SBC only getting 25* total advance. is that wrong? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 05-19-2011, 06:25 PM
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SBC only getting 25* total advance. is that wrong?

I recently built a new motor for my 1960 chev truck. Its a pretty mild 350, 9.5-1 pistons, slightly lumpy cam, edelbrock 600cfm carb.

It pings under heavy (and not so heavy) load, so i started to adjust the timing. Set it at 12* BTDC, and started to work my way back. I got all the way back to 4* BTDC and still pinged (pung?). I decided to put it back at 12, and check the overall timing. At 3500 rpm, its only at 25* BTDC (it actually reaches that at around 2500 rpm and just stays there). I'm thinking this can't be right. Everything I've read says an HEI SBC should be at about 35* total timing?

Anyone have any suggestions or insight?

Thanks!

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Old 05-19-2011, 06:29 PM
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Any chance it's the wrong balancer or timing cover/pointer? Sounds like you're way more advanced than it's showing, especially if it's detonating at 4*, or it's extremely lean. I'd start by verifying TDC and timing tab alignment.
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Old 05-19-2011, 06:40 PM
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More timing would mean that the the ping would turn to hammer!! What octane fuel are you using? Also are these iron heads? What cam are you running? Do you know what your Dcr is? You may be experiencing detonation due to to much compression. Iron heads (especially old tech iron heads) and higher than 9 to 1 comp ratio really need to have a perfect tune to function properly!

As for the timing issue, I will let the more experienced guys on here tackle that one. I refuse to hand out information that may not be true! Just muddies up the water. No sense in that!

Good luck!
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Old 05-19-2011, 06:46 PM
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Make sure your advance moves correctly. If its not you wont get the correct timing. It could also be too strong of springs in your distributor.
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Old 05-19-2011, 07:00 PM
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X2 on making sure your advance is kickin in. Springs could be too stiff or vacuum signal/vacuum advance could be bad.
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Old 05-19-2011, 07:16 PM
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Hmm. Im sure its the right balancer, ill have to check the placement of the timing pointer. Is it possible to put it in the wrong spot? I can't remember without looking at it but seems like there's only one place it can go.
I am running stock 2.02 iron heads. I'll have to look at the box the cam came in again, but its definitely more mild than wild.
im leaning towards possibly a lean condition, so ill chec fuel delivery after I double check that the distributor is advancing correctly. Any tips on how to test this?
also, just to verify, total timingshould be checked with vacuum disconnected right?
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Old 05-19-2011, 07:19 PM
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Quote:
Originally Posted by pearldrumbum
I recently built a new motor for my 1960 chev truck. Its a pretty mild 350, 9.5-1 pistons, slightly lumpy cam, edelbrock 600cfm carb.

It pings under heavy (and not so heavy) load, so i started to adjust the timing. Set it at 12* BTDC, and started to work my way back. I got all the way back to 4* BTDC and still pinged (pung?). I decided to put it back at 12, and check the overall timing. At 3500 rpm, its only at 25* BTDC (it actually reaches that at around 2500 rpm and just stays there). I'm thinking this can't be right. Everything I've read says an HEI SBC should be at about 35* total timing?

Anyone have any suggestions or insight?

Thanks!
I would agree that this isn't right. the total timing should end up around 30-40 degrees that will depend on the heads, piston configuration, overall compression, amount of squish/quench clearance, fuel ratio, operating temp, overall gearing, etc.

The fact a radical change in timing doesn't make the ping go way says something else in a major contributor.

- Lets start at 9.5 against what head, piston, gasket, block deck and or head milling?

- What engine, obviously a 1960 would not be a 350 so what is it?

- Tranny, rear gears and tire size is helpful.

- You changes the cam and carb, OMG can those be a can a worms or what? Do you know where the cam really is to the crank.

- As has been asked what about the damper. Old ones can slip the ring, new ones are year sensitive GM changed the angle between the TDC line and the key position at least a couple times and then moved the pointer around a few more times just ot keep it interesting. Changes on the front of the motor can really get you wrapped around the shaft, so to speak.

- A vacuum leak will lean out the mixture, which will make the engine ping from a little to lot depending on it's size.

- Does it have PCV, if so the valve needs to match the manifold vacuum which a bigger cam will upset so some thought has to go into that choice.

- The carb mixtures are where? I really don't expect you to know, but this needs to be thought about. Not only jet and metering rod sizing but again the cam affects manifold vacuum which acts against the pistons that move the metering rods in your Edlebrock/Carter. So the spring tension the vacuum works against under these pistons has to be calibrated to put the rod into the correct position in the jet. So there's some tuning opportunities here as well. Lean mixtures ping so the spring tension and vacuum need to be balanced, Edlebrock has tuning notes to help you. Holley has a similar tuning situation where the power valve has to be timed to the manifold vacuum. So a simple carb change just nets a different solution to the same problem.

- Operating temperature, too hot makes for pinging. This applies to coolant and intake manifold temps. Coolant should be somewhat self explanatory, but I discuss the by pass briefly, with no bypass and a thermostat there is no coolant circulation when the engine is cold, this leads to the formation of hot spots around the valves especially those paired in the middle and the spark plugs while the gauge temp can still show cold. This also puts steam into the heads above the chambers as does the cavitating water pump. It's hard to get those bubbles out once formed and since they don't do a good job of moving heat out of the combustion chamber some over heating takes place causing pre-ignition which sounds and acts a lot like detonation. This local overheat would not be sensitive to changes in timing but probably would respond to richer mixture to get that force the choke closed a bit and see how the pinging acts, this would also be a way of checking on the effects of a vacuum leak or inadequate fuel flow within the carb. A richer mixture should supress both detonation and preignition.

The intake uses enginat heat to vaporize the fuel that condenses on the bottom of the intake. An aluminum manifold moves in more heat faster so typically a gasket needs to be selected that restricts the flow of exhaust across that bottom heat passage. If the old engine has the old thermostatically controlled "heat riser valve" in the exhaust and it is stuck closed (what old one isn't?) too much heat will be forced in the the crossover and the resulting hot intake mixture will be very ping prone and to a large extent insensitive to changes in timing.

OK there's some stuff for you think about and test for a while.

Bogie
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Old 05-19-2011, 11:04 PM
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Do a compression test and tell us the results
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