ox sensor on sequential efi,high overlap cam....

[Greg Hermann]

Agree except as follows:
>First, high lift on a short duration cam would be very hard on the valve
>train if it could keep the valve at the higher lift long enough to be
>effective.

True enough for a stock valve train. With proper mods, not such a big deal.

>Second, high lift causes several problems in the valve train including valve
>to piston interference (especially with narrow lobe separations), valve
>spring retainer to valve guide/seal interference, rocker arm to stud
>interference, and valve spring coil bind.

If you are not checking and dealing with this stuff, why did you bother
building an engine at all??? Buy one off someone's shelf!!

Adjustable rocker arms are often required to take up any clearance created
when the valve is closed. All of
>this must be checked and corrected before a high lift cam can operate
>properly.

Of course it must all be checked, and what is said is true for a grinder
who just goes to a smaller base circle on the cam. But if you are doing it
right, custom length push rods, and a base circle close to stock are
better.

>Finally, if you are using a cam with enough duration to make a high lift
>effective, the usual limit for noticable improvement is reached when the
>lift equals 25% of the intake valve diameter.

The 25% of valve diameter is all that is needed for maximum flow axiom
(straight out of David Vizard) is quite true for totally unshrouded valves
and straight shot ports--such as with a DOHC hemi chamber. It is slightly
less true for a pent-roof (typical four valve) chamber or a pushrod hemi
chamber. It is well proven that it is BS for a typical wedge chamber and
ports. Take a look at some flow numbers!!

This means that if your intake valve diameter is only 1.84" you will get
the best flow at only .460" lift.

If you have a DOHC hemi chamber. Been there and done that with Jags, Alfas,
and Porsches.
>
Lifts higher than 25% of the valve diameter will add more duration at the
>maximum flow point but excessive lifts will cause more problems than they
>are worth in an engine that is not fully race prepared.

Then why don't they stop the valve at the optimum lift point, and leave it
there longer, instead of just stretching out an existing valve lift curves
duration, and increasing lift by doing so?? Too much work to develop a
really unique profile for each grind??? Yep, I AM a cynic!! :-)

>The bottom line on cam lift is that it is pointless to go overboard.

Duh!!

All newer designs have adequate lift for the operating range of the cam.

Why do I have doubts about this one??? :-)

The minimal gains from an extra high lift cam are not worth the extra work in a
>street vehicle.

I agree if what you are doing is bolting in a cam, manifold & headers, plus
maybe heads. But if you are seriously BUILDING an engine, this stuff is
exactly what you came to the party to do!!

My point is that you should put your heads on a flow bench and flow test
them with the valves at various lifts, and with something round and the
size of your bores under the chamber (so as to duplicate shrouding
conditions as well as possible). Let the flow bench TELL you what valve
lift the flow (both intake and exhaust) maxes out at with YOUR heads,
rather than listening to whatever the cam guy wants to tell you!! If you
have the flow v. lift curves for your heads, any reputable cam grinder will
INSIST on seeing them!! In fact a REALLY reputable grinder will INSIST that
you get them if he is not already familiar with your particular heads!
Picking a valve lift at the point where the flow starts to level out
(rather than where it maxes out) usually makes the most sense for a street
motor, because of durability considerations.

If you are using proper quality valve gear for the level of engine you are
building, valve spring life will be the determining factor every time. And
multiple springs (double, or, even better, triple) will give you the BEST
fatigue life every time for any given level of spring force. So will larger
diameter springs. And so will longer than stock valves and springs with
other factors remaining constant. And roller followers will give you better
performance AND efficiency at ANY chosen RPM range every time, because they
allow higher valve accelerations and velocities at a given level of cam
life, and even make possible valve rates that are IMPOSSIBLE with flat
tappets.

Why put race engine quality work into a street engine?? Cuz it is fun to
do, cuz it runs better, cuz it sounds better, cuz it lasts a lot longer if
you do it right, and cuz it feels GOOD every time you drive it!! (But NOT
because it is cheap!)

Regards, Greg