Bigger is not always better. port sizing.

[DatsunZ31]

I was curious what everyone's thoughts on this are.

for a N/A motor decreasing port size can increase torque, air velocity, and it can make a wider powerband, power falls off at higher rpms...

I haven't read anything on boosted applications, where air is force fed, i believe it can offer quicker spool time but will it bottle neck and cause a restriction? http://www.thumpertalk.com/topic/698...omans-porting/

http://mototuneusa.com/homework.htm

[Dragon Humper]

Not really needed on the RB motors (4 valve anyway), you can gasket match and clean up the ports a bit to get rid of any casting irregularities but actual port alterations are a waste of time and money. About the only thing I have seen to be of any benefit is to remove a hump in the exhaust port that I believe is there to help flow with stock turbo manifolds. Even then it won't do much unless you are 500+ HP. Intake ports should be dressed with a dimple like surface, like a golf ball, to promote wind shear near the surface of the port to pull fuel off the port wall. The exhaust should be polished to keep carbon from sticking to it. Simple.

Edit: I read a bit of the sites you linked and that is a whole different world compared to automotive. The ports they are altering are WAAYY to big and I can see why that is making a big difference in low end torque production.

Engine builds have gone as high as 900hp with bone stock ports and bigger cams. Spool can be enhanced in so many other ways to much greater effect than any port job could ever dream to do.



Jon.

[Skym]

Good post.

Stock ports on RB20DET head have been tested to be good to around 560hp, usually limited by stock cams (that's with adjustable cams gears). I think they are smaller than stock RB25, RB26 ports. With porting, welding, grinding around dome area on head they have made a RB20 head fit on a RB30 bottomend (Tommy Kaira M30).

Excessive porting is not needed unless you decide to go with bigger diameter valves, upgraded intake manifold, exhaust manifold, cams, etc to suit, but what the man above mentioned is normally what you do (including port matching exhaust manifold to head, etc).

I know with 1mm oversized valves, porting to suit, more aggressive cam specs can get another 100hp+ from a 4cyl head. But powerband becomes very peaky (lag then sudden rush of hp). If over port the head you have to fill it, but then it becomes a dragracing engine head.

Also there's adjusting the squish area in head (dome) around sparkplug that helps in preventing knock (raises the limit before knock occurs). Commonly done on 1000hp+ RB, V8, etc engines. Also usually you don't find all of the domes are even at factory if you measure them. Skimming deck of head so they are even, helps to even out each done.

Stock ports, stock valves, stock cams give a wide powerband, loss of peak hp as mentioned above and most just fit cams, porting as mentioned above to take advantage of the stock ports, valves (Poncams or Procams and Procams produce more hp throughout powerband) and when fitting bigger aftermarket turbo's that need more exhaust gas down low (due to running engine richer down low with more aggressive cams) to spool turbo up quicker.

[BenOswald]

What are some ways to increase spool time?

[Skym]

Full exhaust from turbo, boost controller, camtiming, cams, adjustable internal wastegate actuator, ECU tune, increase capacity of bottomend (stroker kit which would be RB 2.2 or RB 2.4 with RB20DET) or fit bigger capacity engine with more cylinders (Nissan V8). Highest hp NA Nissan V8 produces around 430hp. Exhaust housing A/R, exhaust wheel size on turbo, NOS, E85 (can run more ignition timing), etc.