Rattle noise while driving at high rpms only

[Tomsantos@87]

Hi I’m having a issue with my 1987 mustang gt the car was converted to carb supercharged mighty demon blow through 650 carb trick flow stage one cam shorty headers x pipe flow master dumps.the car has been running pretty good since adjusting a few things. today while driving wide open throttle the car seems to have a rattle like a metal tapping noise. The thing is i can sit in the driveway and rev the engine with no noise at all. Any clues to what could be causing this noise while driving?

 

[broncojunkie]

Without hearing it, I have a couple suggestions. If you have the stock fan (or mechanical fan with a shroud), the torque on the engine may be causing the fan to barely hit the shroud. Stock-style (especially worn) motor mounts could contribute to the extra movement.

Timing chain/cover. Slack in the timing chain could cause it to hit the cover.

Engine ping (pre-detonation). Back off the timing a bit and/or use higher-octane fuel. This one is of particular concern.

 

[Tomsantos@87]

Without hearing it, I have a couple suggestions. If you have the stock fan (or mechanical fan with a shroud), the torque on the engine may be causing the fan to barely hit the shroud. Stock-style (especially worn) motor mounts could contribute to the extra movement.

Timing chain/cover. Slack in the timing chain could cause it to hit the cover.

Engine ping (pre-detonation). Back off the timing a bit and/or use higher-octane fuel. This one is of particular concern.

I will check on the other things. Would detonation happen if i where reving the car in neutral?

 

[broncojunkie]

I will check on the other things. Would detonation happen if i where reving the car in neutral?

To be honest, I'm not sure. I know it can happen either way, but I'm not sure if it's more likely to start detonating under load as opposed to no load.

Detonation is the sound of the piston impacting the pressure front of the compression stroke. This shouldn't happen until the piston has already reached its travel to the top of the piston (compression) and it's ready to start its movement back down into the bore. If the piston encounters detonation before it finishes traveling to the top of the bore, they collide with each other. The force can damage the rotating assembly.

Ignition knock or ping is similar. Detonation of the air/fuel mix happens too soon... just before the intake valve is completely closed. The pressure forces the valve closed at a higher rate, causing it to slap against its seat. That is the rattling sound you hear when this is going on. Less timing (delayed spark) or higher octane fuel helps by slowing down the burn process... making the explosion less violent.

I know spark knock is more likely to happen under load. I don't have much experience with detonation, but since the cause is similar, I would guess that it very well may show up under the same circumstances.

If you have a timing light, it's easy to adjust. If your timing is too advanced and kicking off the party before everyone gets to their seat, just back it off a little. You'll be moving the light a couple marks closer to the zero mark on your balancer. If you go the opposite way, you'll make it worse.

If your engine makes high compression and your running fuel with to low of an octane rating, just put some premium in it. If you're already using premium, get some race gas or octane booster and see if it helps.

 

[broncojunkie]

Sorry if the long -winded explanation was stuff you already knew. Just wanted to make sure we're on the same page while discussing this.

High compression engines (aftermarket heads with small chambers and/or larger pistons) are more likely to detonate due to fuel because the air/fuel mix is compressed to a point where it's easier to ignite. Heat from the compression and engine internals can ignite it prior to the spark. Therefore, adjusting your timing may not help.

There's also an in-between area of timing...advance. Timing advance is needed because there's some lag time between the actual spark and the full-ignition of the fuel mixture. So the two are related in that sense. My point is that changing only one of these may not help.

 

[Tomsantos@87]

I back off the timing seemed to stop it. I’m planning on doing heads this winter the stock ones just aren’t doing the engine much justice. Along with the heads i need a good dyno tune.

 

[broncojunkie]

I back off the timing seemed to stop it. I’m planning on doing heads this winter the stock ones just aren’t doing the engine much justice. Along with the heads i need a good dyno tune.

Good deal. A set of aluminum heads will allow more timing and higher compression. They don't retain as much heat and that keeps detonation at bay. I put a cheap set of Flotek heads on my pace car....331 stroker. They seem like they're decent heads for the money. I did upgrade the springs, since my cam was pushing the limits of the ones they came with. I also built a 408w engine using the cnc version of ProMaxx heads...210cc intake runners and 58cc chambers. They also looked like quality heads, but engine is still on the stand, so I haven't tried them yet. They were considerably more than the Floteks but still priced lower than AFR or Trickflow. I have another 351w on a stand with 1970 stock Ford heads. I may eventually decide to upgrade them to Skip White heads, since I've been hearing good things about them. They're under $1k I believe.