[DeTomaso] Minimum Idle Vacuum?

Art Stephens artstephens at charter.net
Tue Nov 27 22:39:00 EST 2007


> I'll ask Glen what his 408C makes for idle vacuum.<

       Thanks Dan.  It would also be nice to know at what rpm that engine 
will idle?  I frequently get stuck in stop and go traffic,  (can you say 
cruising)  :-))),  while in hot weather.  I just don't want to have to idle 
at 1000 rpm to keep it running.

Art





 Since you want to
> keep it under 6000 RPM, the cam that Kaase spec'd for Glen's engine
> should be in the ballpark:
>
> Crane Cams grind number (custom):  HR-232/352-2S-8
> Intake  232 deg duration @ 0.050" tappet lift
>         294 deg duration @ 0.004" tappet lift.
>         0.609" lift with 1.73:1 rocker ratio
> Exhaust 236 deg duration @ 0.050" tappet lift
>         298 deg duration @ 0.004" tappet lift.
>         0.621" lift with 1.73:1 rocker ratio
> 108 degrees lobe separation, intake centerline 103 ATDC
> degreed in at 102.25 intake centerline with new chain
> (should lose about .5 to 1 degree once timing chain loosens up)
>
> On Glen's 408C with 4V heads and dual plane intake, it peaked in the
> 5500 to 5600 RPM range.  For best acceleration, you want to shift
> something like 400 to 500 RPM above HP peak so that cam is in the
> ballpark for your application.  I've not had a chance to drive it
> yet but Glen says it doesn't lack for torque and pulls 5th gear just
> fine at 1500 RPM.  IIRC, we started the dyno pulls at 3100 RPM at
> which point it was making 440+ ft-lbs.  Peak was something like
> 486 ft-lbs.
>
> Specifying the vacuum level will drive the lobe center angle that
> Vizard's cam selection software returns.  If you didn't care about
> idle vacuum, it would return something like 104 degrees LSA with a
> shorter seat duration.  Setting it at Glen's level will probably
> return a lobe separation similar to the one we used (108 degrees).
> Vizard also suggests using a dual patter cam with the exhaust rocker
> ratio about 0.1 shorter than the intake.  For the 351C, that usually
> means running 1.73:1 intake and 1.65:1 exhaust, though 1.7:1 and
> 1.6:1 are also avialable.  Your CHI heads have smaller ports and a
> better exhaust so I'd expect the difference between intake and
> exhaust might be a bit less than Glen's but that might depend upon
> the available lobes.
>
> Dan Jones
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