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Old 12-19-2007, 10:55 PM   #1
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Default Cam overlap

Has anyone with adjustable cam gears played with cam timing?

I usually don't post until after I experiment but I'm getting lazy. I'd like to hear what others have done.

All numbers in cam°, not crank°.

For the last 5 months I've been running the exhintake cam at 2° advanced (most NA people run 5°-6° retarded) from the 180° + three teeth. Not sure how this compares to the stock cam overlap. Exhaust cam has been at 0.

This current setting probably has too much overlap and I feel that there should be much larger power gains as the boost goes up.

I adjusted the gears to -6° exhintake and +3° exhaust. I need to retune and test.

I'm pretty sure at 10psi+ I'm just blowing air/fuel into the exhaust. I don't have an EGT to verify. I'm also looking for a little quicker spool for autox, that'll be a different thread.

Anyone have direct experience? (no 'internet wisdom' please)
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Old 12-22-2007, 10:51 AM   #2
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I did a little bit of non scientific testing. Didn't retune between cam settings, only compensated for the timing changes when I adjusted the exhaust cam.

Result: Cam overlap makes a huge difference, nothing subtle.

2° advance intake
0° exhaust
Boost is linear (and <7psi) until explosive spool at 4500(best SOTP fun but wheelspin from the tires being shocked), very good power at low boost levels, great gas mileage. This setting feels the best but may not necessarily be the fastest at the track.


6° retard intake
3° advance exhaust
Idle is smoother, spool is much earlier and linear but hp is WAY down, possibly 50+whp, top end might be ok but not thrilling enough to sacrifice the low and mid. (7psi at this setting doesn't pull as hard as 3psi at the 2°i/0°e setting) Powerband is smooth but wimpy.


1° retard intake
3° advance exhaust
better than -6°i/3°e


0° intake
1° advance exhaust
early spool, fairly smooth transition to full boost (reduces wheelspin), possibly a good autox setting.


Need to go back to the original 2°i/0°e and do some gps runs then make subtle changes (vs the gross ones that I've done), the butt dyno isn't as calibrated as it used to be.
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Old 12-22-2007, 11:43 AM   #3
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To help people understand what you are doing better that dont know this stuff.
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Old 12-23-2007, 06:00 AM   #4
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Also don't forget that overlap and cam positions are measured in crank degrees and some people like to quote cam degrees (what's written on most cam gears) when quoting the cam advance/retard they did (so the change in overlap (in crank degrees) is double that)
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Old 12-23-2007, 12:01 PM   #5
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Thanks. Interesting work. Most of the cam timing on dyno results I've read indicate that somewhere around 2°retard i and 2°advance e give an overall bump with no loss of power, better idle, and better spool. FM says try 4° and 4° initially. Here's a good quote from Myron
Quote:
Originally Posted by 2/3rd's Cobra
On my 1.8, I have 2 degrees of retard on the intake cam and 3 degrees of advance on the exhaust cam. This set-up gave me 9 hp (2500 to 5800) then peaked at 12 hp @ 5900 rpm through 7200 rpm w/ no loss of torque at all. It was a big difference.
Something I've found by searching elsewhere, but have not seen here at all, is on the 1.6 using a manual intake cam and auto exhaust cam.
Quote:
Originally Posted by bbundy
When I was 1.6 I experimented with all configurations of stock cams and my favorite was to use the automatic exhaust cam with a 5 speed Miata intake cam. It probably didn’t make any more power that way but the shorter duration exhaust cams spooled the turbo noticeably faster.

I'm assuming you're on the stock cams?
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Old 12-23-2007, 10:52 PM   #6
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Quote:
Originally Posted by Ben View Post
I'm assuming you're on the stock cams?
I'm running the exhintake cam. (For those who don't keep up with the NA world..... on a 1.8 you take an extra exhaust cam, lop off the CAS drive and throw it on the intake side for more lift and duration- after 'proper' indexing of course)

The numbers posted above are relative to the 'Josh Cosford Exhintake cam install' and in no way related to indexing a stock cam.

I still need to get the gps in the car and have it spit out real information so I can settle on a cam setting.
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Old 12-23-2007, 11:29 PM   #7
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Got ya. I'd like to see a non FI NA 1.8 run exhintake 99-00 cams.

What I quoted earlier has to do with turbocharged cars.
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