Monday, February 2, 2015

New Camshaft Comparison

<This post is part of the 'Winter Project 2014/15 - New Engine Rebuild' project>

Recently I was shopping around for a new camshaft, now you would think this would be a fairly straight forward affair: "Can I have a camshaft please, this one is knackered" - "Certainly sir, here you go". That's kind of how I expected it to go. But it seems that buying a new camshaft is far more involved than that. Firstly, they don't make the standard 998 camshafts for pre-injected engines anymore, and secondly, of the camshafts they do still make, how do you know which one to choose? Would you like a ST510? Or a ST948? How about a ST731?? No, maybe a ST544? Or maybe even a scatter pattern camshaft!!! None of those, don't worry, there are still dozens of others to choose from!

What Did I Buy and Why?
When I bought the camshaft, because a straight 'like for like' swap wasn't available, I had to choose one of the camshafts on offer. As my camshaft knowledge is thinner than Kojak's hair, I simply went with the advice of the guy at 'Mini Sports' and got the 'ST510 Mini Sport Camshaft - Mild Road - CA1'. The fact it has the words 'Mild Road' in the description means it has to be the right one... right?

It's pretty obvious that I'd no real idea what I was buying and how it differed from the standard factory camshaft, other than the advice that it 'was a bit better'.

But how exactly do I quantify 'a bit better'? Better in what way and how big or small is that 'bit'? Was the 'betterness' even going to be noticeable or would it endow my Mini with magical powers! Who knows! The only facts I have to go on are the camshaft specs taken from the Mini Sports website which are:

ST510 Mini Sport Camshaft - Mild Road - CA1
  • Application: - Mild Road
  • Power Band: - 2000-6500 rpm
  • Valve Lift: - 0.318 "
  • Timing: - 10/50 50/21 
  • Duration: - 240°- 252° 
  • Valve Clearances: - Inlet - 0.015" - Exhaust - 0.015"
  • Lift (TDC): 110°

What the Heck!!!!
What does this mean! What does any of this actually mean and what do I compare it to!! Well if I'm going to build an engine, I guess I'd better find out. And what better place to start than 'Vizard's Big Yellow Bible', Chapter 11, Part 1 - 'Camshafts, Cam Timing & Valve Trains'

Reading through the chapter, it explains in great detail the theory behind valve timing in relationship to the position of the piston in the cylinder and how the camshaft controls it all. It details valve opening times, closing times, the duration, the overlap, the lobe displacement, the difference between cam lift and valve lift and also how all of these factors affect the driving experience. If you need to understand camshafts, this really is a comprehensive and worthwhile read.

Ok, so what does it mean?
So with all that research and new insight into the theory under my belt, Let's look again at the numbers, bear with me as it's difficult to explain. I did my best, but to he honest, I don't think I would understand if not the the book.
ST510 Mini Sport Camshaft - Mild Road - CA1
The ST stands for special tuning, which as far as I can tell from looking at the numbers, is a Mini Sport copy of the AEG510 cam fitted to the 997 Cooper S. Although the lobe profiles are the same, the lobe widths are wider on the ST than the AEG.
Application: - Mild Road
Just a bit of marketing to help numpties like me.
Power Band: - 2000-6500 rpm
Useful info I suppose.
Valve Lift: - 0.318 "
Not massively useful info. Camlift is the important number for me, not valve lift as that is dependent upon the rocker ratio. Valve lift is calculated as 'Cam Lift' x 'Rocker ratio'
Timing: - 10/50  50/21
The first set of numbers refer to the inlet valve, the 10 means the valve starts to open 10° (of crank rotation) before the piston is at top dead centre (TDC). The 50 means that the valve has closed at 50° after the piston has gone past bottom dead centre (BDC). 

The second set of numbers are the same concept but for the exhaust valve.
Duration: - 240°- 252° 
The number of degrees of crank rotation the valve is actually open. The 240° refers to the inlet valve and is calculated as: The point at which the valve starts opening before TDC (50°), plus a further 180° as the crank rotates from TDC to BDC, plus the number of degrees after BDC it takes the valve to close (40°)

Hence 10° + 180° + 40° = 240°

The 252° is for the exhaust valve but is from the BDC to TDC point of view, i.e. 50° + 180° + 21° = 252° - Eh!! that's wrong!!! See paragraph below for reason**.

Valve Clearances: - Inlet - 0.015" - Exhaust - 0.015"
The tappet clearance for standard ratio rockers.
Lift (TDC): 110°
This is the point at which the inlet valve is at it’s maximum open point. It is measured in degrees of crank rotation after TDC. In this case 110° after TDC, meaning that the piston is already quite a way down the cylinder by this point.

** So why doesn't 50 + 180 + 21 = 252? Well apart from being a mathematical impossibility, we have to assume that one (or more) of the numbers in the spec are incorrect. Luckily on page 284-285 of 'Vizard's Big Yellow Bible', it details all the specification numbers for the AEG510 (which is the ST510) and many other camshafts, including the standard cam. Looking at this data I can see that the 50 should in fact be 51, and thus 51 + 180 + 21 does indeed equal 252. Phew, glad that's sorted, I hate loose ends.

What's the Difference Then?
So now we kind of have the meaning of the numbers sorted, we can compare them for the old camshaft v's the new camshaft to see where the differences lie...

Standard 998
ST510 (AEG510)
Inlet Opens Before TDC
1° Sooner
Inlet Closes After BDC
9° Later
Exhaust Opens Before TDC
2° Sooner
Exhaust Closes After BDC
10° Later
Inlet Duration
10° Longer
Exhaust Duration
12° Longer
Inlet Cam Lift
Exhaust Cam Lift
0.015" Higher
Inlet Valve Lift (Std 1.275 Rocker  Ratio)
Exhaust Valve Lift (Std 1.275 Rocker Ratio)
 0.018" Higher
Lobe Displacement Angle* See below
Tappet (Valve) Clearance (Std 1.275 Rocker Ratio)
0.003" More
Full Valve Lift After TDC
4° Sooner
Overlap ** See below
11° Longer
* The Lobe Displacement Angle is the angle between the highest lift point of the inlet lobe and the highest lift point of the exhaust lobe in degrees. This is the only measurement that is degrees of camshaft rotation and not crank rotation.

** The Overlap is the crossover point when one valve is opening while the other is still closing, meaning both are momentarily open at the same time. The overlap is measured in degrees of crank rotation and is calculated as 'Inlet Opens Before TDC Value' + 'Exhaust Closes After BDC Value'

Breaking these numbers down, the only significant differences are the longer duration and the slightly higher lift of the exhaust valve. The longer duration should mean more fuel/air mixture can enter the cylinder, meaning a bigger bang, meaning more go. The longer duration does also mean a longer overlap. Generally a shorter overlap is better for the road, whereas a longer overlap is better for racing. Although the overlap has gone up by 11° from 20° to 31°, I don't think it will adversely affect the road manners too much when you consider the racier cams can have an overlap up to 115°.

The extra lift on the exhaust valve should mean that the exhaust gasses can clear the cylinder more effectively during the blow phase. But to be honest, the difference is so slight that I doubt there will be any noticeable effect, especially with the standard head.

So how does that affect the shape of the cam. Taking a close up photo of the lobes on each camshaft and placing them one above the other, you can see that the ST510 has a slightly wider profile, wheres the standard is a bit pointier.

This is what gives it the slightly longer duration as the cam follower has to travel slightly further over the surface of the ST510 cam as the lobe circumference is longer. The picture below illustrates the difference.

What Have We Learnt Today!
Wow and I thought that camshafts were straight forward, how wrong I was, I think I need a lie down after all that learning. So what have I actually learnt? I have learnt what all the numbers mean when buying a cam, and that you can't always trust them as they can be wrong. I have learnt the difference between the cam I had and the cam I intend to use and I am happy with my purchase of the ST510.

Lastly and most importantly, I have learnt that I am far too interested in camshafts and would be an absolute bore at a dinner party. Unless of course I organised the dinner party and invited only people that were really into camshafts, but I suspect such a party would be just me, in a room, by myself, and maybe those that managed to read this post to the end. Thanks if you did by the way.

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