Robert..
Thanks for the detailed information! I would guess that the huffa-huffa is
coming partly from the cam, and partly from worn valve seats. Also, thinking
about it, the rocker shaft is probably worn pretty bad as well. I am
guessing this because there is a noisy vibration around 3K which smoothes out at
about 3.5 - 4K. Also, around 2K, the variation in the intake pressure seems
to not be as bad. (Hence the cam.)
Solution for now is to make a restrictor for the vaccuume line to the
distributor
vaccuume advance. (And keep working on the rebuild!)
As far as the rebuild goes - I am debating the advantages / dis-advantages of
putting a hot street cam in it, as well as roller rockers. I've read through
Peter Burgess book - Power Tuning a MGB Engine. He seems more concerned with
balancing the engine and porting the head than the hot cam. He also feels that
the stock cam is quite good. Also, he does not like roller rockers.
Personally,
it seems like the bearings in the roller rockers would last much longer than
the normal rockers busings. What do you think? (And all you other listers who
are following this...)
I will probably bore the engine out a bit, but not all the way to the larger
piston.
Balancing all of the connecting rods, and having the block line bored both seem
very valuable. Smooth-balance = free revving.
Thanks again!
Dave Hall
http://www.pacifier.com/~dlhall
72 B GT
74 Midget
----------
From: Robert Allen
Sent: Tuesday, March 04, 1997 4:52 AM
To: David Hall
Cc: 'britcars@juno.com'; 'MGList'
Subject: Re: Vaccuume Advance
Ah, more information.
A "hot street" cam, among other things, will generally have a longer
duration. Thus it is likely that the intake valve is not yet fully
closed as the piston rounds BDC and begins the swing up into the
compression stroke.
At cranking RPM, it is likely that the upwords movement of the piston is
pushing some of the fresh intake charge around the closing intake valve
and back into the manifold. This would result in the "huffa-huffa"
phenomona in the intake manifold and what would seem to be low values
during a compression test. Also explains the vaccum gauge reading
fluctuations.
At above-idle and higher RPM, the "hot street" cam will exploit the high
velocity of the intake charge by waiting to close the intake valve.
Properly timed, the inertia of the incoming charge will pack the
cylinder somewhat higher than atmospheric pressure before the intake
valve slams shut.
The performance cam will exploit valve lift, duration, and overlap to
make more power. (This discusses the 'duration' of the intake valve
only.) There are always trade offs:
1.) Driveability problems at idle. The motor is spitting back into the
intake charge. The SU "constant depression" carbs are rather forgiving
at this. This can be nothing more than the desirable "lumpa-lumpa" idle
of a cammed engine.
2.) Poor vaccuum levels which effect vaccuum-driven devices like the
vaccuum advance distributor and that power-bake booster. Racers might
add "vaccuum cans" which act as storage resiviors for vaccuum devices.
The other performance cam problems are:
1.) Increase valve lift can shorten valve train life and increase the
likelyhood of valve float. Rocker shafts take great abuse.
2.) Valve overlap (when both the intake and exhaust valves are open)
always trails a little fresh charge into the exhaust which raises
emissions.
Often, when switching to high compression pistons, you *must* use a hot
cam as the extended intake valve duration effectively lowers compression
at lower RPMs which helps prevent detonation.
Short Version:
If you have a hot cam, the engine may be behaving normally.
Bob Allen, Kansas City, '69CGT, '75TR6
*****************************************************************
David Hall wrote:
>
> Hi All!
>
> I re-did the compression check last night (the right way - last year I
> didn't turn the engine over enough) and I also hooked up a pressure
> guage. I got 125 lbs in 1,2, and 3, and 120 in 4. The engine has
> 125K miles without a head rebuild. It also has a hot street cam.
> The pressure guage did indeed fluctuate!
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