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rb26 cams into rb25

rb25

By rb25,
in Engines & Forced Induction

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RPMGTR Community Regular

RPMGTR

Posted
Posted

Geoffrey,.... unlike most others offering advice on here, I have actually tested this swap. I've also witnessed the same results on 2 other attempts by people who believed the falacy that this is a good mod.

If you like a lumpy idle, 7-8" idle vacumn, weak bottom end power, flat mid range power and a mediocre top end then this is the "upgrade" for you!

As to why it doesn't work, here is the answer - Cam doctor the GTR cams at .00" valve clearance and you will find the valve is off the seat for over 300degs. That explains the lumpy idle etc. However the lift for 60+ degs of that 300degs is less than .020"(0.5mm). So bugger all airflow and a lot of reversion etc.....basically the engine is now a crap airpump and behaves as such. If Others can overcome these major shortcomings and get a performance increase, then they must be REALLY good tuners .

I did actually dyno a good RB30/25 with GTR cams and it made 250rwkw, however when I tossed them and reinstalled the RB25 cams, hey presto we now have 325kws, smooth idle with 20" vac, spooled way earlier , strong mid range etc, etc.

Maybe I just had a "dud" set.

As for dynosheets, I do have them on the dyno harddrive, but the results were so pathetic I never even bothered to print/compare them to the final result.

Maybe get the people who have such great success to show you the dynosheets of the results they are so proud of. I have yet to see any????? how about you????

I have no axe to grind here, just tired of "internet fallacies" being perpetuated.

I'd much rather see you spending your time on things that actually make your car faster.

cheers,

B.

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Roy Grand Master

Roy

Posted
Posted
...Cam doctor the GTR cams at .00" valve clearance and you will find the valve is off the seat for over 300degs. That explains the lumpy idle etc. However the lift for 60+ degs of that 300degs is less than .020"(0.5mm). So bugger all airflow and a lot of reversion etc.....basically the engine is now a crap airpump and behaves as such.  

Makes sense:) software certainly helps i imagine...any experience with RB20s and cam upgrades? I want to stick with the std valve springs etc but unsure whether i should go 260 or 270deg cams with about 8.5-8.8mm lift...or a combination of both for inlet and exhaust?

Im not worried if i lose bottom end say under 3,000rpm, but between 4,000rpm to 8,000rpm id like measureable gains...even if it means making the same power but with less boost.

That said if a cam will work well enough to help low to mid as well as helping the engine make a bit more power at 6,500-7,000rpm then im happy. At the moment i have about 220-230rwkws with about 1.2-1.3bar. No one ever bothers with RB20s so hard to get a feel for what cam/s works if any?!?!?!!?

  • 2 weeks later...
GTRgeoff Grand Master

GTRgeoff

Posted
Posted

Mate that's the sort of info I like to hear and possubly timely advice. Just as an aside, did you Cam Doctor the RB25DET cams as well? I definitely note a ramped flank on the cam lobe as well, but noticeably lower than the RB26 cams. I am hoping to get away with buying a new dial indicator before the finance minister catches me with the cash tin so I can map the cams and dial in an appropriate overlap in the old schoool fashion before drive on dynos were available so maybe I'll get up to speed on the doctor and take a look as well.

Alternatively I may also have the flanks ground down to get lift without the side effects.

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