View Single Post
Old 08-06-2009, 05:00 PM   #14
Lil bastard
Registered User
Lil bastard's Avatar
Join Date: Feb 2008
Location: Du Monde
Posts: 2,199
Originally Posted by turbo23dog
I don't support or deny what the mechanic said, but one thing I'd like to add is that increase in temp of the intake air will raise the threshold for engine detonation. Turbocharged engines normally have an intercooler set up to aid in cooling intake air to make more power which is related to reducing odds of detonation.

Most newer cars have knock sensors that automatically adjust engine management when the knock sensor is triggered that would prevent dammage.[/QUOTE

Detonation is caused by crap gas in high compression, turbo and supercharged engines. It has NOTHING to do with intake air temp. The intercooler in a turbocharged engine is NOT related to reducing the odds of detonation.

Uh... anything which causes the A/F charge to ignite prematurely is detonation!

Too high a CR for the fuel used, too high a charge temperature (intake temp or fuel temp), too advanced a spark, too low an octane fuel for the CR, too little engine cooling, improper mechanical timing, the wrong heat range spark plug, carbon deposit hot spots in the cylinder, or the cumulative effect of any combination of the above.

And, it doesn't only affect force aspirated engines either (turbo, s/c), today's naturally aspirated engines are just as prone to it.

Modern engine management software will advance the spark to just shy of the detonation threshold to achieve maximum efficiency. As such, it doesn't take too much of an adverse condition to throw it beyond that threshold.

Once the knock sensor senses the increase in vibration, which it attributes to knock, it increases it's base signal to the ECU which then begins to retard the spark either to a pre-programmed level, or it will advance the spark until detonation is achieved and then retard it back slightly from that mark.

Because of this, naturally aspirated engines are more sensitive than ever to such things as the intake temperature.

1990 Porsche 964 Carrera 4 Cabriolet
1976 BMW 2002
1990 BMW 325is
1999 Porsche Boxster
(gone, but not forgotten)

Never drive faster than your Guardian Angel can fly!

Last edited by Lil bastard; 08-06-2009 at 08:11 PM.
Lil bastard is offline   Reply With Quote