L98 flat-tappet cam vs. L98 roller: two questions
 

I may have asked this before, but I can't find the topic in my archives. When Chevy switched to a roller cam in 1987, the timing and lift specs remained almost the same as those of the flat-tappet cam. Does anyone know why?

I know it would be a step back to remove the roller cam from a 1987+ L98 and then install a flat-tappet L98 cam and its related parts, but considering the nearly identical timing and lift characteristics between the two camshafts, I wouldn't think a driver could notice much difference. What opinions do people have about this?

I'm going to hazard a guess by saying that GM didn't want to make too many changes at once... changing the timing/lift would require a whole raft of other mods.

I don't think it's physically possible. The flat tappet lifters can rotate in the bore, so the lifters are round. The roller lifters cannot rotate (the wheel might wind up facing the wrong way) so the bore on a roller engine has flat edges and would be too small for a flat lifter.

Unless you're being purely theoretical, and I never was much good at bench racing.

I would expect you'd have a bit more usable range of power with the roller lifters, but whether that'd be noticeable to the average driver is beyond me...
[RICHR]

Thanks for the reply. According to the GM Performance Parts catalog, a 350 block (part 10105123) designed for roller cams can also accept flat-tappet cams and lifters. I guess that also applies to original '87-'91 engines.

Less friction gave the difference in power only alittle. Same thing as if you were to put 1.5rr on your car. You would see a gain but its the same ration/lift as stock. They did this for drivability mainly.

The the specs of the cam may be the same as Rrubel started to hit on. I believe roller cams give you a more usable power range (they are able to ramp up to open valves quicker). So though the specs of the camshaft are the same, performance results are not.
Also, rollers save some wasted energy that flat tappets lose through friction.
And, race car or street car, how often do you hear of a roller knocking off a lobe.

There is more to a cam than lift and duration. A roller cam can accelerate the valve opening/closing faster (opening the valve quicker and closing it quicker). Therefore, the total area when the valve is open is much larger. Think of the cam profile as being "steeper" than a flat tappet cam. This allows the cam to "act" mild, but because the valves are effectively open further for a longer period, it is stronger.

In other words, don't go back to a flat tappet cam if you have a roller. It would be a big step backwards and you would loose power.

Plasticman

Indeed the lift numbers on an '85 cam are the same as on an '88 cam, i.e..410"/.423"=int/exh. But, as mentioned, the cam intensity may not be the same, i.e. the ratio of durations at .002" & .050" and what about the LSA of the two cams.

When the General went to Al heads in mid '86 this change brougnt 5chp gain, up to 235chp from 230chp.

Then in '87 when the switch was made to the roller cam & lifters another 5chp brought the rating up to 235chp, which remained the rating through '89.

Indeed, IF you could drop an '85 cam & lifters into an '88, the driver would NOT notice any SOTP difference. For most people (even us Corvette guys) it takes over 10chp gain to be detectable on the road, though you will see such a gain at the strip as a trap speed increase.

:yesnod: :thumbs:

There is nothing to be gained from going backwards to a flat tappet, the hydraulic cam will last longer because of the reduced friction anyway.

GM didnt change anything because they couldnt get it to pass emissions in 50 states with alot of changes, they do things incrementally up there in Detroit.

:iagree: BINGO!

Thanks. I understand the advantages of a roller cam, I was just curious to know whether a driver could feel the difference between the two L98 cams. Maybe it's possible to feel the difference if time is spent driving the car at a race track.
Here are two more questions:

1. It seems to me that replacing a flat-tappet L98 cam with an L98 roller is a lot of expense for little gain in reliability and efficiency (I'm only concerned about those criteria), but maybe I'm missing something. Any opinions? Is there anyone who has owned both a flat-tappet car and a roller car and who can provide a comparison?
2. Lingenfelter offers a kit for installing a roller cam in a flat tappet block. It uses a modified retainer plate, three bolts, and what looks like a nylon cam button. Does anyone have experience with this kit? If so, has it proven reliable?

The flat-tappet to roller conversion is a necessity for anyone who wants to put a modern cam in an older (pre-86) block. I was installing an LPE 74219 cam into my 86, and had bought their kit. You need much more than that... the $15 kit only lets you INSTALL the cam. You still need retrofit roller lifters (with tiebars to keep them from rotating) and special-length pushrods (flat tappet are longer than roller, and the retros are different than both). Those parts alone are another $500 if you're lucky.

The LPE kit has a spacer washer, three longer bolts, and the nylon cam button. No retainer plate (no place to bolt it). Several places sell better kits (like Competition Products) with bearing-based cam buttons and they're not much more. The LPE kit was a pain to install, too - the nylon button didn't want to stay and wouldn't tap in.

For other reasons, I wound up getting a rebuilt 91 roller short-block and returning the retro lifters. The cost savings over the retro stuff paid for almost a third of my new shortblock!

I did find that the LPE spacer washer, broken in half, was perfect for helping to install the timing chain crank sprocket...

[RICHR]

OK, if you want to get more out of you early L98 (non-roller cam) install a nice flat tappet hydraulic cam and 1.6:1 or 1.7:1 RRs.

This way you can increase the effective intensity and overall lift of the cam without the conversion issues.

Note: Intensity is the ratio of at .050" to total duration (at say .002"). There is a maximum slope that can be cut into a non-roller cam and this limits the rate at which the cam can open/close the valves. When you go to higher rocker ratio you increase the valve open/close rate as well as the overall lift. For example, going to 1.6:1 rockers will increase vave lift by 6.7% and also increase duration at .050" by 2deg; going to 1.7:1 rockers gives roughly twice these gains.

So long as you keep total valve lift below ~.520" you can increase rocker ratio to improve cam characteristics and still use single springs with dampers and just enough pressure over the nose to control the valves at top RPM. Also, going with Ti retainers will lighten the valve train so you need less spring force to control the valve. This will give you performance benefits while minimizing valve train & cam wear.

65Z01,

Thanks. I appreciate the informative answer.

David