Hi, does anyone know what pressure I should be getting for low compression 8.1 ish, is 150 psi about right?
Early flapper/LPG.
3528 compression pressure
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mateyboy67
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mateyboy67
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Hi
I ask because I had LPG on my range rover and by it had been on for about 7 or 8K miles the valve seats on the exhaust side were so burnt I struggled to get combustion pressure above about 90 psi. It is difficult to interprate cylinder pressure readings against CR with everything in good condition, but on a very good condition 8:1 CR engine 150 would be good on a very tame cam, more than 20% less than that and you are then trying to track where you are loosing it, kn@ckered valve seats and it gets very difficult to be certain that your bores are the problem because even if you seal the bores by squirting oil down them you can just blow the extra pressure out past the valve seats even quicker and see no gain on the gauge. Reading preasures is a good way of finding 1 or 2 bad cylinders but in my experiance really it will eventually come down to a partial strip of the engine to find the problem.
Best regards
Mike
I ask because I had LPG on my range rover and by it had been on for about 7 or 8K miles the valve seats on the exhaust side were so burnt I struggled to get combustion pressure above about 90 psi. It is difficult to interprate cylinder pressure readings against CR with everything in good condition, but on a very good condition 8:1 CR engine 150 would be good on a very tame cam, more than 20% less than that and you are then trying to track where you are loosing it, kn@ckered valve seats and it gets very difficult to be certain that your bores are the problem because even if you seal the bores by squirting oil down them you can just blow the extra pressure out past the valve seats even quicker and see no gain on the gauge. Reading preasures is a good way of finding 1 or 2 bad cylinders but in my experiance really it will eventually come down to a partial strip of the engine to find the problem.
Best regards
Mike
poppet valves rule!
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Rossco
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Just for clarity running LPG does not lead to burnt out valves. What does lead to burnt out valves occuring more quickly on an LPG engine is incorrect settings.kiwicar wrote:Hi
I ask because I had LPG on my range rover and by it had been on for about 7 or 8K miles the valve seats on the exhaust side were so burnt I struggled to get combustion pressure above about 90 psi.
LPG contains about 10% less energy. The usual way of countering the loss of energy is to advance the ignition however it is only the static ignition that needs to be advanced, the all in figure remains the same.
If you only advance the static ignition you'll find that from around 2500rpm you're running too much ignition and therefore generating higher exhaust temps. The result is burnt valves within a few thousand miles.
Hi Rossco
The arguments are long but it boils down to the fact that LPG is a much slower burning fuel that petrol, this as you say can be compensated for by putting in more advance, but only so much before the risk of firing the wrong plug ocours and you start risking back fires. . . still even in this edge you tent to get the exhaust valve opening while the charge is still burning at high revs.
I found that after abour 5K miles the ware on the distributor and cam chain there was so much spark scatter that I was backfiring as I opened the throttle and still had issues of wanting more advance.
You chaps who seem to have success with LPG pretty well all have electonic ignition fired off the crank (megasquirt/ megajolt) and the ones who have most success have dumped petrol all together and gone for LPG only and more compression.
I believe you can build a successful LPG engine that does not burn seats, but it cannot be done as a "duel fuel" engine and definatly not with a mechanical distributor, I understand your point about re curving the distributor, and I fully agree you do not want all the extra advance at idle but to keep it comming in to much higher revs, I just do not think you can do it with a colckwork distributor, to get the necessary accuracy you must have a mapable electronic set up on coil packs (you need the extra amps for get the stuff burning). I also think you really need to dump the cam chain as the cam chain has so much slop in it after very few miles that if opens the exhaust valve too early on over run and causes more damage.
Hopefully you see my point about it not being simple?
Best regards
Mike
The arguments are long but it boils down to the fact that LPG is a much slower burning fuel that petrol, this as you say can be compensated for by putting in more advance, but only so much before the risk of firing the wrong plug ocours and you start risking back fires. . . still even in this edge you tent to get the exhaust valve opening while the charge is still burning at high revs.
I found that after abour 5K miles the ware on the distributor and cam chain there was so much spark scatter that I was backfiring as I opened the throttle and still had issues of wanting more advance.
You chaps who seem to have success with LPG pretty well all have electonic ignition fired off the crank (megasquirt/ megajolt) and the ones who have most success have dumped petrol all together and gone for LPG only and more compression.
I believe you can build a successful LPG engine that does not burn seats, but it cannot be done as a "duel fuel" engine and definatly not with a mechanical distributor, I understand your point about re curving the distributor, and I fully agree you do not want all the extra advance at idle but to keep it comming in to much higher revs, I just do not think you can do it with a colckwork distributor, to get the necessary accuracy you must have a mapable electronic set up on coil packs (you need the extra amps for get the stuff burning). I also think you really need to dump the cam chain as the cam chain has so much slop in it after very few miles that if opens the exhaust valve too early on over run and causes more damage.
Hopefully you see my point about it not being simple?
Best regards
Mike
poppet valves rule!
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Rossco
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Agree with everything you say.kiwicar wrote:Hi Rossco
The arguments are long but it boils down to the fact that LPG is a much slower burning fuel that petrol, this as you say can be compensated for by putting in more advance, but only so much before the risk of firing the wrong plug ocours and you start risking back fires. . . still even in this edge you tent to get the exhaust valve opening while the charge is still burning at high revs.
I found that after abour 5K miles the ware on the distributor and cam chain there was so much spark scatter that I was backfiring as I opened the throttle and still had issues of wanting more advance.
You chaps who seem to have success with LPG pretty well all have electonic ignition fired off the crank (megasquirt/ megajolt) and the ones who have most success have dumped petrol all together and gone for LPG only and more compression.
I believe you can build a successful LPG engine that does not burn seats, but it cannot be done as a "duel fuel" engine and definatly not with a mechanical distributor, I understand your point about re curving the distributor, and I fully agree you do not want all the extra advance at idle but to keep it comming in to much higher revs, I just do not think you can do it with a colckwork distributor, to get the necessary accuracy you must have a mapable electronic set up on coil packs (you need the extra amps for get the stuff burning). I also think you really need to dump the cam chain as the cam chain has so much slop in it after very few miles that if opens the exhaust valve too early on over run and causes more damage.
Hopefully you see my point about it not being simple?
Best regards
Mike
On the supercharged 4.2 I had it ran only LPG due to the high conmpression and used a Unilite and Hyfire for ignition.
I believe that mateyboy67 may be running a megajolt system
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mateyboy67
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No I'm on standard ignition and going to time everything back up to petrol only and see if that helps matters. I'm looking at 4.0 thor and 4.6 gems p38 engines next week (I wanted a cross bolted block) so I'm going to have to look at megasquirt/jolt etc to run the dizzy less coil packs anyway.
Do you know if either will fit to a lt85? I think the 4.0 is a manual and guessing a 380 gearbox so that should be ok but the 4.6 is auto.
I have thought for a while that a mechanical distributor is a bit sloppy for running lpg, petrol being a bit more forgiving due to higher calorific value.
As ever, one thing leads to another.
Do you know if either will fit to a lt85? I think the 4.0 is a manual and guessing a 380 gearbox so that should be ok but the 4.6 is auto.
I have thought for a while that a mechanical distributor is a bit sloppy for running lpg, petrol being a bit more forgiving due to higher calorific value.
As ever, one thing leads to another.
