rocker profile - cam timing - consistency

[mikedsilva]

So, having made myself a digital degree wheel, I can now VERY accurately set my cam timing.

One thing I have have noticed, is that different rockers all seem to open at slightly different stages of crank rotation, and produce different amounts of lift at TDC.

I find it difficult to properly explain, but I will try.

The camshafts I am using, are Webcam 464/465. They call for a valve lift of 4.2-4.45mm at TDC with 0.1mm valve lash. Webcam also supplies another setting of 27degrees on the intake, BTDC, with zero valve lash = 0.05in lift.

So I used my stomski chain tension tools on both sides and set the chain quite tight. (I also noticed a difference between using just the oil fed tensioners compared to using the stomski tools - ie, chain tightness affects the settings).
My rockers had been reconditioned, and my cams are new.
After a short time, I had 4.32mm and 4.33mm on either side of the engine and this was consistent.

But I decided to check all my rockers. To do this, I used cylinder 2, intake position, and installed each rocker, once at a time, in that spot.. Valve lash was set to 0.1mm using a digital dial gauge. I checked the valve opening at tdc for cyl 2... in some cases it was 4.33mm and in some cases it was 3.78 - 4mm... hmmm.

So the amount of lift varies between the rockers. So I then removed valve lash from each rocker, and rotated the engine to get exactly 0.05in lift and checked how many degrees BTDC i was on the digital degree wheel - it is meant to be 27.. in some cases it was 27, but in others it was 28degrees, and in others it was 24degrees..

So.... I am wondering, how do I know which rocker to use to set my cams... is it that critical? Am I obsessing over this? What is best practice?

[Mike Billings]

Somewhere I read the procedure to set the timing on a 993 using the old 911 method. You obtain a NEW 930 rocker and replace the hydraulic rocker on #1 intake with it. Then time the cam.

So it was suggested to use a new rocker.

[mikedsilva]

Got my replacement rockers, but the bushes don't look to have been replaced.. Should I use them?

[Bigtoe32067]

Not if you paid for them to be rebushed

[stownsen914]

It would be odd to not rebush them, and I’d think you’d want it done.. I’d check with whoever did them for you.

Thanks for posting this. I’ve heard that there is complexity to the profile of 911 rockers and the impact on valve lift, but I haven’t investigated it. I guess I’m not surprised, especially for reground ones. Makes me wonder - when a machinist regrinds rockers, does he index them somehow to make them measure similarly after grinding?

A bunch of years ago i degreed a race cam when I was doing the cam timing. I was surprised to find the intake/exhaust separation to be several degrees different between the left and right banks. I attributed it to the cam regrind, but who knows - maybe differences in the rockers (which were new) contributed.

Scott

[Bigtoe32067]

That rocker has definetly not been rebushed.
As fas as the variation I think it can also be attributed to how exact to center the rebushed rods are. If the centerline of the new bushing isn't exactly centered in the rocker then you'll have some variation in the offset and therefore variation as the rocker pivots on the shaft. I don't think these small variations matter that much although that's just my non expert opinion.
Tony

[Bigtoe32067]

Hahaha I just realized I'm responding to a thread from last year. Oh well.

[Walt Fricke]

Well, it is an interesting thread which I missed, so thank you. It looks like not many of the heavy hitter engine rebuilders/machinists etc. responded.

All I know about rockers is that the ratio is given as "approximate" by at least one of the cam grinding companies. I took that to mean that as the rocker moves the ratio changes a bit as the effective lengths between the ends and the pivot of this class 2 (teeter totter) level change. E.g., the elephant foot moves in and out on the head of the valve.

I hadn't spotted checking, much less being able to do anything about it, for cam lift/timing lobe for lobe in order to get that mythical perfect engine, where each cylinder acts exactly like each other cylinder on the crank.

[icarp]

Mike , you have hit on what the best engine builders never over look , when budget allows .
Cam grinders are supposed to make sure the cams are consistent from lobe to lobe.
Rocker arms must have an identical factory specd radius for the rocker pad. If you want your engine to be one of the sweetest running motors , then make sure all these things are blue printed .
This is the only way to make an engine that sings , vale springs , cc of head, comp ratio, BRNG clearance , on and on all through the engine . You are on the right track. JMHO

[Neil Harvey]

A huge part of our work is producing engines that make the best Torque possible. Street or race, it does not matter.

Trying to get all cylinders to produce the same amount of torque is job 1. Difficult under normal circumstances, even more difficult with air cooled engines with distributors, Carburetors, or EFI non sequential and or non adjustable trimming per injector and coil.

If you accept by forgetting about the power gained by lowering all of the losses just turning the engine over and thinking only about the power produced by combustion, then it all is produced within the cylinder head. The valve train has a huge part to play here too. It opens and closes the valves to allow air into and out of the chamber.

Lets accept the manifolding gives equal air flow to each port. Typically does not but to make this point, say it does. If you have cam lobes poorly cut on the shaft, with different LSA's, different lobe lifts and durations, then you should expect different volumes of air to enter the cylinders. The valves will be lifted off their seats at different heights and held open longer or shorter per crank degree and at different crank degrees. Now add in the rocker geometry is all screwed up, you have a big mess.

Let say your engine makes 300 BHP. If you were to measure the pressure in each cylinder, you can work from this data how much torque is produced in each cylinder. I can assure you that in these air cooled 6 cylinder engines, not two cylinders make the same torque. If you could trim the fueling and ignition in each cylinder to match the air mass, you would be on your way to producing more torque overall. If you opened and closed each valve the same amount exactly the same time you would be further towards making more torque.

We are very particular and picking the cams and rockers we use, their construction and or repair. We have measured many cams and rockers that have come through the shop and found some to be of poor quality. Just making the rocker pads nice and shiny is not the only important need. Often cam profiles are copied by companies to short cut their development. From these copied files a master is produced and the cams profile is offered up. These are not original profiles but copies and typically bad ones to boot. Cut and paste designs are also common.

This is why we have decided to offer our own library of profiles. It takes time but we are slowly getting there. So far, we have new high performance race cam for 2.8 - 3.4 L engines, some variations of this profile, a new street version for these size of engine coming along with a replacement for the most commonly used 964 profile. These are in testing now and should be available by August.

So pick your cam supplier wisely and have the rocker arms reconditioned by those companies that know the correct radii of the pad. They should have fixturing to check the rockers before and after repair. If the pad is worn, it needs to be cut, and either hard chromed or welded. Our choice is to hard chrome as this does not put the rocker arm under the stress that welding does. To each his own here. Remember, any rocker pad recut will change the ratio some. This will change the net valve lift but as long as they are all the same, this is a compromise that can be accepted in street engines.

Re bushing should be done before the rocker arms are ground too. Its hard not to re bush and not get the bore in the center. Bushings need to be pin fitted to the shafts, honed for the required oil clearance. The hone will follow the inside hole of the bushing. Checking during the honing should be done for both ID size and taper. The shafts need to be in perfect condition too. Don't turn them around and have the good side facing out. The only thing getting fooled here is you.

Finally a simple check to see where your valve train is, look at the wear pattern on the valve stem tip. It should be in the middle or at least the same per valve.

I'll try to post a photo of some of our finished reconditioned rocker arms and cams.

As stated earlier, the devil is in the details.

[Neil Harvey]

[Bigtoe32067]

Nice work Neil. It's very interesting to hear the amount of work that goes into making our engines run optimally and to understand that a bunch of little gains equals some pretty substantial results. Also to understand that these gains all equal to man hours that add up into a significant investment above and beyond your normal engine rebuild.
Perfection (or damn close to it) comes at a cost. Some get it and some never will. My budget is limited so I try to do every I can do myself but I also realize that some of the more involved processes take an expert to get it right and I'm willing to pay for this expertise and workmanship.

Anyways, that was a long winded way of saying thanks for your and Walts input on all this. It's very interesting and educational.
Tony