Seeking 383 propane engine build advice

Burbling2500 · Sep 19, 2022

I just joined the forum here to learn more after finding folks here seam to have a lot of knowledge and experience running SBC on propane.

The project I picked up is an old '93 2500 Suburban municipal fleet vehicle in great shape. Planning for travel, tow and some overlanding. It was bought with the intention or propane conversion right off the bat. The kit going in is for dual fuel and will automatically adjust mix from unleaded to propane with the tbi and o2 sensor.

The 32 propane gallon tank fits right behind the pumpkin where the 42 was. It has a vapor valve and can run generator, heater grill etc or relief to atmosphere to pull off any LP tank. T Diverter also will allow it to fill small LP tanks. The unleaded is a 27 gallon off an old astro that just barely fits under behind drivers side.

The engine is a 350 vortec with all roller hardware and having it rebuilt by an engine builder with a lot or experience in marine and industrial into a 383. I saw there were a few on here who have already owner and run 350's and 383's on propane and hoping to get some feedback.

This engine builder brought up a bunch of important factors of propane engine builds I found else where. The use of hardened valve seats, not to use chrome piston rings, Sodium stallite valves, oversized or quad core radiator for keeping engine cool with LP's heat potential, wants to use marine vortec heads and he's planning other machining touches in the combustion chamber. Flat or d pistons. Quench he brought up to optimize.

I have confidence in his experience with industrial like forklifts and marine as well as auto, but one thing I am hung up on is most of these forum threads bring up the higher compression ratio being almost essential to gain the benefit of the 108 octane of propane or get the best potential out of the fuel.

He seams to think it should stay at 9.3:1 to 9.4:1 Rather than the 11:1 or higher I see brought up. I know building on engine a balanced feat of engineering and changing one thing effects the other and wondering if I am missing something. I am not an engine builder and don't want to ask for something I don't understand about the 383, quench or other factors.

Use may also be a reason he isn't wanting to raise the compression ratio? This vehicle is intending to be 90% plus propane fuel for daily driving and towing to capacity cross county and overcome the longevity issues engines associated with dry fuels.

Any thoughts or advice from on the compression issue or in general would be greatly appreciated! Lol, aside from go unleaded. We have a 1200 gallon tank with dispenser and looking forward to just buying clean burning fuel once a year that can also power the rest of the home. Unleaded is better as a backup for our situation and just want to get the engine right.

Bebop · Sep 19, 2022

Less compression = less power = less stress on the engine = longer durability.

Which is probably why he recommended you that. Engines are very de-rated when it comes to anything industrial, just to make them live under the abuse of the application.

I'm not a propane guy, I don't even understand why you'd want to run an engine on this fuel, but that's my take on the "why?" question you asked.

extremetownie · Sep 19, 2022

I had a 408 built for propane a while back. I would run a compression ratio up to 12 . After 12 you will have starter problems forever. Then when you get it running turn your way up. Mine ran best at 60 degrees advanced. I know that sounds crazy. The engine was a torque monster, but I would trade it for a fuel injected LS any day.

Burbling2500 · Sep 19, 2022

extremetownie said:
I had a 408 built for propane a while back. I would run a compression ratio up to 12 . After 12 you will have starter problems forever. Then when you get it running turn your way up. Mine ran best at 60 degrees advanced. I know that sounds crazy. The engine was a torque monster, but I would trade it for a fuel injected LS any day.

Burbling2500 · Sep 19, 2022

Here is the work order as it stands.

Thank you @extremetownie. That is inline with my understanding as well. It seams that propane becomes more efficient at the higher compression. Going with stock compression under 10 sounds like it is not optimizing the propane unless there is something in the quench of the d dish Pistons effectively accomplishing this?.. The build is more or less to be a tow for our camper and daily driver so longevity over short term performance.

It will have unleaded as a back up cross country when propane isn't an option. It seams 10:1 to 11:1 would be the sweet spot to run either 92 or premium unleaded or propane...the camper also has a 60 gallon automotive tank that serves the camper so really about a 1400 mile combined range without fill with all tanks.

No doubt unleaded has been more dialed in over the years and has more btu's for squeezing every bit of performance, but our main goal is economy. Being able to buy a whole year of fuel at a time off season for around $1.99 gallon. Equal to about $2.30 gallon unleaded. The idea is it won't really matter how much market instability occured.

The math comes out to about 2-3k in fuel savings annually or maybe 25k in the life of the engine. If the build can be reliable enough to make 200k.

So my curiosity on compression ratio. If there is say a 10% loss of the efficiency of propane going from 11:1 to the 9.4:1 that is around 4k in fuel costs not saved. I don't want to push performance and compromise longevity either. End goal is a dry fuel tow vehicle that lasts.

Looping back to my original novice question. Is there a reason I wouldn't want the engine builder to get at least a 10:1 compression on a 383 with my application? When I ask him he just says 9.4:1 will do fine because of the quench factor. I'm a novice and maybe there is something I am just missing, but paying 7k cash for the build so definitely want it right.

Rock crawler folks like you seam to have the most hands on experience with SBC propane.conversions so I am hoping to either get confirmation that this engine builders plan for compression is sound or be able to share the specific reason of why I want the higher ratio.

I can only assume there is something I am missing about another factor of a 383 quench factor that would make higher compression redundant or unnecessary

I'm meeting with him tonight to go over the game plan once more before the build. Again, I like this guy. He has over 30 yrs building alt fuel industrial and marine engine and I sure don't want to be an ass, but just hung up on the compression ratio...maybe he assumes we'll end up using unleaded more in the end, but with an onsight 1200 tank and 1,000 mile propane range on board I don't plan to ever use unleaded except the first 5 minutes in cold weather start up.

Please advise and thank you again and in advance for any help.

From my

ridered3 · Sep 19, 2022

Nothing to add but cool to get some propane tech/discussion.

Burbling2500 · Sep 20, 2022

I met with the engine builder with more pointed question on why we aren't going with the higher compression that seams more efficient for propane in rock crawling. He said we are accomplishing comparable efficiency and power through quench factor coming to .045" with the d dish pistons in the 383 application. He went back to his original question of my goals and use of the vehicle. Longevity of the engine and as a tow beast.

He said by accomplishing very complete combustion through quench and additional machining including chamfer on the d dish to not trap carbon molecules we are already optimizing the fuel without comprised longevity. He stated that the higher compression ratios that are great for taking a performance and a rock climbing for a couple hours is a completely different application than towing trailers around the country and daily driving and the higher compression ratio would compromise the life of the engine in my application.

He does have over 30 yrs building alt fuel engines including many years teaching and building them in other countries so I trust him. He also has received his same engines back in the early years and been able to examine his own work of what components and machining techniques stand the test of time.

From everything I have gathered from propane got a bad reputation of not producing enough power or premature failure in the early years after the last energy crisis. The design of the builds now have resolved the issues and will last just as long as unleaded and deliver all the HP and torque needed for regular driving. 1/3 the hydrocarbons, 99% less particulate,. 40-50% the cost of unleaded after conversion, storable, versatile.

Every fuel type has its advantage and disadvantage, but given global instability of pandemics wars economic, food shortage and supply chains...propane is the most versatile and stable in our part of the world.

TBItoy · Sep 20, 2022

I appreciate the idea of dual fuel, but it seems awfully convoluted when there are literally no benefits to propane as a fuel for a road vehicle. It's safer and simpler in industrial applications and rock crawlers than liquid gasoline and burns cleaner than diesel, that's why it's used.

Do you have an existing kit/conversion that is for dual fuel with a TBI setup?

Unless there is something I'm greatly overlooking, it seems that you'd be better served as gasoline for you main engine fuel, and maybe use propane as a back up? Although with a Suburban, I think I'd be looking at the larger gasoline tank and a smaller propane tank.

That said, if you trust this engine builder, and he has the extensive experience in alt fuel engines in similar applications, then why in the world are you asking a bunch of rock-donkeys that just crash their **** in the woods about propane engines? Propane was popular 15 years ago when most of the vehicles that people were starting with to build rigs were carbureted, and EFI conversions were considered expensive and less reliable. Nowadays, you'd be hard pressed to find a new build using propane unless it's someone using leftover parts from an old rig.

Burbling2500 · Sep 20, 2022

TBItoy said:
I appreciate the idea of dual fuel, but it seems awfully convoluted when there are literally no benefits to propane as a fuel for a road vehicle. It's safer and simpler in industrial applications and rock crawlers than liquid gasoline and burns cleaner than diesel, that's why it's used.

Do you have an existing kit/conversion that is for dual fuel with a TBI setup?

Unless there is something I'm greatly overlooking, it seems that you'd be better served as gasoline for you main engine fuel, and maybe use propane as a back up? Although with a Suburban, I think I'd be looking at the larger gasoline tank and a smaller propane tank.

That said, if you trust this engine builder, and he has the extensive experience in alt fuel engines in similar applications, then why in the world are you asking a bunch of rock-donkeys that just crash their **** in the woods about propane engines? Propane was popular 15 years ago when most of the vehicles that people were starting with to build rigs were carbureted, and EFI conversions were considered expensive and less reliable. Nowadays, you'd be hard pressed to find a new

Burbling2500 · Sep 20, 2022

@TBItoy enjoyed your input here. He did resolve my question about accomplishing best efficiency through quench rather than compression with life of the engine being priority. He broke down what is being accomplished with the selection of marine and industrial components and machiningto perform and last as expected. I had just seen a lot about the higher compression angle with little on quench. He probably was blowing off my questioning of compression because he thought I comprehended whole the whole system effectively accomplished the same thing.

I don't know what you mean by it not having not benefit on the road? Any given fuels energy potential breaks down to btu's in essence whether it's stick or jet fuel and and combustion chamber engineered to optimize it accordingly right?

A diesel fuel fanatic could say the same of unleaded or a race fuel etc. If a vehicle meets or exceeds the limit of it's legal or intended use then what else is there aside from competition to carry some big brass balls around in a buggy?

I think this Suburban build is finishing around 390 HP and 400 torque after tbi and exhaust mods compared to the stock Suburban's ~210 HP and 330 torque. Sure a sbc engine can go higher and create more power for unleaded or propane, but with lower efficiency for something I wouldn't use towing 10-14k max.

So even after factoring propanes lower BTU it's making significantly more power than it's factory unleaded version and will easily perform past it's it's rated towing capacity and daily driving. There is always making more power and torque that I personally would not use. I'm not trying to roll my ride down a mountain or compete with diesel, just get the job done with ease and efficiency

This current set up would pull a 14k trailer, power a generator, heater grill. Get fueled up onsight at home from a 1000 gallon. Costs almost half unleaded per mile and if unleaded stays the same or triples or just black out and unavailable it makes no difference. Reliable, cheap, versatile and runs everything else on the homestead anyway.

The tractor and mower are converted to so I can just run the truck diverter to fill either or the lp tank for the fish house or jobsight on the spot from the truck.

This dual fuel kit is a new in the box from 15 yrs ago you mentioned. The shop doing the conversion has been doing fleet for the city and commercial and farm from combine to truck since '65. The newer efi computers are efficient and more complex working in sync with the factory computers. More complex means more things that I can't easily maintain myself. The newer NG and propane fleet trucks running on dry gas are going every bit the distance as unleaded for the most part.

What's the down side?

ridered3 · Sep 20, 2022

Purely out of curiosity (limited experience with Propane crawlers and zero experience with Propane street vehicles), what is your potential/expected miles per gallon running the Suburban from just Propane, just Gas, and Dual Fuel when empty/not towing and when towing?

extremetownie · Sep 21, 2022

I think you are incorrect on the fuel mileage. The local propane gas company used to have all there trucks running propane. The most recent engines where 6.0 Chevy with a higher tech propane injection. They used to get 3 mpg with the mixer style we use offroading. They now get 5mpg and are very happy they made the conversion.

ridered3 · Sep 21, 2022

extremetownie said:
I think you are incorrect on the fuel mileage. The local propane gas company used to have all there trucks running propane. The most recent engines where 6.0 Chevy with a higher tech propane injection. They used to get 3 mpg with the mixer style we use offroading. They now get 5mpg and are very happy they made the conversion.

Ouch! I know the 6.0 gas motors didn't get great mileage anyway but that's turrible.

TBItoy · Sep 21, 2022

Burbling2500 said:
@TBItoy enjoyed your input here. He did resolve my question about accomplishing best efficiency through quench rather than compression with life of the engine being priority. He broke down what is being accomplished with the selection of marine and industrial components and machiningto perform and last as expected. I had just seen a lot about the higher compression angle with little on quench. He probably was blowing off my questioning of compression because he thought I comprehended whole the whole system effectively accomplished the same thing.

I don't know what you mean by it not having not benefit on the road? Any given fuels energy potential breaks down to btu's in essence whether it's stick or jet fuel and and combustion chamber engineered to optimize it accordingly right?

A diesel fuel fanatic could say the same of unleaded or a race fuel etc. If a vehicle meets or exceeds the limit of it's legal or intended use then what else is there aside from competition to carry some big brass balls around in a buggy?

I think this Suburban build is finishing around 390 HP and 400 torque after tbi and exhaust mods compared to the stock Suburban's ~210 HP and 330 torque. Sure a sbc engine can go higher and create more power for unleaded or propane, but with lower efficiency for something I wouldn't use towing 10-14k max.

So even after factoring propanes lower BTU it's making significantly more power than it's factory unleaded version and will easily perform past it's it's rated towing capacity and daily driving. There is always making more power and torque that I personally would not use. I'm not trying to roll my ride down a mountain or compete with diesel, just get the job done with ease and efficiency

This current set up would pull a 14k trailer, power a generator, heater grill. Get fueled up onsight at home from a 1000 gallon. Costs almost half unleaded per mile and if unleaded stays the same or triples or just black out and unavailable it makes no difference. Reliable, cheap, versatile and runs everything else on the homestead anyway.

The tractor and mower are converted to so I can just run the truck diverter to fill either or the lp tank for the fish house or jobsight on the spot from the truck.

This dual fuel kit is a new in the box from 15 yrs ago you mentioned. The shop doing the conversion has been doing fleet for the city and commercial and farm from combine to truck since '65. The newer efi computers are efficient and more complex working in sync with the factory computers. More complex means more things that I can't easily maintain myself. The newer NG and propane fleet trucks running on dry gas are going every bit the distance as unleaded for the most part.

What's the down side?

I don't think it's mathematically possible to build an engine that will run on propane and pump gas and get close to equal performance (power/consumption/mileage/however you want to measure "performance"). Propane just doesn't have the energy density.

I'd venture to guess that 'Burb will get about 7-8 mpg on propane unloaded and 3-4 pulling 10-14K. You mentioned daily driving, but do you have a daily commute? or do you just run to town every once in a while? You may live somewhere that fueling up a home is simpler than in town.

I'm not sure where you are, but it sounds like you have the propane thing figured out around the homestead as a primary fuel.

I'd enjoy a build thread.

Burbling2500 · Sep 21, 2022

I am still learning and posted here to learn more.
The factory rated per mile on the '93 suburban unleaded is ~14 gallon. Not great fuel optimization on the TBI design. I believe it was tbichips.com that was recommended here and planning to have him do the final tune and chip burn after Dyno so we'll see.

I chose the tbi for it's simplicity and overall reliability rather than ported fuel injection now for that reason. Same line of thinking with the selected propane kit. It uses the o2 sensor to adjust between fuels and a delete. even the pre '95 Chevy..nearly 30 yrs of on the ground interchangable parts. I'm trading some fuel economy.

The energy density limts mentioned are just a reality as well. A build to optimize propane and unleaded will be different, but each designed equally well for it's intended purpose would come down to it's btu's 91,500 btu propane 112,000 unleaded. A ~20% difference. which can be compensated by making more power through bigger engine.

@TBItoy is correct from my understanding. Assuming each design is optimized The limit of any fuel type whether propane, unleaded or even syngas (wood gasification is next project. The octane of syngas is 104, propane 108. I'll just get a cheap 350 to experiment. Not road worthy? About 1 million wood gasifiers were in operation in WW2 for every form of power generation and vehicles from cars to tanks ). The only limits are the intrinsic energy density or btu's and the ability of the engineer. I am not one for sure.

The value benefit becomes apparent in the cost per btu. Bulk off season propane is 1/3-1/2 the price unleaded per btu which more than makes up for the loss dollar for dollar. I mentioned the 3-5 miles per gallon to the conversion shop and he left saying I better not cut him a check if that's the best he can do.

Once this all comes together I'll be able to share real numbers. It may be that I run propane on a light load. Economy mode and flip to unleaded climbing some hills with a trailer.

Talking with the engine builder the first thing I was asked was what is the use of the vehicle. My priority was getting 250k miles out of a propane engine and make enough power to tow. The 2 things propane get a bad rep for. Both the shop doing the conversions since '65 and the engine builder who's been building Alt fuel engines for 30 yrs would say the fails are poor design and misapplication and gave good specifics of why and how.

Efficiency was 3rd priority on this build. His design focused on atomization and quench for optimizing fuel, but because of my priorities I wouldnt expect more than 30% rather than 20% of potential energy. Still a savings. For me even if propane cost the same as unleaded per mile I would choose it because of versatility, storability and stability. Folks beheld to unleaded are subject to anything and everything hand to mouth everyday.

For example right now today the NG that powers and heats our whole country has doubled over last year and grain drying and heat season increases aren't even begun. Aside from that storage is half capacity of where it normally at right now because of exports. Bidens "emergency reserves" liquidation will expire shortly. Propane is fractioned off right away and barely related to transport. This should make it less effected and more easily available as policies put the hurt hold on Petro.

There are more efficient and powerful propane builds than this, but probably will only have $15k total into an off-grid tow vehicle that is essentially new under the hood and can maintain any situation and place with minimal tools. Maybe light duty rock climbing.

Burbling2500 · Sep 21, 2022

Burbling2500 said:
I am still learning and posted here to learn more.
The factory rated per mile on the '93 suburban unleaded is ~14 gallon. Not great fuel optimization on the TBI design. I believe it was tbichips.com that was recommended here and planning to have him do the final tune and chip burn after Dyno so we'll see.

I chose the tbi for it's simplicity and overall reliability rather than ported fuel injection now for that reason. Same line of thinking with the selected propane kit. It uses the o2 sensor to adjust between fuels and a delete. even the pre '95 Chevy..nearly 30 yrs of on the ground interchangable parts. I'm trading some fuel economy.

The energy density limts mentioned are just a reality as well. A build to optimize propane and unleaded will be different, but each designed equally well for it's intended purpose would come down to it's btu's 91,500 btu propane 112,000 unleaded. A ~20% difference. which can be compensated by making more power through bigger engine.

@TBItoy is correct from my understanding. Assuming each design is optimized The limit of any fuel type whether propane, unleaded or even syngas (wood gasification is next project) is the intrinsic energy density or btu's.

The value benefit becomes apparent in the cost per btu. Bulk off season propane is 1/3-1/2 the price unleaded per btu which more than makes up for the loss dollar for dollar. I mentioned the 3-5 miles per gallon to the conversion shop and he left saying I better not cut him a check if that's the best he can do.

Once this all comes together I'll be able to share real numbers. It may be that I run propane on a light load. Economy mode and flip to unleaded climbing some hills with a trailer.

Talking with the engine builder the first thing I was asked was what is the use of the vehicle. My priority was getting 250k miles out of a propane engine and make enough power to tow. The 2 things propane get a bad rep for. Both the shop doing the conversions since '65 and the engine builder who's been building Alt fuel engines for 30 yrs would say the fails are poor design and misapplication and gave good specifics of why and how.

Efficiency was 3rd priority on this build. His design focused on atomization and quench for optimizing fuel, but because of my priorities I wouldnt expect more than 30% rather than 20% of potential energy. Still a savings. For me even if propane cost the same as unleaded per mile I would choose it because of versatility, storability and stability. Folks beheld to unleaded are subject to anything and everything hand to mouth everyday.

For example right now today the NG that powers and heats our whole country has doubled over last year and grain drying and heat season increases aren't even begun. Aside from that storage is half capacity of where it normally at right now because of exports. Bidens "emergency reserves" liquidation will expire shortly. Propane is fractioned off right away and barely related to transport. This should make it less effected and more easily available as policies put the hurt hold on Petro.

There are more efficient and powerful propane builds than this, but probably will only have $15k total into an off-grid tow vehicle that is essentially new under the hood and can maintain any situation and place with minimal tools. Maybe light duty rock climbing.

Any recommendations on suspension for this? Thinking Tiberons anyway, but haven't even looked into it yet

Burbling2500 · Sep 21, 2022

TBItoy said:
I don't think it's mathematically possible to build an engine that will run on propane and pump gas and get close to equal performance (power/consumption/mileage/however you want to measure "performance"). Propane just doesn't have the energy density.

I'd venture to guess that 'Burb will get about 7-8 mpg on propane unloaded and 3-4 pulling 10-14K. You mentioned daily driving, but do you have a daily commute? or do you just run to town every once in a while? You may live somewhere that fueling up a home is simpler than in town.

I'm not sure where you are, but it sounds like you have the propane thing figured out around the homestead as a primary fuel.

I'd enjoy a build thread.

I'd be content with those numbers 5 and 9 would be great, but still all in the wheel house considering.