Propane 3.8 Draw Thru


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Been lurking for a while...and I gotta say, as far as forums go, you folks are pretty tight knit. Very nice to see. Since my build won't be started for at least a year (81 Monte Carlo Turbo), and until today I've had nothing to contribute, I've held off on posting. This might be a little long, but I ingested quite a bit on the subject. That said, let's talk some Propane.​
A thread in another forum I frequent got the juices flowing , and I've spent most of my day off researching Propane fueled vehicles. The info I found has me pretty stoked at the possibilities for this community. Just to clarify, were talking about Propane/LPG (Liquified propane Gas).​
Okay, The facts: Propane has a comparable Octane rating around 110 (excited yet?), cost varies widely depending on region/state...U-Haul has it for $2.98 per Gallon in town, Federal and/or State tax breaks offered for converted vehicles, Lower emissions across the board when calibrated correctly, Longer engine life, Oil change intervals extended to 13K+ on N/A engine, Zero carbon build-up, lower intake charge temp, and engines have been reported to run much cooler.​
The retrofit kits come in two varieties: Duel Fuel and full Propane. The Duel Fuel kit lets you switch between petrol and propane via switch. It retains the factory carburetor with a "plate" above it to disperse the Gas. The full propane conversion replaces the Factory Carb with a "Propane Carburetor" which bolts up the exact same way as a carb, but from what the off road forums say, is infinitely quicker and easier to tune and rebuild (minutes, not hours). Given the octane differences of the fuels, 91/93 Vs. 110, the off road guys run their rigs at 10:1, 12:1, 14:1 +, and can add quite a bit more advance. Whats the CR of our 3.8's...8, 8.5:1? I'm still trying to wrap my head around it all, surely don't have it all down. Vacum EFE/EFE II system on 78-80 would definitely need to be disconnected, and the Electric EFE/EFE II on 81-83's... I don't know.​
From all I've found today, Draw thru Turbo's were made for Propane...and the off-road community has paved the way. One possible negative I've read about is with exhaust valves and the need for hardened valve seats. I've seen no examples today on it happening to modern engines (modern= Un-Leaded fuel), but it was mentioned several times in forums. I have no idea what ours are made of, someone can chime in? It's a bit of reading, but this looks to be a way to have the built in results water/methanol injection offers, but without the ancillary system. There are several other topics and theories I'm itching to discuss with you guys, but I'll hold thing at a time lol.​
Draw thru propane info​
Compression for Propane w/ turbo​
Propane 101 from an off-roader​
General Valve Seat article:​
Propane conversion FAQ from a merchant​
C/R for propane. detailed discussion on propane theory from people way smarter than me.​
I've bounced back and forth between three different directions for the build: (what most of you have done) modifying the as-is factory system with carb/head/intake porting, cam, and turbo mods....A Megasquirt for ignition and knock sensing w/ factory Rochester....or a Megasquirt with TBI. Looks like there is now a viable 4th option!​
What do you guys think? Any and all opinions/thoughts/comments wanted​

Edited for to make factual correction.
Honestly I think you're about to open a can of worms here. While LP and propane work as fuels I don't know how well it will work on road VS off road. I could see it as an off road car or on a track but the feasability of it for a street vehicle may cause issues.
Thank's for the reply Charlie, I understand your hesitation on the subject. Hell, I never considered it before today...The reason all the offroaders flock to it is simplicity and practicality. With the extreme angles they put their vehicle through, regular carbs can be less than reliable...Doesn't matter of the truck is sideways, on it's tailgate, or upside down...the system keeps on pumping fuel.

Forgot to put it in the first post, but a must read:

These exerts should put you guys at ease.

Post #163

"...I converted my FJ60 a year ago and use it as my daily driver. I paid about $1.66 a year ago to fill up my 500 gallon residential tank and it is now about $2.30. I only use the propane to drive my car.....Mace is right about the PITA on fuel ups if you travel. I went from Park City to Zions and you have to plan a little bit better... But Flying J has propane and most farm communities do as well. I am cool with it but it drives my wife crazy. "I just want to go to a gas station...." My comment is usually #### that... That is why I sold your Denali!
Have you thrown a vacuum gauge on your intake at idle? I think I have a leek. Also, I am thinking of shaving my head to increase compression. I also have and HEI and I totally desmogged but I am not getting the mileage I want...... Thoughts?"

Post # 275

"...Figured I would update my LPG experience....Been months now and lovin the LPG. The gas in the tank is going skunky and been burning it up in the kids mini quad. Never use gasoline anymore. Lube the carb once a week with it as suggested and that's about it. I worked 3 months on the coast and took the 40 and trailer up to the job. Pulled great down the hwy with a 1000 lbs+ in the trailer. Topped up at the last station with 80 liters and headed to the job. Ended up lasting me 30 days. The tank read empty but found out the gauge is NFG. Will read empty but there is still 30+liters left in it. No idea of mileage as the speedo cable is broken and can't find one. Noticed lately my motor is heating up on the uphill climbs. Runs fine on the flat but hill climbs heat it up. Not overheating but spikes near the redline
Took it to a shop and had them dial it in. They leaned it out big time. Wouldn't cold start. passed the the Tec. / smog inspection here with 5 stars.
I have since enriched the mix and it starts 1st try. Still heats up at the top of the hill though. might be the rad, water pump, or timing is off???
Any input??? LPG has been slashed in half here in the past month. about 35 bucks fills the tank and go for a week or 2 on it.
Still less than half the cost of reg fuel.,If any of you can offer up help on the heating issue and head mods. Would appreciate it."

Post #280

"...I have been running my 40 as a daily driver for 3 years or more now. Only good things to say. I have it all dialed in and never lets me down.
Pulled the gas tank out and junked it. I very rarely use gasoline. Never really. We also installed LPG on a dodge Durango using fork lift tanks.
No difference in power and less than half the cost to drive. The gas Co comes to the house and changes out the bottles weekly.

I am considering putting a 300 liter tank at the house and they come monthly to fill it. I can then fill at home.
Restaurant style tank. Some Co will give you the tank if you buy only from them. Kindda leased or a lender.
Will put it out in the blazing sun so the pressure will be higher and fill my cruiser tank.

I average about 200 liters per month. Like I said this is my daily driver. 50-60 liters a week normally. No idea of mileage? my speedo is NFG and all over the map.

Post #53

"...I am running an impco 425 mixer. It is sized reasonably well for my motor. However, that motor is turbocharged remember, so I use a bit more fuel than a typical 2F. Impco model E vaporizer. And use whatever flow controll valve you want (electric or vac operated) The 425 bolts to a Holley 4 barrel baseplate/butterflies. Which makes it simple for me (good thing)..."

Post #61

"I have a gm 350 mated to a rangerII torquesplitter mated to a factory 4 speed, I get 12 miles to the gallon on gas(even before the dual fuel) and 10 on propane( no mater how I drive),this is on 33'S giving you an idea of gearing.propane in oregon is 2.39 just down from my house, and 1.90 about 12 miles away, at this rate I'm up there with diesel at cost per mile. and I hove an extended range of an extra 24 usable gallons of lpg. I think everybody should check it out, its hot hard to do yourself, just research and source your parts through anywhere but ebay! was mentioned here I think..."
you have done your research... how much would all of this cost to do? running nitrous would not be possible right?
it would be cool to see how it works on your project. i love when people get new ideas. good to have you aboard.
Propane works like alky Matt. It needs a lot more fuel than gas. If I remember right the stoceometric ratio for it is below 7:1 where gas is 14.7:1.
Thanks mat, I appreciate it. Went on a binge yesterday. As far as the Nitrous, no idea. Did some searches, but all i turned up is drag racers who run propane as primary fuel...but inject Nitrous alongside injecting gassoline. That's three separate fuel systems!! Not touching that one.

The initial cost depends on where you live, your state requirements, and tax breaks...the state tax incentives/regulatory requirements have all been compiled right here : (this forum won't allow me to insert Hyperlinks :/ ) Some states will give back $500+ on the conversion. I have no definitive cost for our cars yet, but It can physically be done for under a thousand on most cars. Currently researching different mixers/evaporators and their CFM rating. Whats the factory CFM of our non-modified factory Rochester?

It also depends on how your gonna mount them and how legal you want to be, which greatly affects the price. Regular DOT approved tanks aren't legal for "permanently fixed" road operation...but DOT tanks are what most use. Only "ASME Tanks" are legal, which if I remember correctly from reading, are 312.5 psig tanks, not the regular 250 psig DOT forklift tanks. These ASME tanks are offered by two manufacturers: Sleegers Engineering in Canada & Manchester Tank in USA...You gotta call them to get a price. The ASME tanks have Vapor Boxes that surround the valves & vent to the outside in case of a leek, and since Propane is heavier than Air, it vents out the bottom of the trunk. The ASME tanks can be mounted in the trunk or under it, where the gas tank used to be. I'd rather have the weight closer to the center of the car and trunk mount. A couple used and re-certified DOT forklift tanks would run $200.00 or so...which I would not do nor recommend... .. . I'd personally rather have the complete peace of mind on both Safety & legality/liability. Prices on the ASME tanks...Don't know. Not listed on. I ALWAYS site sources of info via hyperlinks, but without the hyperlink feature, it's just a hundred links throughout the post, which would make the post very cluttered. If you guys want me to do it, I'll do it. Fueled Vehicles.pdf

The Stoceometric ratio is less than one point above Gasoline. (
  • Natural gas:17.2
  • Gasoline: 14.7
  • Propane:15.5
  • Ethanol:9
  • Methanol:6.4
  • Hydrogen:34
  • Diesel:14.6
You shouldn't need much more volume of fuel. This is not modern liquid Propane propane specific injectors to retrofit a fuel injection system with. It's old tech for our old ass cars, injected like a carb, it instantly turns to a gaseous state once it leaves the Propane Carburetor before the Turbo. Since this is done Pre-turbo, and since propane reduces the intake Temp, we get a denser charge of Air. The Hot-Air turbo lay-out appears to be an aid for a more complete mixture. Please correct me if my thinking in this is wrong, but an almost Automatic atomization of propane Gas from it's liquid/semi-liquid state to it's gaseous state = more complete A/F mixture, denser air charge, and a lower loss of energy that gassoline. But....You were on to something in your post Charlie. Propane has 72.3% BTU/energy of gasoline....Hmm. But does it make up for it with it's advantages in the combustion engine environment? This website has a lot of tech artcles, just found it:

"The US DoE's Transportation Energy Data Book: Edition 29 (page 296) shows 83,500 BTU/gal LHV for LPG and 115,400 BTU/gal LHV for conventional gasoline but similarly shows LPG to have 72.3% of the energy of gasoline"
And again...we're all learning about this together. Before yesterday...I knew nothing about propane converted vehicles other than the fact that they existed, and the tech has a 70's era Ora around it. It's a completely different animal with some completely different properties from gasoline injection. Some really weird stuff. Please, if your interested at all, feel free to help out with the info digging.
Propane works like alky Matt. It needs a lot more fuel than gas. If I remember right the stoceometric ratio for it is below 7:1 where gas is 14.7:1.

This is not true. I have quite a bit of experience tuning with Propane and we use slightly less fuel then gasoline. On a 750hp application we use $1.70 worth of fuel each pass. I tune on both of the Roush CleanTech Mustangs and can tell you there are pros and cons to running propane. It is a very safe fuel to use and is very dificult to run too lean or too rich. It is very sensitive to temperature changes. From early morning to heat of the day I see almost a 15% fuel diferential due to the temperature of the fuel and the pressure change. Also, watch your oil closely for contamination.
This is not true. I have quite a bit of experience tuning with Propane and we use slightly less fuel then gasoline. On a 750hp application we use $1.70 worth of fuel each pass. I tune on both of the Roush CleanTech Mustangs and can tell you there are pros and cons to running propane. It is a very safe fuel to use and is very dificult to run too lean or too rich. It is very sensitive to temperature changes. From early morning to heat of the day I see almost a 15% fuel diferential due to the temperature of the fuel and the pressure change. Also, watch your oil closely for contamination.
What's the stociometric mix on it Cal? From my school work it's supposed to be about 1/2 that of gasoline like alky.
Welcome to 10 years ago... I used to sell a kit to do this.
Why did you stop? Where there draw backs? Or just not popular?

The kit worked great. I came up with the kit and another company used to manufacture and sell it.

We tried to get two well known board members to do some testing for us.. we sold one of them a kit at cost on his promise he would test it.. he never did. I dont know what he did with it. He has since passed away so I cant ask him.

Basically like everything, people knew and loved alcohol (Guess who the first one was to sell an alcohol system for the Buicks? Yep.. me) and didnt want to try something new. The results were favorable but due to a lot of bullshit I really dont want to get into again, the project ended up being scrapped.
What's the stociometric mix on it Cal? From my school work it's supposed to be about 1/2 that of gasoline like alky.

I never worried too much about stoich since I was tuning for hp. I'm pretty sure it's 15.7ish. To expand on what I posted above, propane is a pretty safe fuel. If you try and run it too lean or too rich it just won't fire. Not much chance of melting a plug or hurting a piston, etc..
After reading Jay's post, I may be confusing things. I thought we were talking about running straight propane not injecting propane into a gasoline engine.
$1.70 for a pass on a 750 HP motor, wow. Sorry to hear it was mothballed Jay.

Learned a little more today. The Impco 425 mixer/carb will flow a max of 460 CFM. Two are needed, and will give more than enough, maxed out 920 CFM. Given it's fuel on demand, it won't effect lower RPM/boost drivability.

We are Cal, "Before Black" non SFI 231's using a single/duel carb style propane mixer
After your response Call I decided to look it up. Looks like LP is stoicich is pretty good compared to gas.:)

  • Natural gas:17.2
  • Gasoline: 14.7
  • Propane:15.5
  • Ethanol:9
  • Methanol:6.4
  • Hydrogen:34
  • Diesel:14.6
Ashley....same here. Primary reason why I post very rarely is because for three years now I've been working on a propane conversion. Long story summarized: It is so easy to do on our Turbo Buicks it's laughable. Safety is the number one rule. Cost factor: The biggest drain on the wallet would be a properly certified tank and plumbing, (certified hoses). IMO the duel system is useless and cumbersome, (full on propane is the way to go). There might be some insurance issues on a full use LPG vehicle depending on the state. One gets less miles per gallon on an LPG engine, but this is offset by the cost of propane in some states and maintenance, (everything last longer because of its clean burning in the combustion chamber). The number one factor for an LPG conversion on the carbed turbo's is that it is more feasible with a carburetor, (EFI is nearly impossible due to cost). And as far as racing, well it may not be fair since LPG runs so cool the engine puts out full power. Finding a service station that is LPG is not easy and infrequent. So the vehicle will be for limited use, or more planning should be done prior to use. It's 15.5 Stoichiometric and octane is 104. These specs are for LPG motor-grade fuel not the type you buy for your BBQ grille, (it's still LPG but it's dirtier).
Yo Freddie. Propane Gas is what is used most with these type conversions. I see many people term "LPG" in their posts, but when you read through...turns out it's regular propane. I'm still not %100 on what LPG is or means. Here is a list of propane grades

Any Details on the system you narrowed too? looks like twin Impco 425 mixers & twin regulators is required from my research. Either a custom Side mount mixer to turbo compressor manifold, or a hole in the hood.
Getting away from propane for a moment and back on over to petrol, though still relevant in context. The thing that captivates me about this engine is it's static nature. Your stuck with the intake manifold it came with, pretty much stuck with the turbo (besides 301 hybrid), stuck with the Quadrajet, mostly stuck with the ignition system, and have very few cam choices. The great equalizer on this engine?...The Turbo. The Turbo is the bottleneck, and it's operating parameters are basically set in stone. The improvements have to be made around it and mostly with existing systems (I'm not talking add on injected liquids or full Drag set-ups here). A perfect test bed to implement a couple different "why bother, just turn up the boost" efficiency modifications at once, and to see the improvements based solely off better design at equal or lower boost. Below are the ancillary modifications that add to efficiency:

Modifying/porting the Quadrajet

Porting: Heads (aftermarket or stock), Turbo housing, Intake manifold, Gasket matching, Port matching,

Turbo exhaust Theory based upon pulse/back pressure/scavenging: Equal length manifolds and their optimal diameter, straight conical diffuser of 7-12° included angle downpipe (at least a mandrel bent pipe in the factory design), Divided wastegate downpipe either vented to atmosphere (screamer pipe) or spliced back in 12"-18" later downstream, and exhaust design with minimal sharp bends.

A modern cold air intake to enclosed filter.

Coatings: Thermal Barrier, Heat shedding, liquid shedding, and low friction.

Read these three many times over:
This may help you to undestand what LPG really is.

As far as bottle necks go, the turbo is 1 and the intake is the other. Just like the HA SFI intakes, if you get a bigger turbo on the set up the intake will have issues flowing all the turbo can do. Going with just a better turbo will be only one issue. The C/T cars work the same as any turbo engine within reason of course. You're not stuck with just a Q-jet unless you've got an E4ME and want to keep the puter and all the equipment. If you want to use a Holley you can go with a square or spread bore design and I think you can even get one made specifically for propane but I won't swear to that.

If you want a different ignition you can go with an after market distributor or if you can find it one of the stand alone C3I ignition units that GM made, but I believe there were some issues with them.