Camshaft: Trying to identify and check

[stevieturbo]

As is often the case, more isn't always better. Sacrificing a few horsepower for better drivability is often a wise decision.

Regards

A great quote in DV's A-series book I think it was, and perhaps others regarding camshafts.

Pick the camshaft you want, then buy the next stage/level down from it.

[GDCobra]

Thanks Steve, I'll give that vid' a proper watch when I get home later.

Rhoads lifters are designed to bleed down. And this would be more so at lower rpm, when there is force over a longer time to eject oil and effectively reduce lift/duration seen at the valve.

I remember reading about these many years ago (are they even available now?), seems to me they give something like the Honda VTEC effect in that they will alter the cam duration and, presumably, at low RPM then full cam defined duration and lift at higher RPM (something like 3000?).
If I've understood correctly this would effectively tone my Typhoon cam down to something more like standard or Hurricane at low RPM while still behaving the same higher up.
Makes me wonder why these don't seem to be more popular, I've heard they are noisy tough, perhaps that is why.

I have used some of the Real steel cams years ago, and certainly don't recall them being crap low down, but it's sooooo long ago not really sure. And my RV8 in various guises was always twin turbo, never n/a
I think I always used the Rhoads lifters too, tickety tick.

Well I wouldn't say the behaviour is crap, probably more like sub-optimal. Anything less than about 1200RPM does not pull smoothly, I know that sounds like very low RPM but it would work very well in my car when at slower speeds which are a fact of life.
I also think this is possibly why (or at least contributing to) my excessive timing requirement at low RPM)

Were the Rhodes really tickety?

[FBW]

One other thing I did wonder about is whether there is any lost motion with a hydraulic lifter. I've never had one of these apart but I'm guessing they have some sort of port in them to let oil in which will close off as the lifter rises (similar to a brake/clutch master cylinder) this could result in the base of the lifter rising somewhat before motion is transferred to the pushrod. Perhaps during early motion when valve spring is not applying too much force the oil pressure is enough to prevent relative motion - Any body know more?

...Let me elaborate a bit.

The main components of a V8's valve train are: camshaft, hydraulic tappets, pushrods, rocker arms, and valves.
The task is to transfer the camshaft's lift to the valves (I'll leave out the topics of valve timing, valve lift curves, etc. for now).
Ideally, this would be achieved with zero play in the assembly of the transmission elements.
The catch is that manufacturing tolerances, wear, and thermal expansion make achieving zero play difficult.

In older engines, this was usually solved with adjusting screws. A specific valve clearance had to be set regularly.

# Adjust >> To compensate for tolerances
# Regularly >> To compensate for wear
# Valve clearance >> To allow for thermal expansion.


Modern engines like the Rover V8 use hydraulic tappets for this purpose. They handle the entire task.

A hydraulic tappet (hydraulic lifter) is most easily visualized as a hydraulic cylinder.
The hydraulic pressure comes from the oil pump.
Inside the hydraulic lifter is a check valve that allows oil to flow in quickly but prevents it from flowing back out.
However, the hydraulic lifter is intentionally not perfectly sealed like a hydraulic cylinder. There are no seals inside, but the parts fit together so precisely that only very small gaps remain, through which the oil can escape very slowly.

The hydraulic lifter is dimensioned so that, in conjunction with the engine's oil pressure, it generates significantly less hydraulic force than the force required to compress the valve springs.

Now, how does it all work?

- The engine has been off for a long time.
- At the valves that were open, the pressure from the valve springs forced the oil out of the hydraulic lifter.
- The hydraulic lifters at the valves that were closed had no external load and are still full.
- The engine is started and fires up.
- The oil pump builds up oil pressure.
- The valves that were closed perform their function from the first revolution.
- The valves that were open start with a smaller lift >> they don't open fully.
- Why? The hydraulic lifter has compressed, and there's play in the linkage, resulting in reduced valve lift.
- The engine rattles audibly for the first few seconds.
- The check valve quickly allows the now pressurized oil into the hydraulic lifter.
- This always happens while the valve is closed (when it's open, the force of the valve spring is acting upon it, and the hydraulic pressure can't overcome that (see above)).
- After a very short time, all the hydraulic lifters have built up pressure, and the valve train operates without play.
- Due to the thermodynamic activity within the engine, the engine warms up.
- If the cylinder head expands faster than the valve train, the hydraulic lifter fills slightly more and compensates for this.
- If the valve train (valve, rocker arm, pushrod, hydraulic lifter) stretches faster than the cylinder head, the resulting load forces the oil out of the hydraulic lifter, causing it to shorten. (That's why it can't be completely sealed.)
- If the camshaft or another component in the valve train wears, the hydraulic lifter also compensates for this.

...so much for the normal operation.

The Rhoads lifters are constructed similarly to those described above. However, they have a slightly increased leakage rate. This means that at low engine speeds and the resulting lower oil pressure, the lifters compress. Consequently, the full valve lift is no longer transmitted to the valve.

In principle, a brilliant idea. Unfortunately, the valve train then operates with play where there shouldn't be any.

Camshafts for solid lifters have a much shallower approach ramp to compensate for valve clearance. Camshafts for hydraulic lifters do not. This creates an overly smooth surface for play. The result is a valve train that runs very loudly and experiences high wear at low engine speeds.

Certainly acceptable if you want to give a racing engine enough slack to be loaded onto a trailer. Not ideal for everyday use.


Regards Frank

[stevieturbo]

Well I wouldn't say the behaviour is crap, probably more like sub-optimal. Anything less than about 1200RPM does not pull smoothly, I know that sounds like very low RPM but it would work very well in my car when at slower speeds which are a fact of life.
I also think this is possibly why (or at least contributing to) my excessive timing requirement at low RPM)

Were the Rhodes really tickety?

It could just be a tuning issue ?

The Rhoads were ticky, but no real annoyance really. As to how much they really changed things, no idea, as don't think I ever ran them against standard type lifters to compare.

[FBW]

It could just be a tuning issue ?

The Rhoads were ticky, but no real annoyance really. As to how much they really changed things, no idea, as don't think I ever ran them against standard type lifters to compare.

In principle, Rhoads lifters cut off the lower part of the cam profile. But only at very low RPMs.

Low valve overlap is one of the factors for achieving smooth engine operation. The Rhoads lifters lower the cam profile by just under 0.5 mm. The graph shows that the overlap of the relatively aggressive Typhoon cams is reduced to something similar to that of the Hurricane cams.
At higher RPMs, the full profile is active.




As I said, the idea is good. However, I believe the advantage comes at the cost of increased noise and, even worse, increased wear.
Therefore, it won't change the performance at higher RPMs at all. Neither better nor worse.


I have no experience with turbocharged engines. Theoretically, "different rules" should apply there than with naturally aspirated engines. I think the valve overlap has to be low anyway.

Is that correct?



Regards

[stevieturbo]

Obviously theory and reality....can be blurred.

It would take special equipment to measure valve lift in real time to see what the Rhoads lifters actually do, and when they do it, as I'm sure even oil types and temperature will playa big role in that too.

As for turbo requirements, depends on the overall install. If it was a full on race deal where you can achieve EMAP/MAP pressure ratios of less than 1:1, then overlap would not be harmful.
But that is less often the case, hence low overlap or sometimes none at all can be desirable.

In general though, it's hard to go wrong with a fairly mild cam profile for a turbo setup. Boost will more than make up for any perceived cam smallness, if that's a way to describe it.

[FBW]

That's what I would have expected.

Theoretically, a larger valve overlap should only make sense on a turbocharged direct-injection engine. It would improve scavenging without heating the exhaust.

Interesting! I'd never thought about that before (due to my problem).

Cheers

[GDCobra]

I remember reading some time back that one one advantage of the injection system used on ME/BF109 engines (Over and above being able to pull negative G) was that, due to being direct injection they could use valve overlap to flow air through the engine to aid cooling without flowing fuel into the exhaust. Handy when your supply of fuel and elements needed to aid octane rating are limited.

[stevieturbo]

That's what I would have expected.

Theoretically, a larger valve overlap should only make sense on a turbocharged direct-injection engine. It would improve scavenging without heating the exhaust.

Interesting! I'd never thought about that before (due to my problem).

Cheers

No.

Overlap can only work when EMAP is lower than MAP. If pressure on the exhaust side is higher than the intake, then whatever is in the exhaust will try and blow back into the intake...not the sort of EGR you want
Whether port or direct injection is irrelevant.

[GDCobra]

OK, thought I'd close the loop on this.
I checked all the lobes on my cam. Positives are:
All lobes gave the same profile within measurement uncertainty which would say my cam is good even if it's not the most suited to me.
The peak positions were in the correct location, don't have the figures in front of me but 110° before and after TDC for Exhaust and Inlet respectively so the timing was correct, I was a bit concerned that it may have altered, I believe there is a vernier pulley on it and thought it may have come loose.

The cam profile (I would say) is consistent with sub-optimal low speed running and possibly high timing requirement at idle.

So the question I have to answer now is do I live with this as-is or pop a new one in there? Not a job I particularly want to do (or pay for) but could make the car a whole lot more satisfying to drive.

Strange thing is though I seem to recall that it used to be better than this when I first got the engine although that could just be that I was less interested in the low RPM manners and more in the high RPM rush back then.

I did install a free-er flowing intake manifold and trumpets a couple of years ago, perhaps that's magnified the issue.

[FBW]

Larger intake funnels shift the optimal airflow upwards, thus tending to lead to more power.

However! They also worsen cylinder filling at low RPMs, thereby reducing torque.

If the smaller funnels are still available, reverting to the smaller ones would improve the engine's versatility.

Swapping them is also much less work than replacing the camshaft.

Regards