Rb25 Head Porting

[zac the muss]

So I've got myself an s1 rb25de which I'm planning to bolt a turbo onto.

is it worthwhile removing these little bumps in the exhaust ports and opening them up a bit?

the head will be coming off for some block restrictors and a few other things anyway but I'm yet to decide how far to go with rebuilding it.

please excuse the noob question I'm new to this whole engine building thing.

[GTSBoy]

They are "extra" material to keep the thickness around the manifold studs "correct", per Whatever spec Nissan were using at the time. You could remove most or all of the bump without negatively affecting engine strength. FWIW though, unless you are going to port the head properly, with a real power and usability target in mind, plus make sure the exhaust manifold is top notch and properly port matched, it's probably not worth your effort doing anything to it.

FWIW, grabbing a grinder and opening up ports, even just removing a little material like those bumps, is not "porting". Shaping the bowls and transitions to the valve seats and getting the velocities right to get the right cylinder fill at the target rev range and all that.....that's porting.

[Mr.H]

A standard RB25 head will flow more than enough power before you need to have any kind of head work done. What I would suggest instead of porting the head is to change the Valve Springs over to something like Supertech Valve Springs from the states. Lets you run higher Rpm before you start burning out your Hydraulic lifters....I wouldnt even change the Cams unless you are after some decent power.

[R31Nismoid]

They are "extra" material to keep the thickness around the manifold studs "correct", per Whatever spec Nissan were using at the time. You could remove most or all of the bump without negatively affecting engine strength. FWIW though, unless you are going to port the head properly, with a real power and usability target in mind, plus make sure the exhaust manifold is top notch and properly port matched, it's probably not worth your effort doing anything to it.

FWIW, grabbing a grinder and opening up ports, even just removing a little material like those bumps, is not "porting". Shaping the bowls and transitions to the valve seats and getting the velocities right to get the right cylinder fill at the target rev range and all that.....that's porting.

Well said.

OP - Also does depend also on aims, you've kinda not said what those are... 300rwkw, 400rwkw, 500rwkw? Street/Drag/Circuit etc.

But by the sounds of it, the above covers everything

[zebra]

Plus 77ty to gtsboy.

In fact just ram rodding ports with a carbide burr. Generally means going backwards.

Port reversion and all sorts of stuff can be messed up

[scotty nm35]

Agreed, the idea is to keep the gas speed up with smaller runners, not slow it down by opening them up. Unless of course it becomes a restriction at 500kw+...

[discopotato03]

Even on a budget turbo pistons and cams wouldn't be a bad idea .

A .

[zac the muss]

cheers guys, looks like I'm best to leave it alone for now.

my goal for this build is around 300kw. the idea is to get a decent turbo setup running on it before switching to an rb30 bottom end built with some decent gear.

it's going into a track car so I want to use the cheap motor as a testbed to get things reliable before throwing some $$ at a decent bottom end.

the head and sump will be coming off so it'd be stupid not to at least do a basic rebuild.

I can get hold of an rb25det neo block and crank (hopefully with proper oil pump engagement) so will probably pick that up and hopefully some turbo pistons if I can get a hold of some.

Edited January 17, 2014 by zac the muss

[discopotato03]

That's all good until you work out Neos have less plateau on their piston crowns because they use smaller chambers to get the same static CR as an R33 spec RB25DET .

If you can add a Neo or same spec Stag head to your list you get as good as it gets in an RB25DET .

An R33 head on a Neo short engine will give you a low CR which isn't real good IMO .

Your call cheers A .

[zac the muss]

As far as I'm aware r33 25det cr is 9 and neo is 9.5.

I like the idea of running a neo head however I already have this motor so may as we'll use it. the r33 head is easier to work with anyway and greddy plenums bolt straight on which is a bonus.

I'm buying the neo block and crank without rods or pistons so may source some r33 turbo pistons or rebuild with the high comp DE ones.

[GTSBoy]

As far as I'm aware r33 25det cr is 9 and neo is 9.5.

Yeah, but that's not the point. The compression difference is partly because of the much smaller combustion chamber, and partly because of the pistons. Put the wrong pistons with the wrong head and depending on which way you do it wrong, you'll get no compression or far too much.

If using the 33 head, then you're best off getting 33 turbo pistons, per your last line.

Edited January 18, 2014 by GTSBoy

[discopotato03]

I would not miss the opportunities to lose the hydraulic valve train and side feed injectors R33s have standard . AFAIK the CRs are the same R33 to R34 but the thing is that tightening emissions legislation forced Nissan to change the design so they could run 34s leaner and cope better with detonation .

Everyone agrees the Neo is the better spec 25DET so worth having .

Side feed injectors are a PITA , Neos went back to 14mm top feed injectors and EV14 eg ID injectors go pretty much straight in . Much more limited selection of SF injectors which aren't as good and a pain to work with .

I would be more than happy to run factory Neo rod pistons and maybe cams in a 300Kw RB25DET , they made more power std than the 33 version did so cope with the extra easily if tuned properly . The cost of improving 33 rods and pistons would more than cover the difference in price of a Neo .

Anyway your call , I know what I'd do .

A .

[Ben C34]

When my head was off when I did my 30 bottom end I hacked off those lumps. I would certainly not call it porting. I know enough about porting to know I can't do it! I had previously hacked the lumps off inside the exhaust manifold.

No back to back comparison for the "lump hack?" power gain,as i dropped a 30 under it at the same time, but I have noticed I sleep easier not worrying about different size lumps in my exhaust ports

[Dale FZ1]

Stock vs stock the Neo spec probably is a marginally better thing. But OP Zac indicated he has a R33 spec 25DE for turbocharging and track use.

If he's wanting a 3 litre, now would be the best time to make that happen.

25DE runs the light(er) duty conrods and 10:1 pistons, also the block does not come fitted with under-piston oil squirters. You might contemplate using that bottom end assembly if going straight to E85 but otherwise it's not destined for long life at 300kW target.

So really it has a good donor head, and hopefully an oil pump with lots of service life left. Perfect for use with a RB30 bottom end.

Stock 30 rods, a nice set of pistons and leave the head alone except for the VCT oil feed modification. Loads of stick, and a longish life if only run up to 6500ish rpm.

Zac if you wanted a reasonably easy turn-key engine and are prepared to play with looms etc then get a 25 Neo complete and start with that. Comments about limited value in porting are accurate because at 400rwhp there's plenty of other areas to spend the money first.

Edited January 18, 2014 by Dale FZ1

[zac the muss]

haha thanks for the input guys. seems like a lot of speculation going on here. as for ditching hydro lifters and side feeds? I'm not building a 700hp competition motor, just want a solid setup that makes enough power to have some fun on track.

clearly a neo 25det is going to be better than an r33 one but I don't have 3k to throw at a drop in motor, nor do I want one as you end up throwing away everything bar the block and head when going for decent power.

il probably end up hacking the lumps out of the exhaust ports as to me it seems completely pointless to have them there, and every little but counts right?

my main focus for this build is to get the oiling 100% sorted and reliable on track before going to a 30 bottom end, hence why I'm picking up a 25det neo block and crank to save having a collar put on the DE one, oil squirters are just a bonus.

this is a project remember, I don't need a built 30det next week. I'm quite happy to build a cheap(ish) motor and run it before going down that track. things like this can serve as a great learning curve.

Edited January 19, 2014 by zac the muss

[Dale FZ1]

Points well made.

Suggest to get oiling, cooling, and fuelling right, and reliability will come.

Increased rpm and boost will make issues with the first two become evident on track.

There's no issues with mix/match to get the engine you want, but you're going to need a set of pistons to give a reasonable static CR plus machining the block to make everything happy. Essentially the same work you'd do to a RB30 bottom end, hence the suggestion.

A healthy unopened 25DET or Neo 25 will give damn good results with turbo and fuel system upgrades until you're ready to spend on internal mods.

It would seem gains can be had simply from minimising those bumps in the exhaust ports rather than completely eliminating them. A bit of smoothing the sharpish edges is easily achieved without much material removed. The factory engineers decided that a bit of strength was needed in the casting to support the exhaust manifold studs, but people have reported no issues by completely removing them (when do they ever really report failures though??).

Either way, good luck you're going to have fun and it will cost a bit of time and $$ whatever you do.