick 300 heads but
I do have experience with the Oldsmobile and Buick versions of the 215 and
also with propane.
It is my understanding that the Oldsmobile head is more detonation resistant
than the Buick head. My reading has also led me to believe that the Buick head
actually flows a bit better than the Olds, thus providing more top end power.
Now for my real world, first hand experience. I built a Buick 215 for my
wife's Chevy Vega. That engine had a mild camshaft, about 8.5 to 1 static
compression, ran premium grade (91 octane) and never detonated. The second engine was
an Olds 215 using the small chamber heads along with high domed Arias pistons
yielding a ratio of 13.5 to 1. The camshaft profile was non-stock but rather
mild. That engine did not detonate at its home altitude of 4,000 FT but did at
sea-level -- even using 93-octane fuel. Initial time for both engine was set
at 10 degrees BTDC but the 13.5 to 1 engine needed to be backed down to 5
degrees initial to forestall detonation at sea level.
Now to propane. The Vega and Buick engine were ultimately sold. The Olds
version of the 215 was rebuilt and sits in my son's Corvair. Here is my logic:
If that high compression engine could survive on 91 octane gasoline -- it would
dearly love 105 octane propane.
Again, I have no experience with the 4 liter or 4.6 liter engines nor do I
have experience with the Buick 300 heads but my personal experience would say
that you should have few problems with propane at the compression levels you
anticipate.
Some of our British and Australian list members may have different
experiences based on the use of propane in the Rover products. But I believe that most
of those applications were duel fuel and not optimized for propane.
http://www.propanecarbs.com/propane.html Propane Facts & Comparison Charts
http://www.patooyee.com/propane/propane_1.htm Propane: Page 1
Darius

Another reply

Date: Tue, 14 Mar 2006
Back in the '80s I used to work for Petrolane, the propane gas distriburor, and we did conversions.
If I remember correctly the max recommended compression ratio was about 10.5:1.
Even though propane is high octane (110) since it is a dry gas, you don't get and evaporative cooling of the
charge, making it somewhat susceptible to detonation at high compression ratios.
I am working from memory here but will try and dig up some info if there is interest.
P Knowles

Last updated
Dec, 2005
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