Adjustable cam timing pulley - optimizing for better FE

[Daox]

Well, my adjustable cam timing pulley for the Paseo will be here within a few days now. So, I figured I'd start some discussion around it and give some background on the engine it is going into. First off, here is the pulley.




The engine it is going onto is a DOHC, but the two cam are geared together. So, they are not adjustable independent of each other. So, if you advance the pulley, the intake and exhaust cams are both advanced the same amount.

Here is a picture of the cams. The exhaust cam is on the top and that is the cam that the pulley is attached to.





From the little I've read, advancing the cam timing helps low end. However, I was thinking that I might be able to get a bit of the Prius/Atkinson like action going on if I am able to retard it enough. Ideas?

[hackish]

You might end up with a slight improvement. Maybe with this you could get the head decked (a lot) and use the pulley to take up the extra cam timing that gives you. Ultimately an engine with a higher compression ratio has a much better BSFC.

-Michael

[moorecomp]

On the ZETEC engine in my ZX2, the exhaust cam has variable timing instead of EGR. Maybe you can retard the exhaust cam to accomplish a similar thing?

[Christ]

Your cam probably doesn't have enough event timing to do this.

If you're pulling against a closed intake valve, you're wasting a good deal of energy expanding air and it'll pull oil into your cylinder from the crankcase until the valve opens. Once the intake valve opens, air will rush in at super-sonic speeds because it'll be under alot more vacuum than normal, making a rather funny intake sound.

On the other side, you'll be doing the same thing w/ the exhaust, except compressing the burnt gasses against the exhaust valve until it opens, which will cause an excess of exhaust pressure at the header opening when the valve finally does open, until the gasses have expanded back to normal density for the exhaust, making a rather interesting exhaust noise.

So to do this correctly, you want a standard exhaust profile, I believe, which means re-meshing your two cams at different timing specs. Then you want an extended intake profile, which stays open for a few degrees beyond BDC-I.

If you go ahead and try it, count the teeth on the mesh gears and see how many degrees one tooth would set the exhaust cam off by. You might be able to come up with some combination of the mesh gears, timing belt, and adjustable gear that keeps your exhaust cam in spec, but alters the intake cam.

I seriously don't think it'll work out, though.

[MetroMPG]

Tim, is the adjustment infinite, or indexed?

[Christ]

Darin - Technically, adjustable cam gears have a basically infinite adjustment, because while they're usually only indexed for 7-14* in each direction, each tooth is typically only slightly more or less than that on the cog.

The result is that you can move the cog a tooth in the direction you want and "back tune" to get those few degrees back.

For instance, one of the Honda engines I had (d series, not sure which one) was 19/32 crank/cam.

360/32=11.25, and with 14* of adjustability, I can make up for an entire tooth this way.

[sc2dave]

Anyone know if they make them for a 5sfe?

[Christ]

If they don't, they're not too difficult to make if you have two of your OE pulley as spares.

You cut the center hub out of one, then cut the outer ring off another, making sure the two pieces are interference with each other if you try to put them together, and then you file/mill the interference section of each piece half way through it's thickness on opposing sides, so they mesh together to give proper alignment. Get yourself a protractor and mark degrees, set the gear at "0", bolt it together in a few evenly spaced places, and degree your OE cam to make sure you've installed the two pieces indexed correctly.

Any time you need to make an adjustment, you'll have to index the gear, unless you accurately marked it to begin with.

[jdgFirefly]

If your building a Atkinson type engine don't forget to increase the compression ratio to make up for the reduced intake volume. The question is what ratio do you need to bring compression pressures back to normal?

I've been wondering if anyone on this site has ever build an Atkinson engine?

[Compaq888]

I'm not a gear head but when I was modifying my last car I had to advance my timing 3 degrees because I planned to run 91 octane. I did that because I ran nitrous, it needs higher octane. I of course retarded the timing 6 degrees when I sprayed. The point where I'm getting is a measily 2-3 degree advance is not going to make any significant low end power. When I advanced the timing 6-7 degrees by accident I had significant improvement in my bottom end power.

If I was going to play with the engine to improve my FE it would be a cam. Even as far as getting a custom cam then playing with the timing.