U guys prob Hurd this enough but I can't seem 2 get one answer, so I'll ask you guys and girls. I have a chevy 350 vortex block with chevy aluminum phase 6 heads, flat top pistons, high rise intake, roller comp cams .510intake .520exhaust, hru distributor and 650 carb. What should my initial timing be? What should my total timing be? Iv got answers like 6-8 initial, then another answer 10-14initial, Any help would be appresiated iv gotta finish this thing. Thanks again
Timing issues
- Thread starter 85cutlass
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alot of this depends on cam duration specs and lobe seperation as well...
... my 350, L98 aluminum heads, 0.495lift, 0.282dur, 0.236dur @ 0.050, 106 deg lobe sep angle... I set the total to 34 deg advance, which comes in at 3740rpm. The initial lands at 18deg @ 850rpm idle (out of gear), about 16deg @ 750rpm idle (in gear).
... shoot for 34-35 deg advance total, set it at where ever the engine stops advancing and settles at maximum. And let the initial land where it lands... it will be around 15-18deg initial.
... I just got myself a Advance Timing Light, it lets you dial in the advance, then you time your engine and watch it land on Zero deg.
I highly recommend this kinda light. But, If you don't have one of these lights, you'll have to do some math and mark out where 34-35 deg is on your balancer...
... my 350, L98 aluminum heads, 0.495lift, 0.282dur, 0.236dur @ 0.050, 106 deg lobe sep angle... I set the total to 34 deg advance, which comes in at 3740rpm. The initial lands at 18deg @ 850rpm idle (out of gear), about 16deg @ 750rpm idle (in gear).
... shoot for 34-35 deg advance total, set it at where ever the engine stops advancing and settles at maximum. And let the initial land where it lands... it will be around 15-18deg initial.
... I just got myself a Advance Timing Light, it lets you dial in the advance, then you time your engine and watch it land on Zero deg.
I highly recommend this kinda light. But, If you don't have one of these lights, you'll have to do some math and mark out where 34-35 deg is on your balancer...
The cam duration 236intake and 240exhaust at .050. I jus looked the the timing it's at 9 degrees and 45 total. The total timing is with mech. And vac. Advance. Vac. Advance is hooked up 2 ported vacuum. When I start the car sometimes it backfires through the intake. Is this timing related?
get rid of the vacuum advance... unhook it, and plug the vac advance valve. With a cam like your's, you need full advance as quickly as possible.
Your total is WAY too high, and it might explain your backfires... And I wouldn't be surprised if you're getting pre-ignition/detonation. A rowdy SBC likes between 34-36 deg of total advance... any advance past that is only benificial if you're running a stroker or a race cam.
Once you get rid of the vac advance, your advance curve will shrink too... which means you'll be getting to full advance quicker. Performance and torque will significantly improve, and HP will still be there.
Do's your harmonic balancer have timing marks past 20 deg? If not, do some math and mark out 33, 34, 35, and 36 deg on it. Or get yourself an Advance timing light.
here's the one I just picked up: http://www.equus.com/Product/Detail/1ED ... 36BC2E44BE
this is a cheaper version with manual dial: http://www.equus.com/Product/Detail/FAB ... 1355B58FD3
it's the best way to set timing accurately... dial your advance on the gun, and point at the balancer and see if it lands on Zero deg, if not adjust distributor and try again.
The RPM readout on the digital gun makes it super easy to tune by yourself... If not, you'll need a friend to read you your rpms while you adjust timing.
Rev up the motor and watch for where the timing stops advancing... then, set your full advance at that rpm. Then let the engine rev down and check where the intial timing sits, should be around 15-20deg.
experiment with which total timing you feel the most torque off the line... Experiment with where it starts to ping under load (try a couple 3rd gear launches, surge into the throttle, don't punch it off the line, try loading the engine more than normal). If it pings, go a degree or two back down.
When you're happy with what you've got there... start experimenting with bringing the full advance at an earlier rpm, say 3000rpm instead of 3500rpm. Check for pingging the same way...
hope this helps...
Your total is WAY too high, and it might explain your backfires... And I wouldn't be surprised if you're getting pre-ignition/detonation. A rowdy SBC likes between 34-36 deg of total advance... any advance past that is only benificial if you're running a stroker or a race cam.
Once you get rid of the vac advance, your advance curve will shrink too... which means you'll be getting to full advance quicker. Performance and torque will significantly improve, and HP will still be there.
Do's your harmonic balancer have timing marks past 20 deg? If not, do some math and mark out 33, 34, 35, and 36 deg on it. Or get yourself an Advance timing light.
here's the one I just picked up: http://www.equus.com/Product/Detail/1ED ... 36BC2E44BE
this is a cheaper version with manual dial: http://www.equus.com/Product/Detail/FAB ... 1355B58FD3
it's the best way to set timing accurately... dial your advance on the gun, and point at the balancer and see if it lands on Zero deg, if not adjust distributor and try again.
The RPM readout on the digital gun makes it super easy to tune by yourself... If not, you'll need a friend to read you your rpms while you adjust timing.
Rev up the motor and watch for where the timing stops advancing... then, set your full advance at that rpm. Then let the engine rev down and check where the intial timing sits, should be around 15-20deg.
experiment with which total timing you feel the most torque off the line... Experiment with where it starts to ping under load (try a couple 3rd gear launches, surge into the throttle, don't punch it off the line, try loading the engine more than normal). If it pings, go a degree or two back down.
When you're happy with what you've got there... start experimenting with bringing the full advance at an earlier rpm, say 3000rpm instead of 3500rpm. Check for pingging the same way...
hope this helps...
That much cam is going to hurt cylinder pressure at low rpm and can promote overheating if the advance is too low. I know it's popular to lose the vacuum advance and in your case probably a good idea, but people that do a lot of highway cruising can really benefit from it because most average compression performance engines will run cooler and get better mpg if they have around 50-55 degrees total timing at cruising speeds. Of course as soon as you touch the throttle the vacuum drops away and gets you back into normal timing ranges. :wink:
85cutlass said:
Typically a backfire through the carburetor means the engine is late. Your timing is too retarted. I usually set my timing at 32degrees or on higher horsepower engines i set them at 36 degrees. I set timing at 3000rpm. Their usually around 12-14 at idle. I also use vacuum advance. I hook mine to full vacuum.
ALight I was messing around with my distributor, my initial is at 9 right now vacuum advance disconnected, when I hook up the vac. Adv. The timing jumps to 33 at around. My total timing with vac. Adv. Disconnected is 25 at around 2700rpms. I think this distributor is no good for my application. Or I need to get an adj. Vac. Advance, and change out some springs for the cintrifical, I would like 36 with initial and mechanical but with the way the distributor is now that would bring my initial up2 20 and with the vac. adv. Hooked up would bring me 2 40 at idle. This distributor seems *ss backwards. what should I do? I would like to keep vac. Adv. Because I take it on the highway often. Thanks for your help.
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