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Old 02-22-2006, 12:15 PM
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Determining Total Timing Needed

Hey all,

I was wondering if anyone knew how to figure out how much total timing you need. I have a '75 chevy small block .30 over, stock iron heads (1.94 intake, 1.50 exhaust), quadrajet, headers, standard hei ignition, standard iron intake, silv-o-lite pistons i think the compression rating on them were 8.5:1. Comp cams 268h grind .454 intake and exhaust. How would i figure out how much total timing i need. Trial and error, or what? Thanks all

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Old 02-22-2006, 02:06 PM
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quoting Marlin Davis: "most small block chevy's with less than 9/1cr like 10-11 centrifugal (20-22 crankshaft) in about 2800 and 16 base with 92 octane (36-38max total)"

BUT!!!!!

"depends on the car weight, rear gears ratio, tranny, tires, altitude etc, etc, etc"

atleast it gives you a starting point....14 and 20?
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Old 02-22-2006, 04:19 PM
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Quote:
Originally Posted by zipfactor
Hey all,

I was wondering if anyone knew how to figure out how much total timing you need. I have a '75 chevy small block .30 over, stock iron heads (1.94 intake, 1.50 exhaust), quadrajet, headers, standard hei ignition, standard iron intake, silv-o-lite pistons i think the compression rating on them were 8.5:1. Comp cams 268h grind .454 intake and exhaust. How would i figure out how much total timing i need. Trial and error, or what? Thanks all
Dynometer time.
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Old 02-22-2006, 04:44 PM
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I do agree with johnsongrass.....dyno is best if there's some bucks available cause once the timings set you tune the carb for the best from both on the dyno
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Old 02-22-2006, 05:30 PM
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I use the seat of the pants dyno and my ear which works pretty well; and cheaper than dyno time

But, for the most part there are acceptable ranges of what you can do. I would say 34-38 total. Although it depends some on the combo and engine, total timing and the power it makes is remarkably similar at redline with your foot to the floor.

The rest depends on the thousands of other factors like red65mustang said, but total timing usually has a range that someone else has already figured out.

You'll need TONS of advance at idle with that much cam and that little compression... like more than 30*. That much cam with those heads will also just kill you. That cam doesn't come alive until 3000 rpms, but those heads (literally) run out of flow at about 3800. You need heads that will support 6500 with that cam.

I'm running ignition locked at 36 with a smaller cam on a big block with 8.6:1, so you have a tough road ahead. Total timing is the least of your worries at this point. You'll have much better luck finding an acceptable curve and a drivable car if you either up the compression or drop the cam duration. I strongly suggest you reconsider those heads, cam, and compression.
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Old 02-22-2006, 05:58 PM
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You have to be careful when it comes to manufacturers specs on piston compression ratio as it really depends on the combustion chamber volume.

It will take a bit of trial and error but just set things one at a time and go drive it, pay attention to any pinging and where it is depending on throttle load and rpm.

If you offer all the specs for your engine I will run it through Engine Analyzer Pro and it will spit out some numbers that should be close for the combination, It will also spit out some torque and horsepower numbers as well.

Tmod
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Old 02-22-2006, 06:47 PM
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Tmod, what kind of specs do you need? curtis: i just realized that my pistons are advertised as 9.2:1 compression, and the heads are 74cc combustion chambers. I have driven the car before, and set the initial at 12* and she seemed to run pretty good (especially for having a 30 year old quadrajet that has not been rebuilt). I rebuilt the carb and had the initial set at about 10* and it seem to run well. I was just asking because I had spoken to a gentlemen that owns a speed shop and for the life of me I could not remember what he recommended. He said he had had a similar setup.
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Old 02-22-2006, 06:48 PM
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Tmod, what kind of specs do you need? curtis: i just realized that my pistons are advertised as 9.2:1 compression, and the heads are 74cc combustion chambers. I have driven the car before, and set the initial at 12* and she seemed to run pretty good (especially for having a 30 year old quadrajet that has not been rebuilt). I rebuilt the carb and had the initial set at about 10* and it seem to run well. I was just asking because I had spoken to a gentlemen that owns a speed shop and for the life of me I could not remember what he recommended for the total timing. He said he had had a similar setup.
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Old 02-22-2006, 07:02 PM
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Bore & Stroke
Rod Length
Valve Diameters
Compression Ratio
Intake Manifold & Carb
Exhaust & Mufflers
Cam Specs (The more you tell me the better)

Tmod
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Old 02-22-2006, 07:49 PM
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bore 4.030
stroke 3.48 (i think that is stock for a 350 chevy)
rod length 5.7
valves 1.94 (intake) 1.50 (exhaust)
cr 9.2:1
stock chevy iron intake , quadrajet carb
duals all the way back with summit mufflers
cam specs .454" lift for both intake and exhaust
Duration At 0.006 Tappet Lift 268 268
Lobe Separation 110
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Old 02-23-2006, 05:17 PM
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Here is the info.

Just look at the bottom of the image for the recommended spark advance.

Tmod
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Old 02-23-2006, 09:21 PM
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Quote:
Originally Posted by zipfactor
curtis: i just realized that my pistons are advertised as 9.2:1 compression, and the heads are 74cc combustion chambers.

That 9.2 sounds perfect for that cam. I would still consider some better heads. In 1975 the most power you could get from a small block was 155, but more than likely yours is the more common 115 hp. Redline was about 3800 rpms, and trust me, they didn't make any usable power above that mark. You've swapped in a cam that really comes alive at 3000. As soon as the cam comes alive, you're out of breath. It will be like trying to run a marathon while sucking air through a soda straw. Just when your body needs more air, you switch to a restrictive breathing tube.

Just swapping on a set of iron Vortec heads from a later model small block will (and I'm not kidding in the least) easily get you an instant 80 hp from that combo. That combo with vortec heads is easily a 300 hp engine, but with your small valve heads is probably only 200-220 hp at best, and the extreme mismatch of flows will make sacrifice power, torque, and MPG all over the place.

I think you can still get brand new pairs of Vortec heads from a GM parts counter for under $700, but there are several sets on Ebay for a BuyItNow price of $250/pr.
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Old 02-24-2006, 10:54 AM
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Tmod, your analyzer results apply probably fine at the strip at WOT, it's plotting approx max timing possible without detonation (and like Marlen says "36-38* depending on the car)

on the street, with the slow slope increase in the 2-4,000 range, very probably very lazy and foot to the floor for acceleration, even a stock dist will have all the timing in by about 3200-3400rpm
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Old 02-24-2006, 01:30 PM
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I'm confused about why you guys are saying this cam (Comp 268H) comes alive at 3000 rpms?

The cam is rated from 1500-5500 rpms.

A cam that would 'come alive' at 3000rpms would be something more along the lines of the comp 292H magnum.

The 268H only has 218 of duration @ 0.050" lift...

Am I right?
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Old 02-24-2006, 04:24 PM
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Yes.

That is agood street cam that will live at 8.5 compression. Although 9.5 is even better. I've got one in my half ton I pull a enclosed race car hauler with all over the country.
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