Hi everyone, been on the road, and back out tommorow, so I've been following
the engine-cam-rebuild parts discussion with great interest via Digest
format.
A few observations;-)
I'd stick with the GM cam or its equivalent (Sealed Power/Federal Mogul) if
you want to be safe. The GM cam is a proven design, and has very well
balanced performance. If anything, its 'too hot' for the GMC, however, a
torque peak occuring between 2500/3000 RPM is perfectly fine, and makes the
most of what engine breathing is availible with stock valves, exhaust and
intake configurations, and there is no good reason to change this. Someone
has mentioned retarding and advancing cams to move the torque peaks up and
down a little in the RPM band for tailoring a cam off the shelf for a
special application- this does work, with very mixed results.
I would not change cam timing by more than four degrees retarded on a
'general performance' type cam under any circumstances. This will move the
point of the torque peak as much as +400 RPM higher in the RPM band,
however, this does not mean you'll get as much torque at peak as you used
to, nor does it mean you'll even have the torque that was present before
correction at the speed of the original torque peak. In short, it may or may
not work at all, but if it does, the point of diminishing returns is usually
circa that 4 degree point. Taming a high performance cam by advancing it is
not possible, usually, it will ruin what idle and vacuum you had in the
first place often, and doesnt generally increase top end, either. VERY low
performance cams can be made highway friendly by retarding them. To my
knowledge, the sole GM engine that ever had this done at the factory was the
140 HP Corvair (4 carb) in Automatic transmission applications using the low
performance (95HP) cam. It cost 18 HP to do this on a 140 HP engine, but
made it idle better in 'drive' range.
Don't be fooled by full throttle and maximum output figures either. Its the
AREA under the HP and torque graphs that you want to increase, not the peak
point. Additionally, the lower the engine speed the output occurs, the more
useful you'll find it to be in a heavy GMC.
Torque peaks represent the point of highest cylinder pressures essentially.
The AVERAGE pressures in the cylinders that push the piston down are
greatest at the torque peak, in other words. These high pressures also are
the point of greatest heat from air compression, and greatest inclination
toward preignition and spark knock; in essence, the point of highest taxing
of octane rating. If you are able to have the same torque at a lower speed,
you generally are making even higher pressures. As the 455 is pretty close
to the limit on Regular fuels as is, lowering the torque peak may create
more problems. Raising it and retaining the original Brake Mean Effective
Pressure will make much more power, but this may not be possible within the
limits of the flow capabilities of the manifolding and valves either. In any
event, the BMEP peak occurs a bit late as it is.
At part throttle, torque peaks occur at higher points on the RPM scale.
ANY change in breathing potential, such as different valve sizes,
manifolding or particularily camshafts will require recurving the
distributor to match the new BMEP curve. If this isn't done, the initial
lead will have to be retarded to a point the advance curve doesn't run over
your new BMEP peak, meaning the rest of the time the engine will be running
slow and hot sucking gas with retarded spark.
Rather than each owner opening Pandoras box with a different untested and
potentially less powerful and more troublesome combination of parts, I'd
suggest sticking pretty close to GM's original design. GM has better
engineers than Edelbrock, Mondello, et al- and they arent engaged in a
secret conspiracy to waste gasoline, and make big gutless engines at General
Motors.
Building an engine to perfectly stock tune, and taking pains to use the best
materials and workmanship is by far the ideal. You can then avail yourself
of the very expensive engineering that GM put into it. Trust me, you and the
guy at the parts counter arent going to ever think of something GM didn't.
Also, regarding ported distributor advance vacuum- engines using this have
it as an emission control, so that the timing does not become advanced under
closed throttle coast conditions. It is perfectly acceptible to hook it
directly to intake vacuum, however you may discover that the VA unit is too
agressive on straight manifold vacuum, and a less agressive unit must be
subsituted.
Either way you connect it, initial timing should be set with the hose off
and plugged. The GM centrifugal advance curve is a tailor made match to the
GM cam, and works immaculately, the only wild card is the Vacuum advance on
a stock engine. Experimenting with several vacuum advance units with similar
specs or even the same part number can often yeild one that REALLY flatters
your particular engine.
ANyhow, great discussion guys, I'm back on the road tommorow, and will be
out of communication for the most part until Tuesday, but I will
periodically check email- send me a personal email if you have a burning
question at
deville
otherwise, I'll see you all early next week!
Happy GMC'ing!
Brent Covey
Vancouver
the engine-cam-rebuild parts discussion with great interest via Digest
format.
A few observations;-)
I'd stick with the GM cam or its equivalent (Sealed Power/Federal Mogul) if
you want to be safe. The GM cam is a proven design, and has very well
balanced performance. If anything, its 'too hot' for the GMC, however, a
torque peak occuring between 2500/3000 RPM is perfectly fine, and makes the
most of what engine breathing is availible with stock valves, exhaust and
intake configurations, and there is no good reason to change this. Someone
has mentioned retarding and advancing cams to move the torque peaks up and
down a little in the RPM band for tailoring a cam off the shelf for a
special application- this does work, with very mixed results.
I would not change cam timing by more than four degrees retarded on a
'general performance' type cam under any circumstances. This will move the
point of the torque peak as much as +400 RPM higher in the RPM band,
however, this does not mean you'll get as much torque at peak as you used
to, nor does it mean you'll even have the torque that was present before
correction at the speed of the original torque peak. In short, it may or may
not work at all, but if it does, the point of diminishing returns is usually
circa that 4 degree point. Taming a high performance cam by advancing it is
not possible, usually, it will ruin what idle and vacuum you had in the
first place often, and doesnt generally increase top end, either. VERY low
performance cams can be made highway friendly by retarding them. To my
knowledge, the sole GM engine that ever had this done at the factory was the
140 HP Corvair (4 carb) in Automatic transmission applications using the low
performance (95HP) cam. It cost 18 HP to do this on a 140 HP engine, but
made it idle better in 'drive' range.
Don't be fooled by full throttle and maximum output figures either. Its the
AREA under the HP and torque graphs that you want to increase, not the peak
point. Additionally, the lower the engine speed the output occurs, the more
useful you'll find it to be in a heavy GMC.
Torque peaks represent the point of highest cylinder pressures essentially.
The AVERAGE pressures in the cylinders that push the piston down are
greatest at the torque peak, in other words. These high pressures also are
the point of greatest heat from air compression, and greatest inclination
toward preignition and spark knock; in essence, the point of highest taxing
of octane rating. If you are able to have the same torque at a lower speed,
you generally are making even higher pressures. As the 455 is pretty close
to the limit on Regular fuels as is, lowering the torque peak may create
more problems. Raising it and retaining the original Brake Mean Effective
Pressure will make much more power, but this may not be possible within the
limits of the flow capabilities of the manifolding and valves either. In any
event, the BMEP peak occurs a bit late as it is.
At part throttle, torque peaks occur at higher points on the RPM scale.
ANY change in breathing potential, such as different valve sizes,
manifolding or particularily camshafts will require recurving the
distributor to match the new BMEP curve. If this isn't done, the initial
lead will have to be retarded to a point the advance curve doesn't run over
your new BMEP peak, meaning the rest of the time the engine will be running
slow and hot sucking gas with retarded spark.
Rather than each owner opening Pandoras box with a different untested and
potentially less powerful and more troublesome combination of parts, I'd
suggest sticking pretty close to GM's original design. GM has better
engineers than Edelbrock, Mondello, et al- and they arent engaged in a
secret conspiracy to waste gasoline, and make big gutless engines at General
Motors.
Building an engine to perfectly stock tune, and taking pains to use the best
materials and workmanship is by far the ideal. You can then avail yourself
of the very expensive engineering that GM put into it. Trust me, you and the
guy at the parts counter arent going to ever think of something GM didn't.
Also, regarding ported distributor advance vacuum- engines using this have
it as an emission control, so that the timing does not become advanced under
closed throttle coast conditions. It is perfectly acceptible to hook it
directly to intake vacuum, however you may discover that the VA unit is too
agressive on straight manifold vacuum, and a less agressive unit must be
subsituted.
Either way you connect it, initial timing should be set with the hose off
and plugged. The GM centrifugal advance curve is a tailor made match to the
GM cam, and works immaculately, the only wild card is the Vacuum advance on
a stock engine. Experimenting with several vacuum advance units with similar
specs or even the same part number can often yeild one that REALLY flatters
your particular engine.
ANyhow, great discussion guys, I'm back on the road tommorow, and will be
out of communication for the most part until Tuesday, but I will
periodically check email- send me a personal email if you have a burning
question at
deville
otherwise, I'll see you all early next week!
Happy GMC'ing!
Brent Covey
Vancouver
