Exhaust Cam Timing Rb25Det

[Kenna802]

Hi All Just a quick question about cam timing.When i got the motor rebuilt the Exhaust cam was 2 teeth out in Advance, This caused the turbo to boost alot earlier on but caused missfire up at top end. I have a rb25det with ATR43G3 installed. I noticed once i reinstalled the timing back to normal Turbo has lost alot of response. I have read in alot of previous comments about cam gears and saw somewhere that the intake\exhaust cam gear advanced\retarded XX Degrees help bring on power earlier but didnt increase top end. Just wondering if anyone could shed some light on this. Yes i know i will need a re tune but any comments would be greatly appreciated.

[GTSBoy]

Basically what you just said, is what you will hear. 2 teeth is quite a long way out. That's got to be at least as far, probably further than you can adjust with vernier pulleys.

[KiwiRS4T]

Get your cam timing right first. With a highflow etc I had an adjustable exhaust cam gear tuned on the dyno - ended up about 3.5 to 4 degree retarded from memory.

[Kenna802]

Thanks for that . I corrected the timing my self when i replaced the head gasket and made sure everything lined up and triple checked before i turned the key. As i said car did give me a lot more lag. I did read somewhere about the adjustable cam gear and them bringing boost on earlier but it was some time ago.

[Mitcho_7]

I know there's no real difference with adjustable intake cam gears even if they preserve vct, but whats the general concensus on exhaust cam gears for rb25dets? Do they produce any worthwhile gains over the standard cam gear?

Edited June 5, 2012 by Mitcho_7

[KiwiRS4T]

They shift the power band downwards - there may be a small drop at max revs but more power in the mid range - I can't remember the number but its worth the effort.

[AngryRB]

They shift the power band downwards - there may be a small drop at max revs but more power in the mid range - I can't remember the number but its worth the effort.

I think your move moving the band upwards, if your retarding it your delaying the closing of the exhaust valve so this is not going to improve anything but mid and top end power, you allowing the engine to breath when the revs are higher,

When your advance it your closing the exhaust valve earlier so more efficient responsive on bottem and some mid but not efficient at the top end cause the motor cant breath as well

My tuner said the cam gears see most gains on engines without cams, if you have cams they dont produce as good a gains, and you will loose bottem end to give to top end, i dont have cam gears and my idle is as lumpy as a wrx,

[wolverine]

engines without cams?

[Old Booster]

aftermarket cams, maybe

[KiwiRS4T]

My RB25DET had 2 cams both stock. I think adjustable cam wheels are not that useful on aftermarket cams such as poncams or special grinds because they are presumably tailored to suit. I have no experience with RB26 but apparently they benefit from tuning adjustable cam gears.

And Sliver/Silver S2 I might have got it the wrong way round (I don't think so) but I can assure you the benefit was to the midrange not the max power. Don't forget that by retarding you are also delaying the opening of the exhaust valve.

[AngryRB]

My RB25DET had 2 cams both stock. I think adjustable cam wheels are not that useful on aftermarket cams such as poncams or special grinds because they are presumably tailored to suit. I have no experience with RB26 but apparently they benefit from tuning adjustable cam gears.

And Sliver/Silver S2 I might have got it the wrong way round (I don't think so) but I can assure you the benefit was to the midrange not the max power. Don't forget that by retarding you are also delaying the opening of the exhaust valve.

haha, silverS2 it was supposed to be, your right it delays the opening so the exhaust valve , but i dont think its shifting power downwards, more like it increasing the filling of the cylinders and there efficiency at higher rpm ( more than 3000rpm) until they reach a restriction in the system, the drop at the top end your talking about is probably the turbo and exhaust manifold and system that start building backpressure which then becomes a traffic jam and decreasing or flatlining the top end, thats with a stock/small turbo, with my dyno graph you can see there is no loss or flatlining upto, its just keep climbing cause theres no restrictions.

advancing the cam would make the combustion more efficient by decreasing the overlap,

[Kenna802]

So can anyone else tell about this? "3.5 to 4 degree retarded from memory" there was a figure i read but i had been looking for it for days with no luck

[Kenna802]

After a bit of research it seems 4 degrees retarded would be the way to go for the exhaust cam on a rb25det

[KiwiRS4T]

After a bit of research it seems 4 degrees retarded would be the way to go for the exhaust cam on a rb25det

I think that's right but ideally you would be doing it during a tune so you could see the effect on the dyno.

[Moodles2]

Best to confirm these changes on the dyno, as you can make things worse (with good intentions)

[Kenna802]

For Sure !! Dont think i will worry as there is another upgrade for my hypergear turbo that allows boost on quicker but im just glad we managed to find it.