DrCharles
Well-Known Member
I think my timing curve isn't quite optimal, but I'd like some recommendations. My combo: 451, 10.3:1 hand-ported iron heads, QF with BLP baseplate 950 double-pumper with 1" open spacer on Edelbrock RPM, .652/[email protected] cam, premium pump gas. 4-speed (2.66 low), 3.91 rear (3000 rpm @ 60 mph).
This cam idles at 1100-1200 rpm, 8" vacuum. I have only manifold vacuum (this carb doesn't have ported), but the vac advance can, with adjustment fully clockwise, doesn't start pulling in at idle. It is all-in at 15" which is my cruise vacuum.
Centrifugal advance is somewhat McGyvered as follows. All figures in crank degrees. I have one superlight spring, and one OEM heavy spring with loop. With the current initial timing, it cranks at 16, immediately goes to 27 at idle (the start of the heavy loop), starts to advance about 1400, and up to 35 deg at 4000 rpm. I am reluctant to get that close to the engine with a timing light at higher revs! The slots of the rotor tower aka cam plate limits the advance to 22 over initial, which is 38 max, less the inherent delay in the electronic ignition and retard unit, so should be 35 or so at high RPMs too. So basically my advance is all-in by 4000. I would like to try all-in by 3000.
Before you ask - the reason for that configuration is that 22 is the maximum initial I can run before it begins to fight the starter when hot. The idle is much better at 27 and could be even higher, but since I street drive mostly, I am wary of too much advance (such as all-in at idle, effectively locked).
Some would say to use manifold vacuum (which I have to use as noted above), and I could get a more sensitive can from FBO that would pull-in at idle, providing 35-40 deg total idle advance. But that also might be too much advance at lower rpm, part throttle operation.
The simplest solution would be a loop spring of identical dimensions except with less tension, so the advance would come in more quickly until it is limited by the slots. But those don't seem to be common. Is it possible to modify one end of the springs in the advance kits so that it has a loop? Or does the spring wire just break?
Or do I have too much low-rpm advance already? My brain hurts. It runs well and doesn't ping, and WOT operation feels good on the butt dyno, but I would like to improve the part-throttle performance below 4000 rpm as much as possible. Thanks for any assistance! @Mattax ?
This cam idles at 1100-1200 rpm, 8" vacuum. I have only manifold vacuum (this carb doesn't have ported), but the vac advance can, with adjustment fully clockwise, doesn't start pulling in at idle. It is all-in at 15" which is my cruise vacuum.
Centrifugal advance is somewhat McGyvered as follows. All figures in crank degrees. I have one superlight spring, and one OEM heavy spring with loop. With the current initial timing, it cranks at 16, immediately goes to 27 at idle (the start of the heavy loop), starts to advance about 1400, and up to 35 deg at 4000 rpm. I am reluctant to get that close to the engine with a timing light at higher revs! The slots of the rotor tower aka cam plate limits the advance to 22 over initial, which is 38 max, less the inherent delay in the electronic ignition and retard unit, so should be 35 or so at high RPMs too. So basically my advance is all-in by 4000. I would like to try all-in by 3000.
Before you ask - the reason for that configuration is that 22 is the maximum initial I can run before it begins to fight the starter when hot. The idle is much better at 27 and could be even higher, but since I street drive mostly, I am wary of too much advance (such as all-in at idle, effectively locked).
Some would say to use manifold vacuum (which I have to use as noted above), and I could get a more sensitive can from FBO that would pull-in at idle, providing 35-40 deg total idle advance. But that also might be too much advance at lower rpm, part throttle operation.
The simplest solution would be a loop spring of identical dimensions except with less tension, so the advance would come in more quickly until it is limited by the slots. But those don't seem to be common. Is it possible to modify one end of the springs in the advance kits so that it has a loop? Or does the spring wire just break?
Or do I have too much low-rpm advance already? My brain hurts. It runs well and doesn't ping, and WOT operation feels good on the butt dyno, but I would like to improve the part-throttle performance below 4000 rpm as much as possible. Thanks for any assistance! @Mattax ?
