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DIY porting/polishing

Alf

By Alf,
in Engines & Forced Induction

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sweetr33 Experienced

sweetr33

Posted
Posted

well i can tell u that u wont hit a water jacket coz on my rb25 head i have taken the lumps rite out of the exhaust side, wouldnt even know they used to be there now, just wondering if it was a bad thing to do or not. Who knows, guess i will find out

Cubes Grand Master

Cubes

Posted
Posted

I remember reading up one this when my head was being ported.

The lumps are there to increase exhaust gas velocity. Unsure exactly as it has been a while now :(

It also looks to me as if they are there to help steer the exhaust gas in the stock exhaust manifold.

proengines Rising Star

proengines

Posted
Posted

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?

proengines Rising Star

proengines

Posted
Posted

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?

Cubes Grand Master

Cubes

Posted
Posted

Its interesting what Jun has done.

Everything being equal (comp ratio) between a quenched and non-quenched motor I can't under stand how the non-quenched motor would have less detonation.

If anything I would think it would be more prone to detonation due to the larger cc head volume. As one increases head volume you also increase the amount of hot exhaust gas that is left in the combustion chamber after every exhaust stroke.

I was under the impression it was always best to have a combustion chamber that had as less volume as possible, lots of nice quench designed around certian rpm bands for which the motor is designed around & a nice dished piston to get the comp ratio you are after.

I've got some more reading to do so I can understand the reasons behind what they have done.. :)

proengines Rising Star

proengines

Posted
Posted

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.

Cubes Grand Master

Cubes

Posted
Posted

I've got nasty pic of an RB20DET head.

Before any one asks. The RB20DET in the pic had its 100,000km cam belt fitted, the tensioner stud snapped due to possible over tightening resulting in smashed valves. I made sure I replaced studs in my RB30DET build. :)

Notice the lack of or very little quench area.

From what I understand a smaller quench area creates a motor that is peaky, the more quench the more mid range style of motor it will be.

Am I on the right track?

Its been a while since I looked in to the whole quench side of things so I may be getting things a little mixed up. :)

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