[DeTomaso] Cam Question

[Art Stephens]

Dan,
     The carb is a Holley 750 double pumper with mechanical secondaries, 
#4779.  The intake will be Scott's dual plane to match the CHI 3V-225 heads. 
The exhaust includes 2" header tubes into 2.25 exhaust pipes into PI's flow 
thru mufflers.  At some point after I get the car back on the road,  I would 
like to upgrade the exhaust to 2.5"  or maybe even 3".  What would narrowing 
the lobe separation angle do for me?  I am also waiting on the heads as well 
as the intake.  I hope to have the engine ready to go in December and be 
back on the road in January.

Thanks,
Art



----- Original Message ----- 
From: "Daniel C Jones To: detomaso at realbig.com List-Subscribe:" 
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Sent: Thursday, November 15, 2007 11:27 AM
Subject: Re: [DeTomaso] Cam Question


>> My thought is that, increasing the cubic inches by 30, and using the
>> same cam, should make it more of a stump puller?
>
> Yes.  The 30 extra cubes should increase torque by 35 ft-lbs or so and
> move your HP peak down.  With that cam, you should peak under 5500 RPM,
> perhaps a little lower but that depends upon the rest of the build (carb,
> intake, exhaust).  In a cast iron 4V, dual plane 408C with a larger cam
> (232/236 degrees, 108 LSA, 0.609"/0.621" lift) we made 440+ ft-lbs at the
> lowest RPM tested (3000 RPM or so).  Ideally, you want a narrower lobe
> separation than the 110 degrees of that Comp cam, particularly if you
> have a good exhaust.
>
> Are you using Scott Cook's dual plane intake manifold for the CHI 3V
> heads or the CHI single plane?  The latter will be soft on torque until
> around 3000 RPM due to the large plenum volume.  This can be offset to
> some degree with the proper carb.  You don't want a Holley with straight
> leg boosters.  You want something with down leg boosters or annular
> boosters.  The ignition curve and exhaust are also improtant.
>
>> I don't imagine the new heads will hurt anything either?
>
> By switching from iron to aluminum at the same static compression ratio,
> you'll give up a bit of effective compression due to the greater heat
> transfer of aluminum (it's the ideal gas law at work: PV=gammaRT).  Not
> a big effect and should be more than offset by the better head flow.
>
>> I just hate to build the engine and regret not going with a different
>> cam.  Any thoughts,  suggestions?
>
> We will have a set of CHI 3V heads with matching single and dual plane
> intakes to test on the dyno.  If you want to spring for a cam, I can
> work up some specs and we can dyno test both your existing cam and the
> custom cam.  Not sure if our time frame fits yours, though.  We won't
> be getting to the aluminum heads until later in winter.
>
> Dan Jones
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