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RB20DET KW's Per Litre is Better than RB25DET? Why?

Cubes

By Cubes,
in Engines & Forced Induction

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Cubes Grand Master

Cubes

Posted
Posted

160kw / 2litres = 80kw Per Litre.

187kw / 2.5litres = 74.8kw Per Litre.

208kw / 2.6litres = 80kw Per Litre.

Why is it that the ol' RB20DET makes more power per litre?

I always thought that the RB20DET had a lesser flowing head etc.

Any one have a logical explaniation?

This probably isn't true in practise but..

If you were to up the capacity and the turbo was able to flow the amount of air with no problems then....

3ltr bottom end with RB20DET head = ~240kw

3ltr bottom end with RB25DET head = ~224kw

3ltr bottom end with RB26DETT head = ~240kw.

Obviously the RB20DET head won't make more power than the RB26DETT head on the same bottom end but why is this?!?!?!

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Warpspeed Experienced

Warpspeed

Posted
Posted

Power depends on airflow through the engine only, engine capacity is irrelevant.

So if you take a given induction system, cylinder head, and exhaust system, it virtually fixes the maximum airflow capacity and hence the power.

If you then place a larger bore and stroked block under the same head it will still be limited to the same maximum airflow. So an RB30DE will make no more power than an RB25DE if everything else remains exactly the same. Think about it.

It will however produce a lot more torque, and the same power at a lower RPM. It will be more grunty and less revy. This might be an advantage, and it might not.

So increasing engine capacity by 20% (30/25) is going to increase acceleration and reduce top speed. You can do the exact same thing by lowering the diff ratio by 20% a lot more easily.

Just to make it even more interesting, your RB30 with tall diff, will have the same piston speed as an RB25 with the shorter diff at the same road-speed in any gear. They will probably also have very similar fuel economy, acceleration, and top speed capability.

Interesting eh !

Revhead Grand Master

Revhead

Posted
Posted

I agree with SteveL. The RB25 isn't being pushed as hard as the RB20 - not that the 20 is on a knife edge or anything, it's just working harder.

I suppose they couldn't make the RB25 too close to the 26 in terms of power; if it had the same kW/L output it'd be a 200kW motor, probably too similar to the GTR engine. Probably the same reason you couldn't get an R33 GTS4 with the RB25DET in it.

Of course this argument doesn't apply to the 34's Neo, which makes a claimed 206kW iirc, so I dunno :)

Cubes Grand Master

Cubes

Posted
  • Author
Posted

Warpspeed.

You mentioned DE motors not DET's....

So.. its sounds correct that on Forced Induction motors, increase the capacity and the peak power will also increase.

From dyno runs I've seen all the RB30DET's still make their peak power at around the mid 6000 rpm.

A RB30DE motor was built and featured in a ZOOM magazine quite some time ago. It was running a low compression ratio of around 8.3:1 and made a final power output of around 110rwkw. This is 160kw at the flywheel territority.

If the Compression ratio was the same as that of the RB25DE it would have surely made another 15kw or so.

In theory what you said sounds correct but for some reason in practise it doesn't work out. Could this be possibly due to the fact that the head isn't actually a restriction when even on a 3000cc bottom end??!?!

hope that made sense i've just finished a night shift... :)

Warpspeed Experienced

Warpspeed

Posted
Posted

H Joel.

I deliberately stayed with n/a motors for the illustration because it is easier to understand. The same holds true with turbo motors as well, BHP per pound of boost will stay the same.

It not just sounds true in theory it is true ! But no one does it.

When they fit the RB30 bottom end they also change a lot of other things at the same time, I certainly would.

Sydneykid is the RB30 turbo king, and It certainly makes a very potent package. The main thing the RB30 bottom end gives you is a lot more off boost torque, and better response from that big bad turbo. All things considered an RB30DET is a very good street package.

But it is also true that there is nothing wrong with an RB26DETT modified to the same level. It all just happens further up the RPM range. Personally I would rather have the RB26 and some shorter rear end gears, but others will disagree.

Steve Grand Master

Steve

Posted
Posted

Warpspeed, does this mean that if you take a RB25, shorten the diff by 20% and you would have a similar package to a RB30 with the original diff? Or does this only apply to NA engines? To regain the the top end would it be as straight forward as increasing the revs by 20%?

Cheers

Steve

Steve Grand Master

Steve

Posted
Posted

It probably would decrease economy and increase wear, but I dont think it would be overly dramatic, because there is less load on the engine.

Enrico, you would have to get a new diff center. I was thinking of going to a auto center, only 4.3 vs 4.1 in the manual, I believe the R32 manual uses a 4.3 diff too? but it might be worth going a bit higher still and get a decent aftermarket one - think I'll be doing a bit more research:)

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