Don Terrill’s Speed-Talk

I'm pretty sure this has been dragged out before but I'll ask anyway.
I've a cam spec. Limited data although it's a Comp flat tappet hydraulic grind.
284/288 of seat duration and 240/244 @ 0.050. 106 LCA. Lift was .541 on the intake (1.6 rr) and .503 for the exhaust (1.5 rr).
How would that translate to a hydraulic roller?
The deal here is I have some output information from a given build and if I were to duplicate it and the only change being a roller (with more lift to suit the heads used), what would get changed? If anything. The overlap at 74 degrees (seat to seat) would remain the same.

Please post a few more details.

Bore, stroke, rod length.
valve sizes and head flow.
Compression ratio.

I'm not looking for a cam spec so to speak. The specification by way of a flat tappet is the best there is. I'm just looking for a way to compare one to the other.

The specs are telling you when the valve is opening and how far. If you had the exact same specs in a roller grind the shape of the cam would be different because of the round roller lifter nose, but the valve should open at the same time and same amount. Some roller grinds cannot be achieved with flat lifter geometry restrictions. I am not a cam expert so maybe one or two on here will give a better explanation than I can.

The velocity and acceleration of a FT cam, is basically limited by the diameter of the lifter. You are limited in a SBC by the .842 size.

Ford has .875 & 1.00 and Mopar, .904, all will out preform the .842 lifter. You can see that, if you compare .200 duration on the 4 sizes. The 1.00 Ford lifter can work with lobe profiles, similar to some roller lifters. The upper end rollers, especially with larger rollers, will produce more lift and duration though.

Most cam makers have lobes in .842 that are maxed out for the lifter size but, they will have less .200 duration and lift, when compared to max lifter lobes, of larger diameter lifters.

The valve doesn't care what raises it, it only cares about how long it's open and what lift it gets to but, there maybe a fiction loss with the FT but, if so, not much.

I'm sure Mike can explain it better than me but, that is how I see it.

skinny z wrote: ↑Mon Jan 11, 2021 7:57 pm I'm pretty sure this has been dragged out before but I'll ask anyway.
I've a cam spec. Limited data although it's a Comp flat tappet hydraulic grind.
284/288 of seat duration and 240/244 @ 0.050. 106 LCA. Lift was .541 on the intake (1.6 rr) and .503 for the exhaust (1.5 rr).
How would that translate to a hydraulic roller?
The deal here is I have some output information from a given build and if I were to duplicate it and the only change being a roller (with more lift to suit the heads used), what would get changed? If anything. The overlap at 74 degrees (seat to seat) would remain the same.

What the engine cares about is the right valve lift curve. If the HFT cam does that then the HRT cam should have the same specs.

Stan

So, if the HFT and the roller have the same seat to seat and .050" numbers (.200" lift notwithstanding but obviously a factor) as well as lobe lift, then they should give similar results from one engine to the next.
Fair enough.
FWIW, I've a successful template to work from but the difference being the template has an HFT cam and I'll be using a roller.

A small hydraulic roller cam with very similar running manners to that flat tappet camshaft would probably have a bit more duration off the seat, about the same at .050" and get larger above .100" lobe lift with more total lift.

If the head can use it ... the extra lift and "area under the curve" is worthwhile.

That much I understand.
While I don't have the flow numbers for the heads in the template (Edelbrock 170cc) I do know what mine are and the FHT cam in question doesn't have the lift that would suit these heads. (Or so say the few that have spec'd a cam for this project).

skinny z wrote: ↑Mon Jan 11, 2021 7:57 pm if I were to duplicate it and the only change being a roller (with more lift to suit the heads used), what would get changed? If anything. The overlap at 74 degrees (seat to seat) would remain the same.

You are correct, you would use the same seat duration, if you don't want to change the rpm band.
Since you are wanting to increase lift, with the change to a roller, the .050" would also increase a little, depending on how much lift you add.

Thanks for that Mike.
The template engine is a bit of an odd duck but it also happens to closely duplicate my own pile of mismatched parts.
Still trying to zero in on the actual build direction though.

pastry_chef wrote: ↑Mon Jan 11, 2021 9:43 pm Please post a few more details.

Bore, stroke, rod length.
valve sizes and head flow.
Compression ratio.

Well, now that I've had a round of answers to the original question, I can throw this out there.
Might get a couple of interesting reactions.
Maybe not.
4.03 x 3.75
6" rod
9.8:1 CR
Head specs attached.



I have peak HP and TQ numbers as well. No printout though.

you are looking at peak power at right about 6000rpm.

lefty o wrote: ↑Tue Jan 12, 2021 10:19 pm you are looking at peak power at right about 6000rpm.

I was digging around when you posted. Found some RPM details.
You're close. Looks like peak HP was flat from 5600-5800.
I'm thinking my somewhat better heads (255@.550") will help output. Cam spec will be very close.

What was so secretive about that build?