They may be a vague question but which has more of an effect on emissions, lift or duration? I've heard not to exceed 220 degrees duration (@ 0.050") to stay emissions legal. I've not heard what lift value to stay under. Can someone elaborate?

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Willie

Supercharged 1987 305 IROC-Z, Daily-Driver, Emissions-Legal.
Former Paxton (6-psig) with 50-hp nitrous: 12.043 @ 112.86 mph.
ATI D1SC (10-psig): 12.056 @ 116.62 mph.
All stats are altitude corrected for 3,100 feet using NHRA's Altitude Correction Table.

http://willie.camaro-firebird.org/

1987 "20th Anniversary Commemorative Edition" Z28 Convertible -- Super Chevy Show Class Winner, 1998.

 

I'll answer this question and the other at the same time...

Lift has essentially no effect whatsoever on emissions. Basically all that you do by ncreasing lift is allow more flow when all other parts of the induction system allow it; this would mostly occur at WOT (obviously the throttle is a restriction!). If you had 2 cams with the same "advertised" (lash point more or less) duration, .050" duration, lobe separation, and intake lobe centerline, but one had more lift than the other, you'd see virtually zero emissions difference, but possibly a considerable HP difference if the other components (heads!!!!) were sufficiently unrestricted to allow for more flow.

On the other hand, valve event timing, including duration, can have a huge effect. Overlap is one area that can be afected; if you have 2 cams, one with a "tight" LS (say, 106°) and one with a "wide" one like stock cams have these days (like 116°), you would observe that during the overlap period, it's possible for raw fuel to pass straight through the cylinder from intake to exhaust and on out. This especially can happen in engines with headers. Obviously such an enginw would have high HC emissions. The wider LS means the exhaust lobe center is far away from the intake lobe center (much earlier), so the exhaust valve is more closed than in the other case.

If the intake valve opens earlier (standard intake centerline, high duration) then more than likely it will open before the cylinder is completely purged of exhaust gases. In that case there will still be pressure in the cylinder, so some exhaust gas will escape back into the intake. This will result in the next cylinder fill having some amount of exhaust in it, which will make it fire more weakly, or maybe even not at all. This is what produces the familiar "lope" of a big cam. If it doesn't fire at all, there's another way to get raw fuel (HCs into the exhaust.

The dilution of the intake charge with exhaust also lowers manifold vacuum. This reduces the "signal" avaialble to pull fuel through the idle circuit of the carb; and it greatly confuses the computer if there is one, because that looks to the ECM like the engine is under load. In the carb, you have to richen the idle circuit to feed more fuel, which usually make the car run rich at part-throttle; in the case of FI, the ECM simply dumps more fuel in. In either case, emissions of both CO and HC go up.

With a flat-tappet cam, there is a practical limit to how steep you can make the ramps of the cam and still expect it to last into the hundreds of thousands of miles. It's a function of geometry. So, to get high lifts, you invariably have to have high duration too. With rollers, the limit is the beating the rest of the valve train and the valve seats take from the jerky valve motion, which is somewhat higher; so it's possible (though the factory hasn't yet discovered it) to have higher valve speeds with a roller and still keep the same reliability.

To answer your other question, that moves the RPM range where the engine produces peak torque. Advancing the cam moves the RPM range down, retarding it moves it up. This is only true on general, and results can vary widely according to the specific cam and engine. In most cases it's not something that I would recommend doing unless you use a 2-piece timing cover and are willing to experiment. The cam mfrs. put the cam timing where they feel it will run the best, and unless you're doing something really unusual with the motor (forced induction comes to mind), they're not far wrong most of the time.

This could be the subject of an entire book easily. Cam design is one of the most complex parts of engine tuning.

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"So many Mustangs, so little time..."
ICON Motorsports

[This message has been edited by RB83L69 (edited October 27, 2001).]

 

Thanks for the info RB. It is appreciated. Currently, my cam has:

214/220 Dur @ 50
482/496 Lift
112 LSA
0 Overlap (good blower cam)

I'm considering:

222/230 Dur @ 50
509/528 Lift
112 LSA
0 Overlap

With my current cam, my emissions are as follows:

Idle
HC: 32 ppm (220 allowed)
CO: 0.13% (1.20 allowed)

Loaded
HC: 27 ppm (220 allowed)
CO: 0.61 (1.20 allowed)

Would the cam I'm thinking of with 222/230 duration push me over the emissions limits?

Willie

 

Willie, Tim (Traxion) just had his 350 Miniram pass emissions with his 230/244 cam. He had to install cats and tweak the eprom to have a higher AF ratio while testing (to lower the HC), and he passed.

So I would say yes; though you may have to tweak the eprom a bit for the emissions test and then set it back to "normal" after you pass.