Race Engine Forum UK

I have a fairly built 8v 2.0 in my Lancia Scorpion. I have 12:1 CR, head done to your specs with stock valves, 4-2-1 ANSA exhaust (high flow muffler, no cat), single DATRA carb. From what I understand, my cam(45/75 - 75/45) has the same 300' duration as the 40/80 cam, but is retarded 5' to produce power at higher rpms. For a street car I don't think I want the power band higher up. In general, are the lobes on cams symetrical (for example, 150' before LC and 150' after LC for a 300' cam)? I have adjustable cam pulleys, could I adjust the LCA to 110' and effectively have 40/80's?

The carb is going to change shortly, I have a dual downdraft setup on its way now.

I have attached a picture of the engine going back in the car... no reason.

Thanks for your input,
Jason Greenwood

Well, generally, if the cam manufacturer cites the timing figures you're safer using them than swinging the timing ad-hoc, unless you optimise it by accurate re-dialling in-situ & dyno testing, see calculations at:

http://guy-croft.com/viewtopic.php?t=14

There is certainly good evidence from work I did last year that running a short lobe separation angle between the two cams tends to yield a the best overall torque/power characteristic, eg: using 102 deg FL inlet and 102 deg FL ex = 204 deg LSA compared with 220 deg LSA when cams are set at 110 deg, a common setting on the F/L TC unit. That said, peak bhp can be higher with a wider LSA, though the bigger area under the torque/rpm curve can, in practice, make the vehicle far more 'driveable/tractable' thru the gears and thus - overall - much faster.

The downside is that such a narrow LSA generate very high lift of valves around tdc and dry build and adjustment of valve reliefs is a MUST; this brings an added problem: are the pistons thick enough to allow deep enough VRs?

Can you alter the full lift position of one cam to make it behave like another? No, not necessarily, because the profiles may be markedly different in terms of the areas under their lift-degree map.

It is quite usual to see two cams of 'same' duration with very different maps, and hence very different lift at tdc, acceleration and dwell. You have to get hold of the cam map to understand what a cam is really doing. Duration is quite wrongly cited by people (all the time, all over the world) as a measure of the performance of the cam, thus it is invariably taken to mean more than it really does. Area under the cam curve at true clearance is rarely mentioned and by cam manufacturers too. If it was, a lot fewer cams would change hands. In reality duration is merely the arithmetic sum of the effective lift and closure phases and nothing more.

I propose to write about this in detail when I get time in 'All the power is in the head', maybe part 7. In the meantime have a look at the graphs below, which demonstrate how different cams can really be - if you buy by duration alone - when you get them home. The Alquati is a good cam for what it is, really it's a 1600 profile, remember, it's lift integral is pretty small. But - I have seen regrind cams with less than 30 in deg area at true clearance, this alone results in huge loss of potential airflow under valve lift compared with a good billet design.

GC

That would be a great idea Guy. The differences between advertised duration, duration @ 1mm and duration @ 0.050" are all well misunderstood. Also acceleration, lift @ TDC, and lobe centrelines.