The characteristics of a fast road cam

[benjj]

Does anyone have any experience of these?

My Mk2 Golf is running (amongst other things) a Kent fast road cam.

I am trying to diagnose a problem I've got which means the car is very (and I mean very) twitchy at low revs. It almost feels like it is being starved of fuel but I know it is not. The throttle is like an on/off switch for the first few % of travel. When you take the slack out of the throttle and apply pressure it is a real 'bang' and you're away. The car performs magnificently at high revs but when in slow stop/start traffic it is like piloting a kangaroo.

This is the first car I've had that has got a lairy cam in. Has anyone else got any experience of these? I read about how racing cams give crappy low rev/speed performance but don't really know what people mean by it.

Any advice very welcome.

Cheers,

Ben

[Rich_W]

As a rule, not what you're experiencing. If anything they are slightly slower at small throttle openings. Fast road's tend to have a deficet of torque low down to transfer the power to the high end of the rev range.

Assuming whoever fitted it did it properly of course. Might find it's too retarded in relation to TDC. But it would need to be a long way out to get "switch like" power under 2Krpm.

Your fault is more likely to be elsewhere.

[benjj]

Thanks Rich.

The only other thing is my fuel lift pump is whining like a b*tch, could be that I suppose though once you are booting it all is well.

[ringram]

Working as designed Id say.

If you want a good cam for around town stay stock.

Generally the issue is that by increasing duration overlap is also increased, this means at lower rpm you get reversion (ie air going out of the engine and not into it!) and the optimal tuning of the intake and exhaust pulses is moved higher in the rev range.

This means less air/fuel in the engine at lower rpm and more at higher rpm compared to stock.

As an alternative you can run ported heads with larger valves and instead of more duration go for more lift, better intake and exhaust tracts. Then there will be little to no compromise only more power.

Often larger cams are also combined with higher compression ratios. This is because less air is trapped in the engine so effective or dynamic compression is less with a bigger cam. This can help crutch the lower end of the rpm range so its not so badly compromised.

[ringram]

That being said as per above you might have an issue with the TPS or some such as well. It depends if the "switch" is rpm based or purely pedal position related.

[stevieturbo]

benjj said:

Does anyone have any experience of these?

My Mk2 Golf is running (amongst other things) a Kent fast road cam.

I am trying to diagnose a problem I've got which means the car is very (and I mean very) twitchy at low revs. It almost feels like it is being starved of fuel but I know it is not. The throttle is like an on/off switch for the first few % of travel. When you take the slack out of the throttle and apply pressure it is a real 'bang' and you're away. The car performs magnificently at high revs but when in slow stop/start traffic it is like piloting a kangaroo.

This is the first car I've had that has got a lairy cam in. Has anyone else got any experience of these? I read about how racing cams give crappy low rev/speed performance but don't really know what people mean by it.

Any advice very welcome.

Cheers,

Ben

You seem to be suggesting you just acquired this car ?

And you also say a fast road cam, then a lairy cam. So more to the point, exactly what camshaft is it ?
Has the car been tuned in any shape or form to accommodate the camshaft, in terms of fuelling or ignition ?
Was it even installed correctly ?


Far too many unknowns, but to me it sounds like something is plain wrong, as opposed to a characteristic of the cam. But without knowing more background etc, impossible to say

[benjj]

Thanks all.

Yes, I've only just got the car. The cam is a Kent item - I have the specific details just not in front of me but the research I have done on it is that it is ideal for top end, poor for bottom. The car has had other work - ported and gas flowed head, exhaust manifold, exhaust and some work on tuning/timing.

None of this was done by me though so I am working it through in order to check things out.

I suspect, as pretty much everyone has said, that the problem is something else, most likely the whine from the fuel pump (rear lift pump in tank) is the cause.

The main question was that of charachteristics of a fast road cam vs normal stock item and that has been answered comprehensively - thanks.

[stevieturbo]

benjj said:

The main question was that of charachteristics of a fast road cam vs normal stock item and that has been answered comprehensively - thanks.

I havent come across a fast road cam that has been that horrific though. Most do still tend to be fairly mild.

But the term could cover half a dozen different options, so it is pretty meaningless without solid information.

[Steve_D]

Have to agree the cam is unlikely to be the problem. If you said it was rough running and generally flat until 3000rpm then took off that would be typical of a performance cam.
The fact it is misbehaving right after tickover points at something else.

Pump is unlikely. If it were failing then it would not perform at higher revs/power demand.

Worn TPS or incorrectly set.
Timing of the cam
Timing of the ignition

Steve

[DVandrews]

It's not unknown for the overlap on after market cams to seriously disrupt the vacuum in the plenum, this in turn disrupts the MAP signal and makes the ECU think there is load on the engine with the consequence that the engine runs on the wrong part of the map/calibration. This is only true for MAP based systems. Things dont get back to normal until the reversion that causes the problem disappears, I.E. when the engine comes on cam. I have monitored this on several engines with real time diagnostics plugged into the OE ECU.

Not knowing the detsils of the ECU or the cam specs/timing I couldn't say for sure.

Dave