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

[Mark S. Riley]

----- Original Message -----
From: "Greg Hermann" <bearbvd at mindspring.com>
To: <diy_efi at diy-efi.org>
Sent: Tuesday, December 18, 2001 9:20 AM
Subject: Re: turbo SBC's... was RE: Thanks for the EFI! (off topic)


> At 9:34 AM 12/17/01, Mark S. Riley wrote:
> >They were legal. Rollers never have been in the "CUP" series. Problem
with
> >mushroom lifter camshafts are that they don't last very long. If you put
an
> >aggressive grind that will need valve spring to maintain control, then
> >lifter and lobe life is very much shortened over "Normal" flat tappet cam
> >grinds. Also the bottom of the block "must" be spotfaced to prevent one
> >point of the lifter touching a casting tit or bump which breaks the foot
off
> >the lifter. Perhaps lifter float cause you were trying to run the
lightest
> >springs to cut down on drag and wear. Then the driver does something
bright
> >like turn it 9600 on a restart. Also they have to be shimmed front to
rear
> >against cam walk cause the opposite lobe will lift the wrong valve with
very
> >little aft to fore movement. Used to see pallets of Crane, Competion
Cams,
> >Cam Dynamics, Reed, and Crower Cams mushrooms at some of the local "CUP"
> >shops. They couldn't run a cam and lifter but once. Wouldn't make two
races.
> >I use to buy the used cams and lifters and  run them in short track
engines.
> >Valve action with a mushroom can be more radical than a roller ever
dreamed
> >about. They just don't last long because of the loading. Oh, the lobes
are
> >thinner to help with the "crosslift problem" because of the big foot. The
> >"CUP" stuff was cut for a 1" foot lifter. When Nascar outlawed the
mushroom,
> >they all went to the ford lifter size, .850" I think. Precisioneering in
> >Asheville, NC does most of the lifter boring and registering for the
teams.
> >They have a factory type lifter boring machine.
> >Duh, that's probably more than anyone would want to know about that "old"
> >stuff. later, Mark
>
> My thought is that with a turbo engine, get the power with boost and don't
> turn it all that fast. MUCH more durable that way, overall, so long as you
> avoid any sign of the devil detonation. Thus, the valve springs can stay
> more like normal "hi-po" valve springs, since spring requirements vary
with
> the square of the rpm. This ought to cure the wear problem. Another
thought
> is to run a "hi-rev" spring kit in the valley--thus allowing slightly
> lighter valve springs, and keeping some of the necessary spring force off
> of the valves and rockers, at least.

with a "rev-kit" on a flat tappet, you force wiping of the lobes on start up
because all the lifters are loaded.
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. The bigger
foot is required because the edge of the lobe wipes past the side of a
normal lifter. This high lift per degree of rotation is where the need for
high valve spring pressures comes from. Also the mushroom lifter weighs more
than the regular flat tappet. Other wise it would be like a "drag stocker"
camshaft and they just cut the top off of a really big lobe and toss the
valve up with weak springs. Again not very reliable for any type of
endurance. If you aren't taking advantage of the lift per degree of rotation
then the mushroom lifter isn't worth putting in the engine. Something as
simple as a bad lifter turns into a major bit of work with a mushroom.
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.

>
> 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.

> They all do it to improve BSFC under load. Of course, none of these are
> high rev items, so the springs are reasonably mellow.
>
> Point is that the wear/durability problem comes from the spring force used
> trying to squeeze out the last few rpm, not  from the mushroom design
> itself.

I'm not aware of any HD diesels that turn over about 2100 rpm. Since the
40's, 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. Valve train requirements are drastically different for any engine
if you double the operating rpm. If the cam profile doesn't require a
mushroom foot, there would be no benefit to using one. They can create more
problems than they fix. later, Mark

>
> In my particular case, 1.375" diameter lifter feet will fit without any
> cross lift problems, even with stock width (about 5/8" wide) lobes, and
the
> cam is properly and precisely located fore and aft in its stock condition.
> 5-3/8" bore centers on a V-8 and good basic design will do that sort of
> thing for you !! :-)
>
> Greg
> >


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