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Old 07-25-2010, 04:42 PM
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Ok, have some questions, little lengthy but hoping for help

OK, Heres the deal, spent the past year building a 2 bolt 400C.i. Small Block Chevy, which has turned in to a creature all its own. It is a standard bore (all i had done was, cut and hone, 4.125), standard stroke, I tossed the old 1.80/1.50 heads and replaced them with a pair of 64cc heartshaped vortec heads( yes i drilled the little steam vents in the heads also), with an aluminum 4Bbl vortec intake, and a 280 duration Elgin cam( .467 int. and .467 exh. 105 int. 115 exh. lobe centers), Gm HEI Distributor, 600 Edelbrock Performer to feed it(sounds a lil small i know but it seems to like it very well, beautiful throttle response). and the motor feels and seems Really Stout

Theres the motor, heres the issue, i timed it out with the vacum advance unhooked and the port pluged on the carb, its about 8-10 degrees base because it WONT run any more retarded than that, i hook up the vacum again and its around 12-14, is this normal? Should i run with out the vacum advance hooked up since the dist. has a mechanical advance anyways?

Also under hard pedal it pings, spark knock im guessing, but easy pedal and "sunday" driving it doesnt seem to do it, im going to be filling her with a nice fresh tank of premium to see if it helps, but other than that im at a loss.

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Old 07-25-2010, 05:06 PM
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Quote:
Originally Posted by 1_Bad_Jeep
Theres the motor, heres the issue, i timed it out with the vacum advance unhooked and the port pluged on the carb, its about 8-10 degrees base because it WONT run any more retarded than that, i hook up the vacum again and its around 12-14, is this normal? Should i run with out the vacum advance hooked up since the dist. has a mechanical advance anyways?
Run the rubber hose from the vacuum cannister to manifold vacuum, not the Fosdick ported vacuum outlet on the carb. Put 16 degrees in at the crank and 18 degrees in the centrifugal advance weights/springs/stop. This will give you the correct 34 degrees total for the very efficient combustion chamber of the L31 heads. Add whatever vacuum advance you want to run on top of the initial/centrifugal. It is not unusual to approach a total of 50 degrees including vacuum advance.

The initial and centrifugal are used for power and open throttle positions. Once you are cruising at a steady rate of speed on the freeway with a light foot on the loud pedal, the vacuum advance will add to the mix and give you increased fuel mileage.

Quote:
Originally Posted by 1_Bad_Jeep
Also under hard pedal it pings, spark knock im guessing, but easy pedal and "sunday" driving it doesnt seem to do it, im going to be filling her with a nice fresh tank of premium to see if it helps, but other than that im at a loss.
Do you have any idea what the piston crown looks like (flat, flat with eyebrows, dished, dished with eyebrows, flat surface for effecting squish, no flat surface, raised ring around the perimeter) or what the piston deck height of the motor is? (How far down in the bore is the piston at top dead center?)Did you re-use the stock 400 pistons or replace them? If you replaced them, what is the compression height of the new pistons?
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Old 07-25-2010, 05:28 PM
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With 64cc heads you're going to have pretty high compression unless you have dished pistons. I have a 350 with flat tops and 70cc heads and it's somewhere close to 10.0;1 cr so yours is probably higher. See if advancing your timing helps, if not youll need higher octane fuel.
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Old 07-25-2010, 06:05 PM
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Techinspector1, the pistons are dished with eyebrows, the stock 400 pistons and rods, machine shop guy said they were in great condition and said run it, it would be a lil loose but sometimes loose makes power, and i know he knows his stuff, been around years and my dad has taken most of his engines to him, i just re-rung it, and im not sure of the deck height, im gonna go with the factory height, of a 1976ish 400 whatever that might be, the crank is ground .010 under, not sure if that makes a diff, but it has the correct over sized bearings in the bottom to compensate
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Old 07-25-2010, 06:19 PM
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What I was getting at is....your static compression ratio is around 10.5:1 as best I can figure it without concrete dimensions from you.....which you don't seem to have. If the pistons are stock 400's with the stock piston deck height, then that's where the problem with detonation is coming from. When the motor was stock from the factory, it was probably somewhere around 8.5:1 or maybe less, so the lack of squish didn't make any difference. The motor would have run on grape juice and turpentine without detonating. But since you have raised the static compression ratio without taking squish into consideration, you have shot yourself in the foot. With the piston down in the bore probably a minimum of 0.025" and realistically more like 0.035", and probably using a conventional thickness head gasket, the piston deck height is probably somewhere in the area of 0.075". This is way too wide to generate any squish, even if you had the correct pistons in the motor.

This is not the first time I have seen a machinist do this. Apparently not all of them know about the effects of squish on a higher compression motor like yours.

You have two choices.
1. Crip the motor on reduced ignition timing and use hi-test gas.
2. Disassemble the motor and do it right with the correct pistons for the job.
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Old 07-25-2010, 06:41 PM
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what do you mean exactly by to wide to "squish" its got great compression and stops boomin the second the key turns off, (doesnt diesel or shut off like my 86 blazer's motor with 131xxx on it)
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Old 07-25-2010, 07:07 PM
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Quote:
Originally Posted by 1_Bad_Jeep
what do you mean exactly by to wide to "squish" its got great compression and stops boomin the second the key turns off, (doesnt diesel or shut off like my 86 blazer's motor with 131xxx on it)
One of the keys to being able to run decent static compression ratio with pump gas is building squish into the motor. You should google squish and read all you can about it so you can teach it to your machinist.

Basically, it is setting up a flat part of the piston crown so that it is 0.035" to 0.045" from the bottom of the cylinder head with the piston at top dead center. When the piston comes up in the bore, this flat part of the crown mates with the underside of the head to "squish" the mixture out of the dead area and over to the spark plug. This creates homogenization and turbulence which minimizes detonation.

Most fellows will use a piston such as this one (see the nice, flat area on the crown to the right of the photo) with a D-cup dish to bring the static compression ratio in at around 9.5 to 10.0 for use with today's pump gas.
http://www.kb-silvolite.com/test/kb_...tails&P_id=210
This particular piston would have provided 9.8:1 static compression ratio in your motor and with the block decked to zero (9.008" with 350 rods), would have made a squish of 0.039" with a 0.039" composition gasket.

Doesn't matter how it shuts off. I'm not talking about dieseling or "run-on" with the key off. I'm talking about detonation because you said it pings on heavy throttle. That's detonation and if you don't fix it, you are headed for disaster.

Last edited by techinspector1; 07-25-2010 at 07:13 PM.
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Old 07-25-2010, 07:43 PM
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ok, i was thinking you meant squish as the overall compression, so supposedly retarded timing and high octane will cure this as well? pulling the motor apart and replacing pistons isnt a readily available option since i dont have the pockets deep enough to do so at the moment.
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Old 07-25-2010, 07:50 PM
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Quote:
Originally Posted by 1_Bad_Jeep
ok, i was thinking you meant squish as the overall compression, so supposedly retarded timing and high octane will cure this as well? pulling the motor apart and replacing pistons isnt a readily available option since i dont have the pockets deep enough to do so at the moment.
Is it gonna be a readily available option when you hole a couple of pistons?
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Old 07-25-2010, 08:07 PM
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nope, itll sit and get stared at like it did the last time i smoked a motor in it
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