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
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