1975 2.7L CIS Cam Timing Question

[AlexGross]

Before I start, I know that there are tons of threads already about timing cams, and let me assure that I have gone through loads of them already to try and figure out this problem.

Back-story: I followed Wayne's procedure for timing the cams...that is, I used clamps on the idler arms to fully tension the timing chains, then timed the left cam; and then timed the right cam. After running the engine through several full 720 rotations, I found the timing readings on the dial indicator were consistent: 0.47mm for the intake valves at overlapping TDC for cylinder #1 and cylinder #4. Note of procedure, this is a 1975 2.7L CIS with stock cams (intake valve overlap range = 0.40-0.54 mm). From my untested understanding of this procedure, I felt that these readings were right-on for the timing and felt ready to install the hydraulic tensioners.

Keeping maximum tension on the idler arms, I slid on the tensioners and let the pistons pop into place. NOTE: that the tensioners are exerting less pressure on the chain than the clamps I was using during the timing.

Here's the question...due to my inexperience and desire to not destroy my engine once it's finished, I have the tendency to overthink and over-check everything. So I checked the timing specs again with the chain tensioners installed and found a discrepancy in the readings...they had changed from 0.47mm on cylinder 1 & 4 to 0.51mm on cylinder 1 and 0.60mm on cylinder 4 at overlapping TDC.



Any thoughts? I am just trying to decide if I have made a mistake somewhere along the line that I need to try to remedy. I tried to keep maximum tension on the chains while timing and installing the tensioners...does the change in timing readings indicate that I need to re-time the cams? Or are these misgivings unfounded? Thanks for helping out this first-time engine builder finish fixing his baby.

[Flat6pac]

You set the timing up with the book. The timing with the clamps in place were in range of spec. Installing the tensioners released the firm spec reading.
The one point to take is, did you load against the direction of rotation against the inserted pin as that can be where there is a flaw in the numbers. Once you set the pin you need to use a 17mm open end and load the cam clockwise to remove slack on the inserted pin.
Bruce

[Walt Fricke]

One thing you can do is keep turning the motor over, and watching to see if these new values move around a bit. The numbers of teeth on the gears and chain wheels are such that it takes quite a number of revolutions for the same chain link to be on the same tooth of any of the gears. A friend tells me he expects to see some change, though he hasn't attributed this to differences in tension.

Did you take your "nicely within spec" readings after you had torqued the big nut to its full book value?

The locating pin system has slop in it. If a guy is lucky, you can hit on a pin location where you can keep the timing where you want it even without tightening the nut. But usually that reading comes with the pin not tight up against the sides of the holes in the direction of chain pull. Which means it is easy for tightening the nut to move things a bit.

[AlexGross]

Thanks guys!

@Bruce: I don't think I loaded the cam to remove the pin slack...but I made so many adjustments that I can't remember every wrench twist exactly.

@Walt: Those readings were taken with the nuts at full torque...in fact, with my first few adjustments, I hadn't read the instructions very carefully, and was torquing the nut fully with every adjustment lol (lot of extra work). Also, I just took measurements again (after probably a dozen full crank cycles) and the readings were the same...

I guess my question is: If you guys were working on this would you take out the tensioners and re-test the timing, or just go ahead with the build? One point of reference; I did run through a meticulous valve-to-piston clearance examination and had ample clearance throughout a full 720 degree cycle on both intake & exhaust valves for cylinders 1 & 4.
I don't mind re-testing the timing, because I know how integral this step is for the safety of my engine, but if I can avoid it, that would be great (obviously haha).

I understand that you guys are working with limited information, but I really appreciate your input.

Thanks again - Alex

And just because it motivates me to see it...here's a picture of the old girl that I'm trying to get on the road by this summer

[Walt Fricke]

I think the 2.7 cams are a lot like the 3.0 and 3.2 cams. Porsche varied the timing from maybe 0.9 to 1.7mm on these motors, with rather little difference in performance, per Bruce Anderson (RIP).

Your new readings are only off, side to side, by 0.09. If within overlap spec, Porsche allowed 0.14 side to side difference. The non-S 2.7s had a cam spec as high as 0.9 for one of their two misbegotten years, per Wayne's fine book's collection of facts. I can't see any serious issues with what you have.

It is a coin flip, I think. I might futz with the right side to get it back down a little. Or I might just think I'd get a smidgen more low end torque out of the motor, and leave it alone. If memory serves, isn't it the right side where you can leave the tensioner in and adjust the cams using it to do the tensioning without having your tools bump into the tensioner?

I set my big nut cams by putting the motor at TDC and rotating the pinless cam (or nudging one with the pin in) until the dial indicator reads what I want, and then tightening the nut some, rechecking, and then going to full 110 lbs/ft torque if it is still what I want. In your case, I'd start with nudging, as pin slop is way more than where you are out of your target range.

Carrera tensioners?

[AlexGross]

@Walt: Thanks again for the help, I'm thinking that I will probably just go ahead with the build as is. And yes, Carrera tensioners.