[DeTomaso] CR, Detonation, and water injection

Wed Oct 3 21:17:44 EDT 2007

Excellent....thanks Danny!

  Wow, 11:1 to 13:1 compression...thats scary ;-]>

  Dawg

Daniel C Jones <daniel.c.jones2 at gmail.com> wrote:
  Mad Dog asked me to comment on the compression discussion...

>> So is 92 really too low for good performance at those CRs?

> So, if my CR was actually over 10:1, I'm guessing it was probably going
> to detonate, under the circumstances, even if I had no oil getting into
> the combustion chambers?

Not necessarily. Many people are running 10+:1 compression ratios without
detonation. I think the oil combined with an improper spark curve is
what probably did you in.

Low compression engines can be just as close to the verge of detonation
as a higher compression ratio engine. Cam design is the key.

> Also, aluminum heads usually increase the liklihood of detonation.

I've never heard that. Conventional wisdom is the opposite is true.
The ideal gas law (PV = gRT) implies lower temperatures result in
lower cylinder pressures which means less detonation risk. Aluminum
heads are thought to run cooler, so are less likely to detonate.
The flipside is you need a bit more static compression to make the
same power.

I like to run all the compression I can safely get away with. Compression
lifts the torque curve across the RPM range. 10 ft-lbs of torque increase
across the RPM band is worth a bunch more acceleration than a 10 HP increase
at the top end of the RPM band. Compression also allows you to offset the
loss of low end torque when running a larger overlap cam. Compression also
increases fuel efficiency. I know of people running 12:1 compression on
pump gas but it takes a carefully coordinated engine package. Also, there
is a point of diminishing return. Going from 11:1 to 12:1 compression ratio
is worth a lot less than going from 8:1 to 9:1. Mad Dog runs 11.4:1 with
his aluminum headed stroker. I went with 10.75:1 on mine for a bit more
margin with bad gas or hot weather on my street car.

Vizard has published a couple of good articles on compression. A summary of
things to do to run high compression on pump gas is listed below:

1. Feed cold air to the induction
2. Keep water as cool as possible (170-180 degF)
3. Keep the air cool in the intake ports
4. Put a heat-reflective shine on the outside of the intake manifold
5. Polish the chamber side of the intake valve to reflect heat or coat
the valve with a thermal barrier
6. Minimize heat transfer through the common exhaust/intake port wall
7. Keep fuel temperatures down (cool can)
8. Run with plugs a little colder than the minimum required
9. Use an ignition system that is gross overkill
10. Utilize as large a spark plug gap as possible
11. Use no more ignition advance than is necessary
12. Maximize quench action
13. Minimize head chamber volume
14. Use flat-top or dished pistons if possible
15. Minimize under-hood exhaust heat--use coated headers
16. Do not ram in but vent out hot air through hood vents
17. Run knock sensing ignition retard
18. Inject water and alcohol mix under high loads

Compression is good but detonation is bad (MMkay). Too much timing always
feels best seat-of-the pants but what you want is the least amount of timing
that doesn't cause a power drop-off. This is known as MBST (minimum best
spark timing). If you are detonating, it will hammer the rod bearings.
If the bearings show distress the crank, rods, and pistons will all need to
be checked. Detonation is death on bearings. They'll lose their crush and
can spin. If this happens at speed, you lose a rod out the cylinder wall
and generally destroy the engine, not to mention the coolant and oil that
finds its way under the tires. If the clearance has opened up enough, there
may be an oil pressure loss but this can be masked by the pressure relief
spring. You should be able to see detonation on the spark plugs. They make
a lighted magnifying glass to check spark plugs for the little flecks of
aluminum (from the piston tops) that get transfered to the plugs under
detonation.

Noted 351C racer Animal Jim is a big fan of that system. My dad used one
on a 12.5:1 427 side-oiler stroker. He had no durability issues but he
used distilled water and alcohol. We had a still in the basement and it
did require periodic cleaning to remove the deposits.

Dan Jones
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Mad Dawg Antenucci 
Team Pantera Racing 
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