[DeTomaso] heads

[Daniel C Jones]

> Since I new to the 351Cleveland engine I got some questions.
>
> What the difference between
>
> a)       4V closed Chamber
> b)       4V open chamber
> c)       2V
> d)       Aussie.

Closed chamber heads will allow higher compression on the same octane fuel.
2V heads have smaller ports and valves and flow less than 4V heads at the
same lifts.  However, the cylinder heads work in conjunction with the intake
manifold.  It's the flow of the pairing that is important.  A good single
plane intake on a 2V head may flow nearly as well as a poor dual plane on
a 4V head.  When we flow tested un ported dual plane intakes (Ford aluminum
and Blue Thunder) on 4V heads, we lost 70+ CFM on the worst runners.  Porting
the intakes got much of that loss back and picked up a lot of HP on the dyno.
Likewise, on the exhaust side, it doesn't matter if your exhaust port flows
240 CFM @ 0.6" lift if you try to push that through a restrictive stock
muffler.  We lost 50 HP through the GTS Pantera mufflers but gained it back
with 3" inlet/outlet Magnaflows.

> I see the difference between the open and closed chamber, but what is
> the real difference in performance, torque  and revs.

Compression is good.  It lifts the entire torque curve.  If you have
the octane available to run flat top pistons and closed chamber heads,
then do it.  On an optimized engine, the fuel economy increase will
roughly offset the premium fuel cost difference.

> The 4V heads have HUGE intake and exhaust ports.

True.  The best fix for 4V heads is adding cubic inches via a stroker kit.

> However, reportedly the Aussie heads are flow-limited

Iron 2V heads (Aussie closed or US open chamber) heads have a poor short
side radius.  We'll be doing some porting to see if we can fix that.

> The problem with that, of course, is that your stock bottom end (crankshaft,
> connecting rods, pistons) will explode at 7000 rpm, so what good is that? :<(

Even under 6500 RPM, a 4V head will beat an unported 2V, assuming everything
else is right.  The biggest qualitative difference in my experience is 4V
heads (on an engine with some cam) will be a bit lazy under maybe 3000 RPM.
After that they come on like someone flipped a switch.  The 2V's are smoother
but lack the outright power.  Extra cubic inches will tame that characteristic
of the 4V heads.

Still it's all about the combination, not any one particular part.
You can make a very healthy 351C combination around 2V or 4V heads.

> And what heads should I prefer to use?

That depends upon how you intend to use the car, what displacement it
will be, whether or not you are willing to use a taller intake manifold,
etc.

> Does the intakes fit to them all or is the difference between them also?

There are matching intakes for 2V, 3V, 4V and high port heads.
Liekwise the headers differ among these heads, though you can use 4V
headers on 2V heads.

> I get the stock intake and was not very impressed about it. Think I buy
> some with a better flow rate.  (Any ideas)

The usual Edelbrock Performer 4V intake is not really any better than the
Ford intake.  The high rise Blue Thunder was also a disappointment on the
flow bench and on the dyno.  Porting these intakes to bring the 4 poor
flowing runners up to match the better flowing runners made a big improvement
(on the order of 40 HP on the dyno).

Are you aware of the email/forum 351C dyno program?  We'll be testing a
wide variety of 2V, 3V, 4V and high port intake manifolds on a 351C
over the next couple of months.

> Unless you make a pretty radical engine, a 750 will probably be too big.
> Although it might deliver high horsepower at high rpm, it will probably be
> sort of 'fat' and 'fluffy' at lower rpm, and actually deliver lower
> performance (while using more fuel).

It doesn't have to be that way but the typical 3310 750 will be that
way.  With a decent booster design and tuning, it's possible to get
good throttle response and fuel economy out of larger CFM carbs.
Regardless of carb CFM, for best fuel economy, you want to run lean of
stoichiometric at cruise and rich of stoichiometric at wide open throttle
(WOT) for power.  14.7:1 is the stoichiometric ratio.  It's the "chemically
ideal" ratio where there is no excess fuel or oxygen left after combustion.
Leaner means there's excess oxygen left after combustion.  Richer means
there's excess fuel left.  Rich of stoichiometric at wide open throttle will
make better power and lean at cruise will yield better fuel economy.  There's
no single ideal ratio that applies to all engines.  Some engines make best
power at 13:1, others closer to 12.5:1.  12.8:1 is a good WOT ratio to shoot
for power and 15.5:1 for cruise fuel economy.  Note that the air fuel ratio
is by weight.  13:1 means 13 pounds of air are mixed with 1 pound of fuel.
The usual target values for normally aspirated 4 stroke engines are about
12.5 to 13 for WOT, 14.0-15.5 at part-throttle cruise and 13.5-14.0 for part
throttle acceleration (or climbing a long hill, pulling a load, etc.).  If
you want to lean out the mixture at cruise for best fuel economy, be aware
that you'll also need to adjust timing.  Combustion gets much slower under
lean conditions and if you don't adjust spark timing, the combustion occurs
much later and exhaust temperature climbs.  That's bad for the exhaust seats
and valves.  However, if you adjust for MBT spark at each A/F ratio, exhaust
temperature will actually decrease relative to stoichimetric (rich will still
be somewhat cooler).  For typical gasoline engines, the range or ratios is:

  A/F  Characteristics
 Ratio
    5  Rich burn limit. Combustion is weak and/or erratic.
  6-9  Extremely rich. Black smoke and low power.
10-11  Very rich. Some supercharged engines run in this range at full power
       as a means of controlling detonation.
12-13  Rich.  Best power A/F for normally aspirated WOT.
14-15  Chemically ideal.  At 14.6 the A/F is at the theoretical ideal ratio
       with no excess fuel or oxygen after combustion. Good A/F target for
       part throttle cruise and light to moderate acceleration.
16-17  Lean.  Best fuel economy A/F ratio. Borderline for part throttle
       drivability (worse than borderline if EGR is used).
18-19  Very lean.  Usual lean limit (Driveability).
20-25  Lean burn limit.  Varies with engine.

To tune a Holley for best power and fuel economy, you usually have to
alter the power valve channel restrictors (PVCR's).  The power valve
only determines the opening point when the additional fuel is added.
It's the PVCR's that how much fuel is added.  In general, PVCR's should
be no larger than one half the diameter of your main jets.  This ratio
of orifice size to jet size will give you a 25% increase in fuel flow,
approximately the difference in fuel flow required to make the difference
between a stochiometric (14.7:1) and a best power fuel air mixture.
Unfortunately, standard Holley carbs provide no easy way to change the
PVCR's so you have to modify the metering block, either drilling the
passages larger or epoxing them shut and re-drilling smaller.  Many
performance Holleys have huge PVCR's which make it impossible to get
reasonable WOT mixtures on milder (and smaller displacement) engines
but some (like the Ford 428CJ 735 Holley) were set up with reasonably
sized PVCR's.  Also, there are aftermarket metering blocks and carbs
(Quick Fue Technologies is one example) that have replaceable PVCR's.

Carter/Edelbrock AFB carbs (sold in Europe under the Weber name) are
easier to tune for fuel economy as easily replaced tapered rods are
used for power enrichment, rather than PVCR's.

> You would get better throttle response with something like a 650.   And
> considering how expensive fuel is in Norway (more than $10 a gallon?!),
> you should seriously consider a 600, if your engine is close to stock.

2 CFM per HP is a good rule of thumb for when a 4 barrel becomes restrictive.
600 CFM is good for 300 HP before becoming too restricitive.  Holley 650's
are typically double pumpers which are set up rich.

Dan Jones