[DeTomaso] CR, Detonation, and water injection

[Daniel C Jones]

Mad Dog asked me to comment on the compression discussion...

> this is what I am thinking;  I go with the CHI 3V aluminum heads with the
> 74cc 4V chambers.  It looks like these heads coupled with SCAT's 408 stroker
> kit including flat top pistons should give me 10.01:1 CR.  It may be a
> little on the conservative side but I do have other things to do besides
> swapping engines and other things I can spend that money on :-)).

It's better to have a small chamber head with a generous quench pad and a
short quench distance.  That will allow you to run a higher compression
ratio on the same octane fuel.

> Anybody out there running this combo?  I'm guessing most of the
> guys running the CHI's have the smaller chambers and more than 10:1 CR?

Actually most of the guys are running the dished pistons with the smaller
combustion chamber to get in the 10:1 range (or more).  A smaller chamber
means a shorter distance for the flame front to travel and less time for
it to detonate.  Kaase sleeved my XE block down and milled his heads down
to further reduce the bore diameter and chamber volume.  He was able to
run 13:1 on pump gas (and win the EMC in the process).

> I've been running 10.2:1 with SVO A-3 heads, an SVO hydraulic cam & Rhodes
> lifters, a tuner Holley 700 DP and 34 degrees total ignition
> advance..... since 1992. I occasionally use 89-octane but if the car will
> be flogged or for going over mountains rapidly, I use 92 octane.

The SVO hydraulic cam that Jack mentions has little overlap and the Rhoads
lifters bleed down further increasing cylinder pressure.  With more cam
overlap, a higher compression ratio could be tolerated.  Alternatively,
a lower octane fuel could be run with the same compression ratio.

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

No.  A 351C with flat top pistons and closed chamber heads will run just
fine on pump gas with a bit of cam overlap and a proper quench distance
(zero deck with a 0.040" gasket is fine).

> Couldn't hear, feel, or otherwise detect the detonation.

On a dyno, you'll notice a power drop when increasing the ignition lead.

> Does anyone know if there are easy ways to detect high RPM detonation?
> Seems like it would be a lot harder to detect than the obvious low RPM
> "ping".

Keep an eye on the spark plugs for fleck of aluminum.  Inspect under a
lighted magnifying glass.

> A later experiment on a chassis dyno (with new block and a Holley carb)
> demonstrated significant torque drop off at 5500rpm when I put the timing
> back in the more-advanced state.

You want to use the minimum best spark timing.  If power levels off at 28
degrees total and holds until 34 degrees total then drops off, you want to
use 28 degrees total.  Glen's mild 408C made best power at 28 degrees total.
Open chamber heads need more spark advance to make best power but also are
less tolerant to higher compression ratios.  Vizard's testing showed that
multi-strike CD ignitions allow the same power with a few degrees less
timing on engines with open chamber heads.

>    The 377 was supposed to be about 10:1 when new.  After crunching some
> numbers,  it looks like the CR could have been as high as 10.5:1 after
> milling the .020 off the heads.

If you were running closed chamber heads, you would need dished pistons
to get that sort of compression ratio?  Was that the case?

> 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.

> While the static compression will drop if the valves stay open longer,
> I don't think that implies the same happens at high RPM when VE may
> exceed 1 on a well designed engine.  If high RPM detonation is the
> problem (which I think Art suspected for his engine) you may be stuffing
> over 100% of the cylinder volume into the engine.

Correct.  However, if you simply shift the torque curve up the RPM band,
you shouldn't be any more prone to detonation (less actually).  Same
cylinder pressure (torque) shifted higher into the RPM range means more
power.

> More advance had been making low and mid range throttle response better.

Sounds like you needed a curve which brought the advance in earlier but
kept the total the same.  Curving a distributor in the car is a real pain
in the posterior.  MSD and others make timing computers that allow this
to be done electronically which is a bunch easier.

> The point is you can make BIG power with a durable low compression
> engine and pump gas.

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.

>    I had the same Spearco system on my 70 MACH1 and had the same durability
> issues as you did.  The aluminum jet disintegrated and the motor gave up.
> The system did as claimed though in preventing pinging.

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