I have been thinking about the way we (we in a general sense, the way most of us with cm gears have been tuning them lately) have been typically tuning our adjustable camshaft gears. And, I am thinking we are doing it backwards for what we are looking for. Hopefully I'll be able to explain why.

Normally, we have been setting the cams to advance (+) on the intake cam, and retard (-) on the exhaust cam. First, let's just think about the intake cam.

You have a certain amount of valve overlap due to the cam profile. Overlap is what gives us more top end (general statement, yes, but applicable in this instance). If you advance the intake cam, you are causing it to open the intake valves sooner than normal. This also means the intake valves are CLOSING sooner than normal. If they are closing sooner, then you are reducing the amount of overlap that normally occurs with the camshafts. That should reduce your top end, and conversely, increase your botom end.

So if you retard your exhaust cam as well, you cause it to open the exhaust valves LATER than normal, reducing overlap even more.

So, essentially we've been tuning for bottom end. The solution is (of course) to tune the cam gears exactly opposite to this. If you RETARD the intake cam (make it open the intake valves later than normal) and ADVANCE the exhaust cam (make the exhaust valves open sooer than normal) then you would have more overlap and more top end (and less bottom end).

So, the next time I head up to the drag strip (it should be this friday) I'm going to try out new settings. I have been keeping the exhaust cam at 0 lately, and I'm going to keep it at 0 for now. So, the intake cam is going to be set at -3 instead of the +3 setting I tried before.

Dyno tuning would be the best idea, but I'll have to settle for the drag strip for now.

Todd,

I am actually aware of this and purposely used those settings to allow for no loss in bottom end and gain in the top end. We in fact did see that retarding both gave more top end but with a definite loss in bottom end. I felt the gain was not worth the loss for what I wanted. As of late, my favorite settings have been intake 0 and exhaust -2 with my turbo, but that is just a preference and not proven.

interesting idea to put into your head.. with the zx2's (i know different car.. but both DOHC 2.0L I4's, so it should be the same dynamics) many of the guys have found out that when they moved the gear so many degrees.. like between 1 and 4 they got a gain.. and then when they moved them farther, like between 6 and 9 they lost power.. then when they moved them even farther, like 10 and 14 they produced even higher gains then the 1-4 settings.
So i'm just throwing that out there incase you guys hadn't thought of adjusting the gears 'PAST' the point where you started losing power and you might be able to start gaining power with an even more agressive timming setting and get better gains then the mild settings before the 'power loss' settings. I hope that makes sense.

Hmm let us know how it goes Todd. Just for the record, I got a better low end on the dyno with both settings advanced two degrees. I was advised not to advance the intake more than the exhaust, but I have tried that setting and it felt slower to me. Of course it can all be my imagination. I'll try to look up to see if I actually dynod intake ret 2, exhaust adv 2.

I did drive around with it. My first impressions at -3 in the intake (compared to +3 on the intake) the car seems to have a bit less bottom end and a bit more top end. Now I really didn't have a chance to open it up too much yesterday, and it was just the initial impression I had. Friday will be the litmus test when I head back up to race.

Ok Im gonna make some guesstimations here cause this all doesnt seem right to me I could be very confused!! I dont know if its true but here it goes...(gonna be long)

First with the intake, I dont believe opening it later would help overlap, if anything earlier would. Now of course if it opens earlier it closes earlier meaning that the airflow thats moving down through the intake at high rpm gets choped off earlier meaning less air gets in there! Also if it opens too earlier at high rpm the pressure of exhaust gases from the piston moving up may cause the air flow in the ports to wait before the overlap achieves a negative pressure balance. Now by opening it later you let the overlap achieve a good balance(my guess is when the piston gets closer to TDC it starts to slow down meaning the gases are moving faster causing a pressure drop to occur) getting the full effect and allowing the intake valve to stay open longer so the the air thats still being cramed in there by the velocity can keep moving.

Now the exhaust, you want it to open before the piston reaches BDC so its open fully when it starts moving up and before the reverse pressure wave(I made that up it sounds good) occurs from the downward moving gases reflecting up and slowing the piston(mostly a guess) and stays open long enough to give a good overlap time with the intake valve. So the exhaust is a hard compromise to adjust with stock cams, between opening to late to give good overlap and opening to early to give good exhaust outflow.

To make this kinda easier if anyone knows the valve events(when they open and close compared to the piston from TDC and BDC) that would be extremely great!!!! My idea for tuning(from someone without adj. cam gears) would be retard the exhaust a 2 and advance the intakelike what brezio said 10-15 or whatever, again the valve events would be really helpful to figure out overlap, how long the intake valve is open and when the exhaust valve opens and closes, etc.
If you've read this much thanks for reading and respond if I was wrong or right or whatever but I hope this helped more than anything, later.
Brent

Todd, I just thought of something... the intake and the exhaust strokes happen next to each other. Say, we, for the sake of simplicity, took the 4 strokes, intake, compression, power, and exhaust, and shortened and simplified them to suck squish bang blow, a normal engine would do this:
suck squish bang blow suck squish bang blow suck squish bang blow suck squish breakdown (this is a DSM, after all)

Now, advancing the intake and retarding the exhaust would make it do this:

suck squish bang blowsuck squish bang blowsuck etc...

whereas retarding the intake and advancing the exhaust would do this:

suck squish bang blow suck squish bang blow suck etc...

So I think that we were right the first time. I mean, the overlap is between the exhaust and intake strokes, not between the intake and exhaust strokes (there are 2 other steps there that prevent keeping the valves open long enough to overlap there). If they were both open, would we be able to have a compression and power stroke? No squish and no bang make a motor not worky worky that well.
Some late night thoughts available for your perusal,
-=B-=

time for ^^^ to get some sleep.....
if that actually works, I think I'm gonna try advancing the timing on my gf.



-Tony '98-GST
eclipzgst@charter.net
http://eclipzgst.webhop.net
turboclipz on AIM

You don't have a GF you dip...
-=B-=

not since Wed. :) ahhh... single again after 2 years. Much relief!
Maybe you should've posted this last week and I wouldn't have gotten rid of her. :)



-Tony '98-GST
eclipzgst@charter.net
http://eclipzgst.webhop.net
turboclipz on AIM

i think u retarded her a bit too much


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thats funny as hell guys. it is 7:30 in the morning and i am trying to figure this out, but i think it is way to early. i'll try later :)
-Josh


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Brent, Bill, I think you both probably have it right, but there are a whole lot of other things I've been thinking about which really throws a monkey wrench into this before I can give a difinitive answer.

Right now I'm assuming I messed up my reasoning, I think I had my valves mixed up, eh, I've made bigger mistakes :)

Actually I messed up my little reccommendation, it should be advance exhaust 2 retard intake 10 or whatever, maybe leave exhaust stock like you were doing. But if you mixed up the valves its no biggy, Ive had worse problems figuring out stuff like this. And I do like the idea of advancing the timing on girlfriends, good idea!!! ;) But is it possible to do what brezzio said, do too little and get small improvements then increase it for shit then it gets extremely better, humans are a little diff. than an I-4 engine.
Brent

^^^ haha...

anyway, damn gears and their 'guess and check' nature.. DYNO shops own your ass when you get cam gears ;) hehe. but i would be interested to see if you did some crazy advance or retard on your gears (mostly intake I think) if ya get a better increase then the very mild/conservative settings. but I've head that's how it is on the Zetec motors. I mean hell.. our Exhaust cam (on the zx2 and contour I4's) have a VCT (variable cam timing) gear and the motor is capable of changing it's exhaust timing up to 30 degrees!!!