What's the difference between them and 'normal' Cams?
Umm, just out of interest really.
Scatter Cams?
Started by
Wiggy
, Aug 24 2005 05:39 PM
4 replies to this topic
#1
Wiggy
-
- Members
-
- 1,036 posts
One Carb Or Two?
- Location: Hampshire
Posted 24 August 2005 - 05:39 PM
It's in the title really. Pro's, Con's.
What's the difference between them and 'normal' Cams?
Umm, just out of interest really.
What's the difference between them and 'normal' Cams?
Umm, just out of interest really.
#2
vasi
-
- Members
-
- 897 posts
One Carb Or Two?
- Local Club: Minis 'n' Oatcakes
Posted 24 August 2005 - 06:10 PM
Because of the 5 port design of the A series engine, it is essentially an unbalanced engine. More like two different 2 cylinder engines joined together. So on a scatter cam, the timings for the centre two cylinders are different than for the outer two. Thus greater power is achieved by designing in the cam a difference between the cylinder pairs.
Better still, would be to use completely independant timings for each lobe (inlet and exhaust).
Better still, would be to use completely independant timings for each lobe (inlet and exhaust).
#3
Dan
-
- TMF+ Member
-
- 21,354 posts
On Sabbatical
Posted 24 August 2005 - 06:24 PM
Well, it's quite simple really.
A scatter cam plays about with the timing of cylinders in pairs. This is in order to acheive greater seperation between the strokes of the pairs of cylinders that share inlet tracts, to hopefully eliminate the charge distribution problems caused by the siamesed ports. So it moves two strokes further apart and two closer together. It's kind of like having a five cylinder engine with one cylinder removed, in that there is a big lump of the cycle where there is no power stroke. But this is more than made up for by the slightly overlapping power strokes happening elsewhere in the cycle which add huge torque and power gains, coupled with the better fuel charge distribution (which basically means you can run a much less flow efficient inlet manifold without such a big penalty, something like a short neck single Weber for example) which all in all means you can get buckets of tractable power out of a scatter cam engine.
The downside is that it's rough. It's really rough at a relatively low (or normal for street use) idle speeds, and basically quite violent for the crank throughout the range as the smooth power delivery is interupted. This is really bad for a three main bearing engine like an A series. You need some proper crank retention (you can't actually buy the all steel multi bolt full main cap set off the shelf any more but it was great for this application) and really good harmonic damping to try to keep the crank straight and stop it failing. Ultimately main bearings and thrusts won't last as long in these motors as they would otherwise.
They are also expensive as it's not possible to grind them from an existing cam due to the unusally wide timing, so a brand new full circle core unit has to be employed which is costly. They must be timed in really well and other mods need to be performed at the same time in order to make them usable. You couldn't just stick one into an engine which had not been assembled with one in mind.
Pretty much a straight race cam.
A scatter cam plays about with the timing of cylinders in pairs. This is in order to acheive greater seperation between the strokes of the pairs of cylinders that share inlet tracts, to hopefully eliminate the charge distribution problems caused by the siamesed ports. So it moves two strokes further apart and two closer together. It's kind of like having a five cylinder engine with one cylinder removed, in that there is a big lump of the cycle where there is no power stroke. But this is more than made up for by the slightly overlapping power strokes happening elsewhere in the cycle which add huge torque and power gains, coupled with the better fuel charge distribution (which basically means you can run a much less flow efficient inlet manifold without such a big penalty, something like a short neck single Weber for example) which all in all means you can get buckets of tractable power out of a scatter cam engine.
The downside is that it's rough. It's really rough at a relatively low (or normal for street use) idle speeds, and basically quite violent for the crank throughout the range as the smooth power delivery is interupted. This is really bad for a three main bearing engine like an A series. You need some proper crank retention (you can't actually buy the all steel multi bolt full main cap set off the shelf any more but it was great for this application) and really good harmonic damping to try to keep the crank straight and stop it failing. Ultimately main bearings and thrusts won't last as long in these motors as they would otherwise.
They are also expensive as it's not possible to grind them from an existing cam due to the unusally wide timing, so a brand new full circle core unit has to be employed which is costly. They must be timed in really well and other mods need to be performed at the same time in order to make them usable. You couldn't just stick one into an engine which had not been assembled with one in mind.
Pretty much a straight race cam.
Edited by Dan, 24 August 2005 - 06:26 PM.
#5
Sprocket
-
- Members
-
- 7,266 posts
Great on Injection faults
- Location: Warrington
- Local Club: Manchester Minis
Posted 24 August 2005 - 10:31 PM
The exhaust note sounds absolutely great though :tongue:
1 user(s) are reading this topic
0 members, 1 guests, 0 anonymous users
