I found another blind spot in my knowledge ...

What is the tuning modification where you uses one set of high lift rockers arms, on the intake I think it is, with standard low lift rocker arms on the exhaust all about?

What the theory and does it have much effect in the practical?

Thanks.

 

A higher ratio rocker opens the valve more, allowing more air/fuel to enter the cylinder.

 

Haven't tried it myself but have heard from others that the 1.75 ratio rocker on the intake is good for a few more HP; not much help on the lower end. I will be trying it on my next build.

 

Obviously I understand that, but that's not the question.

The question is, why do folks go the route of choosing only one set of high lift and one set of standard lift? It's something they used to do in autos too. It may have come from there ... where they have more cylinder, hence more costs.

Perhaps it is to do with the intake valve being bigger and it being more important to get the charge in. Perhaps it just does not make much difference on the rear, as exiting gases are at a lower density than the intake charge, so an standard lift does OK ... and it goes back to the days when they were more expensive and folks more poor so they cut corners. I am just not sure. Sometimes these kind of things just enter into the realm of "folk legend" and people do them without asking why.

Imagine paying for two sets for a V12 ... 6 times more than an H-D!

I have read about it a few times so there must be some 'old school' reason.

There are a number of factors involved. At high speeds, higher lift quickens air intake ... by why not the exhaust?

It's said, the smaller the cam lift, the greater the performance gain from increasing rocker-arm ratio; the better the cylinder head, the more you can expect again; and the larger the valves, the greater the improvement.

For me, the roller tips are almost more attractive ... a case of doing the job properly instead of cutting corners (reducing friction and resistance, in theory increase engine longevity) ... and so I'll use a pair of Crane 1.75s, but I am intrigued and there does not seem to be much discussing of this. I couldn't even find a reference to it in Donny Petersen's or Bill Rook's Bibles. Strange ...

I got a set off a real power junky who wasn't satisfied by how much HP he got on the dyno and was changing cams etc ... from memory, he say they gave him an extra 5 HP/% at top end ... but so much would affect that, it's hard for me to tell what effect they will have.

What sort of performance improvement do folks see with them? There's a fair bit of misinformation on the internet about them changing angles and so on. I have a soft cam so I think it will be safe, easy and worthwhile for me to do.

 

If you don't like valve train noise, don't use roller rockers.

 

Why do they make more noise? Is it all brands? Is it worse than, say, noisy cams?

Thanks

 

High lift rockers make more lift all along the cam profile. Not just more lift at the top of the lobe. Gets the valve open and closed quicker with less cam ramp and effectively increasing duration too.

 

Doesn't increase duration. Closed is closed regardless of added lift from rockers. What it will do is increase lift throughout the open portion of the lobe. Sometimes it can speed up the opening, once the lifter hits the ramp, but it defies physics that it can move the ramp itself. Cam timing will remain the same.

Think about it.... 500x zero is still zero. 1.75 x zero is still zero, etc.

This is confusing because most cam specs are at .050 lift. THAT lift will be greater, hence folks think it opens sooner, but it simply cannot open earlier than the cam allows. It simply cannot open until it sees the ramp of the cam.

This trick doesn't work across the board, OP. It works well if the cam is mid or small and the heads have been worked a little bit.

 

I didn't say it in increased duration. I said it "effectively" increases duration because the valve opens more & quicker on the ramp. If lift hits .050 sooner (and continues over the lobe), "effective" duration is increased. Works on both ends.

 

Crane says effectively 2-3 degrees.
1.725's increase valve lift approx. .030".
Scott