[DeTomaso] CR, Detonation, and water injection

Thu Oct 4 00:31:03 EDT 2007

Dan,

   Knowing what I think I know now,  it sounds like I should have ordered,
my new heads with small combustion chambers,  dished pistons and a zero deck
block?  I already have my forged Scat rotating assembly with flat top
pistons for the new 408 stroker.  I have ordered my CHI 3V heads with the
large 74cc chambers and the plan is to have the piston .035 in the hole to
keep the CR down to 10.0:1.  How much do you think this will hurt me from a
power and detonation resistance stand point?

Art

----- Original Message ----- 
From: "Daniel C Jones" <daniel.c.jones2 at gmail.com>
To: "DeTomaso Forum" <detomaso at realbig.com>
Sent: Wednesday, October 03, 2007 2:34 PM
Subject: Re: [DeTomaso] CR, Detonation, and water injection

> 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
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