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Timing help please
To achieve tickover and driveability I have opted to advance my cam 6 degrees. The cam is a Crower 427 witi 260 intake duration and 266 exhaust duration.
Vacuum at Tickover (900 RPM) has stabilised better at 11 in/HG (it was 8-9 flickering ansd there seems to be a better vacuum at cruise. Re-set the timing to compensate for the cam advance. 18 degrees at tickover (vacuum to distributor blocked off) and 32 degrees at 3000 RPM (wanted 34 to 36 as I had fitted the black 18 degree bushing, but anyway). Problem is when I fit the vacuum to the distributor, its running 50 degrees at 3000 RPM. Obviously the total timing is being increased by the vacuum advance even after a point when I thought it should have stopped. Have I missed something obvious. I want the vacuum to bring the advance in quickly at tickover and light cruse, but I thought it should have stopped contributing at 3000 RPM. Your help would be much appreciated as I am getting to the edge of my abilities in understanding the problem. In anticipation - thanks guys Mike I have heard of people not running with vacuum - is this the reason why. |
Mikey,
I don't like vacumn adv. For a performance street motor, I just don't see any need for it since the timing requirements can be done with mechanical and intitial setting. I run 20 BTDC initial timing. and 32-34 full advance....in by 3000rpm. So I needed my MSD billet distributor to provide only 12degrees of mechanical advance. The bushings that came with the distributor allowed 18 advance (black) which was way too much. So I machined my own bushing tolimit the advance travel to 12. It turned out to be .400" diameter. I'm a strong advocate for running lots of initial timing on hi-po street motors. |
Vacuum advance should not let the timing go beyond the limit set in the distributor. Sound's to me like your timing measurment is wrong or you are about to destroy a motor. Get help, I'm an amateur mechanic.
Chet |
Gut feeling is Chet is right.
That's a bit hot. 36 degrees, maybe 38 degrees. 50 degrees is a bit hot in my opinion. What is "tickover"? |
Thats why I havent taken it off the driveway. The motor would be scrap if i drove it at that. I will probably keep it to mechanical plus initial advance and disconnect the vacuum, but i am still intrigued to know why the vacuum is still taking it further at 3000. Vacuums about 25 in/HG is that usual up there.
Thanks for the bushing size by the way I intended to run at 20 deg initial so it allows me to keep to 34 Mike |
First, make sure you're taking the vacuum off the correct port...second, under no-load conditions (just revving up the motor while sitting) you will get the maximum available vacuum advance, since the manifold vacuum will be high. Under load the manifold vacuum will be lower, maybe varying from close to zero at WOT to 12-15 inches of vacuum under cruise load, (depending on a lot of things).
Do you have an adjustable vacuum pot? you might drive it gently and see if you get any rattle or ping...as you open the throttle the vacuum drops, and the vacuum advance with it, so when accelerating you get no or very little vac advance. You just want to be sure there is no ping under any load/rpm conditions with the gas you intend to use in your car. Vac advance helps cruise mileage, but won't do anything for max power. |
Thanks for all your help guys. I woke up this morning and realised what a twit I'd been. Of course unless the engine is ubnder load or under full throttle, the vacuum will increase the timing.
I'd forgotten that previosly I tested the timing at 3000 with the vacuum disconnected. I will now go and bury my head in the sand for a few days. All the best over there (bet the weather is better than over here). Mike |
trularin,
tickover = idle Terry |
Thanks Terry. I hadn't hear that one.
Ken, do you think anyone can hear a ping on a cobra with side pipes? :D |
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