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Old 07-06-2011, 04:19 PM
oldbogie oldbogie is offline
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Quote:
Originally Posted by z-adamson
I take it a street car with an auto trans with a low stall speed and an engine with a low powerband is not suited to locked timing? This is basically the combo I will have.

I have two choices with my ignition system:

1) Centrifugal advance with vacuum advance

2) Locked out timing with cranking retard that sets back to the locked setting above 800rpm.
A mild street engine with a low stall converter would be best with mechanical and vacuum advance systems. These balance the advance requirements across a broad range of RPMs and differing gear loadings with available pump fuels.

A locked out system that's all in by 800 RPM is likely to experience sever detonation as with typical street gears high ratios netting low RPMs against high loading on the crankshaft will make a very ping prone engine. This can be made to work by lowering the compression ratio, but that kills efficiency very quickly which will having you buying a lot of gasoline for the power produced and miles driven. Assuming the detonation doesn't bust something expensive to replace like a piston beforehand.

20 degrees of retard may prove to be too much leading to great inefficiencies as stated above. This systems are used on race cars to get them started. A proportional advance is eliminated on these as the engine is not expected to operate over a wide power band, it needs to idle and scream, no other pesky requirements. So advance systems are eliminated to reduce a possible failure point. Retard is also used with blowers and laughing gas as these things increase mixture molecular density and a peculiar thing about combustion in an engine is that the speed of the burn is proportional to mixture density. This is why advance systems peak out in a range of 2000 to 3000 RPM beyond that point the burn speed naturally increases and no further amount of advance adjustment is required. In the case of supercharging and nitrous it is common to have a situation where the burn happens so fast that it becomes necessary to pull it back to keep from getting into destructive detonation.

An engine cannot be operated with everything maximized, the best we can do is find sets of optimal solutions which consist of a selection of less than maxed out events. Some events have bigger impacts than others and example would be that an optimal path to power and efficiency would within a confined range trade some compression for more advance. Now this is a dangerous concept so don't go running with it. It's just an example that if you have an engine with a detonation problem compromising on compression ratio will have less a negative effect on power than would reducing timing.

For the most part where street engines are concerned there is a lot of data for best designs and results so I would recommend books like How to Build Max Performance Chevy Small Blocks on a Budget by David Vizard or Small-Block Chevy Engine Buildups: How to Build Horsepower for Maximum Street and Racing Performance by Chevy High Performance Magazine, and there are many others out there.


Bogie
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