turbo SBC's... was RE: Thanks for the EFI! (off topic)

[Greg Hermann]

>The reason for the recent usage of Mushroom cams in race cars, is that the
>lift per degree of rotation can be higher with the mushroom than a roller is
>capable of below .300" lift. Thus at the duration events they were running,
>they had more area under the curve than a roller is capable of.

Precisely.

The bigger
>foot is required because the edge of the lobe wipes past the side of a
>normal lifter.

Yep.

This high lift per degree of rotation is where the need for
>high valve spring pressures comes from.

Nope. Lift per degree of rotation equate to valve VELOCITY. The need for
high spring pressure comes from valve ACCELERATION--and only as the lifter,
valve, and everything else, goes "over the nose" of the cam. One can use
MUCH higher accelerations coming off of the cam and back onto it so long as
the valve gear is rigid enough to do the job. The springs have no serious
limiting influence on the allowable off of and on to  the cam acceleration
rates.

 Something as
>simple as a bad lifter turns into a major bit of work with a mushroom.

But, is also a LOT less likely to happen.

>That's why the OEM's quit using them. Especially with a turbo motor, a
>"killer" camshaft is not needed to make obcene horsepower levels.

I'm not wanting a "killer" camshaft. What I want is a LOT of area under a
HIGH lift but SHORT duration cam profile. Mushrooms are the best, if not
only, way to get there. Why?? because this is a GOOD profile for a turbo,
and will also give good off-boost performance and BSFC.
>
>>
>> No doubt, the thinner lobes contribute mightily to the wear problem, too.
>>
>> There are plenty of HD diesels which use mushroom lifters, stock, without
>> wear problems. So did the IHC "Red Diamond" engines. (404/450/501 I-6's).
>
>I wonder if perhaps the use of mushroom lifters is partly atributable to
>their use of a camshaft as big as a driveshaft and the other side is because
>that's the way we've always done it. Thinking about it, I don't work on
>diesel trucks, but I have several friends who do and they tell me that Mack
>is the only company that still uses mushrooms in the last 30 years or there
>abouts.

Guess they don't know much about Hanomag diesels--- :-)
>

>
>I'm not aware of any HD diesels that turn over about 2100 rpm.

Try all of the Detroit 2 strokes--as MUCH as I hate the screamin' demons !!
\
, I don't think there has been a single engine released by anyone in a
>"passenger" car that wouldn't turn at least 4500 rpm and in most cases over
>5000 rpm.

I think you're a bit "high" with that number, but not much.

Valve train requirements are drastically different for any engine
>if you double the operating rpm.

Yeah--the forces are multiplied by a factor of four (proportional to the
square of the rpm, as I said before)

If the cam profile doesn't require a
>mushroom foot, there would be no benefit to using one.

Of course not. BUT--for a short duration, high lift cam, you gotta have 'em.

 They can create more
>problems than they fix.

Doubtful.

Greg


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