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Hey guys is 38* to much total timing for a 396 with steel heads, quadrajet (SMI), RV cam, flowmaster exhaust,? Not getting any knocking...but it's a new motor and don't want to screw it up.
396 total timing
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You're probably near max for pump gas but if it's ok if it's ok (no ping or other problems). What cr are you at and what are the cam specs?
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9.5:1, and,298/301 lift,218/224 dur,110 lobe...
However Im getting a little bit of dieseling whenI turn the engine off.
However Im getting a little bit of dieseling whenI turn the engine off.
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5,676 Posts
Too much throttle opening will make it diesel .
Low octane will do the same.. when you are on the edge.
Low octane will do the same.. when you are on the edge.
====================9.5:1, and,298/301 lift,218/224 dur,110 lobe...
However Im getting a little bit of dieseling whenI turn the engine off.
36-38 deg total is very common for pump gas motors running on 93 oct pump fuel with moderate compression.
But setting timoing for total only on street is wrong thing to do!
Why,that because doing that often leaves base timing very retarded ,just how much depends on what mech adv setup is in dist at the time.
For example,if your running a stock gm point type dist even one with a point conversion/elimination kit in it often came with 25+ deg mech adv in them .
So if you set total timing to for example 36 deg and the dist had 25 deg mech adv 36 total minus 25 deg mech adv = 11 deg base timing.
And even your mild cam needs a min of 16deg if not 18 deg for best perf so in that case base timing would be 5-7 deg retarded which is a lot and kills perf in all rpm esp comming off idle in rpm below where mech adv is fully in.
Run on/dieseling using todays non leaded lower octane ethenol'd pump fuel in old school sbc/bbc motors is generally caused by motor needing a bit more octane,running a bit too warm overheating carb & fuel with ethenol thats more sensitive to hot temps then higher octane leaded fuel of 40 yrs ago,retarded base timing making you have to set idle screw in so far to get decent idle speed prim throttle plates are out of idle trans circut with motor idling on mains,idle speed set too high,too hot of spark plug heat range to the mix can also help promote run on/dieseling too.
My bbc with mild perf cam little more agressive FT perf cam then yours & approx 9.8 comp with 20 deg base/38 deg total & 10-12 dev vac adv (proper street perf timing curve) doesnt run on at all on 93 oct pump fuel when shut down idling at 900-950rpm in 90 deg outside temp .
But it will at times detonate @ WOT on 93 fuel on hot days so if i know i will playing arround a bit i add real lead booster to increase 93 pump fuel to aapprox 98 octane leaded fuel.
And keep in mind when the 60's chevy muscle cars were sold new like for examply an l34 396/350hp 10.12 comp bbc chevelle came with sticker stating it required a min of 98 octane/Premium leaded fuel and people expect these old school motors to run just fine on a total different formula pump fuel thats lower octane,non leaded with ethenol added.
Your 9.5 comp est might be a bit higher if block and or heads have been decked/cut -resurfaced for flatness and better gasket sealing etc making the est 9.5 per piston mfg closer to 9.6-9.7 and maybe even 9.8 with a lot of decking of block & heads.
Bottom line is with the cam/comp your running in your 396 maybe now 402 if bore .030 for rbld bbc will run very well with approx 18 deg base timing + 18 deg mech adv all in by 2600rpm or so for 36-38 total with a min of 93 oct fuel and will likely need a hit of real lead booster to get closer to 97-98 octane for best perf,most power with no run on.
I have a lot of yrs exp setting up old school sbc/bbc street perf motors in moderat to farily hot perf lvl and they always do well starting with the street perf timing curve i suggested for your motor.
Adding vac adv hooked to full mint vac limited to 10-12 deg max also helps with idle,better off idle throttle responce,more power,better mpg,cooler running esp at idle in traffic ,etc.
I use crane cams vac adv limiter plate avail from summit to reduce/limit stock vac adv from its 20+ deg to 10-12 deg max.
So with 36-38 total + 10-12 deg vac adv you have roughly 46-50 deg timing when at part throttle low load cruise which is good.
Hope this helps you out.
scott
===================9.5:1, and,298/301 lift,218/224 dur,110 lobe...
However Im getting a little bit of dieseling whenI turn the engine off.
Just an FYI,we generally talk gross lift when asking for cam specs so .298 x BBC 1.7 rocker ratio = .506" gross lift on intake & 301 x 1.7 = .511" gross lift for exhuast.
So your cam is still pretty mild with only 218 deg dur/224 deg dur @.05 ,.506"(round to .510") /.511' gross lift,on a 110 deg LSA.
So add mild cam timing and your min of 9.5 comp or maybe a bit more if block/heads were decked for rbld on an old school sbc/bbc street perf motors on todays 91-93 (depending on location/state) premium unleaded pump fuel and thats a rcp that can cause run on and or detoante in right cond,thats for sure & have experienced many times over the yrs setting up/dialing these cars with old school motors on crappy pump fuel.
Thats why i run the real lead booster avail from Wild Bills corvettes online,works out to approx $5.80 per gal to inc 93 fuel to 98 octane vs having to go buy race fuel at $8-10 gal.
Scott
I run 38 deg total (initial + mech) in my 396 on 92 octane with no problems. Dist has an aggressive mech curve and vac advance limited to 10 deg. Engine has 10.0-1 CR, iron oval port heads , iron intake, Q jet, headers, and 221/233 dur cam.
This is a real good artical from Lars
How to Set Your Timing for Peak Performance -- 1 of 1
Date Published: 2001-10-01
Submitter's Name: Lars Grimsrud
Email Address: [email protected]
Corvetteforum.com Member Alias:
How to Set Your Timing for Peak Performance (Non-HEI) by Lars Grimsrud SVE Automotive Restoration Musclecar, Collector & Exotic Auto Repair & Restoration Broomfield, CO Rev. B 4-18-01 This tech paper will discuss setting the timing on a Chevy V8. This procedure also applies to other GM V8s. The procedure outlined here differs from the Service Manual, and is based on my years of experience doing this work in the quickest, least painful, most economical way while keeping the level of quality high. It is recognized that other people will have different methods of doing things, and may disagree with specific methods and procedures that I use. How to Set the Timing When you think about it, setting the timing at idle speed makes no sense at all: You don’t operate your car at idle, and timing changes as the rpm changes. Fact is, the timing spec at idle speed is provided as a simple way for most people to set the timing, and is not a good procedure for optimum performance. Small block Chevys (and most other GM performance V8 engines) perform best when the total timing (full centrifugal advance plus the initial timing setting with vacuum advance disconnected) is all in by 2,500 – 2,800 rpm and is set to 36 – 38 degrees. If you have an adjustable timing light, this is very easy to check. If you don’t, you need to scribe a 36-degree mark on your harmonic balancer. Here’s how: Measure the circumference of your harmonic balancer using a sewing tape measure (or other flexible tape measure). Get it as accurate as you can. Take this measurement and divide by 10. The number you get is the distance to 36 degrees. Measure this distance CLOCKWISE from your existing harmonic balancer timing mark and place a clear mark on the balancer. Remove your distributor cap and rotor. Remove the 2 centrifugal advance springs. Install the rotor and the cap (without the springs). Disconnect the vacuum advance. NOTE: This procedure cannot be used on the HEI ignition systems. Removal of the springs will cause an artificially over-advanced condition that will never be achieved with the springs in place. You can use the basic technique described in this paper with the HEI units (setting timing up to 36 degrees), but to check total timing, you must install a set of soft springs. You cannot remove the springs altogether. With the soft springs in place, rev the engine until the centrifugal advance is pegged out. Adjust for 36 degrees total. Then re-install your original springs. Start the engine. It may kick back a little due to the advance coming in immediately without the springs. If you’re using an adjustable timing light, set the light to 36 degrees advanced. Now rev the engine just a little while observing the timing marks with the light. It shouldn’t take much rpm to peg out the advance without the springs installed. With an adjustable light set at 36 degrees, align the stock timing marks with “0” when the timing is “pegged out.” With the non-adjustable light, align your new 36-degree mark with “0.” Rev the engine a little to make sure the timing will not advance any further. Shut it down. Pop the cap and rotor and re-install the springs. Put everything back together, but leave the vacuum disconnected. Start it up. For future reference, make a note of the timing setting at idle. This is your new curb idle timing spec. Now give the engine a few quick rev’s past 3,000 rpm and verify that the full timing (36 degrees) is coming in. If it’s not, you need to change to a softer set of springs until you get full 36-degree advance before 3000 rpm. (NOTE: A stock set of springs will usually not allow full centrifugal advance to come in before redline rpm. If you have stock springs installed, don’t rev the engine beyond its limits to try to force full advance in.) Shut it down and hook up the vacuum. Now do a road test. The 36-degree 2500 rpm advance curve is optimum for performance, but may require premium fuel. Lug the car around, and punch the throttle at low rpm while listening for detonation (“engine knock”). If you’re getting any audible knock, you MUST retard the timing. Retard the timing in 2-degree increments until engine knock stops. Engine knock will seriously damage engine components if not corrected. If you get no knock, you may see slightly improved performance at 38 degrees total timing. This is particularly true if you’re running at high altitude. If you have no engine knock under acceleration, but the car “chugs” or “jerks” at cruising speed (light throttle application), you are getting too much vacuum advance on top of the mechanical advance. You may need to change out the vacuum advance diaphragm with an adjustable unit available from aftermarket sources. Adjust these units so that you get the most vacuum advance possible without any “chugging” or “jerking” at cruise speed. Your timing is now set for best possible performance. Make note of the new setting, and use this for your future tune-up work. Questions, Comments & Technical Assistance If you have questions or comments regarding this article, or if you notice any errors that need to be corrected (which is quite possible since I’m writing this from memory…)
How to Set Your Timing for Peak Performance -- 1 of 1
Date Published: 2001-10-01
Submitter's Name: Lars Grimsrud
Email Address: [email protected]
Corvetteforum.com Member Alias:
How to Set Your Timing for Peak Performance (Non-HEI) by Lars Grimsrud SVE Automotive Restoration Musclecar, Collector & Exotic Auto Repair & Restoration Broomfield, CO Rev. B 4-18-01 This tech paper will discuss setting the timing on a Chevy V8. This procedure also applies to other GM V8s. The procedure outlined here differs from the Service Manual, and is based on my years of experience doing this work in the quickest, least painful, most economical way while keeping the level of quality high. It is recognized that other people will have different methods of doing things, and may disagree with specific methods and procedures that I use. How to Set the Timing When you think about it, setting the timing at idle speed makes no sense at all: You don’t operate your car at idle, and timing changes as the rpm changes. Fact is, the timing spec at idle speed is provided as a simple way for most people to set the timing, and is not a good procedure for optimum performance. Small block Chevys (and most other GM performance V8 engines) perform best when the total timing (full centrifugal advance plus the initial timing setting with vacuum advance disconnected) is all in by 2,500 – 2,800 rpm and is set to 36 – 38 degrees. If you have an adjustable timing light, this is very easy to check. If you don’t, you need to scribe a 36-degree mark on your harmonic balancer. Here’s how: Measure the circumference of your harmonic balancer using a sewing tape measure (or other flexible tape measure). Get it as accurate as you can. Take this measurement and divide by 10. The number you get is the distance to 36 degrees. Measure this distance CLOCKWISE from your existing harmonic balancer timing mark and place a clear mark on the balancer. Remove your distributor cap and rotor. Remove the 2 centrifugal advance springs. Install the rotor and the cap (without the springs). Disconnect the vacuum advance. NOTE: This procedure cannot be used on the HEI ignition systems. Removal of the springs will cause an artificially over-advanced condition that will never be achieved with the springs in place. You can use the basic technique described in this paper with the HEI units (setting timing up to 36 degrees), but to check total timing, you must install a set of soft springs. You cannot remove the springs altogether. With the soft springs in place, rev the engine until the centrifugal advance is pegged out. Adjust for 36 degrees total. Then re-install your original springs. Start the engine. It may kick back a little due to the advance coming in immediately without the springs. If you’re using an adjustable timing light, set the light to 36 degrees advanced. Now rev the engine just a little while observing the timing marks with the light. It shouldn’t take much rpm to peg out the advance without the springs installed. With an adjustable light set at 36 degrees, align the stock timing marks with “0” when the timing is “pegged out.” With the non-adjustable light, align your new 36-degree mark with “0.” Rev the engine a little to make sure the timing will not advance any further. Shut it down. Pop the cap and rotor and re-install the springs. Put everything back together, but leave the vacuum disconnected. Start it up. For future reference, make a note of the timing setting at idle. This is your new curb idle timing spec. Now give the engine a few quick rev’s past 3,000 rpm and verify that the full timing (36 degrees) is coming in. If it’s not, you need to change to a softer set of springs until you get full 36-degree advance before 3000 rpm. (NOTE: A stock set of springs will usually not allow full centrifugal advance to come in before redline rpm. If you have stock springs installed, don’t rev the engine beyond its limits to try to force full advance in.) Shut it down and hook up the vacuum. Now do a road test. The 36-degree 2500 rpm advance curve is optimum for performance, but may require premium fuel. Lug the car around, and punch the throttle at low rpm while listening for detonation (“engine knock”). If you’re getting any audible knock, you MUST retard the timing. Retard the timing in 2-degree increments until engine knock stops. Engine knock will seriously damage engine components if not corrected. If you get no knock, you may see slightly improved performance at 38 degrees total timing. This is particularly true if you’re running at high altitude. If you have no engine knock under acceleration, but the car “chugs” or “jerks” at cruising speed (light throttle application), you are getting too much vacuum advance on top of the mechanical advance. You may need to change out the vacuum advance diaphragm with an adjustable unit available from aftermarket sources. Adjust these units so that you get the most vacuum advance possible without any “chugging” or “jerking” at cruise speed. Your timing is now set for best possible performance. Make note of the new setting, and use this for your future tune-up work. Questions, Comments & Technical Assistance If you have questions or comments regarding this article, or if you notice any errors that need to be corrected (which is quite possible since I’m writing this from memory…)
And FWIW the shorter the mech advance curve (for more initial), the stiffer the springs will have to be for a certain "all in" rpm. When I shorten the mech curve to where it needs to be for enough initial, many times I end up using the stock springs to get it UP to around 2500 "all in" rpm. If you bring in the mech advance too soon it will start advancing below idle speed, which causes an erratic idle. Also sometimes just installing the bronze bushing that comes in the kits won't shorten the mech curve enough. Then the slot must be partially welded up.
Joined
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345 Posts
66 396 .030 over (402), oval port heads cast iron, weekend warrior for chasing down cheese burgers and chocolate shakes
Crower cam .535/.545 228/232 dur @.050 Advertised duration of 288/298
Base timing 17*- mech puts out 20* for a total of 37*
Vac advance hooked up to manifold vacuum, modified to put out 10* max so I have 47* under a no load condition cruising.
Idle is at 950 rpm which will hold the 17* base timing and the mech is all in at 2500 rpm
You cannot just take any ole distributor HEI, Points, MSD, Mallory, etc throw it in the motor and set your base timing and tell yourself everything is good.
I am right at the line in the sand on my motor with 37* total. A base timing of 17* is pushing things, but this was something I snuck up on from 15*, always correcting the mech not to exceed 37*.
My opinion is 38* is to much. For now I would go back to 36* total, all in at 2500 rpm with a base timing of 14*. Vac advance hooked up to manifold vacuum that has been limited/modified to only put out 10* .
This would be an excellent baseline now to play with. Getting the distributor setup right makes a huge difference. As in this motor is not fun to drive vs this motor is awesome to drive.
Hope it helps
Crower cam .535/.545 228/232 dur @.050 Advertised duration of 288/298
Base timing 17*- mech puts out 20* for a total of 37*
Vac advance hooked up to manifold vacuum, modified to put out 10* max so I have 47* under a no load condition cruising.
Idle is at 950 rpm which will hold the 17* base timing and the mech is all in at 2500 rpm
You cannot just take any ole distributor HEI, Points, MSD, Mallory, etc throw it in the motor and set your base timing and tell yourself everything is good.
I am right at the line in the sand on my motor with 37* total. A base timing of 17* is pushing things, but this was something I snuck up on from 15*, always correcting the mech not to exceed 37*.
My opinion is 38* is to much. For now I would go back to 36* total, all in at 2500 rpm with a base timing of 14*. Vac advance hooked up to manifold vacuum that has been limited/modified to only put out 10* .
This would be an excellent baseline now to play with. Getting the distributor setup right makes a huge difference. As in this motor is not fun to drive vs this motor is awesome to drive.
Hope it helps
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