As quench distance gets tighter, what effect does this have on initial and total timing requirements? I’m assuming less initial is needed, but what about total timing? For example...a low buck rebuilt 350 Chevy with stock rebuilder type 4vr flat tops, .045” in the hole after minor decking. .041” thick head gaskets, vortec heads, .086” quench, 9.4-1 compression, XE268 cam, etc. With 14 initial, 36 total it runs good on 89 octane, starts easily, drives good, etc. It pulls better up top with a little less total (like 32), but low end is considerably more responsive with 36 total all in by 3000 rpms. While trying to figure out what timing curve this thing wants, out of curiosity I cranked the initial timing up to 22* (44 total) and it was a completely different animal...instant tire smoke at part throttle, but also serious detonation on anything less than a 50/50 mix of 92 octane pump gas and 108 octane race gas. With the pump/race gas mix it doesn’t feel any different at 14/36 than it does with 89 octane.
If I was to use a dished piston and a .040” quench, and keep the compression about the same, would I be able to get that instant tire melting response and use pump 89 or 91 octane fuel?
Quench height and ignition timing
Moderator: Team
Re: Quench height and ignition timing
Seems poorly planned & executed rebuild.
either accept it as it is & drive it detuned
or spring for the correct height pistons & enjoy it much more.
? gallons 108 x $ = taller piston job x $
either accept it as it is & drive it detuned
or spring for the correct height pistons & enjoy it much more.
? gallons 108 x $ = taller piston job x $
Re: Quench height and ignition timing
It actually wasn’t planned this way. It was originally a very mild build with 76cc iron heads and a much smaller cam.
It is sitting in my garage to be freshened up now. It needs bored and new pistons...most everything else will remain the same as it was
Re: Quench height and ignition timing
While there are so many variables an exact definition is problematic, anything more than .050" usually isn't really "quench".stock rebuilder type 4vr flat tops, .045” in the hole after minor decking. .041” thick head gaskets, vortec heads, .086” quench,
But to answer your question, yes, best science quench/squish definitely produces more power with less total timing. As the piston almost impacts the head the rapidly reducing space between the two flat areas forces the cylinder volume being compressed violently toward the spark plug, speeds the burn rate and helps limit detonation. Good heads with tight quench today make more power with 28 degrees than old school heads did with 38 degrees.
Yes, No, Maybe - while most of what can be known about the SBC has long since been known, the weight, gearing, torque convertor, ambient air temperature, intake air temperature, carb heat, all are variables affecting a individual combination's ability to tolerate crap gas.If I was to use a dished piston and a .040” quench, and keep the compression about the same, would I be able to get that instant tire melting response and use pump 89 or 91 octane fuel?
You do know it's possible to limit the centrifugal so as to run your 22 initial and 32 total? Pretty soon, someone will be along here telling you just to lock the centrifugal and run 32 total all the time.With 14 initial, 36 total it runs good on 89 octane, starts easily, drives good, etc. It pulls better up top with a little less total (like 32), but low end is considerably more responsive with 36 total all in by 3000 rpms. While trying to figure out what timing curve this thing wants, out of curiosity I cranked the initial timing up to 22* (44 total)
jack vines
Jack Vines
Studebaker-Packard V8 Limited
Obsolete Engineering
Studebaker-Packard V8 Limited
Obsolete Engineering
Re: Quench height and ignition timing
It looks to me like you answered your own question. More initial gives it more bottom end and less total gives it more top end with no detonation. The solution is a shorter advance curve so it can have 18~20 initial and probably a gradual slope advancing from 26 to 32 from 2500 to 5000. Vortec heads don't seem to work well with more than about 32 total, if that much.
You say "vortec heads", what year or casting number. The late '80s TBI heads with the corkscrew intake port are happy with only 25 total, more than that won't work at all, knock, overheat, less power, ugh. For street use they all should have vacuum advance for more timing at cruise.
What kind of distributor are you using?
You say "vortec heads", what year or casting number. The late '80s TBI heads with the corkscrew intake port are happy with only 25 total, more than that won't work at all, knock, overheat, less power, ugh. For street use they all should have vacuum advance for more timing at cruise.
What kind of distributor are you using?
Re: Quench height and ignition timing
I know it’s possible to limit the mechanical advance, but I’m going to fix the quench first. The engine is a little tired so it’s got to be done one way or the other. Hot starting was a definite problem with 22 initial...hopefully with a tighter quench it won’t need that much anymore.
As far as locked out timing with the current setup, if we went that way...is there a way to get about a 18* start retard? I know of an MSD box that can provide 10*, but that wouldn’t help
As far as locked out timing with the current setup, if we went that way...is there a way to get about a 18* start retard? I know of an MSD box that can provide 10*, but that wouldn’t help
Re: Quench height and ignition timing
062 vortec’s, stock out of the box other than a set of comp 981 springs, locks, and retainers.Tuner wrote: ↑Sat Feb 16, 2019 5:46 pm It looks to me like you answered your own question. More initial gives it more bottom end and less total gives it more top end with no detonation. The solution is a shorter advance curve so it can have 18~20 initial and probably a gradual slope advancing from 26 to 32 from 2500 to 5000. Vortec heads don't seem to work well with more than about 32 total, if that much.
You say "vortec heads", what year or casting number. The late '80s TBI heads with the corkscrew intake port are happy with only 25 total, more than that won't work at all, knock, overheat, less power, ugh. For street use they all should have vacuum advance for more timing at cruise.
What kind of distributor are you using?
Distributor is an Accel Blueprint HEI, with 22* of mechanical advance
Re: Quench height and ignition timing
Most just use a manual switch. Get it cranking over full speed before firing the ignition.travis wrote: ↑Sat Feb 16, 2019 6:12 pm I know it’s possible to limit the mechanical advance, but I’m going to fix the quench first. The engine is a little tired so it’s got to be done one way or the other. Hot starting was a definite problem with 22 initial...hopefully with a tighter quench it won’t need that much anymore.
As far as locked out timing with the current setup, if we went that way...is there a way to get about a 18* start retard? I know of an MSD box that can provide 10*, but that wouldn’t help
jack vines
Jack Vines
Studebaker-Packard V8 Limited
Obsolete Engineering
Studebaker-Packard V8 Limited
Obsolete Engineering
Re: Quench height and ignition timing
I agree here with others that at .086" you really have no appreciable "quench"
Better quench in those fastburn Vortec 64 cc. heads will work to help control detonation with a properly curved and limited mechanical advance.
The iron 062 Vortec heads have never really benefited from larger timing as they were designed to increase the burn rate and eliminate added timing advance.
I actually calculate your old static compression at closer to 9.10 to 1 but that compression should not be a problem with 89 octane with a better use of quench and timing control.
On your new build I would use a 2 valve relief flat top piston and set your deck height about -.020" and use a .041" gasket properly sized to the new bore.
This should use about a 10.00 to 1 compression and with a total timing held at or under 32 degrees you will achieve some real "quench" benefits and with good recurved and staged timing advance you should be able to make better power and torque with a good tune. Your MSD distributor can be recurved for a better advance for your needs.
As we know the camshaft will have a lot to do with the effective compression and with valve events timing you can tune out many
problematic detonation issues.
Better quench in those fastburn Vortec 64 cc. heads will work to help control detonation with a properly curved and limited mechanical advance.
The iron 062 Vortec heads have never really benefited from larger timing as they were designed to increase the burn rate and eliminate added timing advance.
I actually calculate your old static compression at closer to 9.10 to 1 but that compression should not be a problem with 89 octane with a better use of quench and timing control.
On your new build I would use a 2 valve relief flat top piston and set your deck height about -.020" and use a .041" gasket properly sized to the new bore.
This should use about a 10.00 to 1 compression and with a total timing held at or under 32 degrees you will achieve some real "quench" benefits and with good recurved and staged timing advance you should be able to make better power and torque with a good tune. Your MSD distributor can be recurved for a better advance for your needs.
As we know the camshaft will have a lot to do with the effective compression and with valve events timing you can tune out many
problematic detonation issues.
The Older I Get, The Dumber I Get 
Re: Quench height and ignition timing
In fact that clearance is into the range which encourages detonation; pinging might well be reduced with tighter squish despite the resultant CR increase.
Felix, qui potuit rerum cognscere causas.
Happy is he who can discover the cause of things.
Happy is he who can discover the cause of things.
Re: Quench height and ignition timing
A factory vortec long block has virtually zero quench due to having a full round dish piston, yet still have typical vortec head timing requirements. 
Re: Quench height and ignition timing
I have seen Accel Blueprint HEI distributors with numerous different advance setups, some using advance curve kits (which are worthless), and some using different combinations of original GM parts, some of which are more useful than others in making the right advance curve for your application.travis wrote: ↑Sat Feb 16, 2019 6:15 pm062 vortec’s, stock out of the box other than a set of comp 981 springs, locks, and retainers.Tuner wrote: ↑Sat Feb 16, 2019 5:46 pm It looks to me like you answered your own question. More initial gives it more bottom end and less total gives it more top end with no detonation. The solution is a shorter advance curve so it can have 18~20 initial and probably a gradual slope advancing from 26 to 32 from 2500 to 5000. Vortec heads don't seem to work well with more than about 32 total, if that much.
You say "vortec heads", what year or casting number. The late '80s TBI heads with the corkscrew intake port are happy with only 25 total, more than that won't work at all, knock, overheat, less power, ugh. For street use they all should have vacuum advance for more timing at cruise.
What kind of distributor are you using?
Distributor is an Accel Blueprint HEI, with 22* of mechanical advance
As you have found by experimenting with different initial and total, your engine (the Vortec heads in general are like this) wants more initial and less total, and wants a slower curve with the advance increasing at a slower rate than the usual old-school open chamber SBC head "all in at 2500" deal, which is probably what you have from Acell.
Too much timing in the mid RPM range will not only knock, it hurts the torque too. Compared to old school head designs the Vortec heads have a faster burn rate so don't need as much advance to get the job done.
Can you post a pic of the advance mechanism? Remove the rotor and show the cam and weights and springs. Do you have any other HEI distributors for parts or any other distributor springs? Specifically, do you perhaps have the heavy springs that come in MSD distributors that are costomarilly removed and replaced with softer springs?
Do you have a dial or digital slide caliper?
About the quench distance, if you deck the block to tighten the piston to head to .040", from the .086" you describe in the OP, I would expect less trouble from detonation even with the higher compression ratio. In any case, the mechanical and vacuum advance curves need to be tailored to the engine's requirements.
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groberts101
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Re: Quench height and ignition timing
I've run and extensively modified several motors with these heads. Even the 4.3L stuff responds in similar.. even if only in 3/4 fashion. So, I will add a couple of my resultant experiences into the mix and add a few blanket statements.
Yes reduced squish heights will help most wedge chamber style motors become more octane tolerant. The lower the rpm's involved(street motors) the more pronounced the effects on detonation rsistance can become. This can be especially true of higher valve angle heads which have less squish pad located on the longside valve/chamber exit. Makes for a more open chamber on that side ultimately putting greater mixture movement requirements on the short side of the chamber. As you move towards a shallower valve angle/stood up valves with dual quench pads(greater squish pad/bore ratio) created from a smaller and more centrally located chamber.. the improvements related to tighter squish clearances are diminished much more quickly since those designs already promote a higher degree of mixture movement compared to say a 23 degree chamber. Laws of diminishing returns entirely based on the squish to bore ratio and expected piston speeds.
Yes total timing can be reduced with larger squish pad size and/or tighter squish heights but it's good to remember that combustion dynamics between lower density mixtures having slower piston speeds is MUCH different than at WOT. Considering it's a street engine I would gladly give up 5 peak to gain another 5 AVERAGE in the powerband the engine see's most often. If you use the right parts though, no loss in peak power will need to be traded off to keep peak output the same.
It's also been my general experience overall that most(maybe even all I've ever tuned) street motors respond very well to increased spark timing under and up to peak torque rpm's and generally give greater throttle response with increased acceleration capability. Maximizing idle and part throttle manifold vacuum will quickly raise cylinder pressures and denote current overall burn efficiency. Also makes the booster more responsive and allows quicker rev's into the cams sweet spot.
Unfortunately, any old mechanical design created in the last 100 years will inevitably become a futile exercise of robbing peter just to pay paul. IF.. you want this motor to become all it can be from a cylinder pressure standpoint?.. you'll need to seriously consider a programmable ignition box with MAP sensor output to mimic light load vac advance created with a vac pot. MSD's programmable 6 series box at 400 bucks seems like a lot but I can assure you that once it's dialed in you will make more power with same or lessor fuel while achieving more average power under the curve.
Going off memory here but IIRC, the advance at idle for motors similar to this one would be around 42-44 base/idle timing(to achieve max manifold vacuum levels).. ramping down to around 38-40 by 2,000 rpm.. and continually pulling another 8-10 degrees by about 3,000-4,000 rpm depending on where torque peak occurs. Those units are retard only devices once the distributor is locked out. Max retard is 25 degrees and allows built in start retard to reduce cylinder pressures enough to keep the starter moving and living long life.
IMO, the biggest consideration towards maximizing ignition timing/cylinder pressure and improving octane tolerance revolves around how quickly an engine can rev while under load. So pretty obvious that an automatic with OEM stall speed pushing 3.23 gears in a heavier street vehicle will result in quite different ignition curve than a lighter 5 speed car weighing 800 lbs less. People who would advise a standard 1 size fits all ignition curve based on the engine design alone will ALWAYS give up some power in a street car. Love those guys!.. and is how and why I've made thousands tuning them over the years. Sometimes average power gained at anything under WOT can make a street engine feel much bigger at part throttle and considerably funner to drive on the street.
Yes reduced squish heights will help most wedge chamber style motors become more octane tolerant. The lower the rpm's involved(street motors) the more pronounced the effects on detonation rsistance can become. This can be especially true of higher valve angle heads which have less squish pad located on the longside valve/chamber exit. Makes for a more open chamber on that side ultimately putting greater mixture movement requirements on the short side of the chamber. As you move towards a shallower valve angle/stood up valves with dual quench pads(greater squish pad/bore ratio) created from a smaller and more centrally located chamber.. the improvements related to tighter squish clearances are diminished much more quickly since those designs already promote a higher degree of mixture movement compared to say a 23 degree chamber. Laws of diminishing returns entirely based on the squish to bore ratio and expected piston speeds.
Yes total timing can be reduced with larger squish pad size and/or tighter squish heights but it's good to remember that combustion dynamics between lower density mixtures having slower piston speeds is MUCH different than at WOT. Considering it's a street engine I would gladly give up 5 peak to gain another 5 AVERAGE in the powerband the engine see's most often. If you use the right parts though, no loss in peak power will need to be traded off to keep peak output the same.
It's also been my general experience overall that most(maybe even all I've ever tuned) street motors respond very well to increased spark timing under and up to peak torque rpm's and generally give greater throttle response with increased acceleration capability. Maximizing idle and part throttle manifold vacuum will quickly raise cylinder pressures and denote current overall burn efficiency. Also makes the booster more responsive and allows quicker rev's into the cams sweet spot.
Unfortunately, any old mechanical design created in the last 100 years will inevitably become a futile exercise of robbing peter just to pay paul. IF.. you want this motor to become all it can be from a cylinder pressure standpoint?.. you'll need to seriously consider a programmable ignition box with MAP sensor output to mimic light load vac advance created with a vac pot. MSD's programmable 6 series box at 400 bucks seems like a lot but I can assure you that once it's dialed in you will make more power with same or lessor fuel while achieving more average power under the curve.
Going off memory here but IIRC, the advance at idle for motors similar to this one would be around 42-44 base/idle timing(to achieve max manifold vacuum levels).. ramping down to around 38-40 by 2,000 rpm.. and continually pulling another 8-10 degrees by about 3,000-4,000 rpm depending on where torque peak occurs. Those units are retard only devices once the distributor is locked out. Max retard is 25 degrees and allows built in start retard to reduce cylinder pressures enough to keep the starter moving and living long life.
IMO, the biggest consideration towards maximizing ignition timing/cylinder pressure and improving octane tolerance revolves around how quickly an engine can rev while under load. So pretty obvious that an automatic with OEM stall speed pushing 3.23 gears in a heavier street vehicle will result in quite different ignition curve than a lighter 5 speed car weighing 800 lbs less. People who would advise a standard 1 size fits all ignition curve based on the engine design alone will ALWAYS give up some power in a street car. Love those guys!.. and is how and why I've made thousands tuning them over the years. Sometimes average power gained at anything under WOT can make a street engine feel much bigger at part throttle and considerably funner to drive on the street.
Re: Quench height and ignition timing
Hopefully that pic works. This, AFAIK, is the original setup to the distributor. I had changed the springs some time back to a light and medium combo that brings in full advance by 3000 rpms. This seemed to give the best throttle response and mileage.
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Re: Quench height and ignition timing
Probably because it's both squish area and clearance among other things that determine squish velocity. Less area required less clearance to generate same velocitygroberts101 wrote: ↑Mon Feb 18, 2019 3:42 pm I've run and extensively modified several motors with these heads. Even the 4.3L stuff responds in similar.. even if only in 3/4 fashion. So, I will add a couple of my resultant experiences into the mix and add a few blanket statements.
Yes reduced squish heights will help most wedge chamber style motors become more octane tolerant. The lower the rpm's involved(street motors) the more pronounced the effects on detonation rsistance can become. This can be especially true of higher valve angle heads which have less squish pad located on the longside valve/chamber exit. Makes for a more open chamber on that side ultimately putting greater mixture movement requirements on the short side of the chamber. As you move towards a shallower valve angle/stood up valves with dual quench pads(greater squish pad/bore ratio) created from a smaller and more centrally located chamber.. the improvements related to tighter squish clearances are diminished much more quickly since those designs already promote a higher degree of mixture movement compared to say a 23 degree chamber. Laws of diminishing returns entirely based on the squish to bore ratio and expected piston speeds.