1jjpop
Well-known member
How do you set timming on a small block chevrolet , with a vacuum gauge hooked to the carburetor ??I know people do it all the time...
set timming on sbc ?
- Thread starter 1jjpop
- Start date
TwoInch
Well-known member
with a timing light and the damper on the crank. base timing is set by turning the distributor. advance timing can be a number of different styles, HEI vacuum advance, etc...
AMCguy
Well-known member
The idea is to advance or retard the timing in order to observe the highest vacuum reading for the least amount of of throttle opening (curb idle).
What this means is that the engine is pulling the hardest against a closed butterfly. Or in other words making the most power for a given throttle opening.
What this means is that the engine is pulling the hardest against a closed butterfly. Or in other words making the most power for a given throttle opening.
gayler
Well-known member
Adjust base timing to highest manifold vacuum. I'd just advance it till it pings and back it off two degrees
AMCguy
Well-known member
I will ad that, usually as spark is advanced (to a point) an increase in engine speed will be observed. You will adjust the throttle opening smaller to bring the engine down to the desired RPM. This will give the strongest vacuum signal.
With the throttle plate more closed, it will also make the the engine most responsive to idle mixture screw adjustments.
With the throttle plate more closed, it will also make the the engine most responsive to idle mixture screw adjustments.
Jazz
Well-known member
To actually change the timing curve you would have to change the springs and weights inside the distributor (assuming it's a mechanical system). I'm guessing what you are really asking however is about base timing which would mean you require a timing light.
To whatever the engine dyno says is optimal
I've dynode all my racing engine and the one constant is timing has ONE SPOT that it's perfect, I've seen huge losses/gains with just 1* of timing. What happens is it may pick up the hp a little bit, but completely throws the curve out of whack so ultimately you make less USABLE power, the graphs go from a nice smooth curve all the way upto peak power to a very peaky in and out spike.
Nice thing with timing is that it is unaffected by jetting, so provided you have the carb in tune a quick way to find where you need to be, go to a drag strip-
Stage in the same exact spot
Hit all shift points the same
Be sure car engine/****** temps are consistant pass to pass, give say 45 minutes between passes to allow things to cool down for the common daily driver...as trying to go round robin will have a dramatic effect on how the car performs.
Make 2 passes where your currently set, say 36* since that's a common sbc starting point..this is your base line.
3rd pass lost 1* of timing and see if your mph picks up or drops off...if it goes faster/more mph...drop another degree of timing and repeat until it slows down again
Same sequence can be done raising the lead also...more mph equals more power, DO NOT use ET as your guide, ONLY mph.
When you stab the dizzy in for the first time, as you bring the engine around so #1 is on the compression stroke, STOP it so you can put the timing pointer on the 20* mark on the damper, then put dizzy into hole and hold it down while spinning to enguage oil pump drive. Bring it back around to #1 compression stroke and stop the damper again so the pointer is on the 20* mark of the damper...now, be sure the rotor is indexed and pointed at the #1 wire on the dizzy cap. Doing it this way will ensure a quick, sure fire start up everytime. I know a lot of guys put damper on TDC and drop dizzy and then have a buddy crank the engine while they manipulate the distributor to get it to light...lots of wear and tear for no reason, when if you do everything as I described there is no drama, just pops right off like your daily driver, then you can go from there and set your baseline at 36* and tune from there.
Granted mine were all race combo's but 5 of them ended up being set at 34*, had a few at 35*, never any higher...but engines are all different since combinations are different so there is no magic number to achieve optimal performance, it takes tinkering...and that is why I gladly paid for dyno time with each new mill since you can learn more in 1hr that you could in 3 months on track testing/changing...plus if there's a problem your not wasting money it took to get it set/hooked up in the car only to redo all that again, and possibly again until the problem is resolved.
I've dynode all my racing engine and the one constant is timing has ONE SPOT that it's perfect, I've seen huge losses/gains with just 1* of timing. What happens is it may pick up the hp a little bit, but completely throws the curve out of whack so ultimately you make less USABLE power, the graphs go from a nice smooth curve all the way upto peak power to a very peaky in and out spike.Nice thing with timing is that it is unaffected by jetting, so provided you have the carb in tune a quick way to find where you need to be, go to a drag strip-
Stage in the same exact spot
Hit all shift points the same
Be sure car engine/****** temps are consistant pass to pass, give say 45 minutes between passes to allow things to cool down for the common daily driver...as trying to go round robin will have a dramatic effect on how the car performs.
Make 2 passes where your currently set, say 36* since that's a common sbc starting point..this is your base line.
3rd pass lost 1* of timing and see if your mph picks up or drops off...if it goes faster/more mph...drop another degree of timing and repeat until it slows down again
Same sequence can be done raising the lead also...more mph equals more power, DO NOT use ET as your guide, ONLY mph.
When you stab the dizzy in for the first time, as you bring the engine around so #1 is on the compression stroke, STOP it so you can put the timing pointer on the 20* mark on the damper, then put dizzy into hole and hold it down while spinning to enguage oil pump drive. Bring it back around to #1 compression stroke and stop the damper again so the pointer is on the 20* mark of the damper...now, be sure the rotor is indexed and pointed at the #1 wire on the dizzy cap. Doing it this way will ensure a quick, sure fire start up everytime. I know a lot of guys put damper on TDC and drop dizzy and then have a buddy crank the engine while they manipulate the distributor to get it to light...lots of wear and tear for no reason, when if you do everything as I described there is no drama, just pops right off like your daily driver, then you can go from there and set your baseline at 36* and tune from there.
Granted mine were all race combo's but 5 of them ended up being set at 34*, had a few at 35*, never any higher...but engines are all different since combinations are different so there is no magic number to achieve optimal performance, it takes tinkering...and that is why I gladly paid for dyno time with each new mill since you can learn more in 1hr that you could in 3 months on track testing/changing...plus if there's a problem your not wasting money it took to get it set/hooked up in the car only to redo all that again, and possibly again until the problem is resolved.
sberry
Banned
Manual or auto but I am also for advancing until it pings and back off a touch.
Gary S
Well-known member
What vintage SB Chevy? If it is a pre-80 and pre-computer, it is easy. Just remove the vacuum advance line from the distributor and plug the line. Then set the timing to the factory specs on the sticker on the radiator cover. After that, put the vacuum advance line back on and set idle using the idle speed screw on the carb to match the sticker and you are good for another 10 or so years.
If it is a computer controlled engine, RTFM.
If it is a computer controlled engine, RTFM.
bigcountry
Member
- Joined
- Aug 9, 2009
- Messages
- 10
Most times you use a vacuum gauge to set idle mixture measuring manifold vacuum, a source below the throttle blades. If you connect to the carb above the throttle blades you are getting "ported" vacuum which is dependant on throttle opening. You set the idle mixture to the highest vacuum reading of manifold vacuum.
The best way to set the timing is with a timimg light. You can set it with a vacumm gauge but the highest reading might put you at very advanced initial timing. Which could lead to problems when the mechanical advance kicks in at higher rpms. Remember total timing is initial plus mechanical. I think IHI is talking about a "locked out" racing distributor, which is fine like he said for an all out drag car, not for a street car. He is talking about total timing not just setting the timimg in a street car. Most old SBC like 10-14 degrees initial time and a total of like he said 34-36 all in by 2500-3000 rpm. The worse the combustion chamber the more total timing it tends to need. Modern LS SBC need only about 26 or 28, Grumpy Jenkins was using something like 40-42 in the sixties in his motors.
The best way to set the timing is with a timimg light. You can set it with a vacumm gauge but the highest reading might put you at very advanced initial timing. Which could lead to problems when the mechanical advance kicks in at higher rpms. Remember total timing is initial plus mechanical. I think IHI is talking about a "locked out" racing distributor, which is fine like he said for an all out drag car, not for a street car. He is talking about total timing not just setting the timimg in a street car. Most old SBC like 10-14 degrees initial time and a total of like he said 34-36 all in by 2500-3000 rpm. The worse the combustion chamber the more total timing it tends to need. Modern LS SBC need only about 26 or 28, Grumpy Jenkins was using something like 40-42 in the sixties in his motors.