peter030371
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| posted on 11/3/22 at 10:04 AM |
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Cam choice
I have a choice of Cams for my 2lt Duratec rebuild. I am looking at moving peak power from the current 6.7krpm to around 7.2krpm. Not so worried what
that peak power really is (expecting 200-220bhp) just so long as it doesn't drop off a cliff past 6.7krpm.
I know that I will sacrifice some power low down (below 2.5krpm) but the only time I am down here is from a standing start and then I can't use
much more than 50 or 60% throttle unless I want to sit spinning the wheels.
I don't want to pocket my pistons and to be honest shouldn't need to for the spec I am looking at.
So I have 2 sets of cams sat on my workbench. Not going to mention brands so I don't get any answers that favour one brand over another, they
are both UK made and both from well known brands 
Cam set 'A' has valve lift of 9.52mm (both inlet and exhaust) and slightly longer duration than a standard set of cams. Cam set
'B' has a little more lift than set 'A' at 10.21mm (in and ex) but at the reduced duration (18°less). Still more duration than
the standard cams.
I have spoken to both manufactures and I am leaning towards 'lift is king' but the other thinks the longer duration will be better to
achieve what I want....but then they would say that!
Any one have any direct experience on this, do I go for more lift and less duration or less lift but longer duration?
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obfripper
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| posted on 11/3/22 at 12:15 PM |
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You will have an amount of lift where no further increase in flow is possible, you want the valve to be at or above this amount for the longest period
to maximise the amount of air into the engine. For the same duration cams, a larger amount of lift will give a longer period in this area, giving more
induced air.
If both manufacturers provide a lift/° graph, scaling and overlaying those will give a better picture of what the cam is doing and which is actually
giving the best airflow.
If neither gives those graphs, you could map out those figures with a dti and angle gauge.
Also some manufacturers quote duration at 0.1mm lift, and some at 1mm lift. This can give a false impression of a cam being weaker than it is.
If you place cyl 1 at tdc, you can measure the distance required to bring the inlet and exhaust valves down to touch the piston, this is the max
clearance at tdc, obviously a safety margin will need to be applied, but most manufacturers give a lift at tdc value with optimum timing which you can
use to check compatibility.
Dave
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peter030371
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| posted on 11/3/22 at 07:40 PM |
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quote:
Originally posted by obfripper
You will have an amount of lift where no further increase in flow is possible, you want the valve to be at or above this amount for the longest period
to maximise the amount of air into the engine. For the same duration cams, a larger amount of lift will give a longer period in this area, giving more
induced air.
If both manufacturers provide a lift/° graph, scaling and overlaying those will give a better picture of what the cam is doing and which is actually
giving the best airflow.
If neither gives those graphs, you could map out those figures with a dti and angle gauge.
Also some manufacturers quote duration at 0.1mm lift, and some at 1mm lift. This can give a false impression of a cam being weaker than it is.
If you place cyl 1 at tdc, you can measure the distance required to bring the inlet and exhaust valves down to touch the piston, this is the max
clearance at tdc, obviously a safety margin will need to be applied, but most manufacturers give a lift at tdc value with optimum timing which you can
use to check compatibility.
Dave
They don't have graphs but I do have a dti at work I could borrow (if I can find it, we don't use it often) and plot them on the same
graph. What am I looking for on the graph though?
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obfripper
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| posted on 11/3/22 at 09:23 PM |
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The area under the graph will give a good idea of the relative performance of each cam, it will also show how much the peak lift is held, as this is
the area where most air is inducted - if you were to map the lift, you could also weight the sites according to flow at that lift to give a theoretic
cylinder filling volume.
I don't know of any direct 2.0 duratec info, but spotted a standard 2.5 duratec inlet port flows 90cfm at 2.5mm lift, 175cfm at 5mm lift, 240cfm
at 7.5mm lift , with small gains from there to 276 at 12.5mm lift, with mention that a high port 2.0 head flows similarly but no mention of actual
spec tested.
It will probably be easier to rough out the readings by first measuring angles for 1mm lift increments, then take additional readings to smooth out
any areas where the rate of change is high, this should be alot quicker than trying to check with fixed angles, once you have your readings you can
offset them to line up the peaks for comparison.
With the lift profile, you can also calculate the useable adjustment range for setting the cams once you get it on the dyno, this will be useful for
your tuner to know the bounds of any adjustments to stay within the safe tdc valve lift clearance.
I'm guessing the cams you have are in the 1.5-2mm lift at tdc range (for the recommended timing), 1mm will likely cover any expansion of
piston/rod/valve, so a max clearance of 3mm would suffice in this example, the piston velocity is slow around tdc but the cam is operating at half
crank rpm which should mean the valve will not catch up the piston unless the profile has huge acceleration at that point which is only likely with a
race cam profile.
Dave
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snapper
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| posted on 15/3/22 at 08:49 AM |
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The Fiesta ST150 head flows more than the standard Duratec head and you won’t run out of flow with what you are planing.
One thing to note is that a 4-1 exhaust manifold will have a dip in torque 3k to 4k maybe wider, a 4-2-1 will fill in that gap
I eat to survive
I drink to forget
I breath to pi55 my ex wife off (and now my ex partner)
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peter030371
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| posted on 15/3/22 at 09:07 AM |
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All the heads I have are 'high flow' so as you say that won't be a limiting factor.
My budget will not extend to a decent new exhaust at the moment. BTB or Simpson both want over £1k and anything lower cost might well only do half the
job for 3/4 of the money.
I am leaning towards the longer duration cam as that is going to move the peak power up the rev range which is what I really want. The shorter
duration/higher lift might give me more mid-range but that is not such a concern to me 
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