Rb30 25 Or 26 & Fuel Economy

[Ben C34]

Really? That's one of the best thought out responses you are likely to get.

[Ben C34]

I will clarify. Yes the neo head would be better than the rb26 for economy based on everything discopotato has written.

[xntrq]

Really? That's one of the best thought out responses you are likely to get.

yeah man, I tend to have this thing where I kind of understand but don't, so need someone telling me a solid yes or no.

but other than that, got complete rb25det ready to be mashed up with the rb30

[discopotato03]

What I meant was using an RB25 Neo turbo head and it's inlet manifold .

It's not an easy job grafting a RB26 inlet system onto any RB25 head and if driving conservatively much of the time I don't know why you'd bother . To me there is only two reasons anyone would ever go with an RB26 head and they are the shim under bucket lash system and the six throttle inlet manifold . The aims being valve train reliability at very high revs and sharp throttle response . Big revs is not a consideration for normal road driving and single throttle inlet systems work fine in these applications .

So basically yes I think an RB30DET based on the Neo turbo head would be easier to get better consumption from - particularly if you can get the static CR around 9.5 to 1 and run it on something better tan basic E10 or 91ULP fuel .

I'd also consider using a taller diff ratio like say 3.7 or 3.9 because doing close to 3000 revs at 110 km/h won't help consumption . A 20% larger engine can in theory pull 20% taller gearing which works out to be around a 3.29 diff ratio , not hard to see how that's a bit extreme so if you halved that to 10% it works out to be a 3.7 final drive ratio . Lighter VL Commodores with RB30Es (SOHC 9.5 CR) from memory had a 3.45 ratio final drive and something very similar to an RB20DET gearbox inc ratios .

A .

Edited August 6, 2013 by discopotato03

[xntrq]

What I meant was using an RB25 Neo turbo head and it's inlet manifold .

It's not an easy job grafting a RB26 inlet system onto any RB25 head and if driving conservatively much of the time I don't know why you'd bother . To me there is only two reasons anyone would ever go with an RB26 head and they are the shim under bucket lash system and the six throttle inlet manifold . The aims being valve train reliability at very high revs and sharp throttle response . Big revs is not a consideration for normal road driving and single throttle inlet systems work fine in these applications .

So basically yes I think an RB30DET based on the Neo turbo head would be easier to get better consumption from - particularly if you can get the static CR around 9.5 to 1 and run it on something better tan basic E10 or 91ULP fuel .

I'd also consider using a taller diff ratio like say 3.7 or 3.9 because doing close to 3000 revs at 110 km/h won't help consumption . A 20% larger engine can in theory pull 20% taller gearing which works out to be around a 3.29 diff ratio , not hard to see how that's a bit extreme so if you halved that to 10% it works out to be a 3.7 final drive ratio . Lighter VL Commodores with RB30Es (SOHC 9.5 CR) from memory had a 3.45 ratio final drive and something very similar to an RB20DET gearbox inc ratios .

A .

thanks mate. my goal is to just gain more rwkw and better response in comparison to my NA RB25DE Neo without compromising fuel economy . no need for high revs as I've never gone over 5k rpm anyway.

[WMDC35]

comparing the de's, I've had cars with r33 type 25de and 25de neo at the same time, driving them back to back for a while seemed to be nothing really in it between them for power or economy

[StevenCJR31]

What I meant was using an RB25 Neo turbo head and it's inlet manifold .

It's not an easy job grafting a RB26 inlet system onto any RB25 head and if driving conservatively much of the time I don't know why you'd bother . To me there is only two reasons anyone would ever go with an RB26 head and they are the shim under bucket lash system and the six throttle inlet manifold . The aims being valve train reliability at very high revs and sharp throttle response . Big revs is not a consideration for normal road driving and single throttle inlet systems work fine in these applications .

So basically yes I think an RB30DET based on the Neo turbo head would be easier to get better consumption from - particularly if you can get the static CR around 9.5 to 1 and run it on something better tan basic E10 or 91ULP fuel .

I'd also consider using a taller diff ratio like say 3.7 or 3.9 because doing close to 3000 revs at 110 km/h won't help consumption . A 20% larger engine can in theory pull 20% taller gearing which works out to be around a 3.29 diff ratio , not hard to see how that's a bit extreme so if you halved that to 10% it works out to be a 3.7 final drive ratio . Lighter VL Commodores with RB30Es (SOHC 9.5 CR) from memory had a 3.45 ratio final drive and something very similar to an RB20DET gearbox inc ratios .

A .

your memory must be failing you :-) rb30e sohc had 9:0 to 1 cr , not 9:5 to 1 due to cats piss 88 octane available at the time Edited August 6, 2013 by StevenCJR31

[chairman]

good ecu that has closed loop and stock cams

[discopotato03]

Yes well memory fades and yes we did start out with urea grade ULP .

Anyway torque is king IMO and getting it in a reasonably free spinning package can only be good . I hear people say that in basic terms the difference between an RB25 and an RB30 with the same lid/CR is about 500 revs . I think at the end of the day the head and manifolds have a large say in the total power potential and the capacity dictates at what engine speed you reach the top ends limits .

Obviously more cubes means more torque at the same revs or same torque at ~ 20% less revs than a 25 .

If you soft pedal most places with small throttle openings and lowish revs then the 30 is good because it will pull higher gears down there more easily than a 25 .

If you give it taller gearing , final drive , you will get a bit more speed in the gears and make the best of the extra low down torque . Life would be good cruising down the Hume at 110 doing 2500 revs and never having to change down . If you sized your turbo so its not trying to boost at these revs every time you touch the loud pedal and tune the engine properly it should get good consumption .

A .