Head Port Size, Velocity Vs Horsepower

[scotty nm35]

Um......Red top RB20 anyone?

Um... No thanks.

[niZmO_Man]

Didn't the red top RB20 come out at like 1985? Variable runners are nothing new :P.

[Ben C34]

Nothing is new.

[discopotato03]

A few Japanese mobs did the two small inlet ports per pot with a butterfly valve to close off one at lower revs/loads . Toyotas was TVIS in early 4AGEs but they like Nissan with early RB20s went away from that idea .

With production engines inc turbo ones the manufacturer does everything possible to maximise the part throttle torque because again that's the state all roads car engines are in most of the time . It goes beyond port velocities to things like combustion efficiencies , in the turbo Neo 25s case Nissan kept the same static compression ratio using a smaller capacity chamber and a lower middle section on the piston crowns . Burning efficiency goes up meaning less fuel for the same heat and the ability to create better heat and therefore higher cylinder pressures at lower revs than an R33 spec R25 can . They obviously found ways to reduce detonation around the torque peak without retarding the ignition timing to the same degree . The extra torque at lower revs would be why they used better R26 rods to withstand greater compressive loads .

Because the engine was making better torque at lower revs they could afford to use a slightly larger turbine housing , which they had from the VG30 single BB turbo anyway , to slightly reduce exhaust restriction which again improves burning efficiency from less reversion pollution .

There comes a time when the turbo needs to boost to make wider throttle torque for greater acceleration or pulling power ie hill climbing . Once you fit larger turbos the boost threshold rises revs wise and you get a gap between engine alone torque and boosted state torque . To get that back you have to find efficiencies the manufacturer couldn't or wouldn't use or increase the engines capacity . Really the only things that generally work are less timing retard from reduced restrictions (eg exhaust) , higher octane rated fuels (eg 98 ULP and or Ethanol) , higher static compression ratios (provided you can beat detonation) , or the capacity increase .

I don't think the gains are there from porting an R33 spec RB25 head . I think everything you do to them is aimed at increasing the engines state of tune and that means extra flow potential at higher revs . I think the way to go with a street R25DET is to do as Nissan did with their Neo version , it costs lots of money to develop production engines at manufacturer level and if they could have got the desired result with simpler changes they would have .

I also like someone's standard parts where possible because someone else did the hard expensive yards to develop them . This includes heads and pistons . I said before if I ever went inside my 33 25T it would get Neo rods pistons and head . The inlet manifold system stays optional but I would stand to gain the greater part of what Nissan did with a bolt in package that keeps the original engine number . In basic terms all you have to cater to is the different CAS wiring if you use the original inlet manifold etc .

In the end most Japanese performance engines from the 1980s-90s didn't make it too far past 2000 . The RBs the SRs the CAs etc were all finished and I think even the 4G63T only went to 2006 , that had a new MiVec head for the 9s only which would have been expensive .

Basically the latest/last revision of most engines is the cheapest and best way to go . You would not use a Red Top NICS RB20DET if you could get your hands on the later R32 GTST version . I doubt there's anything positive a GTS25T engine can do that a GTt one can't do better . Given a choice its always going to be a better basis standard or modified .

A .

[GTSBoy]

That's pretty well written. It does show your usual biases Adrian, but in this instance I don't think there is a position from which to argue against them!