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Cam Card Overlap

How to Degree a Cam

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I have been reading up on the affect of cam overlap on engine performance, specifically in relation to idle. In looking at several different cam cards for turbo engines they all seem to have negative overlap of around -8°, at 0.05" which is no overlap at all. If i do the math and work backwards they do have overlap at the advertised duration of around +34°.

My question is when people talk about overlap are they generally talking about at 0.05" or 1mm or what ever the cam maker uses or are they talking about at the advertised duration overlap?

The reason for asking is I am having idle issues and if I already have "no" overlap, then decreasing it wont help but dropping it from 34° to 32° might help.



Generally the overlap should be taken into consideration at 1mm of lift, so as the duration.

Cam manufacturers can advertise overlap and duration in whatever lift they want but the 1mm is the most honest approach

So basically if we treat overlap as being at 1mm or 0.050" then aftermarket turbo cams should be considered to have 0 overlap? Based on my sample of 6 cam cards for street upgraded cams around 264 to 272 from a couple of manufacturers.

I knew turbos don't like overlap but I did t know it had been completely removed.

What engine is it? Does it have variable cam timing?

Ok the car is a toyota supra with a 7mgte engine. I have installed a set of bc264 cam shafts, car card is attached.

I went through the process to degree the camshafts in but had issues with the cam card duration of 220° as I got 214° for intake and 212.5° for exhaust. I set the center line to match the cam card center lines. I rechecked the valve shims and they are all withing toyota specs so the valves are stuck open.

I have attached an excel sheet with the bc cam card and what I measured in the car.

From what I measured nothing should be too wrong. And if anything it's wrong in a way to reduce overlap. Yet in the videos it wont idle below 1000 and has less than 10mmhg at 1000rpm.

ECU is map based so air leaks shouldn't affect it. I actually added a vac leak to help it idle. The car is crazy rich which I put down to the low vacuum and the TPS telling the ecu the throttle is closed.

The car had good vacuum before i changed cams so i am at a loss as to why things have dropped off so much.

Do you guys think this is a cam timing issue or it it something else mechanical

Attached Files

From the camshafts timing card I really don't see anything radical. You have 8° of overlap and a wide LSA 114° which calls for a camshaft not made for revving.. Are you sure about correct timing them? You should be idling lower than 1000

Are you sure you are actually checking at 0.050" VALVE lift, that you seem to match the spec' at 0.00" but not at the 0.050" valve lift suggests you aren't as that 0.0" is the actual valve lift figure?

The 'standard' comparison of 50 thou" lift was introduced as different manufacturers had different ways and lifts specified for measuring their camshafts and it made comparisons VERY difficult and, when you introduce hydraulics which have negligible effective clearance, well...

While that is often referred to as 'effective duration' and it may be considered so, the valve lifts in the overlap, or transition phase, is still very important as the smaller opening means what gas flow there is in the ports is apparent at these smaller valve opening as higher velocity and hence better scavenging.

From this, you may realise the importance of getting the valve seat shape as optimised at low openings, especially with lower lift cam's, as practical as it can have a big effect on the overlap flow and effectiveness - with long duration and/or high lift cam's it isn't as important because the valves are further off their seats around that time.

That is a relatively mild camshaft so I would expect any idling problems to be elsewhere - you are confident the timing is correct, so perhaps a vacuum leak, ignition weakness or some other problem - don't get tunnel vision 'knowing' xxx is the problem, when it may lie somewhere else you have overlooked, take a step back and review what seems to be happening and what might be causing it - been there, done that... :-(

Thanks guys for the helpful feed back. I can confirm I am 100% measuring from 0.050" lift. In the excel sheet I added the 0.00" lift or the advertised duration numbers that are calculated based on the center line that I have set the cams to and then half the advertised duration ie half of 264. This of course matches the cam cad accurately as I have accurately set the center lines.

I only went through the maths on the advertised duration to see what overlap it theoretically had at the point zero lift because at the 0.050" there is a negative overlap of -8°. A negative overlap means no overlap. I have updated the excel to remove it as it is calculated data and doesnt help with the discussion.

I agree these cams are not wild, more wild than stock but nothing that should give idle issues or kill my vacuum.

I have checked timing is correct, vacuum leaks would just raise the idle above 650rpm and would help with it running too rich. I did a boost leak test and its holding, I also checked fuel pressure and that is also correct.

Running out of ideas other than bent valves... or some other mechanical damage that would be killing the vac. not sure how to test that as with its current vacuum issue compression test would not be reliable.

Attached Files

Don't get hung up on the 50 thou" points - you DO have overlap it is just that during it there is less than 50 thou" lift in the cam(s). Again, this is why it is so important to have good valve and seat design to optimise flow at this point. Heck, in some instances you can double the flow at such low lifts - one of the reasons I am somewhat cynical about people using heads that offer huge flow numbers at high lifts, when not only are they often using cam(s) that don't go that high but the valves spend much more of their opening period at the lower lifts - the engine needs to be considered as a whole package, not as a collection of separate parts.

As for the vacuum - exactly where are you taking it from, that is a VERY low reading? Before stressing out about leaking valves, etc, do a compression test - I doubt you will find a problem there, though, as a bad valve will usually cause a noticable flicker on a vacuum guage. However, consistently low values for all cylinders would suggest a cam' timing problem and a follow up leakage test to confirm.

The engine seems to idle and run smoothly, so perhaps you would be better checking the ignition is correctly timed - I assume you checked the Crank'/pulley TDC was the true TDC when timing the camshafts. I'm not familiar with the engine, but from this (old but still useful) video it suggests it is possible to get the ingnition timing out - https://www.youtube.com/watch?v=Y-smo0jp5bk

Post screenshots of all idle related tables and settings. VE table, idle air valve, timing, etc.

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