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RB26 cams in RB20 didnt work :-(

jnr32r

By jnr32r,
in Engines & Forced Induction

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jnr32r Proficient

jnr32r

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Hydraulic lifters.

The theoretical operation of hydraulic lifters is that they can increase their height (tip of valve to cam rubbing face) but can not reduce their height.  

the lifters will adjust to a point where the valves are slightly open and the car will run like a wrx:

That makes sense and would explain why our cars dont idle. If the base circle is 0.1mm thicker on the RB26 cams then all valves will always be 0.05mm open.

But you said they cant reduce in height and then you said they will adjust themselves after an over-rev, so that means the oil is emptied slightly and the lifter height reduced.

Whats not to say you can reduce that 0.05mm before installation by blowing some oil out and hence making all the valves colsed.

Or are you saying that the oil will fill up to maximum capacity and then push down open the valves. If so i think 0.05mm is such small distance you might get away with it....

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lukevl Collaborator

lukevl

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Theoretically they don't reduce in height. However they do, it just takes a while. So it doesn't matter what size the base circle is, they will not hold the valves open on the base circle. Thats why I said that it will hold the valves open after an over rev, but they will bleed down after a minute or so.

Say a lifter has about 2mm or travel in its hydraulic range, you could have 4mm difference in base circle diameter and the valve timing and operation would remain unchanged.

lukevl Collaborator

lukevl

Posted
Posted

You won't be able to noticeably linish the ramps off the lobe. One option is to get a light cam grind on them to make them hydraulic. If you did do that you would be able to improve the GTR profile by about 0.5mm in lift and 10 degrees or so at 0.050". It would be a goo dimprovement and the base circle would still be like 31.50mm or more depending on how similar the profiles are. This way you could run a hydraulic profile on the lifters and have a larger profile than GTR cams and they would have a new rubbing surface. All on the same hydraulic lifter with no mods. (I am assuming the hydraulics will pick up on 0.25mm travel)

Before you ask, it would cost around $320 Inc. Tax. (Retail).

Sydneykid Grand Master

Sydneykid

Posted
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Hi lukevl, bin a while since we dissagreed, so I'll jump in now...

Am I reading this right...

The theoretical operation of hydraulic lifters is that they can increase their height (tip of valve to cam rubbing face) but can not reduce their height.

Because it seems to dissagree with this....

The lifters adjust their height accordingly to suit valve growth.

Doesn't this mean that the lifters must get both "shorter" and "longer" to take up the hot versus cold clearances?

Or have I missed something :D

Sydneykid Grand Master

Sydneykid

Posted
Posted
the oil will fill up to maximum capacity and then push down open the valves.

That could only happen if the oil pressure was higher than the valve spring pressure. Very unlikely at idle rpm. :D

lukevl Collaborator

lukevl

Posted
Posted
Theoretically they don't reduce in height. However they do, it just takes a while. So it doesn't matter what size the base circle is, they will not hold the valves open on the base circle. Thats why I said that it will hold the valves open after an over rev, but they will bleed down after a minute or so.

Say a lifter has about 2mm or travel in its hydraulic range, you could have 4mm difference in base circle diameter and the valve timing and operation would remain unchanged.

Don't be silly SK. You know how they work. they hold their position for a while but do leak thats why they need constant oil supply.

Being a mechanical seal -Just two bits of steel with maybe half a thou (excuse my units of measurement) of clearance diameter- they leak oil slowly. No meachanical seal is perfect. But this is an important aspect of the lifter, they have to be able to bleed off some of the oil to accommodate valve growth.

Also the force pushing on the lifter is spring pressure, but the force holding the lifter up is oil pressure x lifter internal bore area.

Geez Louise, you have to be careful how you write things around you SK! :Oops:

Cubes Grand Master

Cubes

Posted
Posted

I haven't got time to go googling at the moment due to uni keeping me up all hours of the morning so could you tell me how a cam grind works.

The word grind sounds as if you remove material so no matter what you do it would be impossible to get a higher lift?!?!?

Care to give me a quick run down how a cam grind works? :P

lukevl Collaborator

lukevl

Posted
Posted

Niiice browny. Think of a profile as a series of radii corresponding with angle. If you lay a bigger profile over a cam, and start grinding the base circle grinds first and the last part to clean up (usually) is the nose of the cam. And like browny said, with the smaller base circle you have more lift. This is accentuated by the removal of ramps which instantly gives you 0.5mm more lift without taking the base circle down at all.

discopotato03 Grand Master

discopotato03

Posted
Posted

Fellas I'm no expert but most of the serious efforts I've seen used billet cams . Otherwise they got someone like Ivan Tigh to weld the lobes over the nose and re-grind a new profile . They specifically left the base circle alone so not to alter the geometry . In other words when the valves are closed the inverted buckets are where the designer intended them to be . I dont even want to think about these hydraulic bucket heads and non std base circle cams . Thankfully no sensible cam grinder would do this as they would realise the problems .

It sounds like the only guaranteed result comes from aftermarket RB20 cams . If I'd gone to the effort and expense of buying/fitting RB26 cams that didn't work and were changed back , maybe the aftermarket items were not so expensive after all .

Cheers A .

lukevl Collaborator

lukevl

Posted
Posted

Being a bucket type motor, the profile will be exactly thre same regardless of base circle size (to a certain degree. It makes effectively no difference.) If the base circle is still in the hydraulic range, then there is absolutely no problem grinding the camshafts.

The reason camshafts are hard-faced is so they retain an almost standard base circle (About 0.25mm down in diameter) and they can go back in without any mods to the valve heights and followers. There is nothing wrong with a small grind like I suggested on the GTR cams to make them hydraulic. Nothing at all. Its safer than a lot of other options out there. I dare say that there would be no reduction in hardness of lobes either so its as good as OEM because after all they are OEM cams.

Joel - Spring seat tension is set by the valve height and retainer design only. The 'stops' in the system are the valve on the seat and the retainer and collets.

Cubes Grand Master

Cubes

Posted
Posted

Thanx luke.

Its interesting as a little while back Darren and I had our RB25 heads rebuilt with a little bit of work done to them.

My valve's sat slightly up in the combustion chamber where as his sat lower to the point of almost being flush. The heads were dead stock and the valve heights appeard to vary considerably (they heights were consistant through the individual head).

Upon the valve seat test mine came up with flying colours where as all of Darrens had to be shimmed as there was not enough seat pressure.

Sydneykid Grand Master

Sydneykid

Posted
Posted
Thanx luke.

 

Its interesting as a little while back Darren and I had our RB25 heads rebuilt with a little bit of work done to them.

 

My valve's sat slightly up in the combustion chamber where as his sat lower to the point of almost being flush. The heads were dead stock and the valve heights appeard to vary considerably (they heights were consistant through the individual head).

 

Upon the valve seat test mine came up with flying colours where as all of Darrens had to be shimmed as there was not enough seat pressure.

Not unusual, it depends on how deep the valve seat inserts were inserted and how much cut the seats had on them. Even across all cylinders is import otherwise you get differing combustion chamber volume and therefore different compression ratios. :D

Ahh32 Proficient

Ahh32

Posted
Posted

the onl reason i soaked them in oil was to get rid of the old oil and to make sure they weren't blocked, and that they filled up will oil. The motor was turned over by hand several times, it was also sitting for 2 weeks so that doesn't mean shit about shit.

The head was faced and had a valve grind, using std cams.

i didn't had adj cam gears before, but i will have them when i put it back together. what are the specs used. 2 degrees adv intake. 4 degrees retarded exhaust?

i agree with lukevl about the lifters, but i thought that they bled down under high rpm then take a couple of mins to return to normal.

lukevl Collaborator

lukevl

Posted
Posted

RPM is irrelevant- to a degree. They will bleed down easier with less oil pressure hence low rpm, but I don't think it will make that much difference. They will bleed down becuse they can't seal.

SK - Totally agree about the seat height. I recommend CCing all chambers to check they are the same. Different CR = pain in the shfincter.

Sydneykid Grand Master

Sydneykid

Posted
Posted
RPM is irrelevant- to a degree. They will bleed down easier with less oil pressure hence low rpm, but I don't think it will make that much difference. They will bleed down becuse they can't seal.

SK - Totally agree about the seat height. I recommend CCing all chambers to check they are the same. Different CR = pain in the shfincter.

Hi Luke, actually more like a pain in the ring lands. :shake:

They do pump up because they go past the return oil hole so fast (at high rpm) that there simply isn't enough time to pump. :jump:

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