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Old 06-24-2013, 01:46 PM
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351C Detonation Help

Hi All... First post, as I'm new to the Rod scene and found this forum.

I just picked up a 31 Ford that has a 351C bored and stroked and I'm facing some detonation problems above 3K rpms. The engine was completely built up in late 2011 according to the receipts I received when I purchased the car.

I've turned back the initial timing and even went below 10* intial with all in at 30* and have seen no change at all (it was initially set to 12* initial). Ignition is MSD electronic across the board, but I'm beginning to think this isn't all timing related.

I'm running 93 Octane, and the engine is "supposed" to have 10.5:1 compression, it has a relatively mild cam in it from Doug Hebert Performance.

Advertised Duration: 275/285
Duration @ .050: 215/225
Lift .500/.530
Lobe Center: 112c

In addition I am running CJ steel 4V open chamber heads... Hopefully some of this information is helpful, it's always difficult troubleshooting stuff like this, and I could really use some suggestions.

I understand engines, but I am certainly no expert and I can only go off the information that I received with the car at this point. I'm beginning to wonder, if the smaller cam, open chamber heads and the 10.5:1 compression all together aren't happy even with 93 octane?

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Old 06-24-2013, 02:26 PM
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you need to turn timing back until it quits pinging and then go forward with repairs/tune,,,
get some racing gas if you have to.may have a carbon problem/cooling system/too lean(vacuum leak),,,,,
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Old 06-24-2013, 02:31 PM
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Problem 1. Lack of squish to homogenize the mixture just before the plug fires.
Problem 2. Iron heads don't like much more than 9.5:1 static compression ratio. Aluminum heads will tolerate 10.5:1 SCR.
Problem 3. Cam is waaaaaay too short for 10.5:1 SCR, motor needs intake duration of 227-236 and later intake closing point to bleed off some of the cylinder pressure, which, by the way, is probably off the charts in your motor.
Bad combination of parts, not well thought-out.
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Old 06-24-2013, 03:00 PM
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Originally Posted by techinspector1 View Post
Problem 1. Lack of squish to homogenize the mixture just before the plug fires.
Problem 2. Iron heads don't like much more than 9.5:1 static compression ratio. Aluminum heads will tolerate 10.5:1 SCR.
Problem 3. Cam is waaaaaay too short for 10.5:1 SCR, motor needs intake duration of 227-236 and later intake closing point to bleed off some of the cylinder pressure, which, by the way, is probably off the charts in your motor.
Bad combination of parts, not well thought-out.
Those are some of the things I was worried about. It looks like this is going to cost me a significant amount to change heads and cam and do things the right way...
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Old 06-24-2013, 03:37 PM
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Originally Posted by cayenne View Post
Those are some of the things I was worried about. It looks like this is going to cost me a significant amount to change heads and cam and do things the right way...
It's like the signs that used to hang in speed shops when I was a kid......
"Speed costs money, how fast do you want to go?"
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Old 06-24-2013, 04:30 PM
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Understand that one completely techinspector... I've got a 500 RWHP Mustang sitting right next to her that was done the right way, and it cost big time :-) I just hate inheriting other people's short cuts!

I'm going to go grab a brew and try one more thing. I wanted to change the spring on the MSD distributor and then try a 50/50 blend of 100 OCT to see if it gets better.
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Old 06-24-2013, 06:06 PM
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So a little bit of an update. I changed out one of the distributor springs to a light silver one and one of the heavy silver ones. This should bring the mechanical advance in sooner. After a test drive there were some positive results. The detonation was still there but probably half what it was.

I dropped the spring tension again to the next step which is two blue springs and the power band seemed to drop off and the detonation was worse.

After going back to only one lighter spring, I mixed in a 50/50 mix of 100 octane and 93 octane. This got rid of the detonation. I guess this is only realistically a band aid at best but it is progress
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Old 06-24-2013, 07:30 PM
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There is a fuel somewhere that is compatible with most any internal combustion engine. The trick is having it readily accessible and reasonably priced.
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Old 06-25-2013, 10:37 AM
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Quote:
Originally Posted by cayenne View Post
Hi All... First post, as I'm new to the Rod scene and found this forum.

I just picked up a 31 Ford that has a 351C bored and stroked and I'm facing some detonation problems above 3K rpms. The engine was completely built up in late 2011 according to the receipts I received when I purchased the car.

I've turned back the initial timing and even went below 10* intial with all in at 30* and have seen no change at all (it was initially set to 12* initial). Ignition is MSD electronic across the board, but I'm beginning to think this isn't all timing related.

I'm running 93 Octane, and the engine is "supposed" to have 10.5:1 compression, it has a relatively mild cam in it from Doug Hebert Performance.

Advertised Duration: 275/285
Duration @ .050: 215/225
Lift .500/.530
Lobe Center: 112c

In addition I am running CJ steel 4V open chamber heads... Hopefully some of this information is helpful, it's always difficult troubleshooting stuff like this, and I could really use some suggestions.

I understand engines, but I am certainly no expert and I can only go off the information that I received with the car at this point. I'm beginning to wonder, if the smaller cam, open chamber heads and the 10.5:1 compression all together aren't happy even with 93 octane?
You said the detonation comes in above 3000 RPM, is that in all gears or just cruising in whatever high gear is?

Does this use vacuum advance as well as mechanical?

Do you know how much total advance it has? That would be the addiition of base, mechanical and at certain conditions where the manifold vacuum is high that would pull in the vacuum advance if so equpped. The fairly wide LSA of 112 degrees is going to pull a pretty high manifold vacuum. You can easily get into a situation at cruise where there is way too much advance being put in by a fast mechanical, and the vacuum.

A problem with pinging can be seperating real detonation from preignition. They sound the same but have different causes. Both are sensitve in intake air temp not just what's going in from the underhood atmosphere but also from exhaust crossover heating of the intake if so equipped. Mixture ratio has a big impact, with a high vacuum that doesn't pull in the power valve soon enough, if the main metering is lean you'll get pinging. Engine operating temp is also a sensitive issue, making it run cooler can be helpful. The spark plug heat range can also be an issue especially where preignition is happening, using a colder heat range can help. Deposits in the chamber can also result in preignition, a top end cleaner used before changing spark plugs could help if deposit build up is a problem. Check the plugs if they are very clean to white on the insulator it tells you the chambers are running hot. Iron heads with a lot of compression will want a colder heat range plug simply because iron doesn't take heat out of the chamber as fast as does aluminum. An old trick for plugs is to trim the ground electrode so it abutts the center electrode rather than over-hangs it. This lessens the distance heat has to travel to get out of this electrode cooling it off. Check the total advance carefully too little is as bad as too much, either leads to detonation and preignition issues.

Looking at some of these less invasive changes could solve the problem without having to replace heavy parts or use av or racing gas to make-up some witches brew.

Bogie
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Old 06-25-2013, 10:54 AM
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Originally Posted by oldbogie View Post
You said the detonation comes in above 3000 RPM, is that in all gears or just cruising in whatever high gear is?

Does this use vacuum advance as well as mechanical?

Do you know how much total advance it has? That would be the addiition of base, mechanical and at certain conditions where the manifold vacuum is high that would pull in the vacuum advance if so equpped. The fairly wide LSA of 112 degrees is going to pull a pretty high manifold vacuum. You can easily get into a situation at cruise where there is way too much advance being put in by a fast mechanical, and the vacuum.

A problem with pinging can be seperating real detonation from preignition. They sound the same but have different causes. Both are sensitve in intake air temp not just what's going in from the underhood atmosphere but also from exhaust crossover heating of the intake if so equipped. Mixture ratio has a big impact, with a high vacuum that doesn't pull in the power valve soon enough, if the main metering is lean you'll get pinging. Engine operating temp is also a sensitive issue, making it run cooler can be helpful. The spark plug heat range can also be an issue especially where preignition is happening, using a colder heat range can help. Deposits in the chamber can also result in preignition, a top end cleaner used before changing spark plugs could help if deposit build up is a problem. Check the plugs if they are very clean to white on the insulator it tells you the chambers are running hot. Iron heads with a lot of compression will want a colder heat range plug simply because iron doesn't take heat out of the chamber as fast as does aluminum. An old trick for plugs is to trim the ground electrode so it abutts the center electrode rather than over-hangs it. This lessens the distance heat has to travel to get out of this electrode cooling it off. Check the total advance carefully too little is as bad as too much, either leads to detonation and preignition issues.

Looking at some of these less invasive changes could solve the problem without having to replace heavy parts or use av or racing gas to make-up some witches brew.

Bogie
Bogie,

Thanks for all the feedback, I'll try to answer as best I can, and I certainly agree running her without old witches brew would be nice!!!

The detonation seems to come in when you are getting into the accelerator regardless of the gear. It's an auto as well, but at any time I'm getting into it the detonation comes in.

Total advance has been dialed back to 30* at the moment, 9* Initial + 21 mechanical. The higher the timing the worse the detonation. There is no vacuum advance equiped.

The engine runs cool, even at hard runs she will stay under 200*F... It's equipped with a 55GPM Electric water pump, 2 thermostat controlled fans, and 2 aux fan if needed.

I am running AC Delco R34TS plugs which are already one step colder. These were already installed in the motor, I haven't changed them. I pulled them yesterday and they were not white/clean as I would expect if the chambers were running very hot. Do you think it warrants trying a step colder plug?

I know my total advance it too little at the moment, I can bring it back up by changing the MSD advanced stop bushing to something smaller. If I'm running too little timing, how does this result detonation or pre-ignition?

THANKS! I'm certainly learning things as I go.
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Old 06-26-2013, 04:17 PM
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Originally Posted by cayenne View Post
Bogie,

Thanks for all the feedback, I'll try to answer as best I can, and I certainly agree running her without old witches brew would be nice!!!

The detonation seems to come in when you are getting into the accelerator regardless of the gear. It's an auto as well, but at any time I'm getting into it the detonation comes in.

Total advance has been dialed back to 30* at the moment, 9* Initial + 21 mechanical. The higher the timing the worse the detonation. There is no vacuum advance equiped.

The engine runs cool, even at hard runs she will stay under 200*F... It's equipped with a 55GPM Electric water pump, 2 thermostat controlled fans, and 2 aux fan if needed.

I am running AC Delco R34TS plugs which are already one step colder. These were already installed in the motor, I haven't changed them. I pulled them yesterday and they were not white/clean as I would expect if the chambers were running very hot. Do you think it warrants trying a step colder plug?

I know my total advance it too little at the moment, I can bring it back up by changing the MSD advanced stop bushing to something smaller. If I'm running too little timing, how does this result detonation or pre-ignition?

THANKS! I'm certainly learning things as I go.
All you've said makes this sound like a compresson ratio problem. I rather think the heads need to come off at this point so you can make measurements of the volumes to determine which way to go. If it isn't too high a simple change to the same volume chamber in an aluminum head may solve the problem as aluminum is usually good for a ratio higher than iron. If it's just way high then a largeer chambered head may have to be used to get the ratio down without increasing the squish/quench clearance which would itself take this in the wrong direction for anti-detonation.

Bogie
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