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Discussion Starter #61
My pinging problem is under control now. Just needed to be knocked back a few degrees. No noticable difference in power but it doesn't ping. Not convinced that my vacuum advance is working very well though. It backfires a bit when coming off the throttle at higher revs. Not much just a bit of a crackle through the exhaust. I'd have thought that on such a mild cam that it shouldn't do that if the vacuum advance is doing it's thing. The cam is only 258 degrees intake and 264 exhaust and 440 thou lift.

Is the vacuum advance supposed to create more advance at idle? Or is it just supposed to do it when coming back to idle from revs?

Mine is connected to the carby which is where everyone reckons it should be but it certainly seems to work better off the manifold.

What's the go with that?

There aren't any vacuum leaks.

Chris.
 

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Chris, the vac advance has to go under the carb. It is used to improve fuel consumption when crusing (high manifold vacumn) and yes it does work when you back off because of high vacumn. It shouldnt work at idle unless it is sticking.
 

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Discussion Starter #63
Yeah that makes sense. Thanks for that.

Chris.
 

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V8 GT pilot
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Hey Guys,

What sort of Compression Ratios are you guys running for LPG
I've been advised to keep it around 9.5 : 1 to a max of 10:1 for todays gas mixtures.

I've been told by Gas Research , C.O.M.E Engines , Dynomotive , Sircar , and a couple of other LPG tuning specialists.

What do you guys reckon ??
I'm currently running 9.5:1 351 with 302 CC heads ( dished pistons )

Thanks
 

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Discussion Starter #65 (Edited)
It depends on what kind of cam you're running.

Bigger cam means you can have more static compression but it also means you need more potential flow through your intake and exhaust systems to take advantage of the higher RPM's

If the cam is pretty stock then I wouldn't want to go over 10:1.

The 302 heads with dished pistons is going to allow you to have a bit more compression than if you had stock pistons and open chamber heads but I'd say leave it how it is unless you are going to stick a big cam in it.

Current LPG is usually only 1.6 points higher than optimax. LPG=100 RON ; Optimax =98.4 RON.

Some LPG is worse but it's rarely better.

In the "good old days" when gas had a much higher octane, you could run 14:1 compression with a fairly big cam.

Not anymore.

My 351C is running pretty stock compression (9.4:1). It might be a tiny bit higher as the engine has been rebuilt with stock components but the heads and block might have been machined a bit.

I have just sorted my pinging but I dont want to back the timing off anymore than I have or I will start losing performance big time.

So for my thing with a small cam, it seems that stock compression is about as good as it gets.

Basically, you can't really build the bottom end any different than if you wanted to run it on PULP. The main differences you can make in an LPG motor over a PULP motor are ignition advance, intake manifold and intake ports/valves. Everything else is pretty much the same. LPG takes up more volume getting into the motor as it's already gas so bigger ports, valves and intake manifold can help top end without hurting bottom end torque as much as it would on PULP
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Chris.
 

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Chris

My cam specs :

Crane H-272

Duration @ 50

Intake 216 , Exhaust 228

Lobe separation 112

Gross lift Intake .524 Exhaust .519

Suggested Comp ratio 8.75 to 10.5 ( My comp = @ 9.5 :1 )


Rest of set up = Redline torker Inlet ( single plane )
1/2 inch spacer
Impco 425 Carb LG B2 Converter
4V Exhaust Valves 2V inlet
High Energy extended sump
Crane Hi6 Ignition with LX92 Coil
Magnecor KV85 Ignition Leads
Genie Tri Y into twin 2 1/4 zorst
Factory Electronic Dizzy

170 Rear wheel Kw
14.3 1/4 mile @ Calder 95 mile per hour in an XE Fairmont Ghia ESP

I was hoping to get somewhere around the 190 Kw mark but this is proving to be very difficult.

I will be upgrading the exhaust soon with an X - pipe and better mufflers.
Might also look at different types of Inlet manifolds.

Car goes good but alas need MORE power
The challenge is to get it on the " Dark Side " fuel

" Must use the force , remember to use the force "

Catchya
 
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