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Rb25 Clearance+head mod

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Hi everyone!

I have 2 questions about my R33 RB25 i decided to modify a bit.

The car will be used on the street, and every now and then on the track with 98. pump fuel. Got a big turbo on it which i will be running on 15 psi, with an aftermarket intake + exhaust manifold. Also, it has a standalone ECU and i plan to tune it quite safely. Don't plan to go over 400 hp.

1. My block has a bit of damage, so I have to bore it up to remove the stripes from the cylinder wall. I decided to go with the Mahle 4032 86.50mm pistons for silent operation, and the clearance the manufacturer recommends is between .0016 - .0024 inch. I find that a bit tight. I initially wanted to go with .003 or so, but i suppose i should follow the recommendation. Does anyone have any experience with this thight of a clearance? Also, I'm running Motul 10w60, if that has to do anything with the new clearance.

2. Since it's going to be knock limited, and i plan to run it on the safe side, i have no idea if I could even push it to 400 on pump fuel. About the knock limit, I heard people grinding of, or completely removing the squish pads from the 26 heads to fight the knocking. Does someone know if removing some, or the whole pad would help me push out the knock limit on the 25 too?

Their clearance suggestion sounds reasonable to me for that engine, material, and your power level. I wouldn't go to 0.003.

Oil weight would be more a consideration on the bearing clearance side of things than piston to wall clearance.

With 98 pump fuel, good intercooling, good intake and exhaust flow, coolant and oil temps under control, and an oversized turbo at 15 psi, I think 400 HP sounds reasonable. I would err on the side of caution with coolant and charge air temp ignition comps, boost comp to make things more conservative if it starts to get hot, but I've run that engine to that power level in stock form on similar fuel with success.

Thank you for your reply! This was reassuring. I am planing to changle form the single fan to a dual fan setup on my water cooler, that should take care of it roughly, we'll see. Adding an aftermarket oil cooler never ran through my mind, but i'll be watching the oil temp, and if necessary, i'll be adding one. Thank you! I'll be conservative and pull back a lot at first with the tune and gonna see how it handles on the knocking side with the temps i'll be getting. I want it to be reliable and i dont care if I lose a bit of power.

Have a nice day!

Perfect. If you don't end up needing an oil cooler that's fine too, but with the sensor at least you'll know!

Have a great day.

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