Can you increase OHV Engine Performance by changing the Rocker Arm Geometry to get more Valve Lift?

[6./ZG26_Klaus_Mann]

Question is in the Headline.

[DD_Arthur]

No.  The rocker arm - along with the pushrod - simply transmits the lift and duration of the cam lobe and it's follower to the valve.

 

Edit;  I should also say that as part of overall engine tuning rocker arms can be usefully lightened and then polished.  Reducing the weight of the rocker arm is one of the steps worth taking to help the engine rev more freely and achieve a slightly higher rev ceiling.  Polishing is essential to relieve possibilities of stress failure.

This can also be done to the cam wheels, pushrods, etc.

 

   

Edited August 21, 2019 by DD_Arthur

[MiloMorai]

I think Arthur forgot his geometry lessons. Change the pivot point on the rocker arm and the lift will change.

[DD_Arthur]

6 minutes ago, MiloMorai said:

I think Arthur forgot his geometry lessons. Change the pivot point on the rocker arm and the lift will change.

 

Yep, quite right...but....changing the pivot point actually means a new cylinder head. 

 

Sorry, I thought this was about practical engine tuning,  not imaginary engine tuning. 

[6./ZG26_Klaus_Mann]

22 hours ago, DD_Arthur said:

 

Yep, quite right...but....changing the pivot point actually means a new cylinder head. 

 

Sorry, I thought this was about practical engine tuning,  not imaginary engine tuning. 

Well, that is actually part of the Question. If I was to install a new Cylinderhead on an Engine with different Rocker Geometry, so the Pivot closer to the Pushrod, would the increase in Lift add Power or just result in sooner Onset of Valvefloat? Would my Peak RPM Drop but would I still get a resultant Power Increase at that lower RPM?

The Question is more importantly about a Stock Ported Head going to a Head with Standard Performance Ports for both Sides and Geometry that would mean increased Lift even on a Stock Cam, but is really meant for Boost, Roller Lifters and Aggressive Cams.

[DD_Arthur]

1 hour ago, 6./ZG26_Klaus_Mann said:

Well, that is actually part of the Question. If I was to install a new Cylinderhead on an Engine with different Rocker Geometry, so the Pivot closer to the Pushrod, would the increase in Lift add Power or just result in sooner Onset of Valvefloat? Would my Peak RPM Drop but would I still get a resultant Power Increase at that lower RPM?

 

What engine are we talking about here?   It's difficult to give a general answer for all engines without knowing some detail. If you're increasing the valve lift this way I would have thought valve bounce and valve to piston clearance would be more of a problem initially.

A power increase at lower rpm would be much more easily achieved by a change of cam and/or follower profile but I assume this would mean the engine having to come out and apart. 

 

1 hour ago, 6./ZG26_Klaus_Mann said:

The Question is more importantly about a Stock Ported Head going to a Head with Standard Performance Ports for both Sides and Geometry that would mean increased Lift even on a Stock Cam, but is really meant for Boost, Roller Lifters and Aggressive Cams.

 

I think I see.  You want to fit tuned heads from a slightly different model?  By keeping the stock cam you don't have to split the crankcases - usually a good thing -  but the different rocker geometry should give slightly more valve lift?

If it all fits together and runs - I would imagine you'd have to advance the ignition - then you would see a performance increase of sorts as you'll be passing more gas through the combustion chamber.  

 

 

 

[6./ZG26_Klaus_Mann]

This is the Case in for Example the BMW 2 Valve Boxers. The modern Aluminium Heads change the Rocker Geometry little enough to still work on the Old Iron Engines, but have improved Ports, which would already increase Power and reduce Heat Issues, but I'm wondering wether RPM Resistance and Cam related Performance would improve as well with the increased Lift over the old Heads.

In the Aluminium Engines the Cams are less aggressive than in the Iron Motors, so the Total Valve Lift between Iron and Aluminium Engines is nearly the same, but the Aluminium Heads get more Lift on the Iron Block and Cam.

 

Essentially the Aluminium Engines can pull off a less aggressive Cam than the older Models, because they have so much better Airflow not to sacrifice the Top End, but gain a lot low Down where the Iron Blocks are a not nearly as Steam-Hammery as the modern ones.

That I would see a Power Gain is obvious, but I wonder if I would see a Torque gain as well and how the Curve would shift.

Edited August 22, 2019 by 6./ZG26_Klaus_Mann

[DD_Arthur]

Blimey! I never knew BMW made iron-headed twins

 

When did they cease production? 

Edited August 22, 2019 by DD_Arthur

[6./ZG26_Klaus_Mann]

The 247 Engine went through quite an Evolution. It started off as an Iron Engine and then they gradually started replacing Parts with Aluminium. If you look at the Crank Cases there is an early, rounded one and a later Edgy one. The Rounded one is Iron, the Edgy one is Alu.

The Heads I think came in the late 70s, I think around 78 the Aluminium Heads replaced the Iron.

The one I'm talking about is an R75/5, still fully Iron.

[DD_Arthur]

31 minutes ago, 6./ZG26_Klaus_Mann said:

The Heads I think came in the late 70s, I think around 78 the Aluminium Heads replaced the Iron.

 

Wow!  Astonished in fact.  Even Triumph managed to do away with iron heads by the late 'fifties.

 

Well good luck, this must have been done dozens if not hundreds of times before so there must be plenty of info out there.  An R75/5?  Your not committing an act of vandalism are you?