Jump to content
SAU Community

Recommended Posts

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

Link to comment
https://www.sau.com.au/forums/topic/288589-rb24/
Share on other sites

Why are you so concerned about lowering the comp ratio to silly levels that are of 1980's proportions?

I just don't get it.

Unless you intend to use the chip oil as fuel... there is no reason to in this day and age.

If, as your other thread said, tuning 'isnt the best' or whatever, even lower comp won't save you from detonation necessarily.

Link to comment
https://www.sau.com.au/forums/topic/288589-rb24/#findComment-4834589
Share on other sites

"budget bitsa" is an oxymoron. Trying to mix'n'match parts is just asking for trouble.

Why would it be cheaper - you would have to get a RB25DE, and a (whole) RB20 to get the crank. A RB25DET isn't really all that expensive.

A bit of history. In the good old days - before supertaxis - the Nissan race GT-Rs ran 9:1 c/r and 1.8 bar boost, and were a very tractable race car. The Ford Sierras ran only 6:1 and 3 bar boost. They were an absolute pig of a car to drive - gutless off-boost and uncontrollable on-boost. So, low c/r is not the be-all-and-end-all of safe turbo driving.

Link to comment
https://www.sau.com.au/forums/topic/288589-rb24/#findComment-4834639
Share on other sites

Personally I think the only instance of a shorter stroke crankshaft in an RB25 being useful would be to increase it's rev potential or to possibly if legal suit a class/capacity limit .

I reckon I'd do the cylinder capacity calcs because guestimate thinks you may be less than 2400cc .

To seriously destroke an engine you have to look at the shorter stroke and possibly longer connecting rods to improve the bore/stroke AND rod/stroke ratios - if you want all the benefits available .

I think destroking an engine to reduce the compression ratio is a bit out there somehow , I wouldn't be aiming for anything less than 8.5:1 in anything that saw much part throttle road type usage .

The manufacturers had all sorts of reasons for using silly low 7.5 or 7.7:1 ratios in the late 70's to mid 80's era OEM turbo engines . It was the era of "power kerosene" octane ULP and turbochargers/engine management systems were crude by todays standards . They also hadn't gotten around to realising that intercoolers AND more sane 8-8.5 CR's gave much better overall results .

Lastly having the piston further down the cylinder at top dead center is probably the worst way to reduce CR because the piston is further away from the heads quench pads so you may reduce the CR and raise the detonation threshold . Lose lose situation for no very good reason .

Imo cheap and quite reasonable = ex VL RB30 , close to equal bore and stroke and good rod length to give very acceptable rod angularity . Arguably better than RB25 or 26 because can make the power with 20% less revs in some cases .

Just my thoughts , cheers A .

Edited by discopotato03
Link to comment
https://www.sau.com.au/forums/topic/288589-rb24/#findComment-4834644
Share on other sites

8.7:1 is not that low. Factory rb25det is 9:1. See the difference? It isn't much. The 8:1 figure was for comparison purposes, (ie if I started with an rb25det). 8.7:1 also more compression than what japanese (8.5:1) and australian (8:1) Legacy RS's use. I wouldn't call it rediculously low.

One of the questions was about the quench pads. I asked you whether they have them or not, and noted concerns about the combustion chamber shape. I know if they use them it's not ideal to pull the pistons away from them, but if they don't use them it doesn't have nearly the same effect.

I have done the calculations. here.

Just so you know, the difference in stroke is 2mm, the major displacement change (RB20-RB25) comes from the bore change. I am moving the pistons down 1mm from where they previously sat. Nowhere near as bad as the spacer plates they used to use in the 80's on 202's and so on, and it's only a little worse than using a thicker head gasket (as some people do to drop the compression).

Another question - Do the pistons in an RB30E come all the way to the top of the bore (or very close)?

Link to comment
https://www.sau.com.au/forums/topic/288589-rb24/#findComment-4834909
Share on other sites

Hay mate, i think its a great idea. I found the nissan article were the rb25 block uses 20 crank and sohc head and carby lol.

Im a big fan of the 20 and only dont like the 25 because of its slow revving nature without hi rpm compared to the 20, revs isnt everything but its fun an sounds good lol.

If you find out anymore about what bearings, rings,pistons sizes etc throw us a pm as im interested in doing the same thing sometime.

Stupid question, but does a lower compression turbo'd car help make it rev quicker ?

Good luck with it all.

Edited by bbe
Link to comment
https://www.sau.com.au/forums/topic/288589-rb24/#findComment-4835456
Share on other sites

i agree with ash it just sounds like a lot of hassles, effort and mish mash parts and costs

it would end up cheaper, easier, quicker and a better result to just get an rb25det

too often we see people "trying to be different" or "trying custom" and its just a waste of time, effort and $

Link to comment
https://www.sau.com.au/forums/topic/288589-rb24/#findComment-4835468
Share on other sites

Stupid question, but does a lower compression turbo'd car help make it rev quicker ?

Good luck with it all.

From my knowledge lower comp will allow you to run higher boost with out fear of detonation , but its the shorter stroke that will allow it to rev faster. On paper it seems like you could be on a winner. In practice, well it would be interesting to see.

Link to comment
https://www.sau.com.au/forums/topic/288589-rb24/#findComment-4835493
Share on other sites

It all depends on whether I get a car that can easily take an RB. If I get something like a VL or a cefiro, we will probably find out. But I don't like my chances of finding one in good nick for next to nothing.

School holidays... :O

Link to comment
https://www.sau.com.au/forums/topic/288589-rb24/#findComment-4835816
Share on other sites

i agree with ash it just sounds like a lot of hassles, effort and mish mash parts and costs

it would end up cheaper, easier, quicker and a better result to just get an rb25det

too often we see people "trying to be different" or "trying custom" and its just a waste of time, effort and $

And too often on this board people aren't willing to accept new ideas or try new things just because they are different. "Just go with something proven" gets thrown around a LOT on here. 8.7 to 1 isn't that low of a compression ratio. The change in quench height is a valid concern, but he already noted that and that's the main point of his post I think. I do think Discopotato is right on though that the only real meaningful advantage is the ability to rev higher, but the 25 hydrualic head is going to be the limiting factor anyways, so unless he is planning to do something about that, it's kinda pointless.

there's no shortcuts in the car modding game unfortunately...

Wrong, but by all means, keep spending all that money on overpriced jap namebrand parts and stay slow. I'll find better deals, new things, and cheaper parts as long as the work well and go faster for a lot less thank you very much.

Link to comment
https://www.sau.com.au/forums/topic/288589-rb24/#findComment-4837147
Share on other sites

Nah I'm not still in school. Although I might mentally still be 15 (at times) I'm 22 and have done a few things with cars. Mainly auto-manual conversions and engine swaps (for identical engines and factory gearboxes though). But I am trolling for information before I go out and buy a long term project car (if I'm building an engine for it, it's going to stay with me for a while).

Incidentally I found a picture of an RB25 head, and it does have squish pads. But they cover a very small portion of the bore (at least compared to engines like the later 20v 4A-GE's, and f-a-r less than most 2 valve per cylinder engines) so I'm not sure what to think.

Link to comment
https://www.sau.com.au/forums/topic/288589-rb24/#findComment-4837168
Share on other sites

Buy a shagged rb25det, (ringlands gone etc) then purchase forged pistons at desired CR.

Get it bored OS, crank work done etc and you have a cheap rb25det with the CR you want.

That would probably end up costing the same as a rb25de/rb20crank purchase as you will need to have machine work done on that too more than likely so that it all works properly.

my 2c.

Link to comment
https://www.sau.com.au/forums/topic/288589-rb24/#findComment-4839002
Share on other sites

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 .

Link to comment
https://www.sau.com.au/forums/topic/288589-rb24/#findComment-4839293
Share on other sites

  • 3 weeks later...

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now


  • Latest Posts

    • Update 3: Hi all It's been a while. Quite a lot of things happened in the meantime, among other things the car is (almost) back together and ready to be started again. Things that I fixed or changed: Full turbo removal, fitting back the OEM turbo oil hardlines. Had to do quite a bit of research and parts shopping to get every last piece that I need and make it work with the GT2860 turbos, but it does work and is not hard to do. Proves that the previous owner(s) just did not want to. While I was there I set the preload for the wastegates to 0,9bar to hopefully make it easier for the tuner to hit the 370hp I need for the legal inspections that will follow later on. Boost can always go up if necessary. Fitted a AN10 line from the catch can to the intake hose to make the catchcan and hopefully the cam covers a slight vacuum to have less restrictive oil returns from the head and not have mud build up as harshly in the lines and catch can. Removed the entire front interior just shy of the dashboard itself to clean up some of the absolutely horrendous wiring, (hopefully) fix the bumpy tacho and put in LED bulbs while I was there. Also put in bulbs where there was none before, like the airbag one. I also used that chance to remove the LED rpm gauge on the steering column, which was also wired in absolute horror show fashion. Moved the 4in1 Prosport gauge from sitting in front of the OEM oil pressure gauge to the center console vents, I used a 3D printed vent piece to hold that gauge there. The HKB steering wheel boss was likely on incorrectly as I sometimes noticed the indicator reset being uneven for left vs. right. In the meantime also installed an airbag delete resistor, as one should. Installed Cube Speed premium short shifter. Feels pretty nice, hope it'll work great too when I actually get to drive. Also put on a fancy Dragon Ball shift knob, cause why not. My buddy was kind enough to weld the rust hole in the back, it was basically rusted through in the lowermost corner of the passenger side trunk area where the wheel arch, trunk panel and rear quarter all meet. Obviously there is still a lot of crustiness in various areas but as long as it's not rusted out I'll just treat and isolate the corrosion and pretend it's not there. Also had to put down a new ground wire for the rear subframe as the original one was BARELY there. Probably a bit controversial depending on who you ask about this... but I ended up just covering the crack in the side of the engine block, the one above the oil feed, with JB Weld. I used a generous amount and roughed up the whole area with a Dremel before, so I hope this will hold the coolant where it should be for the foreseeable future. Did a cam cover gasket job as the half moons were a bit leaky, and there too one could see the people who worked on this car before me were absolute tools. The same half moons were probably used like 3 times without even cleaning the old RTV off. Dremeled out the inside of the flange where the turbine housing mates onto the exhaust manifolds so the diameter matches, as the OEM exhaust manifolds are even narrower than the turbine housings as we all know. Even if this doesn't do much, I had them out anyways, so can't harm. Ideally one would port-match both the turbo and the manifold to the gasket size but I really didn't feel up to disassembling the turbine housings. Wrapped turbo outlet dumps in heat wrap band. Will do the frontpipe again as well as now the oil leak which promted me to tear apart half the engine in the first place is hopefully fixed. Fitted an ATI super damper to get rid of the worn old harmonic balancer. Surely one of the easiest and most worth to do mods. But torquing that ARP bolt to spec was a bitch without being able to lock the flywheel. Did some minor adjustments in the ECU tables to change some things I didn't like, like the launch control that was ALWAYS active. Treated rusty spots and surface corrosion on places I could get to and on many spots under the car, not pretty or ideal but good enough for now. Removed the N1 rear spats and the carbon surrounding for the tailpipe to put them back on with new adhesive as the old one was lifting in many spots, not pretty. Took out the passenger rear lamp housing... what do you know. Amateur work screwed me again here as they were glued in hard and removing it took a lot of force, so I broke one of the housing bolts off. And when removing the adhesive from the chassis the paint came right off too. Thankfully all the damaged area won't be visible later, but whoever did the very limited bodywork on this car needs to have their limbs chopped off piece by piece.   Quite a list if I do say so myself, but a lot of time was spent just discovering new shit that is wrong with the car and finding a solution or parts to fix it. My last problem that I now have the headache of dealing with is that the exhaust studs on the turbo outlets are M10x1.25 threaded, but the previous owner already put on regular M10 nuts so the threads are... weird. I only found this out the hard way. So now I will just try if I can in any way fit the front pipe regardless, if not I'll have to redo the studs with the turbos installed. Lesson learned for the future: Redo ALL studs you put your hands on, especially if they are old and the previous owners were inept maniacs. Thanks for reading if you did, will update when the engine runs again. Hope nothing breaks or leaks and I can do a test drive.
    • No those pads are DBA too  but they have colors too. I look at the and imo the green "street" are the best.
    • I’m not sure what happened I told them about sonic tunes free OTS tune and the next the I know .. I was booted..   To funny 
    • Yea - I mean I've seen my fuel pump which is decades old and uh, while I'm not saying this with real knowledge... but I sure get the ick at using anything in the fuel system that produced the state of that pump. Many years ago I went through multiple pumps (and strainers) before I dropped the tank to clean it out with extreme violence. I'm talking the car would do maybe 50km before coming to a halt, which resulted in me cleaning out the filter with some brake cleaner and going on my way. None of my stuff ever looked like what came out of your fuel tank. I don't think I'd be happy with it unless every single component was replaced (or at least checked/cleaned/confirmed to be clean here).
    • I'm not going to recommend an EBC pad. I don't like them. Just about anything else would suit me better. I've been using Intima pads for a while now.
×
×
  • Create New...