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Hi guys.

I have electronic ignition with vacum disconnected, PI cam, triple webbers on my TR6.

I have a strobe light. 

Would i be in the correct ballpark of 11° static and 33° total at 4500rpm 

Thanks guys. No need to be exact, but would this be a sensible starting point. 

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IIRC 33 all in at 3500.

However unlike the PI you can use butterfly-edge vacuum to add advance at cruise, maybe up to 10 crank degrees. If the mixure is leaned at cruise mpg should improve. If the mixture is not set lean at cruise no point in vac-advance.

In other words optimum spark timing needs AFR to be known, mixture and spark interact.

Peter

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Peter as ever is completely correct. On my supercharger setup I run a Holley carb which supports vacuum advance. It runs lean already at cruise (gives 30-35mpg at motorway speeds) so some more advance is good. I also have a problem with the setup which means I am a little lean at cruise around town which leads to the odd backfire when cold (going leaner as the fuel condenses in the cold manifold I think). Vacuum advance seems to have sorted this as well..

Tim

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45 minutes ago, Tim D. said:

going leaner as the fuel condenses in the cold manifold I think).

Tim, I think that might be "wall wetting" on the cold manifold that does lean the mixture but also leaves it enriched in the more volatile low-octane components. The end result (until the manifold and blower warm up?) is a need for more advance to compensate for slower combustion from leaning and more retard to compensate for the faster-burn of low-RON. I can see how electronic ignition can find the  sweet spot far better than mechanicals.

30-35 mpg is brilliant, and thats despite the ca 1-2hp taken by the blower (roughly a 10% penalty) Contributing to the mpg as well as spot-on AFR and sparks will be the quality of the mixture, almost all vapourised and spread evenly between cylinders. eliminating cylcle-by-cylcel variability and thereby allowing that precise tuning. Supercharging need not necessarily mean excessive consumption as you have shown. 

Peter

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2 hours ago, Peter Cobbold said:

Tim, I think that might be "wall wetting" on the cold manifold that does lean the mixture but also leaves it enriched in the more volatile low-octane components. The end result (until the manifold and blower warm up?) is a need for more advance to compensate for slower combustion from leaning and more retard to compensate for the faster-burn of low-RON. I can see how electronic ignition can find the  sweet spot far better than mechanicals.

30-35 mpg is brilliant, and thats despite the ca 1-2hp taken by the blower (roughly a 10% penalty) Contributing to the mpg as well as spot-on AFR and sparks will be the quality of the mixture, almost all vapourised and spread evenly between cylinders. eliminating cylcle-by-cylcel variability and thereby allowing that precise tuning. Supercharging need not necessarily mean excessive consumption as you have shown. 

Peter

Makes sense Peter. Remember with the eaton setup the supercharger is taken out of the circuit at cruise by a vacuum operated bypass valve. Not sure if it reduces parasitic losses. 

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31 minutes ago, Tim D. said:

Makes sense Peter. Remember with the eaton setup the supercharger is taken out of the circuit at cruise by a vacuum operated bypass valve. Not sure if it reduces parasitic losses. 

Tim, dunno for certain but Mercs used a clutched pulley on thier Eatons maybe to eliminate that loss. Peter

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Yes they used a magnetic clutch. Sadly not on the TR installation. 

Tim

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5 minutes ago, Tim D. said:

Yes they used a magnetic clutch. Sadly not on the TR installation. 

Tim

Less to go wrong on the Moss installation, I like that. What happens to mpg  if you inactivate the bypass ? Peter

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Peter,

that’s an interesting one. Only deactivated it once and not for a long run. You could feel an improvement in throttle response. I suspect that there are some interesting mixture changes that occur at the moment of transition when the blower comes back into play.

tim

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1 hour ago, Tim D. said:

Peter,

that’s an interesting one. Only deactivated it once and not for a long run. You could feel an improvement in throttle response. I suspect that there are some interesting mixture changes that occur at the moment of transition when the blower comes back into play.

tim

Tim,  My expectation is that as well as improved response the cruise mpg can be improved, as further leaning of the mixture and advancing spark will be feasible. The reason is the blower will thrash and heat fuel droplets to vapour that burns more consistently than a fuel mist. Lab engines fed by a vapour tank  run smoothly down to AFRs of 18. Peter

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Interesting.. I thought that having the supercharger running at cruise would mean a denser charge than standard so less retard..

Is this not true?

Tim

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2 hours ago, Tim D. said:

Interesting.. I thought that having the supercharger running at cruise would mean a denser charge than standard so less retard..

Is this not true?

Tim

Not true with a blower , could be true if it  were a compressor-type supercharger eg Lysholm screw, Sprintex screw, Shorrock, Judson.  Eaton, Wade, the big GMs have no inbuilt compression, hence "blower".

There is no built-in compression in the Eaton, it is a "positive displacemnt" device that generates boost by moving uncompressed mixture towards the inlet valves faster than they can use it. When the carb is throttled back at cruise the supply to the blower is restricted such that the inlet valves are starved and the boost vanishes. If anything the heat from the blower at cruise would tend to lower density and need more advance. Given the better evaporation of fuel in that heat it should be possible to lean the cruise mixture on the carb to ca 18:1 and optimise the sprak for that. Explt engines run on vapour in a tnak ( fatally so when a tank exploded, sadly) show good lean burn combustion. Its in Heywood's textbook.  A neat "green" project !!

Peter

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:-) always forget it is a "blower" absolutely right!

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