Rb24

[spaztik24det]

Have you completed the build yet i hope u went through with it as im wanting to build one myself

if you could pm me that would be good

also is there a difference between the rb25 crank and rb 20

from what i understand isnt the ability to rev higher with the same power alow for faster 1/4 mile times eg hondas

and f1 cars

[Dobz]

After some research, I discovered the existence of the RB24S, a combination of RB25 block, RB20 crankshaft, RB30 SOHC head, a carby, and unknown rods and pistons. Which leads me to believe that the RB20 crank can fit into an RB25 without dramas.

I want to do this for two reasons, the stroke-rod length ratio improves slightly, and it drops compression significantly. I won't go into the maths (epic long calculation) but if the compression was 10:1 to start with (I think that's what the n/a R33 RB25's have) it drops to 8.77:1. If I start with 9:1 (the turbo version) it drops to 8:1. The only issue that might prohibit big boost is the combustion chamber shape.

So three questions:

1. Does the RB25 combustion chamber use squish (also known as quench) in it's design, or is it closer to a hemi style of head?

2. Has anyone done this before?

3. Are there any reasons (such as oil pump drive, or similar) why this wouldn't work?

Yes I'm aware that it's ideal to use lower compression pistons (or a genuine DET engine), but for a budget bitsa approach it makes sense, especially when the combined cost of an RB25DE and an RB20 crank is far less than the cost of an RB25DET. And I only give away 70cc for the lower compression. And even if it's not ideal, it's probably still way better than an RB20DET.

Thoughts?

Ben

Destroking an rb25 for what ever reason is pointless, planning to use an rb30 head and a carby on it just makes it worse.

[Nee-san]

I'll say this then I'm out because this could go nowhere forever .

IMO torque is what drives you down the road and the more you have the better off you'll be to a point (traction) .

I think it's brave to say that an engine that revs (as in picks up revs) faster makes for a better car .

In the real world compromises have to be made and no one drives everywhere in 2nd gear at 4000 + revs .

Money is always an issue to most mortals and chasing small changes in stroke length IMO is not worth the effort unless your hands are tied by class regs .

If I was that way inclined I'd put any any extra money into RB25 head work and cams because I reckon that would work better than a de stroked one with a lesser CR .

The theory around using a "decompression plate" on an archaeological find (Olden 202) was not a good one and while it got the static CR down it killed all the bottom end torque as well .

The only reason CR's are lowered is to fight off detonation and there are better easier ways to do that anyway .

While ever you have an engine that has to be run at part throttle it is stupid to have real low CR's , the fact that the engines throttled (strangled for want of a better term) means that the cylinder filling ability is lower so the "dynamic" or effective compression pressure will be as well . If you like having no torque and doughy throttle response this is the way to go about it .

I really wish people could grasp the fact that if you make an engine breathe better by removing avoidable restrictions you can often make lots of power/torque without high boost pressure . "Big boost" makes for lots of problems so if you can make adequate power without it then you're not making problems that are difficult/expensive to solve .

Your call .

People don't realise boost is a measure of your engines inefficiency not "my car holds 30 pound fully sick bruz"

[VB-]

use RB30 bottom end

/thread

PS- RB20 with 26 crank comes out around 2.4l doesnt it?

[spaztik24det]

use RB30 bottom end

the rb30 has even bigger crank pins massive rods massive stroke compared to the 25 if u put a 20 crank into that block then compression would be a problem

[zebra]

Main and Rod journal sizes are the same across the whole RB range.

He wasnt saying use a 30 block with a 20 crank he was saying use the complete 30 bottom end

[spaztik24det]

Main and Rod journal sizes are the same across the whole RB range.

He wasnt saying use a 30 block with a 20 crank he was saying use the complete 30 bottom end

like what making just a rb30de/det instead of a 24

[zebra]

yeah, less stuffing around, more power, more torque blah blah blah

[VB-]

yeah zebra knows whats going on

rb30det - cheaper with a result thats better in every way.... the bit i added on the end, is a completely different combination of parts that results around 2.4l from memory, the only reason i could imagine for doing this though is for a particular class of racing that may be restricted by capacity

[zebra]

Not many race classes I know of need 2400cc, its either under 2000cc or under 6000cc

[GTRNUR]

Lowering the capacity to achieve a "better" rod ratio is a step in the wrong direction. People stroke engines to increase displacement at the expense of rod ratio for a very good reason. Displacement increases make a larger overall improvement to engine performance than any mythical and ideal rod ratio. Eg... 2.7 and 2.8 stroker cranks. Both add displacement at the expense of a little rod ratio.

There are production engines out there with ratios ranging from 1.44 to 2.1. Whoever said 1.75 is perfect for an Rb engine has never also said precicely what it is perfect for. Did Nissan get it wrong? In reality there is no such thing as a perfect rod ratio. It comes down to 1. What will fit, and 2. What will work well for the given application and survive with reliability.

The only reason you would chase the "perfect" rod ratio is if you are after that ultimate tuned package, which you can not ever hope to achieve with a bitsa engine and especially a limited budget.

If budget comes into the equation even slightly the goal should be as much displacement as you can cram into the block as you can, and everything else is secondary. That being rod ratio, squish efficency, compresion ratio etc. In the end raw displacement makes the biggest overall difference. So in the RB world this means an RB30 bottom end.

[spaztik24det]

Lowering the capacity to achieve a "better" rod ratio is a step in the wrong direction. People stroke engines to increase displacement at the expense of rod ratio for a very good reason. Displacement increases make a larger overall improvement to engine performance than any mythical and ideal rod ratio. Eg... 2.7 and 2.8 stroker cranks. Both add displacement at the expense of a little rod ratio.

There are production engines out there with ratios ranging from 1.44 to 2.1. Whoever said 1.75 is perfect for an Rb engine has never also said precicely what it is perfect for. Did Nissan get it wrong? In reality there is no such thing as a perfect rod ratio. It comes down to 1. What will fit, and 2. What will work well for the given application and survive with reliability.

The only reason you would chase the "perfect" rod ratio is if you are after that ultimate tuned package, which you can not ever hope to achieve with a bitsa engine and especially a limited budget.

If budget comes into the equation even slightly the goal should be as much displacement as you can cram into the block as you can, and everything else is secondary. That being rod ratio, squish efficency, compresion ratio etc. In the end raw displacement makes the biggest overall difference. So in the RB world this means an RB30 bottom end.

an increase in displacement does not allways acheive more power once again f1 engines big bore short stroke high revving torque they are able to make 900hp without a turbo

by shortening the stroke of the 25 decreases the overall stress on all the parts at a given rpm wich is also what the rod ratio is good for hp is torque times rpm /5252 a direct relationship between torque and rpm yea

being able to generate a higher revving toque due to the increased ablility to rev eg better rod ratio and shortened stroke is that not a good thing

Edited November 18, 2010 by spaztik24det

[zebra]

an increase in displacement does not allways acheive more power once again f1 engines big bore short stroke high revving torque they are able to make 900hp without a turbo

by shortening the stroke of the 25 decreases the overall stress on all the parts at a given rpm wich is also what the rod ratio is good for hp is torque times rpm /5252 a direct relationship between torque and rpm yea

being able to generate a higher revving toque due to the increased ablility to rev eg better rod ratio and shortened stroke is that not a good thing

For starters you are around 200hp optomistic on the F1 motors.

Sounds like you have just read a book, I know so many people who have tried Long rod and Short Rod combos on motors, they all gave up and built strokers and went faster.

The only real expection to that is nascar where the motors need to survive at 9000rpm for hours on end with no rest, and only recently have they gone to a 4.125" bore and a 3.25" stroke, have they made anymore power over the old 4.030" x 3.5" combo...well no they havent, do they rev any harder...no

There is still enough room to throw a 6" (RB30) rod in an RB, just just needs custom pistons...does anyone do it...well no.

Plus more cubes = being able to spin a larger turbo = more power.

[GTRNUR]

an increase in displacement does not allways acheive more power once again f1 engines big bore short stroke high revving torque they are able to make 900hp without a turbo

by shortening the stroke of the 25 decreases the overall stress on all the parts at a given rpm wich is also what the rod ratio is good for hp is torque times rpm /5252 a direct relationship between torque and rpm yea

being able to generate a higher revving toque due to the increased ablility to rev eg better rod ratio and shortened stroke is that not a good thing

Perhaps if your building an F1 engine but seriously the materials being used in F1 are vastly different than an offset ground Rb25 crank with a set of cheap rods and pistons that are still based on a conventional RB design. You cannot compare an F1 engine to an RB. There only common ground is that they are both 4 stroke engines. And you cannot hope to rev an Rb24 anywhere near whats needed to get any benefit from the rod ratio change.

Yes longer ratios reduce some forms of internal stress on internal components. Specifically acceleration rates from BDC and TDC are less, and overall piston velocities are reduced, and inertia loading on the rod journels is reduced.

Standard Rb25 stroke is 71.7mm with a 48mm rod journel and a 1.7:1 ratio. Presuming you go for an RB20 rod you can reduce the journel to 45mm and at the same time reduce the stroke 2.8mm to about 68.9mm. The ratio would then be 1.77. I dare say that with such a small stroke and only a tiny increase in ratio you would barely pickup a 2-3m/s^2 benefit in piston acceleration reduction at the top end of 9-10K rpm. At those engine speeds your well into requiring all kinds of radical forged parts, good oil system and most importantly a bullet proof valve train.

[VB-]

F1 cars only make 900hp, top fuelers make 7000. cruise lines make 100,000+

cubes win

[Dobz]

I gotta get me a cruise liner! All i've got to do is slap some chrome 22's on it and go in the autosalon dyno comp FTW!