Need help with Propane dynamic compression ratio

[calpyro]

I am building a Chevy 454 bored .060 over for a street driven pickup intended to be a daily driver, with no towing or real hauling ever. I have done all of the machine work and have ordered parts with the exception of the pistons. I have question about what compression that I should shoot for.

The cam that I have picked out is a Comp Cam designed for economy and low end torque:

http://www.compcams.com/Company/CC/cam-

... d=392&sb=2
Comp Cams 11-230-3
RPM range 600-4800 (This should fit the RPM range of an Impco 425, right?)
Lift 250/260
Duration @ .050 206/212
LSA 110 degrees
Valve lift .47/.474

My thought was to maximize the propane's high octane with high compression pistons and a camshaft with little overlap, high vacuum that has a power-band under 4500 RPM.
Would static C/R 11.7:1 and dynamic C/R 10.25:1 be an appropriate compression on a camshaft with fairly low lift and duration that has little overlap?

Engine Data:

466 Cubic inches/7.65 liters
11.7:1 Static compression
10.25:1 Dynamic compression

Other vehicle data:
3.5:1 rear gear ratio
Turbo 400 trans (1:1 final drive)
35" rear tires
+/- 5000lbs vehicle weight

Comp Cams CamQuest camshaft selection program estimates the power at: Peak torque = 657@2500 RPM and Peak HP 445@4500 RPM.

Is my dynamic C/R in the ballpark and do you think that detonation will be a problem

 

[berkshire]

Have you discussed this with the people that sell the conversion kits such as,

http://www.propanecarbs.com/methods.html

They have a vested interest in you getting the best performance out of your conversion.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[calpyro]

Yes, I have talked to three of the largest vendors. The cam profile that I want to use maintains a somewhat higher dynamic compression that most builds that they see. They have not said that it would not work, just that they don not have enough experience to give an educated opinion.

 

[miningman]

I can't comment as to the actual compression ratio question but having had a Dodge truck converted to propane many years ago (conversion done professionally ) I can offer the following.

You are wise not to be towing... I forget the reasons why, but propane engines tend to self destruct under towing loads. You need to seriously consider use of a low temperature thermostat. I used a 160 F which certainly reduces the interior heat in a Camnadian winter and again I forget why this is advisable. From memory you go two grades cooler on the spark plugs. I suspect the reasons for all these mods are related to temperature of combustion.

Altho I didnt did do it, serious conversions ( and yours sounds real close) get the distributor recurved.

And the main positive is untill you own a propane powered unit , when in cold weather, you dont realiose how poorly a gas engine starts compared to propane. The reason is gasoline is still a liquid as it passes thru the carb into the cylinders. Propane is of course a vapour down to and below minus 35. Easy starting!!!

 

[calpyro]

Thanks for the reply.

My plan with the timing was to set the dist to: 30 degrees total, base, vacuum and centrifugal combined. All in at 2500 RPM. From that I am going to get a wide-band O2 monitor and knock sensor to fine tune the air/fuel and timing.
That being said, despite searching through the archived posts of this site and others, I have been unable to get a feel for what is a safe maximum for a dynamic C/R.
My preference is a bit lower C/R, but one of the major suppliers of pistons at an affordable price won't manufacture a lower C/R piston for a few months. My choice is to pay several hundreds more for forged or wait a couple of months or run the 11.7:1 static pistons.

 

[winfieldblue]

There are many things to consider when running propane, I had a couple of performance propane books when I was younger, here's a good guide to building an engine for propane fuel.

http://www.amazon.com/Economy-Perfo...52930303&sr=8-15&keywords=propane+performance

I only paid $30. for my copy so you might want to hunt around for a better price than what is listed.
A lot of the gasoline rules don't apply to propane so it's easy to kill an engine with pre-ignition if one isn't careful (I killed 4 0r 5 engines over the years through inaudible pinging) . Generally speaking the temp. of the inlet mixture has to be kept as low as possible because the octane rating of propane goes down with higher intake temps, and with higher inlet velocity too (according to the author). Running a cooler thermostat helps to get a cooler inlet charge if the inlet manifold is heated by coolant, and also helps by supplying the converter with cooler water resulting in cooler propane temps leaving the converter. The distributor needs to be reworked for decent performance too, with more initial mechanical advance (somewhere around 20*btdc) and less total(around 25*btdc), not to mention a custom vacuum advance curve on top of that (an adjustable vacuum advance unit is a big help). As far as compression goes, I can take a stock ford engine with a 9-1 c.r. and make it ping by using too much ignition advance, so your compression ratio question is a difficult one to answer, Jay storer (the author of the book above) suggests around 10-1 from memory, but one size doesn't fit all. From experience a closed chamber head generally works better than a large open chamber (tolerates more ignition advance before pre-ignition), also employing a ping sensor is a great idea because you can't always hear it happening. As mentioned by Miningman, drop a few heat ranges when choosing spark plugs, the cooler the better (assuming the same reach), gas doesn't wet the spark plugs so fouling isn't usually a problem. Here's a link to a local ford lpg forum with some ideas on how to lower inlet mixture temps

http://www.xfalcon.com/forums/index...s-to-stop-pinging-when-running-on-gas-part-2/

 

[Lou Scannon]

Generally speaking the temp. of the inlet mixture has to be kept as low as possible because the octane rating of propane goes down with higher intake temps, and with higher inlet velocity too (according to the author).

Since this is an engine and fuel Engineering forum for Engineers, I can't let that pass without offering some corrections and observations:

Generally speaking the temp. of the inlet mixture has to be kept as low as possible because the autoignition, and hence the knock margin (other things being kept equal) of any typical Otto cycle fuel goes down with higher intake temps, and with higher inlet velocity too (according to the author).

There is a plausible mechanism of higher inlet velocity affecting auto-ignition or knock, which is the resulting increased turbulence in the combustion chamber promoting small scale combustion reactions and bulk flame speed.


"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[calpyro]

Winfieldblue and hemi,

Thanks for the helpful advice.

 

[winfieldblue]

Quote (winfieldblue)
Generally speaking the temp. of the inlet mixture has to be kept as low as possible because the octane rating of propane goes down with higher intake temps, and with higher inlet velocity too (according to the author).

Since this is an engine and fuel Engineering forum for Engineers, I can't let that pass without offering some corrections and observations:

Generally speaking the temp. of the inlet mixture has to be kept as low as possible because the autoignition, and hence the knock margin (other things being kept equal) of any typical Otto cycle fuel goes down with higher intake temps, and with higher inlet velocity too (according to the author).

There is a plausible mechanism of higher inlet velocity affecting auto-ignition or knock, which is the resulting increased turbulence in the combustion chamber promoting small scale combustion reactions and bulk flame speed.

I am a bit inarticulate Hemi, the linked book explains the process much better as you do. I used the term "according to the author" after mentioning inlet gas velocity and its effect on combustion/pre-ignition to avoid counter arguments from folk who can't get their mind around the concept...

 

[dicer]

Since its octane rating is over 100 you should be just fine. Average driving except for hill climbs and from stop to speed, is a low HP demand deal. Done correctly 11.7 would work fine for pump gas, and racing.

 

[calpyro]

dicer,

Thanks for the help.

 

[calpyro]

This weekend the pistons will arrive. "Discere Faciendo, Learn by Doing"

 

[calpyro]

One more question. Are there any current books available on LPG conversions, engine building? The book shown above is quite expensive.
Thanks.

 

[winfieldblue]

I'm not aware of any specialized books other than the linked one above, but have come across numerous magazine articles over the last 30 years, more info is available via google. Be aware that some countries fuel suppliers(like Australia's for example) spike their propane with butane lowering the octane rating, so performance engine specs from these people won't necessarily apply to your build. An interesting development in propane systems in recent years is the liquid injection system where propane is injected into the manifold as a liquid, giving better performance than the mixer set-up, however the last time I checked it cost around $Aus 4500.for this system. I'm not sure if your mixer will flow enough air/fuel either, here's a link to some local guys swapping info on their builds:

http://performanceforums.com/forums/archive/index.php/t-67116036.html

 

[miningman]

Look for " Propane Fuel Conversions" by Jay Storer . published in 1986 by S-A Design Books

 

[calpyro]

Thanks for the tip about different countries LPG and engine specs.
I do have two complete Impco 425 systems that I can combine on my engine. However, I was going to start with one because I already have an Edelbrock Performer manifold.
Mining, I have been looking for that book for less than the going price, so far with no luck and surfing the web for any info.
I have searched the archives of this board and read every thread with "propane and LPG" and so far have not found a really solid answer to what dynamic C/R is the max.
I am going to install the 11.7:1 pistons and if I run into detonation problems, I will install an alchol/H20 injection system.
Hope for the best....