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

[Warpspeed]

Yes, the disadvantages of a shorter diff are decreased fuel economy, increased noise, and increased engine wear.

Fitting a longer stroke crank is also going to decrease fuel economy, and increase engine wear exactly the same.

It all has to do with piston speed with relationship to road speed. You can change the relationship to give more piston travel for each meter along the road, you increase the leverage. A longer stroke will do it, and shorter gear ratios will do it as well.

It really makes no difference if the engine operates at atmospheric pressure, or some boost level, exactly the same principles apply.

But there are other factors that come into it as well. One is rotating inertia, particularly of the flywheel. You are not just accelerating the car, but also the flywheel and the whole drive-train as well. A lower gear ratio is going to make this effect worse.

Another factor is piston ring friction in the bore. A long stroke engine is going to add a lot more friction, whereas a shorter gear ratio is not going to add any extra sliding friction.

Since yesterday, I have been experimenting with different stroke combinations on my Dyno 2000 software engine simulator. It is really interesting, I can vary the stroke from 70mm to well over 100mm while leaving every other engine parameter exactly the same, including the compression ratio.

What happens is that engine torque changes exactly as engine capacity does, but there are absolutely huge differences in the shape of the power curve.

A long stroke engine produces very high torque low in the RPM range, and that torque stays fairly flat, then falls off very quickly as RPM rise because the induction system can no longer feed the engine. The power rises to a sharp and narrow peak at mid RPM and then drops like a stone.

The identical engine with a short stroke has very poor low RPM torque, that smoothly rises to a shallow hump at mid RPM, and continues on up to quite high RPM, tailing off only gradually.

The power curve is much flatter and broader than the long stroke engine, but occurs at much higher RPM, and continues on. It does not drop off anywhere near as fast as with the long stroke engine.

Interestingly the short stroke engine also produces about 8% more peak power. This can only be because of reduced ring friction, because I did not change anything else.

As I kept reducing the stroke, the power kept rising, but peaking at a higher RPM, with less torque everywhere.

The engine I have modeled is a supercharged and inter-cooled RB26 running 15psi boost with special blower cams and ported head. I used that only because that is what I am playing with at the moment.

The power maximum I was getting was 499 BHP at 8500RPM, and 405 ft/lb at 4500 RPM.

Power drops to 488BHP at 9,500, and 452 BHP at 11,000 to give an idea of how well this thing breathes at the top end.

Now all I did was increase the stroke to make it an RB30.

Torque went up to 485 ft/lb at 2,000 RPM and fell away after that. Power peaked at 491 BHP at 6,500 with 454 at 8,500, and 353 at 11,000.

so at 11,000 RPM the power fell from 452 to 353 just by increasing the stroke by 14mm. Not that you could actually run either engine at that speed, but it does illustrate a trend.

While these power figures are not high compared to what some of the turbo guys claim, the supercharger produces far more low and mid range torque, in fact the torque is as flat as a ruler between 2,000 and 5,500 at 400 ft/lb, and without lag. So on road performance should be far better than a peaky high power turbo engine of similar or higher power. Easier to drive as well.

[EnricoPalazzo]

Very interesting. Ive been wanting that Dyno 2000 program for ages now, just didnt know whether it was any good and where i could purchase it.

[Cubes]

Warpspeed...

So theoretically the best way to increase capacity is to increase the bore size?

[meggala]

wellI'm doing some thing interesting I'm fitting an sr 20 to my r32

shock horror I hear you say but the proof will be in the driving and in the testing I have been very curious about this conversion for ages as most of you who have been to my site will realise.

yes I love rb's but wanted more torque with out increased capacity the car will be running a stock turbo gtr cooler and exhaust I cant wait for the inital dyno run and I cant wait toget it on to the track to see how it goes. as I find the 180's etc seem to have an edge on us I seriously considered putting the sr into the cefiro but decided against it because I feel I would liek to see the difference in the r32. Ihave a feeling this wont be the last sr conversion I will do

meggala

[MM2death]

Finally...i've been waiting for someone to do the SR transplant. I haven't seen it done to a R32 before. Should be very interesting and when complete, make for some great dorifto action.

Scott.

[EnricoPalazzo]

Megs: How will a SR20 produce more torque then a RB20??? 4cylinder vs 6 cylinder...

[Warpspeed]

Enrico, I first purchased Desktop Dyno a few years back it cost only $45 then. I later purchased the Dyno 2000 package which is far better for $145 (it does turbo and supercharged engines). There is now Dyno 2002 available that I have not tried yet.

Motion Software have a website with free demo download programs. The software is distributed worldwide by Mr Gasket Company. Your local speed shop will have the Mr Gasket catalogue, and will order the software in for you.

There are better engine software packages available than Dyno, but they also cost a lot more. Dyno is a very good entry level package though, and exceptional value.

There is an instruction book that comes on the CD that explains a lot of theory on engine operation that will completely change your perception of how a lot of things actually work. You can simulate any engine, and try out a lot of ideas to see what works and what does not. It is particularly good for choosing camshafts.

Joel, I have not experimented with changing the bore size, but will certainly give it a go and get back to you on that one.

[Warpspeed]

Enrico, engine torque is completely independent of bore and stroke or number of cylinders, believe it or not !

Engine capacity is the single main factor that decides torque.

Compression ratio will only have a small effect.

The curious thing about it, is that the state of tune will alter the shape of the torque curve a lot, but peak torque value always remains about the same for a similar capacity engine.

Supercharging or turbocharging makes a big difference to torque, but that is a quite different thing.

[Sydneykid]

Hi guys, this is a really interesting thread. A couple of observations.

Firstly with the longer stroke engine and fuel economy, we find that we can run one gear higher in most cases with a 3.1 litre engine. ie; with the 2.6 litre we used 2nd and 3rd a lot, but with the 3.1 litre we use 3rd and 4th. By lowering the rpm this improves the fuel economy. This is particularly noticeable when driving off boost, which is much easier to do with 3.1 litres.

Secondly Warpseed is OK with the torque, rpm, bhp comparisons. But if you feed in a port and/or a valve size increase, that suits a 3.1 litre, you will find the torque drop off in a 2.6 litre at low rpm is extreme.

You get a similar result from a larger turbo with more airflow capability, but the same maximum boost. I suspect this may also be the case with a supercharger change (either capacity or gearing).

In summary, you could specify a 3.1 litre RB engine to have roughly the same shape power curve as a 2.6 litre RB engine, but with more torque and more bhp everywhere in the rpm range. But you can't do the reverse, in that regard there is no substitute for cubic inches.

Hope that adds to the discussion

[Mase]

I've seen the sr20 put in a 32 before. Looked alright and you could fit a family of four in there as well. Heaps of room left in it. Yes they do tend to have it over us in the take off etc but i find on the freeway the rb's are king. I've got a silvia with an rb20det. I was hoping that with the drop of weight the lack of low down torque wouldn't be as bad in the s13. It goes well, but with the car being completely stock it definately has a lot left in it when you give it some. Will see if i can find that pic of the sr20 skyline.

[Warpspeed]

Yes, I take your point about running taller gearing with the RB30 on a road car. I also agree totally with your comments about valve size (cams) and head work restoring the top end revability of an RB30D conversion.

The point I was trying to make was that putting a 20% bigger displacement engine under a stock head is not going to get you 20% extra power. But if you do the extra work on increasing airflow as well as displacement, that is certainly going to give you a bunch of extra power and torque.

Some people are unhappy after fitting shorter rear end gears because of the increase in noise and fuel consumption. A good way to find out for yourself if it suits your style of driving is to try driving around for a week or two without using fifth gear. If it drives you totally nuts staying in fourth gear after an hours run down the freeway, forget fitting the shorter diff gears because it is going to be like that all the time afterwards.

[Mase]

Actually, with this diff stuff. My car was originally an sr20de, it was fitted with an lsd but i think the ratio may have been different to the sr20det silvias. I find that doing 60klms i generally have it in 5th. What speed are you fellas with r32's generally going into 5th gear?

[Cubes]

So how would we actually determine a good cam shaft profile for a RB30DET with the RB25DE head? As it could be a costly exercise playing with different profiles as in AUS or SA even it isn't something that people really have experience ith except for a small minority.

Mase... I sit in 5th at 60kays.

My diff is slowly starting to get a little loose and open wheel every now and again in the wet when doing U-Turns so after the new motor is in I might either look at a mechanical diff or one from a GTR. I need to look or find out the difference with the GTR diffs compared to the GTST diffs.

[Cubes]

Here's the Desktop Dyno Link

http://www.motionsoftware.com/purchasejac.htm

[Sydneykid]

The original question "RB20DET KW's Per Litre is Better than RB25DET? Why?"

RB20's run more boost.

RB20's have more agressive camshaft timing.

RB25's had to pass more stringent emmission rules.

Try comparing an R34 RB25DET with 206 kw.

Hope that answers the original question

[GTS20X]

Warpspeed, your camparision wasn't a true RB26/RB30 camparision as the RB30 not only has a longer stroke, but also a longer conrod which also affects engine power and what rpm's it's delivered at.

Carlos

[Cubes]

Sydneykid...

RB20's do run more boost but they also run a lower static compression ratio of .5. Therefor the RB25DET doesn't require as much boost to make the same power.

I might have to dig out that engine dyno proggy and have a fiddle to see how the static cr affects power when on boost.

[GTS20X]

Are the R32 GTST diffs plate or viscous LSD? I know the R31's are plate, mine had a couple of broken plates, i replaced those and added a couple more and now it works awsum!!

Carlos

[Steve]

Joel, I would have thought that CR wouldnt really affect peak power on a turbocharged engine, as power is a function of torque and rpm, the main deciding factor would be air flow (or boost, or air and fuel, as this provides the force, once burnt) at a given rpm? Or am I completely missing the point yet again?

[Cubes]

That sounds logical Steve however in a thread I think it was the RB30 one Sydneykid said with a higher CR you don't need to run as much boost to make the same power. Maybe its only 1 psi not sure.

Unless I mis-understood him.