Tuning LPG on Chevy G20

[Dunksterp]

Hey Guys,

I'm tuning my LPG system on my '92 Chevy G20 van with a 350 TBI. I've got an Impco 425 mixer on top of the TBI unit, which is fed by an Impco Model L Vaporiser. I've also got an Ostrich 2.0 from Moates, so I can tune the spark timing table on LPG.

I've now fitted a wideband O2 sensor (that's set to LPG, not Petrol) and it's all running quite lean, idle at 18:1, full throttle at about 15.5:1. So I'm going to add some fuel at both idle and to the power circuit. Should I also add some timing into it? More fuel equals a bit more timing right?

This is my current timing table, which I get almost zero knock from now, just occasionally for 1ms going up long inclines, which I'm currently trying to tune out of it.

Any advice appreciated. Cheers

[Tom68]

Hey Guys,

I'm tuning my LPG system on my '92 Chevy G20 van with a 350 TBI. I've got an Impco 425 mixer on top of the TBI unit, which is fed by an Impco Model L Vaporiser. I've also got an Ostrich 2.0 from Moates, so I can tune the spark timing table on LPG.

I've now fitted a wideband O2 sensor (that's set to LPG, not Petrol) and it's all running quite lean, idle at 18:1, full throttle at about 15.5:1. So I'm going to add some fuel at both idle and to the power circuit. Should I also add some timing into it? More fuel equals a bit more timing right?

This is my current timing table, which I get almost zero knock from now, just occasionally for 1ms going up long inclines, which I'm currently trying to tune out of it.

Any advice appreciated. Cheers

Optimise the timing after you get your air fuel ratio where you want it.

Lean A/F can tolerate more timing, so you may already have too much.

What are seeing on the X axis of that graph, the 30 to 100 labelling ?

[storm]

When tuning for timing for LPG I usually (99% of the time) use the petrol timing curve until I have AFR/Lambda where I want it with between 1-2% correction (or <1% if possible). You will never get perfect AFR/Lambda in a data log as there are to many variables to deal with.

"More fuel equals a bit more timing right?" No, not right. Putting more wet fuel in enables you to cool the cylinder charge and by extension the EGTs which in some circumstances provides room to add more timing. LPG is a dry fuel it doesn't cool the charge down like petrol so the science of adding timing doesn't quite work, unless of course you are injecting liquid LPG.

For timing I generally do fuel as above then after you have the fuel correct adjust the timing to suit your desired outcome. FYI the lower the timing for the same result the more efficient the engine will be. To put it in a simplistic way, there are more variables but the general idea works, an engine may be able to take 50+ degrees of timing and feel great but if you are producing the same amount of torque and using the same amount of fuel at 40 degrees then there is no benefit in using more timing. You are looking for maximum, without knock, cylinder pressure at approximately 16-18 degrees ATDC so set your timing BTDC to achieve that and it will give you as close to or on MBT as is possible.