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Discussion and questions related to the course Practical Engine Building
So i currently have the engine on stand, crank assembled with oil clearances roaming the 0.0022"-0.0026" range. Using mics and dial bore.
For an engine that will be drifted and maintaining constant rpm, targetting 8k or 7.5k to be safe. Its a NEO engine with RB26 crank, cp pistons, eagle rods, JUN oil pump, Oil restrictors to head etc.
ACL Race series bearings
What kind of clearances are to be better? 2.5 thou is supposed to be on the looser side but i keep seeing 2thou comming up a lot. Cant decide which range to go and why
I personally tend to aim for 0.0025" on the mains in an RB26. There isn't a specific clearance however that you MUST have to ensure reliability. It comes down to so many aspects such as power and rpm range, as well as the quality of your oil. One problem with the RB engines is that they do suffer from oil surge and this will destroy the engine very quickly regardless what your clearances are. I'd recommend a quality baffled sump and a little bit more oil volume to try and safeguard against this.
What about rod clearances, do builders usually go tighter or they go the same clearance on mains and rods?
The rule of thumb is that you'll need around 0.001" per inch of journal diameter. With this in mind, since the rod journals are smaller then the main journals, you'll normally target a slightly tighter clearance. I'd suggest aiming for 0.002" in this instance.
I would have never ventured into engine building or at least gotten this into it. I gotta say i enjoyed a lot the videos etc you guys make, finding about you guys and the affordable entry to these topics was such a surprise.
Keep it up, a lot of locals really like what you are doing and im really getting deeper into engine building and tuning.
Thanks for the feedback Oliver!
Sorry to revive an old post - Andre, what would you recommend max RPM for a stock RB26 crank (without taking the rest of the engine into consideration)?
I have a new RB26 crank, built with 2 thou mains and rod clearances. All balanced etc, billet girdle, and high quality oil pump and system (baffles, larger sump etc). Head has been rebuilt and has 272 JUN cams, springs, guides, lapped valves, etc. Ross Harmonic balancer and an 8 inch clutch.
But my question is specifically aimed at the RB26 crank alone though. HP will be about 700rwhp. (limited where it stops making power of course)
Just curious of your experiences on the strength and RPMs of the cranks themselves.
Most of the serious RB26 builds I've been involved with have used aftermarket cranks so I've honestly never pushed a stock one to breaking point. I know there's plenty of examples of stock short blocks making 1000 hp or thereabouts but I haven't been there myself. As for rpm I can't give a definitive recommendation for the same reasons. 8500 is almost certainly going to be fine but you may be able to get away with 9k if the crank is in good condition to start with. Personally I'd tend to aim a little more cautious and limit yourself to around 8500.
Hi Andre. It's a 10,000km old R33 GTR N1 crank. (Was purchased new on the second rebuild from the previous owner). The build was poorly done so we rebuilt it again with better components and repaired the head (guides were badly worn).
One thing I would like to add that is interesting - The engine has a Holinger 6 speed sequential behind it. I've been told the shock loading (hard gear changes etc) isn't good for the crank. Is this something you have heard of before? Do you think it could potentially reduce the life of the crank?
Also due to being a close ratio box and having only a BW8374 turbocharger (1.0x split pulse rear), I think the RPM will be limited to as low as 8,000rpm because that's where I calculate it will run out of puff and the close ratio will keep it well inside the torque band.