Yelofvr

I have been considering the merits of a roller versus non-roller hydraulic flat tappet cam. The two biggest benefits I read about with roller cams are the fast opening and closing ramp and the reduction in friction. When I view the lobe profiles of each, the roller cam has clearly more lift area than the non roller. At first I thought oh yes, it is easy to see how this roller lobe will let more air in the cylinder and make more power, for equivalent 0.050" duration cams the roller will have a shorter .005" duration which reduces overlap and improves idle, plus the reduced friction make it an easy decision….assuming $300 for the cam, and $250 for roller lifter is "easy". Then I looked a little deeper with respect to actual valve position versus crankshaft position and piston speed. I will discuss the intake cycle. For a "typical" performance TPI cam you are somewhere around 215/220 duration at 0.050". This means with 0 deg cam advance, the intake valve will open to 0.050" at 5 deg after TDC. At this point the piston speed is almost ZERO and thus getting the valve open quicker will not really help you cause you are not trying to draw air into the cylinder yet. Maximum piston speed occurs at a crankshaft position near 90 degrees past TDC(depends on stroke and rod length) and reaches 50% of this speed at approx 45 degrees past TDC. Mathematically, the piston speed follows a sin^2(x) function, where x is the crankshaft degree of rotation. My point is that your ramps can be slow, because your piston speed is slow at the early point of the intake cycle. Having your valve open more at this time is just not necessary. Now the end of intake stroke. As the piston approaches BDC it is slowing radidly, decreasing the need for more air and the same argument would follow with one exception. At this point in the engine cycle the air column moving into the cylinder has inertia and even though piston speed is very slow, the inertia continues to pack air in the cylinder. This would argue in favor of the roller cam since the valve is at higher lift. In actuality it is not that simple and the dynamics here get very complex since this inertia is proportional to air velocity squared and valve opening affects velocity. To summarize my point, the roller cam profile looks very promising as it approaches an "ideal" valve action ie. immediate opening, immediate closing, max area under the curve…… BUT our pistons do not move that way, they follow a natural sine wave, which is exactly what a non roller cam lobe looks like. To back up my argument, look at the TPI L98 when it switched to roller cam from flat tappet. The horsepower only went up by 5, I would argue that there is that much HP gain in friction reduction alone…… so what did the roller profile itself actually get you???? Nothing???? What did I miss here??

Any thoughts??


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Dave Zelinka

R.O.B.

OK, I can't argue with all of that tech knowledge, but reduced friction = more hp and lower oil temp and water temp. This alone sounds good. Plus I think the roller has an advantage for higher rpm's without float, but I could be wrong.

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Rob
'86 TA 305 TPI

RB83L69

1. Your analysis of piston motion, while technically accurate, minimizes the effect of inertia, which as you point out, tends to continue packing the cylinder even after the static conditions would indicate that the cylinder should already be full. Then on top of that, the intake charge is moving down the pipe, and at some point the valve closes; the charge keeps moving, and there is usually still some amount of forward motion left in the charge at the next intake valve opening, so the cylinder starts to fill (assuming the exhaust gas is fuly evacuated - big assumption) even before the piston goes down. The cam people will tell you that the one single valve event that has the greatest effect on the motor's output is the intake close, because the longer you can hold it open, the more time you have for (a) the charge to keep on moving in, and (b) the column of air in the intake to keep moving longer and longer.

2. The factory failed to take advantage of any of the roller's potential for performance improvement. They simply copied the old cam grinds onto a roller blank. How incredibly dense. That's why it was only good for 5 HP.

Even if the increased valve opening at the closing event results in only a 5% increase in cylinder fill, that's still 15 HP in a motor like most of those on this BBS. For somebody with a high-RPM setup the gains can be far greater even than that.

Another advantage is that far higher spring pressures can be used. This can more than compensate for the tendency for the valves to float due to the steeper ramps. Again, the factory missed it.

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"So many Mustangs, so little time..."

Yelofvr

Thanks RB, After posting this also on the corvetteforum, I was reminded that you are limited in peak valve lift on a hyd flat tappet due to the ramps. For the same duration, the roller gives higher lift at peak and near peak lift. So in a performance application where you can take advantage of the higher lift, ie. heads that flow well at high lift and springs that dont bind, you can make a big difference in airflow with a roller cam. In a stock application where the heads dont flow better with more lift and you are limited by valve spring bind, you cannot get much benefit. The benefits do not come so much in the opening and closing, but the shape of the cam lobe around peak lift. The roller lobe is much higher and broader and this is the point in the intake cycle where your cylinder is trying to pull in lots air due to the high piston speed!

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Dave Zelinka

Vader

Yelo,

Just to play devil's advocate, in addition to greater cost, the roller cam scheme tends to create a slightly higher overall inertial mass of the valve train, probably due to the extra parts of the lifter and retaining mechanisms. They also tend to create a little more mechanical noise when the engine is cold. The first problem can easily be overcome with stiffer springs or a rev kit, and the second is merely a nuisance.

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Later,
Vader
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"No matter how hard you try you can't stop us now"
Adobe Acrobat Reader 4.0

Guest

A roller cam to some extent allows the cam manufacturer some leeway in the lobe profile to do as they please, whereas with a flat tappet it was much more limited. The newer cams I see are going to an asymmetrical shape on the lobes, which is much easier to deal with on a roller profile from some of the crazy ideas I have seen.

D_Amlee

Yelo, the ramps don't really limit your valve lift, that limit is normally set by the springs. Too much lift gets into coil bind conditions. The one thing you seem to have overlooked in your analyis is the fact that the roller cam allows faster valve lift rates all along the entire profile, not just at the area between .005 and .050 lift. If you look at the amount of lift between .050 and say half way up the cam profile (by the way... even though cam profiles are vaguely sinusoidal, no cam profiles are true sine waves) the valve will be opened faster by the roller cam and will have a greater amount of lift at all points between .050 and max lift. That is the key benefit provided by the rollers. The comparison of the lift at .005 and the lift at .050 is just one way of measuring some of this, but the cam designers keep the rest of the profile a closely guarded secret. And THAT is where the power comes from.

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89 IROC with lots o' stuff

-=ICON Motorsports=-

Box of Rocks

Fever,

RB83, Vadar, and D_Amlee all make valid points - so I'll just report the dyno comparison I just review in CHP.

Flat tappet hydraulic Comp 244 duration @ .050 - 468 HP @ 6,800 RPM.

Same spec (nearly) Comp solid roller - where idle vacuum was comparable - and the Comp dynoed 527 HP - at 400 fewer RPM. The increase in low RPM torque was astounding - more so than the HP.

Uh, I'll be using a Comp solid roller in my 406.

BOR

Pablo

iTrader: (1)

well now thats not totally fair they shoulda done a heads up of roller solid vs flat tappet solid. Hydraulic lifters have a nasty habit of soaking up some lift on the upstroke, especially on a high rpm motor like that, which probably compounded the less aggressive ramp of the flat tappet cam in comparison to the solid roller

FastBroker

not if you adjust the hydraulic lifters to the bottom of their bores...