Cam Timing Question Options

[Borderline]

I'm in the middle of upgrading my AX engine. I'm putting in a 78 mm stroker crank to go with the 103mm pistons giving me a 2.6L T4. I have a Scat cam that has about 262* of duration on my 2366 cc engine that I've been running for the past 4 years or so. I finally decided that I would go with a slightly more aggressive cam with 272* duration and bought a Web Cam 86c. the timing card is shown below. Now the question is when I install this cam and check the valve timing at the valve using .050" valve lift, how many degrees of duration should I measure? I was expecting 272* plus or minus a couple. I guess the real engine builders will know what I got. What do you think I should have expected?

[Borderline]

I guess I should have been more clear. when I bought a cam that said it had 272* duration @ .050" lift at the valve I was actually expecting to measure 272* of duration at the valve. What I got was 282+* of duration at the valve. My buddy with a bunch to T1 experience did some calling and came back with "everyone knows that the .050" duration is measured at the cam". The rocker ratio changes the duration at the valve and as the cam grinder doesn't know your rocker ratio he can only spec the cam.

The Scat cam that I installed 4 years ago was specified as 262*. When I installed it, it measured 262* at the valve. I just checked the timing at the cam and it is 252* at the cam. So now I'm a little frustrated not knowing what I'm going to get. My buddy gave me a Scat 271* cam about 6 weeks ago. It checked out at 282* at the valve. So I'm even more confused. I just think that 282* is too much cam for an AX.

The people at Web Cam have been very nice and they are going to regrind the cam with the 86b profile. That should have a 262* duration at the cam and give me 272* at the valve.............I hope. (IMG:style_emoticons/default/headbang.gif)

If anyone would like to share their experiences.........feel free (IMG:style_emoticons/default/biggrin.gif)

[Joe Ricard]

Back in the day I had a 86b +5

Pretty dang good with 2.0L and a bit of port work and Tangerine EVO header. 40 IDF's 8.5:1 compression.

[Woody]

I'm building a 2414 with an 86b/86c right now but haven't made it to the cam timing yet.

[stownsen914]

Well, technically ratios rockers only change the lift, not the duration. But I guess when you measure .050 at the valve, it WOULD change the duration a little on each end since the .050 would happen earlier on the opening end and later on the closing end of the cycle.

For what it's worth, I carefully measured the Elgin camshafts I put in my 911 race motor when I first built it years ago, and was surprised to find that the specs were off a bit from spec. Basically one cylinder bank measured different from the other by a few degrees. Apparently they ground the lobes on one camshaft such that the spread between the lobes was slightly differently on one compared to the other. So I could have either the intake valve timing the same on both sides, or the exhaust timing the same, but one would have to be off. I think I wound up setting it in between so split the difference.

Scott

[Borderline]

Scott, years ago when I built my first T4 I had an Elgin cam and he wouldn't give me a spec card just told me to use the stock timing marks on the gears. I didn't degree it and to this day I have no idea what it was (IMG:style_emoticons/default/biggrin.gif) It was a low compression street motor so it didn't make much sense to push the issue. The engine ran great but the head temps got pretty hot.

I'm finding that the stock rocker ratio (~1.30 to 1.35 : 1) adds about 10 degrees to the duration at the valve

[ChrisFoley]

I've always looked at the zero lift timing first, then compared 050 lift numbers.
If total duration and lift are close to expected I'm happy.
I only use the 050 numbers to compare one cam grind with another on paper.

86c seems pretty aggressive for AX unless you plan your gearing for a higher/narrower rev range. That cam craves rpm - 7000+.

[stugray]

I'm finding that the stock rocker ratio (~1.30 to 1.35 : 1) adds about 10 degrees to the duration at the valve

Regarding the rocker ratio:
I found that the Intakes tend to be ~1.36-1.38 while the Exhausts are 1.30-1.32.