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Old 08-08-2022, 07:42 PM
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Default Engine Stroke And Its Affect On Total Timing?

Hi,
Curious to know what the 4” stroke of my 421 wants.
Does it prefer 32,34,36 or 38 degrees total timing or does cam design and head material affect it more?

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1966 GTO

1966 421-9.3-1 comp-Race Tec 23cc Pistons
1966 Tri-Power
1967 670 Heads
Pontiac "Highlift" TriPower Cam by SpeedPro
DUR 214 Int 224 Exh @.050 - 107 ICL
LIFT .445" Int .465" Exh
Tri-Y-Headers by Tribal Tubes w/ Goerlich Mufflers
1966 Muncie Wide Ratio 4-Speed
1968-72 Chevy 12 Bolt Rear End w/ 3.73 rear gears

Last edited by SD421; 08-08-2022 at 07:50 PM. Reason: Make title make sense
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Old 08-08-2022, 07:58 PM
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Shorter stroke perfers more timing generally. About half way between what a 455 and a 400 like

Combination of parts used makes a big difference.... I've seen 455's like anywhere from 28 to 42 degrees total.

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Old 08-08-2022, 08:12 PM
Schurkey Schurkey is offline
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You won't know what timing it likes until you experiment. There's ENORMOUSLY too many other variables to predict based just on stroke length.

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Old 08-08-2022, 08:20 PM
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Start with 34* Total in at 2800 rpm.

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Old 08-08-2022, 10:40 PM
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Nowhere did I see exactly what cylinder head being used.
So as some have said, need a bunch more info.


So yeah, that alone by itself makes or could make a runner or some melted ring lands…

Smart money is start conservative ,say 30 if your using modern combustion chamber heads for instance and proceed carefully.

If using stock type iron heads , numbers will be different.

Then the rest of the stuff figures in. Compression, gearing, car weight, fuel type used etc.

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Old 08-08-2022, 11:49 PM
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Quote:
Originally Posted by PAUL K View Post
Shorter stroke perfers more timing generally. About half way between what a 455 and a 400 like

Combination of parts used makes a big difference.... I've seen 455's like anywhere from 28 to 42 degrees total.
Dear god...28 to 42..? Guess u can't be wrong with that spread...

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Old 08-09-2022, 12:42 AM
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Quote:
Originally Posted by Sun Tuned View Post
Nowhere did I see exactly what cylinder head being used.
So as some have said, need a bunch more info.


So yeah, that alone by itself makes or could make a runner or some melted ring lands…

Smart money is start conservative ,say 30 if your using modern combustion chamber heads for instance and proceed carefully.

If using stock type iron heads , numbers will be different.

Then the rest of the stuff figures in. Compression, gearing, car weight, fuel type used etc.
Sun tuned, I have the info posted in my signature.

Running 670’s on it now.
9.3 to 1 compression
3.73’s
1966 GTO

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1966 GTO

1966 421-9.3-1 comp-Race Tec 23cc Pistons
1966 Tri-Power
1967 670 Heads
Pontiac "Highlift" TriPower Cam by SpeedPro
DUR 214 Int 224 Exh @.050 - 107 ICL
LIFT .445" Int .465" Exh
Tri-Y-Headers by Tribal Tubes w/ Goerlich Mufflers
1966 Muncie Wide Ratio 4-Speed
1968-72 Chevy 12 Bolt Rear End w/ 3.73 rear gears

Last edited by SD421; 08-09-2022 at 12:44 AM. Reason: Add info
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Old 08-09-2022, 06:32 AM
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This is the kind of timing profile that was commonly used in the mid 60s on 4 bbl motors of 350 hp or more and stated to need to run on the high octane fuel of back then.

I would start off with 3 degrees less total timing then this at full throttle ( no vac advance) between 4000 and 4500 rpm.

For vacuum advance these days it seems that a max of 12 degree should be observed.

As posted above it's different for every combo, but one thing you can count on is that the longer / closer the stroke of a motor is to making it square, and or over square as in the 455 the slower you will find you need to have the mechanical advanced kick in and not detonate.

Your 3.73 gears will help a lot in that the motor will not lug as much at low rpm's until it comes on the Cam.
This should help you with getting more responce to take place by putting in more timing faster.

Those heads on 3.750" stroke motors and the the proper octane seem to do will with 40 degrees total , but take your time and find out what your combo wants.

All this being said I would invest in a add on knock senor to guide you.
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Old 08-09-2022, 09:11 AM
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Quote:
Originally Posted by Scott Roberts View Post
Dear god...28 to 42..? Guess u can't be wrong with that spread...
Didn't want to back myself into a corner

28 degrees was good on a 16:1 4.25 stroke engine with welded chambers, 42 was best for a Super Stock 455 with SD-455 heads.... The fuel used makes a difference also.

What kind of timing did the Funny car like?

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Old 08-09-2022, 02:37 PM
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Quote:
Originally Posted by PAUL K View Post
Didn't want to back myself into a corner

28 degrees was good on a 16:1 4.25 stroke engine with welded chambers, 42 was best for a Super Stock 455 with SD-455 heads.... The fuel used makes a difference also.

What kind of timing did the Funny car like?
Never see one run best at 28°....due to high compression and efficiency of the chamber or many other factors?
Funny car ran much more than u would think a blower car would.. I ran it as high as 40° with 40% overdrive..it ran hard for a couple passes..lol

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Old 08-09-2022, 05:59 PM
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Quote:
Originally Posted by Scott Roberts View Post
Never see one run best at 28°....due to high compression and efficiency of the chamber or many other factors?
Funny car ran much more than u would think a blower car would.. I ran it as high as 40° with 40% overdrive..it ran hard for a couple passes..lol

Yes that I quite a bit more than I would've guessed. What fuel?

A good chamber and high compression usually doesn't like a lot of timing.... I think most serious Hi-port builds end up running best with 28-30.

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Old 08-09-2022, 07:13 PM
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Track or dyno would answer your question with certainty

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Old 08-10-2022, 02:11 PM
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Quote:
Originally Posted by PAUL K View Post
Yes that I quite a bit more than I would've guessed. What fuel?

A good chamber and high compression usually doesn't like a lot of timing.... I think most serious Hi-port builds end up running best with 28-30.
Methanol and the occasional splash of Propylene oxide..

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