How do you control Pre-Ignition
Thread Starter
Pro
Joined: Aug 2000
Posts: 708
Likes: 21
From: Montrose CA
How do you control Detonation - Engine Knock
I know the obvious... premium gas, retard timing, keep engine temp within reason... My wonderful 383 has all these under control... still has a slight tendency to knock when mildly advanced and under hard acceleration... what have you done to go the extra mile to control detonation...
LA gas stations recently all lowered octaine rating from 92 to 91.
My ignition is currently set at 32-degrees BTDC maximum with no vacuum...
Can I brag a little... 24.5 mpg on a hundred mile road trip on Sunday. Wow.
[Modified by Carl in LA, 9:14 PM 3/25/2002]
LA gas stations recently all lowered octaine rating from 92 to 91.
My ignition is currently set at 32-degrees BTDC maximum with no vacuum...
Can I brag a little... 24.5 mpg on a hundred mile road trip on Sunday. Wow.
[Modified by Carl in LA, 9:14 PM 3/25/2002]
Burning Brakes
Joined: Nov 2001
Posts: 1,189
Likes: 2
From: Austin TX
Re: How do you control Detonation - Engine Knock (Carl in LA)
That's great gas mileage for your setup. What are you using for
fuel delivery and induction? What's your compression ratio?
Aside from the obvious things you've already mentioned, here are a couple
other factors, although they start to get much more complicated.
One is "activity" in the combustion chamber. Some newer heads have intake
ports and combustion chambers that promote a "high swirl" of the air fuel
mixture in the combustion chamber which has several benefits, including
a reduction of the chances of detonation or pre-ignition to occurr. Piston
design can have a positive or negative impact on this too.
Another is the quench area (the area between the flat side of the cylinder
head and piston that gets "squeezed" when the piston is at TDC. Too little
quench area and the piston can hit the cylinder head. Too much quench area
and you're left with "dead spots" where pre-ignition can occurr. The "ideal"
quench area for a street engine with steel rods is .035" to .040" and is
controled by machining the deck of the block, the deck of the pistion, and/or
the thickness of the head gasket.
The factory is getting better about both of these things which is why we've
seen compression ratios come back up in recent years.
I don't know there's much you can do (very easily). If you're only experiencing
"mild" pre-ig or detonation and only under very heavy throttle, I would'nt
worry about it as long as you're aware and can listen for it and back off if
it gets worse.
Sounds like you've got a good combination and you're "right on the edge".
Dan
fuel delivery and induction? What's your compression ratio?
Aside from the obvious things you've already mentioned, here are a couple
other factors, although they start to get much more complicated.
One is "activity" in the combustion chamber. Some newer heads have intake
ports and combustion chambers that promote a "high swirl" of the air fuel
mixture in the combustion chamber which has several benefits, including
a reduction of the chances of detonation or pre-ignition to occurr. Piston
design can have a positive or negative impact on this too.
Another is the quench area (the area between the flat side of the cylinder
head and piston that gets "squeezed" when the piston is at TDC. Too little
quench area and the piston can hit the cylinder head. Too much quench area
and you're left with "dead spots" where pre-ignition can occurr. The "ideal"
quench area for a street engine with steel rods is .035" to .040" and is
controled by machining the deck of the block, the deck of the pistion, and/or
the thickness of the head gasket.
The factory is getting better about both of these things which is why we've
seen compression ratios come back up in recent years.
I don't know there's much you can do (very easily). If you're only experiencing
"mild" pre-ig or detonation and only under very heavy throttle, I would'nt
worry about it as long as you're aware and can listen for it and back off if
it gets worse.
Sounds like you've got a good combination and you're "right on the edge".
Dan
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Re: How do you control Detonation - Engine Knock (Carl in LA)
If it's TPI, what year.
On the '85-'89 L98 an upgrade (see C4 Tech Tips) to the ESC module will stop false knock counts.
On the '85-'89 L98 an upgrade (see C4 Tech Tips) to the ESC module will stop false knock counts.
