Retarded Exh. cam
 

So, after copious amounts of searching I am still short on info concerning the use of retarded exhaust cam timing for a boosted application. Without the use of adj. cam gears it is possible to run the stock exhaust cam retarded 1 tooth (12deg.). This in turn reduces the TERRIBLE 20 deg. stock overlap to a much more boost friendly 8 deg. I am curious if any one happens to have a dyno plot of any gains/losses using this method. Please forgive me if my search skillz are weak and there is an obvious answer, Im a noob.:noob:

Why do you think that retarding the exhaust cam will REDUCE your overlap? Does your engine work differently than mine?

Quote from solomiata's cams page:

"There are a couple of options available for factory based 1.6 performance camshafts (I have not dyno'ed any of these and they are just pure speculation, try at your own risk). The 323 GTX turbo exhaust cam can be installed for 1.6 aftermarket turbo applications or it can also be used for performance NA motors by installing it 1 tooth off (12 degrees) advanced (-->). This gives timing numbers of EVO 57 ATDC and EVC 13 BTDC and an overlap of 18 (-5-13) with the stock NA intake cam. The stock 1.6 NA camshaft can also be made more appropriate for boosted motors in theory by installing the exhaust cam 1 tooth off in the retard direction (<--). This gives timing numbers of EVO 65 ATDC and EVC 3 BTDC and an overlap of 8 (-5-3)with the stock NA intake cam."

Because it keeps the exhaust valve shut longer. Advancing would make the exhaust cam open sooner, increasing overlap. There's a lot more info on this via miata.net- though JasonC here is has played with overlap on the dyno and believe he's present in these threads on m.net. I say just try it. It's going to make everything out of boost worse, but if the benefit in boost is worth it, maybe you can live with it.

The guy you are quoting has it backwards. The exhaust valve is closing as the intake is opening. If you retard the exhaust cam then when the intake valve opens, the exhaust valve will be open longer (more overlap, not less) before it closes and you will have intake charge escaping through the exhaust valve.

You've got that backwards. Overlap is when both valves are open. This only occurs at the end of the exhaust stroke and beginning of the intake stroke.

Yes, six has it right.

You'd want to retard the Intake cam to reduce overlap for a boosted application.

Go- retarding the intake cam sets the action back in the timing event so that it is completing later. That means the exhaust valve is opening earlier in the intake valve's event.

Overlap is controlled with both cams since it's relative. Get on a dyno and roll both of them to tune. I got started this at a dyno day and witnessed retarding my exhaust cam 1 cam degree produce 2 hp.

If you retard (roll backwards, counterclockwise, left) the exhaust cam, it will open later. If you advance the intake it will finish the valve action sooner- before the exhaust action begins. Look:

Retarding the intake cam will also have the effect of moving the powerband of the engine slightly up in RPMs, but the effect is minimal.

Taken from this explanatory article:http://www.circletrack.com/techartic..._duration.html
"Overlap is the point in crank rotation when both the intake and exhaust valves are open simultaneously. This happens at the end of the exhaust stroke when the exhaust valve is closing and the intake is opening."

You are incorrectly saying "That means the exhaust valve is opening earlier in the intake valve's event." The exhaust valve never "opens" during the intake valve's "event". It is closing when the intake valve is opening. The time that they are both open is overlap. Then there is an entire power stroke after the intake closes when both valves are closed before the exhaust begins to reopen for the exhaust stroke. The intake begins to open at the end of the exhaust stroke while the exhaust valve is still open (overlap) to take advantage of the scavenging effect of the exhaust rapidly flowing out of the combustion chamber.

There are two ways to reduce overlap with cam timing on our motors. Retard the intake cam or advance the exhaust cam.

Yeah- you're right. I just went through all my stuff from over a year ago and I retarded the intake and advanced the exhaust (1* cam each). I should have just searched my post here.

A couple of quotes from boosted Miatas that I referenced prior to the dyno run cam tuning- and what really compelled me to pick up the adj gears.

Results from the above changes (fat lines) vs. stock cam timing with stock normally aspirated cams:
http://i158.photobucket.com/albums/t...comparison.gif

This diagram really helps to understand whats happening during valve events.

Attachment 195309

Visualize retarding the exhaust event (opens later in crank rotation) moves the red curve to the right, more overlap. Retard the intake cam, moves the blue to the right, less overlap.

There's ALWAYS a retard behind the exhaust cam when I'm driving

as a vvt guy, i agree with half and disagree with half

retarding the intake cam on a boosted car makes a big change. remove 10-15% of your power from 3500 RPM and put it at 7000 RPM. consequently, advancing the intake cam will do the opposite. both depend on what you start with.

max power at redline, retard intake cam 12 deg.

best midrange, retard it 3 degrees.

compromise is somewhere in between there. advancing it beyond 0 nets you only a loss in power above 4k and no real improvement below.

Yes, retarding the intake makes a huge difference. I found mine liked between 4-6* of retard, making up a little top end for what my tiny blower was lacking.

Dumb question. If I retard my intake cam, do I have to break out the timing light and verify timing afterwards?

cam :ne: ignition

depends on where your cam sensor is located.

if it's on the crank, then you're fine.

if it's on the exhaust cam, then you're fine.

if it's on the front of the intake cam gear, then you're fine.

if it's on the back of the intake cam, then you'll want to verify timing because the crank pulley has moved relative (via the intake cam gear) to the CAS.

That's about what I thought, thank you.

Thanks for all the input guys. I've actually already retarded the exhaust cam one tooth (12deg) the butt dyno is seeing a bit more power up top, though i've noticed quite a difference down low. It appears a worth while investment to snag some adj. cam gears though. If anyone has some before and after dyno plots of cam timing I would be interested in seeing them. Hopefully I'll have some to show myself within a month or so. I've just finished swapping in a fresh motor with a rebuilt head and i'd like to see what she is laying down with my cam tweaks.

12 degrees!