Ignition setting w/T56 transmission?
04-29-2019, 03:21 PM
#1
Member
Thread Starter
Ignition setting w/T56 transmission?
Hi
I would like a second opinion about some advice I got on another forum about my engine set up and the ignition adjustment.
It's not that I think the advice is wrong, but the knowledge about the T56 overdrive transmission is better here.
My setup:
Chevy small block, roller block,
Vortec heads,
Beehive springs,
1.5 ratio roller rockers.
Howards Cam 183215-14:
Valve Lift Intake: .495
Valve Lift Exhaust: .500
Duration Intake: 270
Duration Exhaust: 278
Duration at 050 Intake: 217
Duration at 050 Exhaust: 225
Lobe Separation: 114
Intake Centerline: 110
Skip White HEI distributor.
Very low crusing RPM beause of 6 speed manual trans, but that's not a problem. When stepping on the pedal, the engine pulls smooth from app 1000 rpm and up. Fuel consumtion app 26 miles/gallon. No overheating problems.
QFT DP 670 carb.
Rear axle ratio: 4,09:1
This is the advice I got, that I would like your comments on:
"The cam and DCR compression ratio dictate the base setting and the centrifugal curve and amount. This also has to be balanced with the gearing which in your case will be affected by the gearing in that there is a two ratio overdrive in there which put the engine at a significant mechanical disadvantage, read that it is operating in a range considered to be 'lugging'. This is a place where it is easy to get the engine into pre ignition and/or detonation so the general rules about a fast curve with a big cam are out the window. You're really operating in a zone that is better suited to port if not direct injection with load active, computer managed ignition control rather than carburetor combined with mechanically controlled ignition advance. The reason for that is the loading on the crankshaft and thus cylinder pressures are much more variable as to what spark timing is needed by gear selected to throttle position to manifold vacuum and RPM than with more conventional gearing especially with a manual transmission as there is no converter slip nor even modulated slip with lock up converters. This is not to say you can't find some compromise that works for all of your conditions, but does say the workable compromise is just that it will not optimize any and all conditions.
The detonation limit will determine the curve the engine will live with, to a great extent this will be a function of your driving habits, if you sit on the edge and pay constant attention to how the engine is operating and take immediate corrective action to the engine's struggles with throttle and gear changes you can run more aggressive timing with the assumption that you're on top of the situation risks. If you drive more laid back, ears on music or conversation than engine then you need to be more conservative with the ignition timing to avoid a sudden and unpleasant surprise."
Is this a well known problem? If so, how do you solve it?
I would like a second opinion about some advice I got on another forum about my engine set up and the ignition adjustment.
It's not that I think the advice is wrong, but the knowledge about the T56 overdrive transmission is better here.
My setup:
Chevy small block, roller block,
Vortec heads,
Beehive springs,
1.5 ratio roller rockers.
Howards Cam 183215-14:
Valve Lift Intake: .495
Valve Lift Exhaust: .500
Duration Intake: 270
Duration Exhaust: 278
Duration at 050 Intake: 217
Duration at 050 Exhaust: 225
Lobe Separation: 114
Intake Centerline: 110
Skip White HEI distributor.
Very low crusing RPM beause of 6 speed manual trans, but that's not a problem. When stepping on the pedal, the engine pulls smooth from app 1000 rpm and up. Fuel consumtion app 26 miles/gallon. No overheating problems.
QFT DP 670 carb.
Rear axle ratio: 4,09:1
This is the advice I got, that I would like your comments on:
"The cam and DCR compression ratio dictate the base setting and the centrifugal curve and amount. This also has to be balanced with the gearing which in your case will be affected by the gearing in that there is a two ratio overdrive in there which put the engine at a significant mechanical disadvantage, read that it is operating in a range considered to be 'lugging'. This is a place where it is easy to get the engine into pre ignition and/or detonation so the general rules about a fast curve with a big cam are out the window. You're really operating in a zone that is better suited to port if not direct injection with load active, computer managed ignition control rather than carburetor combined with mechanically controlled ignition advance. The reason for that is the loading on the crankshaft and thus cylinder pressures are much more variable as to what spark timing is needed by gear selected to throttle position to manifold vacuum and RPM than with more conventional gearing especially with a manual transmission as there is no converter slip nor even modulated slip with lock up converters. This is not to say you can't find some compromise that works for all of your conditions, but does say the workable compromise is just that it will not optimize any and all conditions.
The detonation limit will determine the curve the engine will live with, to a great extent this will be a function of your driving habits, if you sit on the edge and pay constant attention to how the engine is operating and take immediate corrective action to the engine's struggles with throttle and gear changes you can run more aggressive timing with the assumption that you're on top of the situation risks. If you drive more laid back, ears on music or conversation than engine then you need to be more conservative with the ignition timing to avoid a sudden and unpleasant surprise."
Is this a well known problem? If so, how do you solve it?
Last edited by Hotrodder; 04-30-2019 at 12:13 AM.
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