Well i did promise so here we go a brief describtion on cams and what makes a
Rover v8 work in certain rev bands.
PS please excuse my spelling i was expelled
from school at a young age so started building Rover
engines at 13.
I will come to cams and what I think are good and bad in a bit, firstly what are
you trying to achieve is to get the engine to fill its total cylinder capacity
throughout its rev range, easy said impossible to do unfortunately.
The first place to start is the cylinder heads, a really good flowing pair of
cylinder heads means you don't need such a viscious cam to make decent horsepower!
Unfortunately this is what lets the
Rover engine down big time, even in the 500 TVR
heads the valves are just no where near big enough to flow the air that is
required, so we are left with trying to make some decent bhp and driveabillity
with a cam.
Basically as much lift as you can get the better your
engine will perform through the whole rev range regardless of cc's, the
Rover head flows most cfm when the valve is
something like 700 thou from it's seat.
here is no Rover cam lifts that far because it just
can't, 600 is about its max, lift is related to
duration, you can't have lots of lift with little
duration as the valve will be being opened so quick that the cam follower will
just dig into the side of the cam profile and break, so
with a 600 though lift you are going to need 320 deg of duration, what you now
have is a full circuit race cam that will only make power from 5000 rpm upwards!
Duration is the time the valve is lifted of its seat. Too little and you will have no
power - too much and the car will drive like a pig. However duration, LCA (lobe
center angle , and overlap are all related, its not so much the duration that
kills driveabillity it's the overlap, just 10 deg can make or break the engine!
So duration make bhp lift makes both, as i said in the other post i am only
going into road engines!
The lca is the angle between full lift on the inlet and full lift on the exhaust
lobe, by making this wider you can tame the cam for the same given duration,
however what will happen is the came with the wider lca will idle better drive
smoother make more bhp and have a wider torque curve, the cam with the smaller
lca will only have a benefit in the mid range, around peak torque.
So in my honest opinion for a road cam I like to go
for around 285-290 deg of duration with a wide lca around 114, this to make a
good road sports cam, you can drive it down the shops with no hunting and use it
on a track day as well.
The same cam on a 108 lca will make it more peaky in the
mid range, but you will have to keep changing gears in slow traffic and it will
drop of the cam quicker too, peak power will be around the same but where the
114 will still be making good power at say 6500 rpm the 108 will be dead and
buried and need a gear change.
A race car is totaly different - I would specify a a camshaft to work in the rev
range I wanted and make the most of that rev range.
1 Falls off
the cam very quickly
These are all based on the fact that you will be running standard management
systems and a plenum, there are of course many more cams out there but these are
the general ones you will buy.
So to sum things up a bit, if you want a good low down nice driving torque cam
go for a cam with a max of around 270 deg duration and a lca of around 112 deg.
If you arent bothered with idle qualities and want a
good mid range cam go for one with a low lca and around 280 deg of duration.
If you want a track day cam or fast road cam go for
around 300 deg and around 110 lca
If you want a good all rounder go for around 285 deg and wide 114 lca,
both Kent and Piper will make cams to what you want within reason so give them a
The Piper 285 for instance, loses nearly 25 bhp over the 404 at the top
end, this on a controlled dyno not a rolling road, but it gains around 10 ftlb
at around 2000-3000 rpm, but if you asked Piper to grind it on an lca of say 110
you would only be losing around 10 bhp at peak, but gaining around 20 ftlb at
Also please note cams cannot be judged against other cams on rolling roads,
especially different rolling roads, there are too many variants,
all the way from the air filter to the oil in your gearbox and diff!!!!!
Throttle bodies against plenums and cams!!
What I breifly said earlier with overlap cams, this
will murder an engine running a plenum, whereas on throttle bodies it tends to
smooth things out a bit, take a single plenum all 8
cylinders draw from this one opening, not a proplem there in fact it is good as
each cylinder can draw as much air as it wants with no restictions as far as air
flow is concerned.
The BIG DOWNSIDE TO A PLENUM, on the overlap period part of the cam, this is
where both exhaust and inlet valve are open at the same time, i.e. at the
end of the exhaust stroke where the piston is forcing the burnt gas out the
exhaust port the inlet valve opens before the exhaust is shut, instead of the
unwanted rubbish going out the exhaust it is sent back up past the inlet valve
and into the plenum.
This is aggravated by the fact that the next cylinder is sucking
hard and will suck the waste out of the disposing cylinder into the good one,
the problem here becomes that it is not fresh and lacks oxygen, so instead of
the new cylinder getting a good charge of fresh air and fuel, it has 20 or 30%
of nothing that can't be ignited, hence poor combustion poor idle and low
This is only at low-ish rpm, at high rpm you have the advantage that the
exhaust manifolds are "should" be scavenging the fresh inlet charge into the
cylinder, if you have the money to go to throttle bodies then this is where the
biggest gain is going to come in the low to mid range.
On the overlap period each cylinder can only contamite its own cylinder, and
part of this will be lost to the atmosphere anyway, so genearly lets say at
below 3000 rpm a plenum is giving a cylinder 70% of fresh charge to be ignited a
set of throttle bodies will be giving 90% at the equivalent rpm all this = more
ftlb of torque, just be carefull you dont go too small on the throttle bodies or
this will hurt the top end breathing, you have gone from a 72 mm plenum and as I
said each cylinder will see 72 mm down to what ever the size of your throttle