camshafts

[ianthomson72]

Hi All,

This is a query for the tech experts on the forum.
Why do the inlet and exhaust camshafts lend to have the same timing and lift as they each operate under very different conditions. The inlet is sucking at generally less than atmospheric pressure and the exhaust has high pressure expanding gases forcing their way out.
I hope confession is good for the soul as I have to admit to managing to break a camshaft through a serious bout of brain fade ( don't ask). I bought a pair of cams on E bay which have different lift and timing and am thinking of fitting the higher lift cam ( 360 thou) to the inlet and leaving the exhaust as standard (s)

Any Thoughts?

Cheers

[rgh0]

HI

You can run different camshafts on inlet and exhaust but for normal road style cams it makes little difference and there is no need for a more restrictive exhaust cam duration. Thus manufacturers tend to make the inlets and exhausts the same to reduce complexity of manufacture and maintenance.

For race cam shafts it is sometimes desirable depending on the rest of the engine design to run an exhaust cam with less duration ( and thus typically less lift) than the inlet to increase mid range torque but with at a small loss in top end power.

If you have one 0.360 inch lift cam ( a D - sprint cam profile ?) and one standard 0.350 inch lift cam ( a B - standard or C - SE cam profile) putting the .0.350 lift cam on the exhaust may make a small difference in top end power versus two D cam sprint profiles but it should not be noticeable

cheers
Rohan

[vernon.taylor]

Salut Rohan

Thanks, that was a concise and interesting reply.

Is it possible to explain as briefly why ? From a noob's knowledge of fluid mechanics it would seem that less lift/duration would mean that less exhaust gases would escape (though more quickly and I suppose the quicker gas expansion would cool things more?) so why would keeping more of the exhaust gases in the cylinders improve any performance parameter?

On the inlet side the logic seems clearer - the more lift/duration, the more inflamable mixture can be aspirated - is that right?

Merci

Vernon

[rgh0]

As you increase duration of the exhaust cam three things happen :

1. The start of the exhaust valve opening occurs earlier on the power stroke, this increases time for the exhaust gas to blow out under their own pressure and reduces the pumping losses especially at high revs but it reduces torque and power especially at lower revs by reducing cylinder pressure earlier in the power stroke

2. At the end of the exhaust cam duration during the overlap with the inlet cam opening increasing duration increases the overlap which is typically beneficial at high revs to complete chamber emptying but not beneficial at lower revs leading to loss of torque.

3. Increased duration allows increased lift and increased lift is generally beneficial in most circumstances on an exhaust cam as exhaust blowdown is supersonic for most of the period and rate is governed by the flow area and valve opening area alone during this period. But due to the factors above the increase in lift possible with increase in duration becomes a net loss at longer durations.


For the inlet cam a longer duration allows higher lift and a greater intake period in a similar way. However again to long a duration reduces mid range torque as it increases the overlap period at the start and and increases the reversion period at the end when inflow inertia is meeting a rising piston which reduces effective compression ratio especially in the mid range.


Thus in a Twin-cam racing engine in an Elan (whose exhaust is never the best due to limited space) a combination of something like a 320 degree 0.5 inch lift inlet and 290 degree 0.460 lift exhaust can give the best performance. With a Twin-cam in a light open wheeler where mid range torque is less critical and a better exhaust system can be fitted having the same inlet and exhaust at say 320 degrees and 0.5 lift will give maximum peak power and better lap times generally ( but not always)

cheers
Rohan

[ianthomson72]

Thanks Rohan,

That 's a very informative reply - still haven't decided on the way forward yet.

Cheers

Ian T

[vernon.taylor]

Merci, Rohan. I'll try to digest that.

@+

Vernon

[AHM]

Why do the inlet and exhaust camshafts lend to have the same timing and lift as they each operate under very different conditions.

Add to your thoughts that the inlet and exhaust valves are different sizes.

From a noob's knowledge of fluid mechanics it would seem that less lift/duration would mean that less exhaust gases would escape

The mass flow into and out of the cylinder is, within reason, equal. The exhaust gas has a much greater pressure differential, so has the ability to leave through a smaller hole in a shorter time.

Under some circumstances the exhaust pressure wave can create a depression in the cylinder that sucks the air/fuel in.

[Mick6186]

Isn't that the reason for a tuned length on the exhaust primaries to create a low pressure wave into the cylinder to get more fuel in, hence a valve overlap.
Mick