CAMSHAFT SELECTION AND A BIT MORE

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 real good flowing pair of cylinder heads means you dont need such a viscous cam to make decent horsepower!, unfortunately this is what lets the rover engine down big time, even in the 500 tiv 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.
LIFT,
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 it most cfm when the valve is something like 700 thou from is seat, the reality here is no rover cam lifts that far because it just cant, 600 is about its max, lift is related to duration, you cant have lots of lift with little duration as the valve will be being opened so quick that the cam follower willjust 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
Duration is the time the valve is lifted of its seat, to little you will have no power too much 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 its 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 dood 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 spec a a cam to work in the rev range i wanted and make the most of that rev range.
mc1 good idle, excellent drive ability, reasonable bhp.
mc2 poor ish idle, poor ish drive abilty good mid range, good bhp.
h404 very poor idle, crap driveability, good mid range, excellent bhp
stealth very good idle, very good drive ability, ok ish mid range, good bhp.
Piper 270 ok idle good mid range crap bhp
piper 285 crap idle, poor drive ability, very good mid range,ok bhp, falls off the cam very quickly
piper 300 crap idle crap drive ok mid range good bhp
kent 200 very good idle good drive poor every where else
218 good idle good drive good mid range poor bhp
214 ok idle ok drive ok mid range ok bhp
224 poor idle, poor drive, good mid range, good bhp
234 crap idle crap drive, good mid range, excellent bhp,
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 call, 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 200-3000 rpm, but if you asked piper to gring it on an lca 0f say 110 you would only be losing around 10 bhp at peak,300but gaining around 20 ftlb at 2000-3000.
Also please note cams cannot be judged against other cams on rolling roads, espiecially different rolling roads, there are too many varients, 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, where as on throttle bodies it tends to smooth thigs 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, ie 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 more agrivated 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 cant be ignited, hence poor combustion poor idle and low power, This is only at low ish rpm, at high rpm you have the advantage that the exuast 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 size your throttle bodie is,
Thats enough for tonight hope this kind of helps and if it does i will do a proper one for the sprint magazine
regards Rob

Bloody good read that.

Nice one Rob, I shall make this a Forum FAQ for everyones future reference.

BB

Rob,

Thanks for the great explenation. I was wondering if your remarks apply to the 500 or also to the 430/400 engines? Are there any differences between them reacting on the cams?

Regards,

Peter

Yes, fanks. four thatt Rob" verry inrmoative

thanks Rob, very good artical. Looking forward to seeing it in sprint.

Nice one Rob I can see now why cam design is such a black art, with so many variables and compromises needed to balance it out.

Yes your absolutely right this is where high hc's come from along with the fact that with a performance cam your compresion is dramaticaly affected at low revs so the mix inside the cylinder is getting squashed properly between the head and piston hence even poorer combustion, this is one of the reasons you should raise the static compresion ratio with a wilder cam.

Hi Rob,

Great stuff. So if I understand correctly, to make a RV8 (5.2 in my case) run smoothly (in town) but yet very powerful (e.g. with a H404) it will need a different head/valves or seperate throttle bodies.

Hi gerjo
If you put better flowing heads on your engine now but kept everything else the same cam,induction etc.. it will drive the same in the way of manners, but it would make a lot more power, so what you can do is put a tamer cam in it so it drives like a pussy cat but it will go on to make the same bhp as you have now with an increase in low down torque, unfortunetly there arent any off the shelf heads you can buy for a road car to acheive this, i have a pair here i have been messing with for years welding them all up and changing the port angles and have cost me around £4000 so you can see how expensive it could get! you would be looking at around £2500 to replicate them...
Throttle bodies will significantly alter the way your car drives, if you ever get the chance to drive a car on either individual carbs or injection you would be amazed in the difference. but again by the time you have bought all the kit and installed them dont expect anychange out of around £7000 for an omex set up.