PRE IGNITION

[Anonymous]

Hi i have an XD falcon 250 Crossflow, its been worked, 30 thou over pistons its stroken out to close to a 270, 3.3 rods compression is 10.5:1. I started off with a 600 holley. that was way too big i was getting flat spots getting bogged down low.. and lots of pre ignition, it would turn off sometimes. The block was deck to 0.

I then put on a 450 holley, i have had better economy and performance out of it but still down low it boggs, but i think thats cause of my gear ratios 2.92, im going to go to 3.5s.

Anysways i still have pre ignition, the motor has been on a gas analyser its been tuned properly what can i do to stop this??? Can anyone help me?

 

[Anonymous]

Hi i have an XD falcon 250 Crossflow, its been worked, 30 thou over pistons its stroken out to close to a 270, 3.3 rods compression is 10.5:1. I started off with a 600 holley. that was way too big i was getting flat spots getting bogged down low.. and lots of pre ignition, it would turn off sometimes. The block was deck to 0.

I then put on a 450 holley, i have had better economy and performance out of it but still down low it boggs, but i think thats cause of my gear ratios 2.92, im going to go to 3.5s.

Anysways i still have pre ignition, the motor has been on a gas analyser its been tuned properly what can i do to stop this??? Can anyone help me?

I'm assuming the zero deck gives you .040" quench height with the head gasket. What octane are you running? Do your head and piston crown have lots of matching quench surface? Most older heads aren't optimized for high swirl and hence a faster burn, so even if you do have good quench, the mixture may be taking too long to burn and is detonating prematurely. Spark plug placement is also frequently placed towards the edge of the combustion chamber and often away from the exhaust valve on older heads. Over exposure to the heat of the exhaust valve can frequently cause preignition of your charge. This is why most newer heads place the spark plug towards the exhaust valve, to burn that part of the mixture first before it gets a chance to detonate.

It could be that the compression is just a little to high for the octane you're running. Fast burn heads allow you to get away with higher static compression ratios before preignition and detonation occur. If you're not getting your fuel/air mix burned quickly enough, this could be your problem.

 

[Anonymous]

Too much compression ratio with not enough fuel octane, along with too much timing advance can create the detonation scenario.

You may want to look into the water injection posts that are in the archives, or consider some ways to decrease the CR.

 

[Asa]

i'm gonna move this to the Aussie six section

 

[Anonymous]

Im using 98 octane....

 

[L6]

Anysways i still have pre ignition, the motor has been on a gas analyser its been tuned properly what can i do to stop this??? Can anyone help me?

The motor is more likely to be suffering from detonation, not pre ignition which will cripple a motor in a matter of minutes.