Variable Duration Flat Tappet Durability

[strokersix]

The Comp Cams catalog states that their variable duration tappets should not be used for extended street duty. I can imagine that increased valve seat wear might occur from valve velocity at closing. Is this the reason for Comp's warning or is there some other reason?

Thanks.

 

[victorymotorsports]

What does the variable duration tappets do? Or specifically, how do they work?

 

[strokersix]

Variable duration tappets (Rhoades lifters) bleed down more quickly than standard tappets. This effectively closes the valve sooner. At high speeds there is not much time for the tappet to bleed down so you get the whole cam lobe action transmitted to the valvetrain. At low speeds the tappet bleed-down effectively shortens the duration. The effect on engine performance is similar to having a 10 degree shorter duration cam at low speeds to boost torque. The bleed-down can be achieved by a bleed hole or slot or by increasing the clearance between the tappet body and internal tappet piston. Nowadays with roller cams I don't think the variable duration tappets are as popular.

I would be interested in some actual engine performance data using variable duration tappets versus standard tappets.

 

[evelrod]

If anybody is using them today, they are keeping it a secret. Haven't seen them in a while and with most of the SBC V-8 group around here it is more 'monkey see monkey do'. Your right about the roller cams. Seems to be the hot setup these days.
As to why not to use them for street? I have heard a muffeled engine with them at idle and it just sounded like when I would 'lash' my hydraulic lifters on my 1964 Corvair turbo Spyder to get a few more revs. LOUD!

Rod

 

[patprimmer]

I think part of their appeal to the hot IMAGE group was the clackety clack, like solids.

They worked for me in a everyday high performance street sleeper, but I now mainly have OHC and buckets, Hydraulic or Roller in anything I play with.

I would never consider useing them in a roller, for fear of cracking a follower. I don't know what a rev kit would do for them Regards
pat

 

[evelrod]

Pat, what if you used them on a cam with clearance ramps ie, solid lifter type with acc and dcc ramps?

Rod

 

[patprimmer]

Do you mean with a roller or flat tappet.

I guess a rev kit might bleed them down, even on the base circle until the tappets were wide opened, depending on oil supply rate and bleed of rate at the rev kit spring pressure.

I also guess that if the clearance is taken up on the base circle, there is no need for opening ramps, just closeing ramps.

I guess they would have some effect as the opening rate and total lift would bleed down a little at low speed, but the ramps would reduce the bleed down effect on the closeing side, not eliminate, but reduce the effect.

Now you got me thinking, a dangerous move at any time.

With a few experiments re oil supply rate, bleed down rate and rev kit spring pressures, I might only lose 3 or 4 cams before I got it to work. Then again, I don't get enough time to sleep as it is Regards
pat

 

[victorymotorsports]

The lack of opening and closing ramps on a hydraulic cam (compared to a solid) is probally the reason why. Vizards book on SBC cams and valvetrains discuss this specifically.

Reguardless, ANY HYDRAULIC lifter will decrease the natural frequency of the valvetrain, and thus will limit how fast you open the valve and the upper rpm range.

 

[Metalguy]

I have run Rhoades lifters in quite a few engines in the past (BBC, SCB and Corvair 6), and it seemed to me that they didn't retain the essential convex shape on the rubbing surface very long. Seems like every time I had occasion to pull an intake manifold I'd pull a few lifters and look at them, and they'd be flat or even worse, slightly concave. The really weird thing is that they seemed to hold up better in the BBC, even tho the lifters are the same as SBC. The loads in the BBC were certainly higher (stiffer springs, 1.7 vs. 1.5 ratio).

I finally gave up on all flat tappet/pushrod setups.

 

[strokersix]

Thanks to all for the comments. Sounds like none of you think too highly of Rhoades lifters! I do not have any experience with them but I will soon. I'm trying them on a custom Comp Cams grind 270-5 lobes intake and exhaust on 110 degree centerline, installed 2 degrees advanced in a 9.5:1 292 Chevy six in a street car. I accept and appreciate all of your comments but I'm kind of hard-headed and just have to try it to see for myself.

Has anyone done a roller tappet setup on the 230/250/292 Chevy? If so, I would appreciate any comments on what parts to start with and what alterations might be required.

How much can the variable duration effect be altered with oil viscosity? For an extreme example- how would 0W30 weight compare to 60 weight racing oil with regard to variable duration tappet action?

 

[Metalguy]

All of my experience with them was in AZ, and I only used 15-50 Mobil 1 oil. The lifters do NOT bleed down until the oil gets good and warm, and when they do "start to work" the effect is fairly mild. They do make a cam a little milder, and I was never bothered by the clicking-it's not that loud.

IIRC, I measured the hardness of the rubbing surface of a few lifters, and they were all about the same-close to Rc 60, I think. I suspect that Rhoades buys stock Chevy lifters and then mills the small bleed-down flat on them.