2.7rs MFI cam timing at 4.7mm, no zing to redline

manbridge 74 · 08-16-2017, 07:38 PM

Been going through CMA on my personal car. Baselining really, as it starts and runs well with good mixture readings per LM-1. Good correlation and synched TBs at 3000rpm. I didn't build the engine but I do have the build sheet. It shows cams were timed at 5.3 and 5.28 mm. Unsure just how many miles it had when I got it but leakdown is excellent at around 1-2% on all cyls. Advance curve measures per RS specs with 38 degrees at 6000 rpm. Pump has the correct 2.7rs space cam. A 73 S engine so cams are correct.

I decided to check cam timing because it doesn't have a noticeable rush coming on the cam. From 4500 to redline is about the same pull. It measured 4.7 mm on cyls 4-6 so I stopped there. Seems it should have less low end and more top end as this would be a retarded setting.

I'm going to set it back correctly but how could this be so off? Wear? Did the chains get over or under tensioned when set?

manbridge 74 · 03-08-2018, 05:03 PM

Finally got around to looking deeper. Checked timing on both banks with the Stomski tools and left bank was 4.89 and right was 5.15. So while not off too much and wanting to look at the idler bushings for binding I was able to get both banks dead on to 5.3mm. Results were a smoother idle and a hair deeper engine tone with snappier rev up and no loss on top end.

Camgrinder mentioned in a post that exhaust duration on S cams is fairly short so retarded setting can lead to piston pumping loss. The breathing resonance makes a difference. I should’ve did a before and after dyno run.

Matt Monson · 03-10-2018, 12:09 PM

Cool.

Peter M · 03-10-2018, 01:27 PM

Can you really tell the difference of a 0.15/0.41 mm cam change with your butt?

If you overlayed the before and after dyno curves, what would the maximum difference be? More than the dyno error range?

Serious questions and would love to see any solid data anyone has on this topic.

Thanks

manbridge 74 · 03-11-2018, 01:02 PM

Well I doubt peak power changed at all but it does seem to idle smoother, the S cams with larger overlap than most cams can make smooth idling a challenge. I can push the hand throttle all the way down and it will idle at 500rpm without dying where before it would stall.

So of importance is getting both sides the same. One must suppose that Porsche designed the cams and MFI space cam to work together, (being in the “window”) so this was mainly ruling out more variables on a somewhat crude fuel delivery system.

This car operates mainly at 6400-8600ft elevation so power being some 20% less and having the intake valve closing later can mean dynamic compression would be lower. Hence trying to optimize engine breathing was also a goal.