VX Ignition Advance Curve

Road-race engines and ancillaries - general discussion
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Renier RSA
Posts: 3
Joined: February 22nd, 2009, 4:46 pm

VX Ignition Advance Curve

Post by Renier RSA »

We recently started our Lancia Beta project car which also involved installing a Volumex engine and gearbox for the first time. (A 5 year project)
The engine has the following specifications:
• Crank assembly, balanced with piston and conrods weight matched
• 8,4:1 Compression ratio. Modified head (Matched and ported) with 1mm over size Inlet valves
• Inlet Cam 10.0mm lift (Effective) 270º Duration (0.05mm off seat) timed at 110º ATDC (Lobe Centre)
• Exhaust Cam 9.8mm lift 270º Duration (0.05mm off seat) timed at 114º BTDC
• DCOE 45 (09) with 32mm Venturis with start of jetting being:
55F8 Idles, 150 Mains,180 Air jets on F16 Emulsion tubes
• Marelli SM 808 PX electronic distributor’s (Exhaust Cam end-driven)
I manage to borrow a similar distributor as a guide. It turned out that the distributor we got with the engine was fitted with a wrong vacuum advance mechanism.
A loose fit to the pick-up base caused erratic timing and the advance unit was removed all together. I modified the pick-up’s base so that it can be externally adjusted and locked in place.
Both centrifugal bob weights’ springs seems to be of equal tension and length. If I advance the exciter the springs pulls it back to its stops. The springs compare well with that of the borrowed distributor.
The timing is steady through-out the test range (4500rpm)
I have mapped the distributor’s centrifugal advance curve after fixing and servicing and found the following
1000rpm - 8º BTDC - Initial as set
1500rpm - 11º
2000rpm - 16º
2500rpm - 22º
3000rpm - 28º
3500rpm - 36º which is also the maximum
I have no experience on a forced engine’s ignition curve, but compared to the normal aspirated Lancia engine’s curve the VX curve seems to be linier and not tapering off compared to the Bosch JGFU 4 which is on the natural aspirated Lancia.
I am afraid that 36º BTDC at 3500rpm might be too high and cause detonation.
Can anybody offer advice on the curve and max advance
Renier Nelspruit RSA
Attachments
Then still on the stand
Then still on the stand
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Just before the first start (15mnths later)
Just before the first start (15mnths later)
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Guy Croft
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Re: VX Ignition Advance Curve

Post by Guy Croft »

your spec worries me a bit, you have not mentioned (considered?) the octane rating of the gasoline.

The VX adv curve is the same as the atmo units so you could (I stress could..) use a distr with 34-36 deg total adv (peaking at anything from3500 to 5500) but whether - for any given ignition timing- the engine will detonate - depends on the firing pressure and octane rating. And with your setup incl non-standard cams and higher CR even than standard you may have problems. Sure you can have the distr recalibrated to reduce the advance but I definitely would not recommend simply setting it at a lower than normal (10 deg) static to achieve that.

Let me have the RON gasoline rating and I will guide you best I can,

G
Guy Croft, owner
Guy Croft
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Posts: 5039
Joined: June 18th, 2006, 9:31 am
Location: Bedford, UK
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Re: VX Ignition Advance Curve

Post by Guy Croft »

Your can timing is completely unsuitable for supercharging by the way! GC
Guy Croft, owner
Renier RSA
Posts: 3
Joined: February 22nd, 2009, 4:46 pm

Re: VX Ignition Advance Curve

Post by Renier RSA »

We have 95 octane available (Lead replacement and unleaded)
I have mapped both cams and will attach the graphs later the week

Thank you in advance for your guiding
Renier
Guy Croft
Site Admin
Posts: 5039
Joined: June 18th, 2006, 9:31 am
Location: Bedford, UK
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Re: VX Ignition Advance Curve

Post by Guy Croft »

inescapable reality is you will have to use octane additive capable of raising the RON rating to at least 97 and likely 98.Plugs will need to be NGK B9EGV.

If you do that you can run the distributor you mentioned above with 10 deg of static advance.

The use of ducted air to the carb from atmosphere will be vital.

The exhaust header needs to be a big bore (1.5" + ID) 3ft 4-1 and one large or two smaller straight thru silencers with a large colume ot bore ratio are a MUST HAVE!

You may need to come down on chole size to 30 or even 28mm. The acid test is the engine must pick-up off load when you snap the throttle open. If it does not the chokes are too big or the exhaust has too much back-pressure.

Your cam timing needs changing, blown engines don't like late inlet valve opening. Go to inlet 103 and ex 107. Same as the standard VX and as close to perfect as it comes.

Follow my lead and you won't wreck the engine. Don't follow (exactly) and all your hard work will be wasted.

GC
Guy Croft, owner
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