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flat tappet specs to hyd roller

542 Views 6 Replies 5 Participants Last post by  johnsongrass1
do the specs of a flat tappet cam equate to the same specs of a hydraulic roller cam in terms of engine Output?
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1979 Chevrolet Malibu 496-TH400-9" (cruiser). 1992 Chevrolet S10 355-700r4-7.625" (daily driver).
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The short answer is no. If the consideration of "the same specs" is based on using only the commonly used advertised duration @.006" lift, duration @.050" lift numbers and the total valve lift numbers typically used to identify and/or compare camshafts to one another would look like this:

Rectangle Slope Font Plot Parallel


The 2 compared cam lobe profiles here would share the same often used numbers seen in the retail catalogs (i.e. Summit Racing, Jegs).

This illustrates the point that the basic numbers don't tell the whole story because of the difference in the shapes of the cam lobes. Technically you're not going to find a flat tappet and a roller cam that truly share the exact same specs. From a catalog listing, you can see the .006" and .050" durations will be the same. The total lobe lift is also the same. As shown in the graph, if you try to compare specs anywhere outside of the catalog listed numbers...the specs won't truly be the same.

The "area under the curve" shows the additional flow the roller profile has compared to the flat tappet profile sharing the .006, .050 durations and total lift. The roller profile will outperform the flat tappet.

It's worth noting that the additional "area under the curve" with the roller profile makes the cam "act bigger" than the flat tappet profile. If both profiles share the same "212 degrees @.050 duration" the roller cam acts 6 to 8 degrees longer than the flat tappet would. Sources vary on the difference but 6 to 8 degrees @.050" seems like a commonly used comparison. So, if you had a 220 @.050" flat tappet and you wanted to keep similar RPM ranges, you would choose a roller with 212 to 214 @.050

Keep in mind that the graph is still very crude in its demonstration and this is a generalized viewpoint. It's made the way it is for simplicity. That graph, while good for this discussion, shows the relationship of long ago outdated symmetrical cam lobe profiles.

The folks that really know this stuff better may chime in and correct anything here. I'm at a hobbyist level of understanding and not even close to what the pros know.
 

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79 malibu nicely done.
sandwich it would not be worth the change on an older mild build engine. If you are stepping up the cam in a milder engine I would use a pressurised solid lifter cam. If you are stepping up a fair amount with a cam change and perhaps
a bigger fuel management system then a bigger and roller might be in order? pre roller block with small cam I doubt
deserves the $k roller change over. You do the math for your purpose.
 

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Discussion Starter · #4 ·
Sandwich?? WTF...
Thanks guys I was just thinking about the the cam specs of a flat tappet that I have in an older crate 350 in the garage to a roller cam selection for a 880 block under the work bench.
 

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That graph shows hydraulic flat tappet vs hydraulic roller, it looks a bit different if you compare solid flat tappet vs hydraulic roller.
Mechanical flat tappet is actually faster off the seat up to about .150" lift ....so the mechanical has more area under the curve at low lifts.

Once you are looking at cams with a 3000 rpm and up power range, the hydro roller vs solid flat picture gets very cloudy. Now your getting into upper end rpm high enough you have to start worrying which hydraulic roller lifter to buy so you can rev it above 6000 rpm and not lose control of the valvetrain(float).....whle the solid flat is happily zipping on up to 7500 rpm without a hitch.
 

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1979 Chevrolet Malibu 496-TH400-9" (cruiser). 1992 Chevrolet S10 355-700r4-7.625" (daily driver).
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I freaking LOVE solid lifter cams. Good stuff. Some hydraulic flat tappets are faster off the seat too. Very often flat tappet camshaft lobes have a higher primary intensity (base circle to cam lobe) than roller profiles. Then the roller profile sees more secondary intensity (going along the cam lobe).

The "area under the curve" at lower lifts seems to be a less than desirable place to have it. A number of folks actually try to avoid having it there. In fact, I think the "modern" inverse radius roller lobes were at least somewhat (maybe more) aimed at trying to eliminate that. I've always wondered how much that was something to do with that. For all I know, the reduction in overlap was an unintended consequence that turned out beneficial.

The beauty and simplicity of a solid flat tappet set up has to be one of my favorite things about hot rodding. They work!
 

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NO matter what, that pic indicates the LOBE SHAPE is whats most important and the specs given NEVER give you all the information.
Even between cams with the same lobe shape, measured at .006, .010, .050 etc will give you differing numbers making a person THINK it's a different cam but it's really not.
This happens all the time with the so called white box generic cams. The cam designed 40 years ago, might be the same cam listed today as "tight Lash", "fast ramp" "hi intensity" or some other marketing BS.
Remember that on the next question on here of "Which is better?"
Likely the same......just measured different.
 
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