Rev Kit Please Answer Question!

Discussion in 'High Tech Retrofits' started by lluciano77, Jun 25, 2006.

  1. lluciano77

    lluciano77 Guest

    I need to know some info so I can order my rev kit and springs. If I run a rev kit will it change the spring pressure requirements? Since the rev kit will keep the lifters on the lobe more, will it require less spring tension at the valves? If that is the case, how do I figure out what valve springs I will need once the rev kit is installed?

    Right now my cam company says to get 140# seat, 360# open springs. What would the new numbers be assuming the rev kit will reduce the requirements?

    Or, does the rev kit add spring pressure in addition to the springs being used?
     
  2. Rick WI

    Rick WI Veteran Member

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    The rev kit does not affect the spring requirements required of the camshaft. The manufacturer will be able to supply the preload of the spring on the rev kit. For a camshaft as small as yours there really is no benefit. A rev kit basically keeps the valve train intact if a lifter fails. With a Hyd roller as mild as yours lifter issues are rare.

    A proper spring keeps the valvetrain in control.
     
  3. lluciano77

    lluciano77 Guest

    So the AFR Hydra rev kit is unecessary? It is not like other rev kits. It puts the spring pressure on the lifter body, not the plunger.

    You are looking at the new cam specs right? The new hyd. roller has pretty steep ramps.
     
  4. Rick WI

    Rick WI Veteran Member

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    Have run the AFR kits since 1992 or 93 on various applications. In your application not necessary. If you run a big solid roller on the street it's a nice to have in case you explode a lifter.

    For your application you'd be best served by installing a set of beehive style springs after having the spring heights measured. Then have the spings setup by a shop so all the installed heights are checked and shimmed as needed. That's how I'd set those heads up. Nice and simple, no fluff and it works.
     
  5. Goat

    Goat Veteran Member Lifetime Gold Member

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    "The rev kit does not affect the spring requirements required of the camshaft."

    Why is that? The valve spring and the rev kit spring are in parallel, so their respective forces are added. That certainly is the case with solid lifters and I would think a similiar scenario when the hydraulic lifter is "locked".

    So, if you had a valve spring with 300# on the seat and a rev spring with 100# on the seat, it would be the same as having a single valve spring with 400# on the seat.

    Tell me what I'm missing.
     
    Last edited: Jun 26, 2006
  6. Rick WI

    Rick WI Veteran Member

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    The purpose, from a valve control standpoint, of a rev kit is to put additional preload, spring pressure, on the lifter. If you compensate the pressure on the valve spring with the added sping pressure of the rev kit spring in theory what did you gain? Nothing.
     
  7. Goat

    Goat Veteran Member Lifetime Gold Member

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    I disagree. To keep things simple, lets look at a solid lifter for a minute.

    1)The main purpose of a "valve" spring (kind of a misnomer) is to keep the lifter in contact with the cam lobe, right?

    2)The valve is just the end link of a multi-link system that includes the rocker arm, pushrod, and lifter. The valve doesn't really care what the spring load is (edit, see below) as long as the linkage stays intact and the valve opens and closes in sequence with the cam, right?

    3) The "valve" spring exerts its force against the lifter, right?

    4) The rev spring exerts its force against the lifter, right?

    5) So, they are in parallel, right? I'll agree that it may not be a one for one swap, because the springs are in different parts of the linkage, but they are in parallel and thus add up.

    6) My reasoning then is the rev spring "force" (not pressure) can take the place of part of the "valve" spring "force" since they both act at the same point (and in parallel).



    What parts of the above do you disagree with?

    edit. I have only been addressing the dynamic portion of the valve spring. Yes, the spring is preloaded to close the valve and keep it shut when its supposed to be, but its main function is to keep the "linkage" intact, in other words, the lifter in contact with the cam lobe.
     
    Last edited: Jun 26, 2006
  8. Rick WI

    Rick WI Veteran Member

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    Based on your theory then why use a rev kit at all?
     
  9. Goat

    Goat Veteran Member Lifetime Gold Member

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    In the case of a solid lifter. Say you had a cam that required 400 lbs. spring force on the nose. Instead you used a spring with 300lbs and installed a rev spring with 100lbs. You still have the required open force, but you have reduced the load on pushrods,rocker arm studs, and rocker arms.

    There are obviously alot of things going on with spring harmonics and such at high rpms that I am not aware of, but I am just looking at the system as sketched in a free-body diagram and analyzing the forces and their vectors.

    Of course in a hydraulic roller, rev kits are used to help overcome the limitations of the hydraulic valving inside the lifter that tend to float the valves at higher rpms. But, that spring force is still part of the overall force (that includes the valve spring) that is pressing down on the lifter when in motion.

    I don't know all of the specs of rev kits when sold as hydraulic roller fixes. Maybe the extra force added is not that great that it worries the manufacturers and they don't spec a lower force valve spring. But I say again, in theory, if you were designing a valve train system from scatch and you wanted to be truly accurate, all those forces should be taken into consideration and you could probably benefit from using a rev spring to supplement the valve spring.

    I do know that Isky uses rev kits for solid roller cams. And I could be wrong, but I would be surprised if it was just to hold the lifter in place in case of an upper end valve train failure.
     
  10. MikeM79

    MikeM79 Moderator Lifetime Gold Member

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    I miss the point of a rev kit entirely. Assume that they work as claimed.....say an extra 75 horsepower at 6,500 rpm was the result of simply adding the rev kit. Everything I have seen suggests that all of this extra power is produced way past the rpm at which the peak torque occurs. In other words, the rev kit slows the rate of "torque decay" to the right of the torque peak (as seen on a two axis chart). As a practical matter, I just don't see the engine operating in this range for very long anyway because the driver may have already grabbed the next gear (or he is going to in the next couple of tenths of a second).

    Am I missing something? Perhaps it is useful when in situations when every last bit of power must be wrung out and the parts budget is big enough to allow it. I would think that the average street machiner would be better off spending his money on parts that push the whole torque curve up the chart rather than just part of it.

    I ran into this issue on dyno day for the my 454 BBC with a small Comp Cams roller cam (218/224). The torque peak on that engine is at 2,800 rpm (580 ft-lbs). Torque production fell off a cliff at about 5,200 rpm due to the heavy hydraulic roller lifters wanting to bounce off the bump stick instead of staying in contact. We were able to push the "cliff" up to about 5,600 rpm by shimming the springs as Rick suggested earlier in the thread. However, picking a cam is always a tradeoff.....and it doesn't take too long for a 218/224 cam to "nose over" anyway. Had I wanted to make big torque over 5,000 rpm I should have gone with a solid roller.
     

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