From: Yvan on
Nedavno Athol pi�e:

> If the Impco is big enough to do the job and starts reliably when
> cold, I'd forget the atmo ring setup entirely.

I noticed that IMPCO 100 has quite complicated design (compared to amos
ring or mixer plate). Is there an explanation how does it work
somewhere on the net? A link would be great.

I like knowing how stuff works, it helps tuning and maintenace.


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From: Yvan on
Nedavno John McKenzie pi�e:

> That should clear up completely with an lpg specific ignition curve.
> There's certainly nothing wrong with your plan to run edis, but if you
> can recurve the existing distributor (or another factory one that was
> from a model with hei if it doesn't currently have it) and get very
> good results. It's quite surprising how much difference the right
> ignition curve makes. You'll definitely notice a big improvement in
> lower rpm ranges. The big danger is that with a 'stock' distributor,
> if you just dial in more advance, as mentioned, it improves the lower
> rpm, but it makes it more of a problem at higher rpm with even more
> advance at higher rpm (and it had too much to start with). The biggest
> problem with that is actually that it's often very hard to tell that
> it's got too much advance up higher. It doesn't ping like most petrol
> engines would with too much advance, it just seems to drop off power
> above 3000rpm or so. What's even worse - a lot of people think 'oh
> well, it's just the loss of power because it's on lpg' - when it's
> actually the timing. If you find yourself doing a lot of heavy towing
> with the car, it's even more important to limit the amount of total
> advance.


Can you post a link to the some explanation of how should I tune
ignition timing for the engine povered by lpg only? I know I need more
advance in low rpm, and less at high. But I do not know if I can tune
it by "ear" (as we say over here). At least I need some starting point,
to know (in a bit more detail than what I wrote) the difference between
petrol and lpg specific ignition curve.



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From: Yvan on
Nedavno Yvan(a)office pi�e:

> Can you post a link to the some explanation of how should I tune
> ignition timing for the engine powered by lpg only? I know I need more
> advance in low rpm, and less at high. But I do not know if I can tune
> it by "ear" (as we say over here). At least I need some starting
> point, to know (in a bit more detail than what I wrote) the difference
> between petrol and lpg specific ignition curve.



If anyone is interested I found some links about LPG ignition timing:

http://www.acl.com.au/web/acl00056.nsf/0/359683e8a538a3e64a2566c0007bb33e?OpenDocument
http://cars.rasoenterprises.com/Propane-Ignition.htm
http://edwardatherton.110mb.com/Megajolt.htm (scroll down to "Tuning")
http://www.dualcurve.com/8790_8791_8792%20for%20Web%20viewing.pdf 2.5MB



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From: John McKenzie on
Yvan(a)office wrote:
>
> Nedavno Yvan(a)office pi�e:
>
> > Can you post a link to the some explanation of how should I tune
> > ignition timing for the engine powered by lpg only? I know I need more
> > advance in low rpm, and less at high. But I do not know if I can tune
> > it by "ear" (as we say over here). At least I need some starting
> > point, to know (in a bit more detail than what I wrote) the difference
> > between petrol and lpg specific ignition curve.
>
> If anyone is interested I found some links about LPG ignition timing:
>
> http://www.acl.com.au/web/acl00056.nsf/0/359683e8a538a3e64a2566c0007bb33e?OpenDocument
> http://cars.rasoenterprises.com/Propane-Ignition.htm
> http://edwardatherton.110mb.com/Megajolt.htm (scroll down to "Tuning")
> http://www.dualcurve.com/8790_8791_8792%20for%20Web%20viewing.pdf 2.5MB
>

I've seen them, I've not had enough time to reply. Out of those links
(And not just because it's an Austalian link either!) the ACL one is the
one I think is the closest to the truth. There's some damn good info on
the acl site about engines.

My own experiences generally have shown that any time an engine liked
more than around 30 degrees total timing, it was running a _very_ low
static compression ratio AND had an lpg system that couldn't support
enough airflow for it.

In terms of initial timing, I've run as much as 20-25 on one engine, but
this had a 2500rpm torque convertor stall speed, so never spent any time
below 2500 with high throttle openings (it would simply flash that
high). If it's a manual (I'm not sure if yours is auto or manual?) or an
auto with a tighter torque convertor, you'd want to put the initial
advance somewhere more like 16-20. On the example of mine that runs 25 -
It actually runs a locked distributor - no mechanical advance, only vac
advance.

One of the things mentioned in the old SA design book by Jay Storer
(it's a publisher that used to put out a bunch of peformance related
books - mostly USA based stuff) - on propane conversions, I remember
they suggested setting initial timing using a vacuum guage - whatever
showed the highest vacuum would be a good place start, or just a couple
of degrees short of that to make it easier to start esp when cold.

Generally whilst lpg likes less total advance, it can usually tolerate
that mechanical advance coming in a little earlier. I tend to find
around 2600rpm is about as low as you'd want max advance to have come
in by. In some applications where the compression ratio is high enough,
you often find it might run better softening the mech advance curve to
come in by 3000rpm

A last comment - vacuum advance - I don't know why, but it seems a lot
of the engines I've played with varied more significantly in how much
vac advance they wanted on lpg. As a starting point I'd look at
approximately 8-10 and then try a couple either side and see which makes
it happier (that's peak vac advance). I'd generally take the car for a
drive with a vac guage, and look at what vac it has in the manifold at
whatever your highway cruise speed is, going up a slight hill. It'd want
some vac advance in that situation, and the peak amount at whatever vac
is present for that speed on a flat road. This will work for most
situations, or give a good starting point. But I am basing it on the
idea that you are running around 100km/h highway cruise speeds. If it's
higher than 125-130 or so, you'd have a somewhat different scenario and
wouldn't want peak vac advance under those conditions

--
John McKenzie

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From: Yvan on
Nedavno John McKenzie piše:

> I've seen them, I've not had enough time to reply. Out of those links
> (And not just because it's an Austalian link either!) the ACL one is
> the one I think is the closest to the truth. There's some damn good
> info on the acl site about engines.


I've searched, and posted few links that seems to show most info on the
topic. Most were useless.



> My own experiences generally have shown that any time an engine liked
> more than around 30 degrees total timing, it was running a very low
> static compression ratio AND had an lpg system that couldn't support
> enough airflow for it.


So max advance should be less than 30 degrees. OK.



> In terms of initial timing, I've run as much as 20-25 on one engine,
> but this had a 2500rpm torque convertor stall speed, so never spent
> any time below 2500 with high throttle openings (it would simply flash
> that high). If it's a manual (I'm not sure if yours is auto or
> manual?) or an auto with a tighter torque convertor, you'd want to put
> the initial advance somewhere more like 16-20. On the example of mine
> that runs 25 - It actually runs a locked distributor - no mechanical
> advance, only vac advance.

I have manual gearbox. Initial advance 16-20 degrees. OK.



> One of the things mentioned in the old SA design book by Jay Storer
> (it's a publisher that used to put out a bunch of peformance related
> books - mostly USA based stuff) - on propane conversions, I remember
> they suggested setting initial timing using a vacuum guage - whatever
> showed the highest vacuum would be a good place start, or just a
> couple of degrees short of that to make it easier to start esp when
> cold.
>
> Generally whilst lpg likes less total advance, it can usually tolerate
> that mechanical advance coming in a little earlier. I tend to find
> around 2600rpm is about as low as you'd want max  advance to have come
> in by. In some applications where the compression ratio is high
> enough, you often find it might run better softening the mech advance
> curve to come in by 3000rpm


Max mechanical advance at 2500 - 3000 rpm (on the ACL graph it's at
~2000 rpm) . OK.



> A last comment - vacuum advance - I don't know why, but it seems a lot
> of the engines I've played with varied more significantly in how much
> vac advance they wanted on lpg. As a starting point I'd look at
> approximately 8-10 and then try a couple either side and see which
> makes it happier (that's peak vac advance). I'd generally take the car
> for a drive with a vac guage, and look at what vac it has in the
> manifold at whatever your highway cruise speed is, going up a slight
> hill. It'd want some vac advance in that situation, and the peak
> amount at whatever vac is present for that speed on a flat road. This
> will work for most situations, or give a good starting point. But I am
> basing it on the idea that you are running around 100km/h highway
> cruise speeds. If it's higher than 125-130 or so, you'd have a
> somewhat different scenario and wouldn't want peak vac advance under
> those conditions


We have highway limit of 120 km/h, and I do not drive faster (usually :)
I drive most of the time in the city traffic, and on the roads with a
60 - 80 km/h speed limit. Does that change anything? Or should I tune
vacuum advance at 100 km/h regardless where I do most of my driving?

All this (very helpfull) info I have to adapt for use with EDIS. It has
programable 3D ignition map (I need laptop), and connection for a
manifold vacuum for indication of engine load. Good thing about it is
that I can change ignition timing while I drive :-)

Again you have been very helpfull, thank you for your time.




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