Nissan 240SX Forums banner
Cancel
Create thread New Garage Forums

Rb20 Vs Rb25

Jump to Latest
Status
Not open for further replies.
1 - 20 of 44 Posts
A

anthonyO3

· Registered
Joined
·
19 Posts
Discussion starter · #1 ·
i have used the search button and i really couldnt find anything that will help me. I seen on different sites that the RB20 is alot cheaper than the RB25. What are some of the big diff. in the two motors? For a weekend drag car what would be the best choice to go with?
Also i noticed that the RB20 has like a 8K redline compared to the RB25 6.5K, so would the RB20 be the better motor to drag in?
 
#2 ·
It's simple. A bigger motor will make more power. The RB25DET doesn't rev to 8k because it doesn't need to. It makes its power down low. No need to rev high.

RB25DET > RB20DET

I personally would never consider an RB20DET. If I wasn't doing the RB26 I'd just build the stock KA and turbo it. It's got tons of displacement for a 4 cylinder and is decently strong.
 
#4 ·
my ka hates me, she gets mad and spits timing chains at me..lol
 
Save
Like Like
#5 ·
I like the RB20 a little more than the 25. The main reason the 25 makes more torque is b/c of compression which is 9.0-1 where as the 20 uses a 8.5-1 ratio. Accept for the head gasket being a little weak the RB20 is a better high boost option imo. With a better head gasket you can push 400-450rwhp out of the 20 farily easily. I wouldn't want to do the same thing to the 25 without lowering the compression. Now if you really want to step it up and you don't mind doing some building you can have 500+ reliable HP from a 20. When I say building you would need to upgrade the cams I would use 260 or 270 for drag. Then disassemble the block and have the crank, rods, and pistons cyro treated and have the pistons swain coated with GC on top and PM on sides and have everything balanced. Along with a bigger turbo and injectors and a way of controlling everything you could push over 600RWHP on a 20. The process above will cost around $4000-5000 for rods, crank, and pistons coated and balanced the cams a new headgasket, injectors, turbo, manifold, and gaskets for everything. I am leaving out ECU controll due to various costs for different options. Also the cost is with someone else installing everything. If you do it yourself take about $1500 off.

Don't get me wrong I like the 25, but for the money I would rather keep my 20.
 
#6 ·
the RB25 can make over 550HP at the wheels on stock internals if tuned properly, and do it fairly reliably too....

Lower compression means poor off boost performance also, you gotta remember that too, until a point where boost kicks in, you arent going to have as much power.

Not an arguement, just stating fact, larger displacement plus higher compression=more off boost power & more on boost power.
 
#8 ·
JonPowell said:
the RB25 can make over 550HP at the wheels on stock internals if tuned properly, and do it fairly reliably too....

Lower compression means poor off boost performance also, you gotta remember that too, until a point where boost kicks in, you arent going to have as much power.

Not an arguement, just stating fact, larger displacement plus higher compression=more off boost power & more on boost power.
Click to expand...
Yeah but you are forgetting something high compression with more boost= detonation

I would much rather have a 8.5-1 compression when I start turning up the boost than a 9.0-1. Detonation will become a bigger factor sooner for the 25. Plus the smaller displacement equals out to lighter and smaller rotational mass which in turn allows for a higher rev without worrying about breaking something.
 
#9 ·
like I said, not to start an arguement, but you forgot something also....with higher compression and larger displacement, you dont need as much boost to get the same power...and I did have the disclaimer in there....with proper tuning.
 
#10 · (Edited)
No I totally understand I am not trying to start a war either.

However you will still have to worry about detonation when you start to make power. Many engine builders believe it is better to have lower compression with higher boost than it is to have a higher compression with boost. Also I am sorry to say, but the 25 is going to be much more prone to loose reliability when you start uping the boost. The 20 is in a builders stand point a better enging to boost. Yes you will have better off boost response but you loose some topend with that trade off also...

You know on second thought Jon how about we agree to disagree...;)
 
#11 ·
one question for you guys, you seem to know what's up with the rb's. I'm thinkin of getting one. But this is a general question. Will adding more fuel help to prevent detonation? say if timing is advanced any, do you think adding fuel would make it safer? or if timing is retarded can you run less fuel?
My guess is no but what do you all think?
*don't know if this question is typed the way i want it to come out or not haha
 
Save
Like Like
#12 ·
If you start advancing the timing and it is knocking, more fuel will compensate and save an engine....

If you retard timing it will run rich unless you lower fuel.

If your engine starts to lean out and knock while timing, the engine can, in most cases, detect knock and pull timing back to preserve the motor, thus making it less lean as the fuel doesnt have as much time to fully detonate.
 
#13 ·
uno said:
one question for you guys, you seem to know what's up with the rb's. I'm thinkin of getting one. But this is a general question. Will adding more fuel help to prevent detonation? say if timing is advanced any, do you think adding fuel would make it safer? or if timing is retarded can you run less fuel?
My guess is no but what do you all think?
*don't know if this question is typed the way i want it to come out or not haha
Click to expand...
Well it depends on how much you advance or retard the engine. You can use fuel to help with detonation, but only to a certain extent. You don't want to add to much fuel which will in turn cool the cumbustion chamber too much causing loose of power. On the other hand you deffiently don't want a lack of fuel which as everyone knows will cause detonation. Now if you do advance the timing then you will most likely need to add a good bit of fuel depending on how much the advance is set. However you very rarely want to take away fuel from the engine the way it runs without knowing exactly what the A/F is. You might be able to get away with slightly less fuel but you would also loose a good bit of power if you retard it to where you could use less fuel. If you are worried about fuel conservation then I would reccomend a risng rate fuel pressure regulator and a timing retard kit. These two units will allow you to run slightly less fuel when crusing and on the highway, but as soon as boost is applied they will compensate the fuel and timing to your needs...
 
#14 ·
DLSracing said:
No I totally understand I am not trying to start a war either.

However you will still have to worry about detonation when you start to make power. Many engine builders believe it is better to have lower compression with higher boost than it is to have a higher compression with boost. Also I am sorry to say, but the 25 is going to be much more prone to loose reliability when you start uping the boost. The 20 is in a builders stand point a better enging to boost. Yes you will have better off boost response but you loose some topend with that trade off also...

You know on second thought Jon how about we agree to disagree...;)
Click to expand...

Yeah well my Dad can kick your dad's ass!

There are just 2 different schools of thought here, 2 different, both correct ways to make power....

9:1 is still not a high compression ratio, most NA motors are 10 or more:1 but yeah...not saying you're wrong, just saying you have a different attitude than I do about it.

The Rb20 is a great motor, as are all RB motors...just gotta tune responsibly
 
#15 ·
Thanks JonPowell and DLSracing you answered it totally...that was fast too. You all knew what i was talking about somehow, even though i'm running on 2 hours sleep. Thanks again guys.
 
Save
Like Like
#16 ·
JonPowell said:
Yeah well my Dad can kick your dad's ass!

There are just 2 different schools of thought here, 2 different, both correct ways to make power....

9:1 is still not a high boost ratio, most NA motors are 10 or more:1 but yeah...not saying you're wrong, just saying you have a different attitude than I do about it.

The Rb20 is a great motor, as are all RB motors...just gotta tune responsibly
Click to expand...

Yeah I hear ya I just don't want to start another war on the boards. I know you and me both believe in having a good tune. There is no way around having your engine tuned to get the most out of it. Believe me I have respect for you since I know you have similar views on how to tune and only using the best parts for your car as I do.

I am done high-jacking




But the RB20 rules alll....
 
#18 ·
uno said:
Thanks JonPowell and DLSracing you answered it totally...that was fast too. You all knew what i was talking about somehow, even though i'm running on 2 hours sleep. Thanks again guys.
Click to expand...

Well that is why we are on here. Plus as a Mod I try to addhere to higher standards when posting and give useful information...
 
#20 ·
US RB25's have done 585 and 586RWHP with stock bottom ends. The 585 being totally stock long block. Aussie JMS RB25 has done 601RWHP with a stock botom end.
The RB25 has a much stronger tranny.
On a budget? get an RB20 otherwise the RB25 is obvious for dragging.
 
1 - 20 of 44 Posts
Status
Not open for further replies.
You have insufficient privileges to reply here.
About this Discussion
43 Replies
16 Participants
Last post:
DLSracing
Nissan 240SX Forums
posts
1.6M
members
94K
Since
2002
A forum community dedicated to Nissan 240SX owners and enthusiasts. Come join the discussion about performance, builds, engine swaps, turbos, modifications, classifieds, troubleshooting, maintenance, and more!
Full Forum Listing
Explore Our Forums
S13 240SX (89-94) New Member Introductions General Tech S14 240SX (95-98) KA24DE & KA24E