Cam tuning

[Nim]

If you had adjustable cam gears, and wished to advance your intake cam a little, would you then retard your exhaust cam? Or would you advance the exhaust cam aswell?

edit: this is assuming the intake and exhaust are independantly adjustable

[ViPeR_NiPPleX]

depends. if your intake and exhaust have seperate cam gears(joined with a belt), then its more than possible to alter one without changing the other.

[Nim]

I know it's possible, my question is, what should you do? Advance intake and exhaust? Or advance intake and retard exhaust?

I mean generaly. I'm sure there are some instances where you want to leave your exhaust right where it is.

Perhaps a better question is what does VVTi that works on intake and exhaust do? Does it advance them both? or retard the exhaust while advancing the intake cam?

[myne]

VVTi advances the inlet in stages, in the 1uz VVT also retards the exhuast for the simple reason that the exhaust is geared off it.

Usually you can adjust inlet and exhaust independantly on a twin cam.
I suggest you do some reading on how cams affect engine air flow since that'll answer most of what you want to know.

[ed]

vvti typically acts on only 1 gear (intake) and advances/retards it with respect to the exhaust gear, opening and closing the timing and duration of overlap dynamically through the rev range.

for a guide to what playing with the timing does, read:

http://www.billzilla.org/engvariable.html

ed

[ed]

VVTi advances the inlet in stages, in the 1uz VVT also retards the exhuast for the simple reason that the exhaust is geared off it.

read this
http://www.toymods.net/forums/showthread.php?t=3156

[myne]

Oh oops.
So the overlap doesnt change at all - interesting. Very suited to FI then

[The Witzl]

Rather than asking vaigue questions expecting this audience to teach you, perhaps try the following link and learn all about it yourself.....

Pick me teacher, I'm ever so smart!

.... i know it's the first place I visit when I dont understand something!!

[Nim]

I already went there Witzl. Couldn't find anything. I searched howstuffworks.com too. Also, the question has already been answered by others.

I figured it was an easy question, as anyone who has put adjustable cam gears in their car and had a hand in tuning them will know.

So, the answer to my question is:

when you advance the intake cam, you want to retard the exhaust cam to improve overlap, and make for better mid-range? I guess it gets a bit more complex after that, reading Bill's site.

Thanks.

[oldcorollas]

when you advance the intake cam, you want to retard the exhaust cam to improve overlap, and make for better mid-range? I guess it gets a bit more complex after that, reading Bill's site.
Thanks.

it depends on too many factors, like how far "out" are the cams to begin with, where you want your engine to work best, what other problems are you trying to cover up by moving cams...

ideally, you don't ever need to move cams. set and forget.

[ViPeR_NiPPleX]

is vtec yo not enough for you nim?

[Nim]

Unless you have VVTi... then you want to constantly move cams.

But yes, I see your point.

[Nim]

is vtec yo not enough for you nim?

LOL, nope, I was iVTEC yo! Too bad it doesn't come on any decent engines. umm... 'cept the Euro engine... thats a nice bit of work.

[tricky]

What you wanna do is have a read of joel's (turboRA28) thread on cam timing. All the basics, complexities and optimal measuring details are in there. Basically, if you advance or retard both cams (in unison), the overlap is constant but the powerband moves up or down the rev range respectively. The best results are achived by leaving the exhaust cam and advancing the inlet cam a SMALL amount.

Of course, this is a pointless excercise with a stock engine because you'll gain little power and lose bottom end driveability... And of course a lot of R & D went into the stock cams and stock cam timing anyway.

[oldcorollas]

gosh, look what i found bookmarked....
http://www.sportcompactcarweb.com/in...technology01a/
not great, but not bad