Race Engine Forum UK

I'm about to start building an engine for my Skoda Felicia rally car, using the 136 engine. Due to the Group A rules, I will be using the standard MPi inlet manifold and standard exhaust manifold, although (within the port size limits allowed, which are very close to standard dimension) I will clean up the ports and manifolds.

I am building from a reputedly ex-works engine (whose piston failures I described on another thread), but the camshaft is one area where I am unsure how to proceed. On initially seeing the "Grp A" cam, it looked a dramatically different profile to standard (see picture below), with much "wider" lobes, and therefore a much greater time of opening. It also has more lift.

I also have a Kent "SK6" cam (in a poor state of repair, with some damage to some of the cam lobes), and a standard one.

Last night I measured the cam profile for inlet and exhaust for each of the three cams (using a degree wheel, the cam in the block and a follower and pushrod), and was surprised to see that the GrpA cam was quite similar to the SK6 profile in terms of the follower movement.

Although my measurements were only taken every 10 degrees (I think 5 would have been better), the standard cam seems to have a Lobe Centreline Angle of 110 degrees, while the GrpA cam and SK6 are more like 105.

I was also surprised to see that the valve acceleration rate for the standard cam's inlet is actually more than the other two, and the exhaust rates are all quite similar - this had been one of my initial concerns when looking at the GrpA profile, but it would seem not to be the case, and I think that I'm right in saying that this will be good for long-term reliability of the valvetrain.

Kent quote the usable range of the SK6 as being 3500-8000 rpm, so would it be fair to assume a similar range from the GrpA cam?

What may not be visible in the picture is that the GrpA cam has a bigger base circle and the shaft itself is thicker - 23mm as opposed to the 20.5 of the SK6.

Any comments or thoughts on this are appreciated - I have attached a picture showing the three cams side-by side as well as an excel spreadsheet of my measurements with graphs of the lobe shapes, acceleration curves in it as well. I realise that my measurements aren't relative to engine TDC (I just took timing from the keyway on each cam), by the way.

Thanks for an interesting post and I am sorry I was unable to devote time to it till now.

I have looked at your well-presented data and plotted off the inlet characteristics, since this is primarily what I'm interested in. The ex is far less critical and one can assume quite reasonably that the ex will match the inlets well enough in both the GpA and Kent cases.

As far as LCA is concerned I will simply say that all will work OK but the Kent/GpA LCA will tend to give a stronger top-end characteristic. Closer LCA (lobe centerline - or separation - angle) has been proven to do this, but so much depends on the inlet and ex length that one could never say it will be better for sure on any particular engine than a wide separation. You might be lucky and it would be more powerful, you might not and it could be worse. 106 is a good all-round figure.

On the basis of the graphs of inlet below I would say the GpA has the edge and that is the one I would use. Plus you can be sure it has been thoroughly dynoed by Skoda. Is the Kent one the same? No, if your measurements are spot-on. I'm honestly not sure (sorry, my posting seems always to get done in a bit of a rush) whether I'm looking at cam lift or valve lift, assume valve?
For those who do not know - with engines with rocker valvetrain, the rocker ratio is generated by the mechanical advantage of the rocker and controls the valve lift from a given cam profile. In determining true valve lift the length of the rocker, the rocker centreline position and valve tip have major influences on both the valve lift and cam timing.

I hope this helps a bit and feel free to report in again.

GC

Hi Guy

Sorry, I should have stated it - it was cam lift - I ran a DTI on the cam follower in the block. The rocker ratio is 1.44 to 1.

I'll fit the GpA cam (with new followers, just to be on the safe side), and see how it goes - the rest of the engine seems to have been well modified, the head has had a considerable amount of porting work done and what looked (at a brief examination) to be a nice valve job too.

Thanks for your comments - they're all appreciated!

Darren