Track application RB20 vs RB25

[droptopsf98]

Hello gang, I need some friendly advice on what to do in this situation.

I have an S13 coupe race car I'm building (streetable track car). Car will be primarily serving as a track car, so that takes priority. I have an RB25 motor/trans lined up. I have a friend with an RB20 trans/mounting setup for sale.

End goal, I do not see this car crossing 350whp. In the short term, I plan to leave the RB25 stock for this year.

I know the RB20 trans is lighter, but isn't as strong.

So here's how I see it, correct me if I'm wrong.

RB20 Pro's:lighteruse stock driveshaft

RB20 Cons:uses cable drive speed sensorwon't hold as much power

RB25 Pros:StrongerSpeed sensor plugs in

RB25 Cons:HeavierHave to use custom driveshaft

My RB25 setup has the trans coming with it, however a friend of mine has an RB20 trans, and mckinney mounts (which I need to buy anyways, so just trying to buy the right set)

I was pondering just buying the RB20 trans, using a stock driveshaft, and just running that until it dies.

Just looking for some feedback. We want to save whatever weight we can, but not at the expense of it dying in 3 weeks
Modified by droptopsf98 at 6:29 AM 11/21/2009

[Joe]

niether.

parts are a b**** to find and RB's are very tempernmental on a racetrack.

if you HAVE to go rb for your "race" car get a 25 only because its YEARS newer than the 20.

[Coolwhip]

Also to consider... if you are saving weight where ever possible, you'll want to look into replacing your factory driveshaft anyways to shed some weight. So regardless of transmission used you'll be going with an aftermarket driveshaft meaning it would omit itself from both RB20 pro, and RB25 con.

I forget the exact weight differences between the two transmissions.

What type of track use is the car going to be experiencing? As the RB20 transmission is well equipped to handle 350whp. But reliability of a track car is also a conern as you rather not be down for a track event as you're working on a transmission after every other event.

Others will probably have better input than me but I figured I would throw those thoughts in for the juice.

[droptopsf98]

Thanks for the input guys.

Why do you say they're temperamental? Cooling issues?

Valid point on the driveshaft, I had already considered a 1 piece from mckinney anyways. Will keep that in mind.

I am not drag racing, I will be mostly circuit racing.

midamericamotorplex.com

That is the website for the main road course I will be on. Simple, no elevation changes, so a good beginner track. My KA got me to 105 mph on the front straight, and a friend's slightly modded Z06 hit 128mph. Just to give you an idea what speeds I'll be hitting.

[danman04x]

Oh also eliminate the speed sensor as a con for the rb20..Just swap in the ka speed sensor and plug it into the lower ka harness

[Shocker]

No offense to the rb20 owners on here, but I wouldn't even waste my time on one, the extra low end torque you get from the 25 is well worth it, especially on corner exits.

[Cjmartz2k]

No offense to the rb20 owners on here, but I wouldn't even waste my time on one

[DriftingisLame]

I'm pretty sure he's not talking about the motors. I know it didnt sound very specific in the first post, but I think he's weighing the pro's and con's of using the rb20 trans vs the rb25 trans for weight reasons. In which case the rb20 trans would be a better option provided the gearing matches the circuit you're on (I assume the gear ratio's are different between the 20 and 25 boxes), and it will hold the power you're putting down.

Not saying the power characteristics of the rb25 are not better or anything, but you must consider that the RB25 was never used in any of nissans race cars. The ring lands on the rb20's and 26's both are much lower down the piston, and the rods are much thicker than rb25's. I cant confirm this, but the added stroke on the RB25 and the thinner rods dont seem like they would be very RPM friendly for hours of non stop abuse with a high power output. I believe the RB25's internals were built with emissions and street performance in mind rather than being made to withstand the rigors of circuit racing.

Just some food for thought, not bashing the rb25 or anything, it obviously makes more power.

-Max

[Cjmartz2k]

Yeah, I see what you mean about the fact he might be talking about the transmission's and not the motor. If that's the case (a) I agree with you about using the 20 transmission for only 350hp and (b) the OP sucks at getting his point across (if he is indeed only talking about the transmission). 20 rods are tiny just like 25 rods BTW. The pistons don't appear to be as resilient, but to be honest, all the people I have ever seen with popped RB25's did it with stock or smaller turbos that just had a garbage tune. My 600hp stock bottom end'ed RB25 took TONS of abuse, and when it came apart, the pistons looked great. My buddy who owned the shop I went to all the time (and tuned my car) bent the rods in his stock bottom end'ed RB25 at well over 700hp, but the pistons looked perfect. I do agree they look weak, but I don't think they are as weak as everybody thinks.

OP--I'm not sure if you are talking about motors or just trannies, but bottom line, a 20 transmission will handle 350hp just fine (but that's getting up there for it), and the two trannies are interchangable. You could put a RB20 behind an RB25/26 for example to save weight if the power didn't get much over 350hp. With transmission's, it actually all about how much torque, but nobody uses those figures with RB's for some reason.

[eh?]

I haven't seen an RB20 rod in person But from ever pic I've seen they look to be as beefy as the RB26/RB25 neo rod RB20 left/RB26 right

That said I don't think rods should ever be a problem in the lower power levels. These motors have a tiny stroke.

As for overheating problems? Run a clutch fan. I will guarantee it will fix 90% of the RB/s-chassis cooling problems. There's a reason why The Aussies rarely (if ever) speak of cooling problems and people with 240sx's do.

[droptopsf98]

haha my bad...TRANSMISSIONS. I'll be running the RB25 motor for sure. I got the whole setup for 1300 dollars, so I couldn't say no.

If I were to sell a used RB25 trans, what is an acceptable price?

This is very much a budget race car, but I don't want to cut the wrong corners. I pondered just keeping the 25 trans around in case i ever decide to go bigger on HP. Flipping the trans would likely allow me to get the rest of the stuff I need though.

[EVILS14]

free if its a fellow niconaut

[RustspecS13]

I'll trade you transmissions. Mine works great I would rather just have more strength because I drift.

What do ya say?

~Alex