proengines

Joel, exactly what I was thinking. Everything I've ever read and seen says that the smaller the combustion chamber (without shrouding the valves too much) and the flattest piston top you can get will give less detonation. I know its a long way from an RB engine, but the old ford clevelands detonate less at the same compression with a closed chamber and flat top piston than an open chamber and domed piston. With the engine design of the RB25/26's theres really no way to use a flat top piston and run the compression you need. With my 30 I'm going to have some pistons made by Mahle to suit, my aim is 8.5:1. I much prefer the SR20 piston and chamber, dished piston and less valve angle but with the RB you're stuck with the angles nissan made them at.

I remember now where I saw it, on their website, this is what they say: The occurrence of a knocking is controlled by removing squish area. Squish area exists in order to bring combustion speed forward. The role of squish area is pushing out the inhaled fuel-air mixture to near a spark plug. However, in the case of tuning engine, the squish area causes natural ignition and a knocking is caused. In order to pull up limit of the occurrence of a knocking , squish area is deleted and adjusted. I know these guys have the runs on the board but I'm a little lost. As far as I know, the squish area is there to provide a shockwave to force the fuel and air into the combustion chamber and towards the sparkplug (which is what they say above in engrish). I could understand raduising the edges of the two flat areas to remove hotspots but wouldn't removing it altogether mean you ended up with a pocket of unburnt fuel on each side of the piston dome?

Ace_nz, I take it you're talking about the graph on the website? The inlet started to pick up at around .200" through to about .420". From what I remember, it flattened off around there which was fine, the cams were step 2 HKS, around .410" lift(10.2-3mm) from memory. The exhaust picked up right from .050" and stayed between 15-20CFM over the standard port right through the range. I was subletting my flow testing at that stage and Ive looked around for the original sheet but cant find it. The air speed was up over standard, noticeably on the intake side from memory. I should have some new really small pitot tubes soon so I can get them in the port to measure velocity without disturbing the air as much. I think in a smaller port most of them are far too big. Joel, I put the head on the block and scribe the bore onto the head surface then use a radius cutter to open the chamber to this line and blend it into the chamber, it also removes the step on the transition of the valve seat into the chamber. When I test the next head I'll try it first without it and test and then unshroud the valves and check again. I did a B6 mazda head the same way recently and you could measure the difference before and after on the flowbench. It also removes a couple of hotspots on the exhaust side as it removes a couple of sharp edges. Also, thanks for the compliment. Sweetr33, thanks, thats good to know. It's interesting, I remember seeing a RB26 head done by Jun i think, and they machined out the entire quench area of the head, I'd like to know the theory behind it and whether they use a different shaped dome on the piston. Anyone know?

dnb, I can't work out how to upload photos or i could show you the profiles here, if you want, have a look at: http://www.proengines.com.au/images/radius.gif and http://www.proengines.com.au/images/4angle.gif The profile of the cutter is what is cut on the valve seat. These aren't the exact profiles I use but are pretty close. The valve seats on the 45deg section. Sweetr33, I can tell you in a few weeks, I haven't been game to take the entire lump out on a customers head as I'd hate to have to call them and tell them I had hit the water jacket porting their head or lost flow and needed to weld it back in there. I'm starting on a head for my own engine, it'll be a RB30/RB26 head and I have the head here ready to start when I get a chance. I'll take the hump out a bit at a time and see what it does on the flowbench. I think it will work better as the exhaust port only flows around 73-75% of the inlet with the hump there and it would be nice for power to get it up around the 78-82% mark. I'll also be trying some bigger valves so I'll let you know.

Alf, I normally use carbide burs for shaping and stock removal, you're probably safer using a double cut bur as theyre not as agressive. Next I use commercially available porting rolls to remove the die grinder bur marks. Once you get close to the shape you need I'd use 60 grit emery on a stick in the die grinder, turn the speed down a fair bit and it will leave a beautiful finish. You can also use flapper wheels in the bigger parts of the port. I normally remove the guides before I start to give access to the entire port then fit new guides once the porting is done and its time to cut the seats. Dont just cut the guides off flush with the port, theyre pretty short to start with and you need them to keep the valve positioned properly on the seat. Ive found that a 4 angle inlet seat and a slight 30deg back cut on the inlet valve along with a radiused exhaust seat (it does have a 45deg part for the valve to seal on) and no back cut on the exhaust valve work really well and with a basic port job will pick up around 20CFM flow@28" of water. 2 things though, first, be very careful around the short turn radius, you need it to remain a radius and blend smoothly from the valve seat into the port. Second, don't make the port too big by hogging it out, theres no need and you will lose air velocity.

Interesting reading guys. If everyone had the same opinions about what is best we would all still be driving around in model T Fords. I don't know if this should be a new thread or not but I have an engine here for my own car and one for a customer, both are RB30 block, RB26 head, neither have any intake manifolding at all. Both will be single turbo, t55'ish. Aiming for around the 700hp or so mark, mine will be auto, mostly for drag use. So, starting with nothing, what would the people who know use? I'm not so interested in what someone read in a magazine but more some real world knowledge. I have a flowbench here and can test myself but I'd like a starting point which will save quite a few dollars.

I've heard all sorts of bad things about these manifolds but then I had a customer buy one of the SR20 manifolds a couple of weeks ago and it definitely looked the part. Welding was beautiful and all the ports were hand blended. I had to grind one bolt boss off the head to clear #3 pipe and the factory cast dump pipe interferes slightly on the bellhousing requiring a little grinding, would probably be no problem with a dump pipe they also sell for US$30 or so from what I can remember. The flange plates are mild steel but the pipes are not magnetic. One pipe comes into the collector at almost 90deg to the turbo flange but I suppose this is a compromise for space, still, I'd say it would have to flow better than the standard cast manifold and for the price of one you couldnt buy the flange plates and bends to make your own from steam pipe. Some people seem to be worried about the manifolds cracking eventualy, but really for that price, buy 2 and keep a spare for when it does! I haven't seen an RB25 manifold but if it's similarly made I'd be happy to use one for the price they are on my own car, maybe not for a customers where I'd be up for the cost of replacing it if it did crack. It's like most things, Quality costs and you cant expect a $250 manifold to last as long as a $2000 one!

Off the top of my head, .004" is the specified clearance. I normally add .0005" (half a thou) when I hone them just in case one day it overheats and picks up. They use file back rings so this isn't a problem, even without file backs it would only add .0015" ring gap. I was talking to one of the guys from ACL pistons a couple of months ago and he was saying with their forged pistons (Ross currently makes their forged pistons for jap engines) you could probably run .0035" as long as you ran a good 5w30 or 0w30 synthetic oil. The thing is, the clearances most piston makers give you are the minimum specified clearance that shouldn't cause any problems for them or you, not an absolute figure. The only problem with increasing the clearance too far is that the rings will lose some stability if the clearance is really big. with a forged piston though, once it warms up it expands and takes up most of this clearance, thats why they're noisy when they're cold and quieten down when the engine warms up. By the way, once ACL/Mahle start producing forged pistons for jap turbo engines you wouldnt use anything else. Their V8 pistons are beautiful, run really tight clearances and are ultra strong. They do Subaru's and Hondas so far I'm sure, more to follow. 6 or 7 of the F1 teams use Mahle pistons so thats a pretty good recomendation.

Sydneykid, we had a look at that today with some 12mm id aluminium washers, I'll try it tomorrow and see what difference it makes. Ive been trying to think why the pistons would move forwards and the only thing I can think of is maybe under really heavy braking (might be those brembo brakes... lol). I suppose they only have to touch once. I rang the piston supplier today to ask if they had heard of this happening with any others using the same pistons and he wasn't sure but then not many have probably been apart yet either. Regarding the rb30's, is there much metal to drill and tap to fit the squirters to them? I'm currently starting one for my own car and wouldn't mind fitting a set but heard they are a lot thinner below the gallery than a 20/25/26. Oh, and thanks guys for the nice comments, I thought it would make sense to try to explain what happened, it might save someone else the same problem. (I'm still worried about the guy who wanted to rape me though...)

Just to clarify things, I assembled the engine and just had a look at it about an hour ago. The side of the squirter can touch the piston on the pin boss area if you lever the piston towards the front of the engine (the squirter is on the side of the piston towards the rear). When the engine was assembled, I dummied and checked the skirt to squirter clearance which is fine. (I have had Wiseco pistons for RB26's where the relief milled in the piston skirt wasn't deep enough and they interfere, which means having to mill the reliefs deeper, thats why I dummy them to check). With the piston centred in the bore, there is also enough clearance between the squirter and the piston pin boss. It's only if the piston moves right to the front side of the bore that the squirter touches the piston. The piston can move a fair amount sideways (as in front to back of the engine) as all piston skirts are machined slightly oval with around a .020"-.025" (.5mm) smaller diameter in line with the piston pin than across the skirts. The piston to bore clearance was .005" (5 thou) across the skirt which is where it is measured. I've never seen this problem before but there is a first time for everything. It's just a matter now of fixing it. To fix the problem it's a matter of machining something to give the clearance needed. It's a choice of machining the piston pin boss or machine the squirter. I think it would make more sense to machine the squirter slightly along one side rather than machine a piece out of the piston pin boss. The squirter is not under any load and it is only a matter of linishing the parting line from when the squirter was cast. I would prefer to do this than machine the piston at all as they are obviously highly stressed. Really, it's up to Nik what he wants to do. The engine wouldn't have lost any oil pressure due to the broken squirters as the bolts that holds them in place have a plunger and spring inside that only lets oil to the squirters once oil pressure is at a certain pressure. Also, the conrods are fitted the right way around. I just thought I'd clarify some things to save people having to guess. Theres no sense in trying to blame anyone else, ultimately I'm responsible and thats why I always give 12 months/20 000km warranty on my performance engines and will fix the problem.

You wouldn't be able to do a straight swap for the rb26 buckets as they will sit down too deep in the lifter bore. The 26 also runs a different retainer as it has a recess in the top to locate the adjustment shim. The 25 has the HLA built into the bucket and the tip of the valve sits a fair bit deeper in the head in relation to the base circle of the cam than the 26 valve does. There are also 2 different spring setups on the RB25 so you need to be careful that you order to suit what you have. I would say the aftermarket solids for the 25 are designed to take up this extra distance with a longer "foot" inside the bucket that sits down onto the shim. Do they also sell the retainers and shims with the cam and buckets? The standard retainers have no way to locate the shim. Yep, the CA18 and VG both run hydraulics.