Follower bleed down rate

sidecar · Post by **sidecar** » Fri Oct 14, 2011 7:09 am

mgbv8 wrote:
Its not really oil related... Its time / rpm related depending on the viscosity of the oil you are using and the temperature of the oil at that time as this effects viscosity.

The lifters take oil in and let oil out in their up and down cycles. When the cycle gets too fast for them to drain they cant let oil out fast enough and they stay pumped up. As the engine rpm increases so does the oil pressure. This means that each shot of oil from the lifter gallery contains oil at a higher pressure. When the amount of oil going "in" to the lifter equates to more than the lifter can drain at that rpm it stays full !!!

Like Devonman says, if this is the case how come half of the lifters don't drain way when the engine is switched off? I know that knackered old lifters can drain but mine don''t.....Actually I wonder if they are designed so that the can only drain whilst they are sat on the base circle of the cam? This would stop open valves draining the oil when the engine was switched off but would allow pumped up lifters to drain back to their pre-load setting whilst the engine was running. This could be done with a port arrangement a bit like the ports in a 2-stroke engine.

You are saying that the relief in tension on the lifters due to valve bounce allows the lifters to fully prime and give full lift which in turn allows the valves to stay more open than they should ??

Yes that is what I'm saying, and it is by design in order to stop the nut behind the wheel from revving the engine any harder!

Maybe thats the way Rover designed it ??
Either way, on the stock setup the valves being held a bit open at this rpm does the job of limiting rpm??

Yep, I think it does. But I guess you can still bounce the engine around at this sort of RPM and hold that RPM. The recent post with the broken spring retainers and the crackling sounds from the exhausts I reckon was due to the valves staying open, the chaps valve train was not designed to run at 7500 RPM and it was letting him know.

sidecar · Post by **sidecar** » Fri Oct 14, 2011 7:27 am

DEVONMAN wrote:I think we need to do the Math on this one.
The internal area of the lifter plunger is about 0.2 sq inches. Therefore at say 50 lb/in2 gallery oil pressure, the upward force is 10 lb. The valve spring fitting load is 75 lbs which gives pushrod resistance load of 75 x 1.6 = 120 lb.
Therefore the normal pushrod load far exceeds the pump up force/ability of the lifter and pump up therefore cannot occur until the push rod load is removed/reduced by valve float action at high revs.

An effective rev limiter designed by Buick for the RV8 back in the 50/60's and we still depend on it.

Cheers Denis

Hi Denis,

Thanks for agreeing with me!

Your maths is sound but you have missed out an important point.

The pressure that the valve spring can produce is used to control the valve train, at low RPM there will be an excess of spring pressure lift over and not required but as the revs increase more of the springs available is required in order to keep the lifters in contact with the cam. At the point of valve float all of the springs pressure has been used up, there is none left over to hold the pre-load in the lifter.

Actually as I write this its making me think that if the oil pressure is high enough there will be an RPM where the valves are not yet floating but there is not enough 'spare spring pressure' left over to hold the pre-load down in the lifters, THIS will be the point at which they pump up and start to hold the valves open. (This may well be within a hundred PRM of the actual valve float RPM anyway at a guess)

The other fly in this ointment is that according to Des Hammil the oil pressure in lifter gallery is not constant, it is lower than what the gauge is reading and the further away from the oil pump you get the lower the pressure gets due to the various lifters and bearing all taking a share of the oil and its pressure

Interesting stuff!

Pete

wokingwedger · Post by **wokingwedger** » Fri Oct 14, 2011 7:38 am

mgbv8 wrote:[As your issue seems rpm related. what oil pressure do you read at the problem rpm ??

I will check, but from memory its about 30 psi at idle and not discrenably any more at 1200 rpm when it happens.

It is much more noticable when the revs are on the way down (falling oil pressure)

DEVONMAN · Post by **DEVONMAN** » Fri Oct 14, 2011 8:12 am

Hi wokingwedger.

It would be helpful to have a good recording of the noise.
I have listened to the one on Piston Heads but I could not distinguish the noise from the rest.

Cheers Denis

wokingwedger · Post by **wokingwedger** » Fri Oct 14, 2011 8:22 am

DEVONMAN wrote:Hi wokingwedger.

It would be helpful to have a good recording of the noise.
I have listened to the one on Piston Heads but I could not distinguish the noise from the rest.

Cheers Denis

Yes I know what you mean.

My noise is more distinct than that one, but I dont have a video recorder (or the knowledge how to get it on the computer)

TVR Beaver · Post by **TVR Beaver** » Fri Oct 14, 2011 8:24 am

Hi All

Just to add my 2p's worth... and with regards to oil pressure drop as the oil gets further away from the pump etc... as Peter says.. its hard to tell where its coming from... but in general area terms I would say its more towards the front / dizzy side (where youd ecpect the oil pressure to be best??...
There is a cap on the lifters (also a clip of this being moved on the TVR post)... Can this float due to high pressure in the lifter as the valve train pressure comes off?....

Have a look.. see what you think?

All the cars that I've seen doing this (about 60-70% of the 500's) all make the same noise and same area?... but why only 500's??

wokingwedger · Post by **wokingwedger** » Fri Oct 14, 2011 8:28 am

TVR Beaver wrote:There is a cap on the lifters (also a clip of this being moved on the TVR post)... Can this float due to high pressure in the lifter as the valve train pressure comes off?....

I have been sitting at my desk with a pen in the lifter cap and wriggling it as John says, and it does sound just like that

wokingwedger · Post by **wokingwedger** » Fri Oct 14, 2011 8:33 am

TVR Beaver wrote:but why only 500's??

More aggressive cam, speed of drop on the back of the cam ?

sidecar · Post by **sidecar** » Fri Oct 14, 2011 8:35 am

wokingwedger wrote:
TVR Beaver wrote:There is a cap on the lifters (also a clip of this being moved on the TVR post)... Can this float due to high pressure in the lifter as the valve train pressure comes off?....

I have been sitting at my desk with a pen in the lifter cap and wriggling it as John says, and it does sound just like that

I don't don't where the clip of this is on the TVR post but if you mean the cup that sits inside the lifter then I know a bit about those. Basically a company called 'Bhere' or something like that had loads of lifters made for the Rover lump, unfortunatley the cup was slightly too small in diameter, as a result it could flip over which jammed the pushrod with bad results. This info came from RPI who I normally take with a large pinch of salt but I guess that they had no reason to make this up. They told me that they had to repair many engines as a warranty claim due to this fault.

wokingwedger · Post by **wokingwedger** » Fri Oct 14, 2011 8:40 am

sidecar wrote:
wokingwedger wrote:
TVR Beaver wrote:There is a cap on the lifters (also a clip of this being moved on the TVR post)... Can this float due to high pressure in the lifter as the valve train pressure comes off?....

I have been sitting at my desk with a pen in the lifter cap and wriggling it as John says, and it does sound just like that
I don't don't where the clip of this is on the TVR post but if you mean the cup that sits inside the lifter then I know a bit about those. Basically a company called 'Bhere' or something like that had loads of lifters made for the Rover lump, unfortunatley the cup was slightly too small in diameter, as a result it could flip over which jammed the pushrod with bad results. This info came from RPI who I normally take with a large pinch of salt but I guess that they had no reason to make this up. They told me that they had to repair many engines as a warranty claim due to this fault.

link attached

http://www.youtube.com/watch?v=XbCEYmtLPoQ

sidecar · Post by **sidecar** » Fri Oct 14, 2011 8:46 am

I see! Well that is what RPI were on about!

I guess its up to you whether you believe them or not, you could ring them but be prepared for Chris at RPI trying to sell you an ignition box!

As to whether you can put new lifters on an old cam, there's a can of worms, I think that you can but I know that many others think that you can not!

wokingwedger · Post by **wokingwedger** » Fri Oct 14, 2011 9:01 am

sidecar wrote:
As to whether you can put new lifters on an old cam, there's a can of worms, I think that you can but I know that many others think that you can not!

Interestingly, the Range Rover workshop manual say if a tappet is worn, replace it (dosnt mention replacing cam)

DaveEFI · Post by **DaveEFI** » Fri Oct 14, 2011 9:12 am

wokingwedger wrote:
sidecar wrote:
As to whether you can put new lifters on an old cam, there's a can of worms, I think that you can but I know that many others think that you can not!
Interestingly, the Range Rover workshop manual say if a tappet is worn, replace it (dosnt mention replacing cam)

I've replaced the odd noisy tappet before now without replacing the cam. Assuming the cam looks ok, of course. They are so cheap there's nothing really lost by doing so.

wokingwedger · Post by **wokingwedger** » Fri Oct 14, 2011 9:18 am

DaveEFI wrote:
wokingwedger wrote:
sidecar wrote:
As to whether you can put new lifters on an old cam, there's a can of worms, I think that you can but I know that many others think that you can not!
Interestingly, the Range Rover workshop manual say if a tappet is worn, replace it (dosnt mention replacing cam)
I've replaced the odd noisy tappet before now without replacing the cam. Assuming the cam looks ok, of course. They are so cheap there's nothing really lost by doing so.

Did you 'break the cam in' again. (run at 2000 rpm for 20 mins etc)

I have a set of brand new 'old stock' lifters made in the 80s still in the oily wrapping paper, and I am tempted to fit them !

My cam has only done approx 2500 miles anf the lifters have no significant signs of wear

DEVONMAN · Post by **DEVONMAN** » Fri Oct 14, 2011 9:33 am

I think we should try and eliminate a few items from the equation in an attempt to find the cause of the noise.

Lets start with the dizzy.
Has anyone got the noise on a 500 engine that has been converted to crank trigger? If so then the dizzy is out of the equation and therefore forget the following.

Grasping at straws, my thoughts on the dizzy !!!!
Did TVR fit lighter advance springs in the 500 dizzy to give greater initial advance? Weights may rattle at 1200 -1400 rpm.
Slim argument I know but possible as the cam pulsing may chatter the dizzy drive at these revs.

Second thought, It it possible that TVR do not remove the oil pump drive toggle an the dizzy shaft when installing a dizzy in a serp engine.

Cheers Denis