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Discussion Starter #1
OK, so I know this is a 4x4 Forum, but I found a couple posts here with some members who seem to know a thing or 3 about the topic I am dealing with, so please don't beat me for asking about a 2WD truck... I'm just trying to make the truck work better and you guys seem to know your stuff...

I have an '83 GMC Dually Crewcab with a (as far as I know) stock 454 on propane, with a presumably low-compression setup (I did a compression test and cylinders range from 105-120psi, so I'm assuming that its your average smogger 7.5:1 BBC with nothing more done to it than getting the valve guides and maybe seats done for the propane)... it runs decent (once it warms up a bit, when you first start it, it's a little lumpy, and my father says he can hear wristpin rattle) and has HUGE oil pressure - 7/8 of guage at idle from a cold start and 2/3 guage at idle in gear at full operating temp... now I bought the truck this way, and the motor has been in the truck long enough for the propane certification to expire, and it shows absolutely no signs of any smoke, so I'm assuming the heads were done properly to last this long...
my questions are this:
firstly, I'm somewhat familiar with propane and LPG systems, as I spent almost 3 years dispensing it for a local fuel station chain, I know that LPG likes big compression and timing, but to what degree or limits, I'm not positive of, so I'm asking here in hopes of a definitive answer...
I have a couple of possible solutions to the compression ratio right now: on Kijiji (Canadian equivalent to craigslist) right now, there is a guy with a set of 101cc closed chamber 396/427 heads (casting # 3917215, for specific info), which I estimate would put the motor around 9:1 (this is a rough estimate based on unreliable info, so if anyone has more accurate numbers, I would welcome the info), however, even though they are oval-port heads, I'm not sure if my 1983 intake will bolt to them and work properly, and also, they need 1 valve guide and seat done, not sure why... asking price: $275 + whatever it costs me for head repairs, gaskets, etc...

my second option is a set of 12.25:1 .030 over LS7 pistons with wristpins and new rings, but no rods... pics are posted and they look used, but still in good shape and they have been cleaned up... asking price $250
down side is: I would imagine (although I obviously haven't had the motor apart to verify) that my motor is still stock bore, so I will have to spend money on boring the block, and I assume that in spite of the wild oil pressure, that with that drastic of a compression change, that I should be doing at minimum new rod bearings, possibly more... also, although I've read that these 1-ton blocks generally come with cast crank and pistons, they use forged rods, but I can't verify this, so again, any info is welcome...

given these bits, I'm wondering what kind of actual RPM this motor would be capable of...? I've always assumed that a smogger BBC would be good to about 4000rpm (like my stock 400SBC), but some of the info I've read lately has indicated that this motor may be easily good to 5 or even 6000+rpm (not taking into account head flow or cam limits), so I'm just curious...

so I'm looking for opinions/info/options as to which would be the better choice to bump up the compression to be more suitable for LPG in this truck...
I'm not looking to make big power, this is not an off-road or track truck, it's a hiway tow unit/commuter that I use to haul my 2 Fiero's to shows around Canada and the US and I use to commute to work 50miles a day, so I'm just looking to get better towing power and maybe better fuel mileage, if possible... I know that both can be achieved, and one of my old customers had an old ElCamino with a built BBC on LPG that he claimed was 600+hp and getting around 20mpg, but obviously I never was able to get in-depth into the car or the motor to get details or info on the build or the LPG setup, but he claimed to know a top-notch LPG tuner...

secondly, I'm looking for more info on timing and tuning for LPG... I know that when I initially step on the throttle, the truck pulls off the line very responsively, but then immediately falls on it's face unless I am right into the throttle, wondering if it could be a timing issue, or possibly mixer settings, or even something else... I'm not scared to tune a carb, and give me a Q-jet and I can tune it better than most mechanics, but I've never messed with LPG, so any info is welcome (even arguments over techniques, lol) because I don't have a clue where to start with a mixer, lol...
now I realize that this is a 7500lb truck, but my 81 3/4ton crewcab (6500lbs) with 3.73 gears (VS 4.10 in the dually) on a stock 350/4bbl would absolutely annihilate the dually with nothing more done to it than removing the pollution pump setup and manifolds and going headers and dual 2" exhaust... the dually has stock manifolds and dual 2.5", so it's not like it's super-restrictive either, however a set of headers and possibly dual 3" are on the agenda, but again, most LPG trucks I see have tiny exhaust anyways, so I'm not sure if I'm working against myself here...??

Again, any info on any of the above questions or comments are welcomed...

Anthony
 

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To be honest, the other propane thread you posted it has alot of bbc guys saying about a single 425 is fine up to about 4500 rpm. Your better off calling one of the propane guys that sells the kits.. both altfuel1991(buddy) and got propane(cary) can help you a ton. For a worked up big block i can see you prob will need 2 of everything. You can do that with the dual carb adapter or even use the dual carb intake. I think if you did work to the bbc and still want to daily drive it with lpg and want to save on fuel mileage the progressive linkage will help. This needs to be tuned with a wideband afr or something close to that. Then you will know if its too rich or too lean and can adjust accordingly. For the mods you want to do to the bbc i cant help with that. Hopefully some of the big block gods will chime in.

for instance:

I have a sbc thats 12.5:1 with big valves and small chambers(about 450hp). Using one mm x450 i noticed that it limited power in higher rpms. I added another x450 carb with 2 model e's on an impco dual carb adapter...it revs thru 6,000 crisp and clean with really good throttle response. The timing limited also to 30 degrees all in by 2500 rpm. Right now my initial is in the 12-14 range. For exhaust i have big tubed headers with 3 inch collectors and 3 inch exhaust and it seems fine.
 

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Discussion Starter #4
kool.. thanks :)

right now, like I say, the main goal is to get the compression up (I have a set of .030 LS7 pistons coming on Sunday, should take care of that, numbers say it should be around 12.25:1, but it will depend on what heads are on the motor right now), and to tune it up a bit in the meantime, which means learning how to tune the mixer, which I have honestly never done before... Carbs (specifically the Q-Jet) I'm good with, but mixers is totally new to me, so any information on how to tune them and/or set them up would be great...
as it runs now, when you step on the gas pedal, the motor has the initial torque to lurch forward with authority, however, 5 or 8ft down the road, it falls on it's face (unless I am right hard into it, but even then it seems gutless even compared to my old 350/4bbl crewcab on gas, and the 350 had taller gears and WAY less motor, lol...
as far as I know, I have a 425 Impco on there, but I will have to look closer to verify that, for the sake of adjustment instructions...
as for the rpm's, I'm not really looking to build an 8000rpm race motor, I want a strong tow motor that will still get respectable (or even impressive) fuel mileage and be happy towing 10,000lbs worth of trailer down the hiway right around the 21-2400 rpm mark in overdrive (once the 700R4 goes into the truck)...
 

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Usually the only adjustment to the mixer is for the idle mixture. The power mixture is preset from Impco for mileage and is normally close as is unless someone has already moved it. If you are going to use this as a tow vehicle then 12 to 1 compression might be a little too much. You will need a very good cooling system to keep the temperature down.

Also you need to check out the timing.

On most applications, the timing should be a total of 30 degrees(base and centrifugal) with all of it in by 2500 rpm. What we normally do is disconnect the vacuum advance, run the engine up to 2500 rpm, set the timing at 30, lock it down, then let it idle with the vacuum advance disconnected. See what your base timing is running. If it is low,4-10 degrees for example, you can alter the centrifugal to lower that number and increase the base, still keeping a total of 30 degrees. As you increase the base timing check to see if the engine spins and starts smoothly when hot. If you reach a point that the engine bucks or loads the starter, back off about 3-4 degrees and that is your base timing. Subtract that number from 30 degrees and that will be the advance you need to have. This will provide a good start, strong idle and proper advance curve. We have had some engines run as much as 20 degrees base timing but 14-16 degrees is normal. The vacuum advance should be connected to ported vacuum and have no more than 10 degrees. This helps fuel economy at light throttle positions.

As a rule set the timing first, idle mixture second and idle speed last.
 

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Discussion Starter #6 (Edited)
ok, kool... that's the info I think I was looking for... thanks a bunch...
just out of curiosity (again, as I'm new when it comes to building propane), as I understand it, when it comes to propane and compression, the higher the better (up to about 15:1 or so), and I know that the higher the compression the better the torque, so how or where am I gonna have issues with 12:1? as I understand it, that would be relative to about 9 or 10:1 on pump gas...?? or am I out to lunch? lol

and as for cooling, the truck has the stock BBC 4-core rad and a 7-blade fan, and still has the full fan shroud... will I need more than that?

also, I am picking up a set of headers, and I have the option of either the 2.5" dual exhaust on the truck, or I have 3" pipes I can use (already set up for headers), is there any advantages or disadvantages going either way? I liked the 3" on my 350 truck, but I had to put a 2.5" restrictor in right at the collector flange to keep the backpressure up because the 3" was too much pipe for the stock motor... so again, being new to propane and being my first BBC, I'm not positive how much of my previous knowledge applies here... also, the 2.5" pipes drop to 2" after the mufflers, so I would imagine that could be an issue as well...
 

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Compression ratios with propane are similar to using gasoline in that the higher the compression the higher the cylinder temperatures that can cause pre-ignition. Propane is ready to ignite as soon as it enters the cylinder whether compressed or not. Gasoline enters the cylinder as tiny liquid molecules and must be compressed and heated before it converts to a vapor to be burned. The vaporization process actually cools the cylinder temperature. But either fuel can only handle so much compression and temperature before it will auto ignite. We converted a rail buggy several years ago that had 13to1 compression but we are talking about a 1200 lb rig that is seldom under a hard load for more than a few seconds. A truck being used as a work vehicle towing or hauling a load is going to be pulling for a longer period and building up a lot of heat that will not dissipate quickly. The heads and block must be free of any rust and scale inside the cooling passages that could slow the temperature transfer to the coolant and the water pump and other components must be in optimum shape as well to transfer the heat. A working truck should run around 9.5 to 10 to 1 compression to be safe and make the motor last.

I would stay with the 2.5" pipes and run small tube headers to help low end torque. Also go 2.5" all the way out.
 

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Discussion Starter #8
ok, so in a truck that weighs 6000 or 7500lbs (depending which truck I keep this motor in), am I gonna have serious heat concerns just running it as a commuter or weekend toy type unit? I wasn't planning to make big changes to cam or heads or anything, although I was considering an RV cam, but havn't bought one or anything yet... advice good or bad?
 

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If you are running 12 to 1 compression you can have issues in a vehicle that heavy. If you are going to run that high of compression ratio you may want to talk to a cam manufacturer about which cam to use. The RV type cams generally close the valves early which further raises the effective compression ratio. I learned my info from the pioneers of propane, Ak Miller and Don Bass. These are the guys that spent years learning what to do or not do with propane.
 

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Discussion Starter #10
absolutely, from the responses I've read on here, I understand your knowledge base... that's why I opted to post my questions on here, lol...

ok, so with the high compression like this, should I be just keeping the stock cam? would it cause further issues, or would it limit the air flow enough to keep the compression within reason? I mean the mixer is pretty limited as far as flow anyways, but it should be more than enough to do what I'm looking for, being as I'm only looking at about 4000, maybe 4500rpm tops...
is it just heat issues I would be looking at or would there be other concerns to this high of compression? is detonation a serious concern? would I need to back the timing off with a setup like this?
 

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If I am going to the trouble to build the engine I would replace the stock cam with a more efficient aftermarket one. The cam grinders have the technical knowledge to take into consideration all the components being used as well as the intended use of the vehicle and come up with a proper cam for it. If you are going to limit the rpm to around 4500 then a cam designed for that range should be available. As far as temperature I would run a 160 degree thermostat to keep the base temperature as low as possible. Big blocks tend to run hotter anyway, around 200 to 205 on good days from my experience.

As far as backing off the timing, if you have to do that to run the engine then the temperature then dictates how the engine will perform because the timing will be low and the engine sluggish when cold. The combustion process will then be unpredictable and performance will suffer. Any engine is designed to operate efficiently with proper timing for any given fuel regardless of the temperature.

The single mixer will work fine at the rpm you are talking about and get the best mileage.

Is it possible to have some of the dome milled from the pistons you are getting to drop the compression? This would help the situation.
 

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Discussion Starter #12 (Edited)
I wasn't originally planning to rebuild the whole motor, lol... the plan was just to bump the compression up so that the propane wasn't fighting against itself at 7.5:1... changing the cam is an expense I wasn't really including in the limited budget, nevermind a custom grind... and I didn't realize that 12:1 was gonna cause me issues like this... I've towed with 9 or 10:1 on gas and didn't have an issue, so considering how much propane likes more compression, I figured right around 12 should have been good, lol... maybe I should have taken the LS5 pistons that buddy had instead of the LS7's, lol

as for milling the dome, I probly could, but these are factory .030 over LS7 pistons, with the GM casting number right on the top of the piston... if I machine them, they lose any value they might have had, lol...
 

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Discussion Starter #14
hmmm... well, I think the point may have just become moot... I've been having oil pressure issues with the truck the last few days (it used to almost bury the guage on cold startup, but since I hauled my 4x4 home from the shop a few days ago, it will barely make 1/4 of the guage, even on cold start), and apparently I inadvertently used synthetic oil to top up the normal oil that was in it, so it looks like I may have burnt out some bearings, and I don't have the budget for a full rebuild right now... so I guess the SBC stays in the 76 and my diesel goes in the 83 afterall...
sad situation, but thanks for all the help anyways... not sure what I'm gonna do with the motor or the pistons, but if and when I get around to doing something with it down the road, I know who to talk to... :)
 

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back in business, hehe

well, a bit of a hiatus, but with some good news, lol...
apparently I didn't hurt anything too bad, as I did an oil change on the motor and flushed out the synthetic, and the oil pressure came right back to what it was (took a little run time to get it out, but it did, then I changed the oil again to be sure it was all cleared out, lol)
so long run, the project is back on, with a little more defined goals (although a completion date is far from set, as finances are crap right now)...
after a rather long discussion with a well respected engine local engine builder, the plan is to use the LS7 pistons, stock 1983 oval port heads, a Comp XE268H cam kit, and so far, stick with the single mixer (although the cam is capable of considerably higher RPM than the mixer will handle, so I may import the mixer from the crewcab that the motor is coming out of to add to the one in my old 76 that the motor is going into, but that's a much later project, lol)
also, being as the power and torque should be substantially improved, I will be keeping the TH400 and 4.10 gears that are in the 76 and just using it as a cruiser with some jam... the 700R4 just wouldn't cut it with what the motor should be putting out (my guy estimated 425hp and well over 500lbs torque, even on propane)
 
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