LPG advance

[Richard93Vogue]

So I was talking socially to a mechanic today and the talk got around to LPG and tuning. I said that I had my V8 at about 12deg. before.
I was a bit taken aback when he said Oh no, you need to set it right on zero. TDC... he was quite adamant that this was the best setting for gas.:eek:

Didn't take the conversation any further than that....

[jazzaD1]

that is absolute bull****! enough said!

[bee utey]

There is a school of though amongst some mechanics that all vehicles must be tuned to OEM petrol specs regardless. This is so that the vehicle meets emissions specs. Otherwise they risk being fined if the vehicle fails a subsequent roadworthy test. I actually put this to a couple of boffins who came to interview me WRT my LPG licence and workshop. To put it simply, they had no useful answer, other that that a future AS1425 standards review might actually address the issue. So in the meantime I consider not advancing LR timing/changing plug specs for LPG to be a capital crime. Also I am yet to hear of someone who was failed on petrol emissions when fitted with dual fuel on a older vehicle (pre 2004 at least)

Now that all newer vehicles have knock sensors the issue becomes less important, I mean, how many % of the vehicle population on gas now has a mechanical advance dissy? A fraction of 1%? it's hardly going to change the size of a city's smog cloud.

[Blknight.aus]

at TDC, yep sure, If I was about to rebuild the engine prior to installing the gas.

All dizzied Rover engines start at book spec and then need to be "eared" in. 2 supposedly identical engines could have upto 5 degrees difference in the timing they want for best running, Guys up on their engines will redo the timing about every 3 months or so.

[Davehoos]

you would have to bag test a car to have it fail-and then it would have to be way out.

P76 ran TDCand it was recomended that it was run on 92.[not 91].
it ran better on 10 and would cope with almost 20 with reduced mechanical advance.i often slide the dist back and forth,try to find a sweeet spot on the disco,not had any luck.

UK has an electronic retarder for using dual fuel-and ive seen kits years ago that had a detonation sensor and boost sencor for old carby turbo that retarded timing when needed.

[Davehoos]

There is a school of though amongst some mechanics that all vehicles must be tuned to OEM petrol specs regardless.

this line was pushed heavily when i did my lience and when i spocken to people in industry the powers that be are pushing the point that new conversions are aproved and the install completed to the aproval spec.

no aproval no conversion.

I did say this was the line pushed--but maybe not followed.

[Richard93Vogue]

Resurecting an old thread here but it has to do with the topic.
Amongst other things I am installing a new standard spec cam into my 1993 V8. The question to the brains trust is, Should I set the cam to TDC or as I'm already running injected LPG, should I install it with some initial advance?

Once I get it running and broken in I will be changing the dizzy to wasted spark with Megasquirt controller,so perhaps that would make any change to initial cam timing irrelevant being as I will be able to control advance from the computer???

Perhaps I should be asking what the relationship is between cam timing and spark timing?

The more I think about it the more confused I get. I had not thought about cam timing as a variable before being faced with installing a new cam and having to get it right.
Any advice/knowledge greatfully received.

[Richard93Vogue]

Bit more reading and I understand more about valve timing and ignition timing. Different but related beasts.
So the question still is, with a standard cam running LPG mostly, will I benefit from any advanced valve/cam timing?

[Richard93Vogue]

Here is a quote from eHow.com
In summary advancing a cam will increase compression pressures, requiring the use of higher octane fuels. LPG is what? 115 octane?


Effects of Cam Advance



Advancing a camshaft from its original position causes all of these valve events to happen earlier in the cycle. A camshaft advance of 4 degrees will cause each opening and closing event to occur four degrees sooner than before, changing the ability of the cylinder to build pressure. For example, if the intake closing event is designed to happen at 55 degrees after bottom dead center (ABDC) it will now close at 51 degrees ABDC, or 4 degrees earlier. The same is true of the exhaust events--they will happen 4 degrees earlier, even though they don't happen until after top dead center (ATDC) of piston travel.

Effects on Cylinder Pressure



Although all valve events occur earlier, the greatest impact of an advanced cam is to close the intake valve sooner in the compression stroke. This means that a greater volume of air and fuel gets trapped and compressed before being ignited and has the effect of creating more torque and power. However, if too much pressure builds up, extreme heat can cause pinging (detonation). Advancing cam timing can require the use of higher octane fuels in order to avoid this.



Read more: What Happens When You Advance Camshaft Timing? | eHow (http://www.ehow.com/about_6547919_happens-advance-camshaft-timing_.html#ixzz2Z6yxF3S8)

[Davehoos]

ignition timing--ignite fuel mix so that combustion is complete at given crank angle,[memory] 15-20 deg after TDC.before that it could cause pinking.

exhaust valve opens at a point to allow scavenging of the chamber,uses enertia to drag all the burnt gas out and reduce pressure to allow the clean gas to race in.

intake cam timing ALLOWS the quantity of air into the chamber without pulsing back into the intake.

P76/rover V8 lean burn style engines use reduced exhaust flows to reduce maximum compression at higher rpm but are good at low RPM[volumes]

NSW[Newcastle] LPG is 91-unless you get the good stuff from up north.

[Davehoos]

general argument is that LPG produces less tourque across the rev range--so reduce cam timing to get more tourque.

but all that does is reduce the air flow[volume] and if you use a vapour LPG system then you get less burnable fuel.

or you can fit a wilder cam with extractors and rev the engine harder using gears to create tourque at higher rpm.this will use lots more gas.

turbo or liquid injection is different story.