256 RB25 Poncam experiences plz .

[discopotato03]

It was retarded not advanced . The idea being the exhaust valves open and close 2 degrees earlier so the cylinders blow down fractionally earlier in their cycles and the overlap timing is a little shorter .  So instead of the valve timing being 8-68 63-13 it would be 8-68 65-11 and the overlap duration 19 rather than 21 degrees . 

I imagine they set the cam where they wanted it with the crank at TDC , then moved the crank with the modified pulley so the cam would be in the normal position but with the crank 2 degrees after TDC . Tighten the four bolts and cam timings finished .

A .

4 hours ago, Dose Postman STFUU said:

brap brap brap bro

 

Edited August 12, 2016 by discopotato03

[Dose Pipe Sutututu]

On 12/08/2016 at 8:17 PM, Dose Postman STFUU said:

brap brap brap bro

Strong name 

[discopotato03]

Can confirm that my 33 25 now idles fine with Poncams @ ~ 850+ . ATM back on ULP98 and running std base pressure .

Need a better pump and people say these injectors spray better with higher base pressure .

 

Cheers A .

[Dale FZ1]

Presuming you've had something done with the head?  Pretty economical with the detail Adrian.

Do a bench test and see for yourself what happens to injector spray patterns with bigger base pressure.

Takes the guesswork out of whether you need to listen to what "people" say.

Probably going to be hard to beat a Walbro 460 if you need a new pump.  Contact Mafia for some comments/experience on what works well and how to control the pump to best effect on a road car with Link/Vipec ecu tricks.

[discopotato03]

No extra head mods this time aside from the slightly retarded exhaust cam timing . Just freshened up . 

Will go to the fuel pump thread and talk to Mr Mafia .

 

A .

 

[Dale FZ1]

So... head off, lifters given a birthday, and reset the cam timing?

[discopotato03]

I think the buckets were fine , just a few RS exhaust valve guides . If they don't line up with the seat they don't seal too well . Was ported first time round and had Ferrea (sp) over sized exhaust valves fitted . Head was O ringed to make the OE gasket reliable with extra heat and pressure . Std springs shimmed for a few extra pounds .

A .

[hardsteppa]

ok so cams are in, fitted to standard positions, no adj. gears...due to other work also being done to car over last few weekends, haven't done heaps of driving as yet or fully retuned, but impressions so far - idle is a little bit rougher but nothing overly noticeable. If you're giving it low throttle, standard cams win as there's a slight loss in pickup. If you're giving it medium or more throttle, the 256's are noticeably better although haven't even given it full throttle yet.  Looking forward to retuning and seeing the end result.

[Dale FZ1]

^^ Andy the truth is for most cases there's far more gain in having optimised inlet, exhaust, and cooler piping than there is cams.  Comments from Stao in his Hypergear thread bears this out.

But changing to these baby upgrade cams sure isn't going to hurt.  Going to be interested to see your results.

[discopotato03]

The best explanation I have is that aftermarket cams in a street car get you the last 50 Hp , not the first (over standard) . I think there are many better mods to make initially than cams in either flavour RB25DET . Better easier bang for buck .

I just happened to go that way since I opted for a second head to modify , saved downtime , and had it all done together . I also opted to have that head decked to get the highest practical compression ratio without making it unreliable , supposed to be around 9.2 - 9.3 to 1 . Not really a relevant comparison but I was told at the time that the GMS GTRs ran about 9.6 to one esp after CAMS forced them to reduce boost to make it "fairer" for the iron ox taxis .

I now think that lack of exhaust valve sealing and fuel pressure issues were ganging up on me , first issue fixed and a new pump shortly (no time atm) .

A .