Timing on a 360 stroked to 373
- Thread starter mmiruzzi
- Start date
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swinger340
Well-Known Member
Is this a .070 360 (not stroked) or was crank offset ground?
Woods74
Broke Senior
Read this:
http://www.hotrodders.com/forum/chevy-305-timing-w-mods-1268957-singlepost2.html
Timing advance is mostly a function of the camshaft timing. More cam begets more ignition advance and conversely. At issue is the density of mixture in the cylinders, that is molecules of fuel and air per volume. The lower the molecular density the slower is the speed of the burn, therefore, the more initial timing is required.
Long duration high lift cams or changes that increase rocker ratio resulting in mixture lost out the exhaust on overlap or blown back into the intake by a late closing intake at low RPMs. To overcome this, the compression and the timing are increased to force more work from a less dense mixture. However, as the engine gains RPMs the mixture achieves a ram velocity to where the inertia of the incoming mixture exceeds the losses and the density within the cylinder climbs very rapidly. At this point further advance is not required as high density mixtures naturally burn faster and faster as the density increases, which means advance at Wide Open Throttle (WOT) about the range of 3000 RPM give or take, doesn't need any more advance than an ordinary engine. So most of the ignition advance problem is at idle and RPMs under about 3000 with part thorttle. The typical solution is to raise the base setting, but since the total should not be any more, however, much the base is raised has to be subtracted from the variable. So if a stock engine needs about 34 degrees total and has 8 in the base, it has 26 in the distributor cam. If the best idle and low RPM performance is with 12 degrees in the base, then the distributor cam's travel has to be reduced to 22 degrees or detonation may be encountered. This is done by filling the counterweight slot with an obstruction. This can range from welding the slot end and filing it to the needed length, to using bushings that MSD and others sell, or putting a screw in the end of the slot.
The other issue will be how fast the advance comes in and whether vacuum advance is needed. The speed of the centrifugal build up is controlled by the springs and the mass of the counterweights. There are kits out there to do this. Generally a low compression engine will tolerate more advance coming in faster. A high compression engine will like the advance to build slower. Combine these features with the camshaft. A mild cam will want a slower rate of advance, a hot cam will take more, faster. This is a place where mixture ratios and exhaust efficiency also come in. A rich mixture burns slower and will tolerate more advance. A lean mixture burns hot and fast and will not tolerate as much advance. An inefficient exhaust system increases back pressure leaving burnt product behind, this reduces burn speed so the engine will tolerate more advance higher into the top RPM ranges. An efficient exhaust pulls more mixture into the cylinder increasing its density so less advance will be required.
Vacuum advance can be used to trim the mid RPM power range. This is additive to the base, so being sure that vacuum plus base isn't too much requires the use of an adjustable advance can. Typically with a hot cam the idle vacuum is low which causes us to increase the base, but as the RPMs start to come up, the manifold vacuum may increase which will pull in advance from a vacuum system this is good with high gears that make for low RPMs to where it isn't practical to start the centrifugal advance such as with an automatic transmission. As the throttle advances more the vacuum falls and the vacuum advance drops out as the centrifugal begins to come up.
Getting all this coordinated for best power everywhere can be a real exercise. But when done right, you get the most power from the fuel you're running thru the engine.
Bogie
http://www.hotrodders.com/forum/chevy-305-timing-w-mods-1268957-singlepost2.html
Timing advance is mostly a function of the camshaft timing. More cam begets more ignition advance and conversely. At issue is the density of mixture in the cylinders, that is molecules of fuel and air per volume. The lower the molecular density the slower is the speed of the burn, therefore, the more initial timing is required.
Long duration high lift cams or changes that increase rocker ratio resulting in mixture lost out the exhaust on overlap or blown back into the intake by a late closing intake at low RPMs. To overcome this, the compression and the timing are increased to force more work from a less dense mixture. However, as the engine gains RPMs the mixture achieves a ram velocity to where the inertia of the incoming mixture exceeds the losses and the density within the cylinder climbs very rapidly. At this point further advance is not required as high density mixtures naturally burn faster and faster as the density increases, which means advance at Wide Open Throttle (WOT) about the range of 3000 RPM give or take, doesn't need any more advance than an ordinary engine. So most of the ignition advance problem is at idle and RPMs under about 3000 with part thorttle. The typical solution is to raise the base setting, but since the total should not be any more, however, much the base is raised has to be subtracted from the variable. So if a stock engine needs about 34 degrees total and has 8 in the base, it has 26 in the distributor cam. If the best idle and low RPM performance is with 12 degrees in the base, then the distributor cam's travel has to be reduced to 22 degrees or detonation may be encountered. This is done by filling the counterweight slot with an obstruction. This can range from welding the slot end and filing it to the needed length, to using bushings that MSD and others sell, or putting a screw in the end of the slot.
The other issue will be how fast the advance comes in and whether vacuum advance is needed. The speed of the centrifugal build up is controlled by the springs and the mass of the counterweights. There are kits out there to do this. Generally a low compression engine will tolerate more advance coming in faster. A high compression engine will like the advance to build slower. Combine these features with the camshaft. A mild cam will want a slower rate of advance, a hot cam will take more, faster. This is a place where mixture ratios and exhaust efficiency also come in. A rich mixture burns slower and will tolerate more advance. A lean mixture burns hot and fast and will not tolerate as much advance. An inefficient exhaust system increases back pressure leaving burnt product behind, this reduces burn speed so the engine will tolerate more advance higher into the top RPM ranges. An efficient exhaust pulls more mixture into the cylinder increasing its density so less advance will be required.
Vacuum advance can be used to trim the mid RPM power range. This is additive to the base, so being sure that vacuum plus base isn't too much requires the use of an adjustable advance can. Typically with a hot cam the idle vacuum is low which causes us to increase the base, but as the RPMs start to come up, the manifold vacuum may increase which will pull in advance from a vacuum system this is good with high gears that make for low RPMs to where it isn't practical to start the centrifugal advance such as with an automatic transmission. As the throttle advances more the vacuum falls and the vacuum advance drops out as the centrifugal begins to come up.
Getting all this coordinated for best power everywhere can be a real exercise. But when done right, you get the most power from the fuel you're running thru the engine.
Bogie
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