Ability to rev reduced after installing roller rockers

adamnreeves · Post by **adamnreeves** » Sun Jun 16, 2013 2:15 pm

So I have installed my roller rockers and increased my RPM limit from 6000 to 6500. However I am a little disappointed.

Previously on the standard rockers I could accelerate right up the 6000 without any sign of let up. Now I accelerate and power seems to be down and starts backing off around 5600 and by 6000 power drops right off so much that I cannot hit my red shift light which I could do before!

Valve lash set at 60thou as per the insructions for rhoads. I know oem ones are more like 30thou.

I suspect pump up but I thought I'd initially try the setting I had before, i.e. keep everything else the same. I could try and reduce lash down to 40 thou maybe.

Also the engine does sound a little more grunty at mid range, where it feels slightly down on power.

My son drove the car back and he is as familar with the car as me and he made the same observations.

Rover 4.6V X-bolted
Piper 285i high lift cam, power upto 6500
Rhoads lifters
Cat when last dyno'd before made 291 BHP at 5,800 and 305 lbft.

I didnt take any data logs and may do this next week.

sidecar · Post by **sidecar** » Sun Jun 16, 2013 2:42 pm

I wonder if the roller rockers are heavier than the standard rockers?

If so your valve gear might be starting to float, what happens then is that the lifters pump up and this causes the valves to be held slightly off their seats until the lifters bleed down. holding the valves off the seats acts as a rev limiter. I think this process does not happen at an exact RPM, it probably starts to happen over a range of RPM gradually getting worse until the engine just won't rev any higher.

I guess with the rockers it is not just the weight but it is how far the weight is away from the rotation axis that is important.

I run standard lifters with a Piper 285 cam, I run adjustable pushrods and I have set the pre-load to 20-25 thou. My lump makes max power around 5500 but it will buzz round to 6k easily enough. As my pistons, rods and rocker gear are standard I don't won't it to rev more than 6k anyway.

Like you said you could try reducing the pre-load and see if that helps.

kokkolanpoika · Post by **kokkolanpoika** » Sun Jun 16, 2013 7:51 pm

I agree..

sidecar wrote: Like you said you could try reducing the pre-load and see if that helps.

stevieturbo · Post by **stevieturbo** » Sun Jun 16, 2013 7:59 pm

Are your springs up to the job of handling heavier rockers ? Assuming they are heavier.

As above though, you could try messing with lifter pre-load too.

Debatable whether too much or too little is better.

mgbv8 · Post by **mgbv8** » Sun Jun 16, 2013 8:18 pm

From memory my original 3500 v8 got rev limited by the lifters at 5800rpm.

Are these rockers 1.6 ratio?

Post by **katanaman** » Sun Jun 16, 2013 11:09 pm

Same as Perry, are these rockers the correct ratio?

kiwicar · Post by **kiwicar** » Mon Jun 17, 2013 9:07 am

Hi
Can I clear something up in my mind. . when you say lash do you mean preload? Is this a hydraulic cam?
I think your issue is valve train mass and valve float but I just wanted to check.
Best regards
Mike

adamnreeves · Post by **adamnreeves** » Mon Jun 17, 2013 8:47 pm

Thanks for the replies. I did this as an upgrade!

Anyway I do mean pre-load. Heavier possibly when I think about it, the OEM ones are ali and this are squarer and have adjusters built it so could be. Ironically I thought I would be able to increase RPM as my max power could be around 6200RPM. I shall take some datalogs and if they do not show anything else to blame then I shall drop the preload down to 40thous?

I'll also try and estimate the ratio but these were off a pair of rover heads but this is no guarantee is it.

adamnreeves · Post by **adamnreeves** » Mon Jun 17, 2013 9:32 pm

sidecar wrote:I'd drop the pre-load down to 20-25 thou!

If you are using adjustable pushrods just tweak them up so that is no preload AND no play then turn the adjuster 4 'flats' of the nut then lock them off at that.

They are adjustable push rods. I spent an awful long time adjusting them and the rocker arm adjustors so that the lock nuts and the allen adjustor were flush with exactly 60thou preload! I think I have OCD!! One turn of the allen adjustor just happens to be 40thou so I can achieve the required adjustment in minutes.

adamnreeves · Post by **adamnreeves** » Mon Jun 17, 2013 9:39 pm

Oh, the valve springs by the way have 75lbs seat pressure and 200lbs at a lift of 12.7mm which if I remember correctly is pretty much on spec to what my cam manufacture says to use with the cam.

sidecar · Post by **sidecar** » Mon Jun 17, 2013 10:02 pm

I'd drop the pre-load down to 20-25 thou!

If you are using adjustable pushrods just tweak them up so that is no preload AND no play then turn the adjuster 4 'flats' of the nut then lock them off at that.

sidecar · Post by **sidecar** » Tue Jun 18, 2013 4:34 am

adamnreeves wrote:Oh, the valve springs by the way have 75lbs seat pressure and 200lbs at a lift of 12.7mm which if I remember correctly is pretty much on spec to what my cam manufacture says to use with the cam.

I guess the cam manufacturer is assuming that the weight of the valve train is going to be standard when they spec up the springs required.

Even reducing the preload is only going to mask the problem in a way (assuming here that valve float and thus lifter pump up is the issue). Reducing the preload just means that the lifters can pump up less which holds that valves open less, the problem has not really gone away. Its worth a try though because it won't cost you anything other than some time to make the adjustment. (You don't need to remove the valley gasket and use wire gauges if you use the 'flats' method).

I must admit I'd be disappointed if I was in your shoes, I would have hoped that the job would have been a simple swap out of the old rockers to the roller rockers.

EDIT...I was thinking that you are using adjustable pushrods but of course the adjuster is part of the actual rocker! In that case I don't know how many 'flats' you will need to turn the adjuster nuts, it will depend on the pitch of the thread.

bigaldart · Post by **bigaldart** » Tue Jun 18, 2013 6:51 am

Set preload at 20 thou, rockers will be heavier, but the ease of setting preload makes up for it, the preload makes a big difference to max rpm.

Alan

adamnreeves · Post by **adamnreeves** » Wed Jun 19, 2013 1:14 pm

I measured the ratio as best as I can whilst installed and I make it 1.55 however just 1mm error means it is 1.45 to 1.59 I think. So I am thinking these are 1.60.

kiwicar · Post by **kiwicar** » Wed Jun 19, 2013 1:35 pm

Hi
They could well be SBC rockers "repurposed" for the rover engine, in which case they very well could be 1.5-1.55:1 ratio.
Best regards
Mike