# Rod angles vs Rod lengths...RB26, RB28, RB29, RB30...??



## Sub Boy (Jan 28, 2008)

I noticed there has been a bit of talk on rod angles of late...

http://www.gtr.co.uk/forum/125167-10-6-140mph-i-need-suspension-help-please.html

It is common knowledge that the RB26DETT was spose to be a 2.4lt, and Nissan stroked it to 2.6 at the last minute, So in that the rod angle has been said that it's not ideal.

So...If you stroke the RB26 even further to a 2.8, this will make the rod angle even worse?

The RB30 (and the couple of people that are running RD28 cranks to make a RB29) these are a better angle than the 26 or 28.....But I have read they have higher loads on the big end bearings due to the longer stroke?

What are peoples thoughts on this?opcorn:


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## Sub Boy (Jan 28, 2008)

....55 people have looked, but no opinion? 

What about the engine experts....come on guys


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## infamous_t (Jul 9, 2007)

It is easier to search for rod/stroke ratios, the lower the figure (shorter the rod), the worse the angle and the higher possibility of poking a leg out of bed, due to higher loads on the cylinder wall..












> So...If you stroke the RB26 even further to a 2.8, this will make the rod angle even worse?


Yes, there is no way around the maths, unless you use a riser plate, ala os30 or hks hi deck.

rb26 rod/stroke ratio 1.64
rb30 rod/stroke ratio 1.76
os30 rod/stroke ratio 1.61

About 1.60 is limit as far as most engine builders are concerned, with 1.80-2.00 being closer to most engine builders preferred rod stroke ratio. 
Most of the jap stroker stroker kits reduce the rb26s rod stroke ratio further, with the os30 being the worst of the lot.

Sorry if that's a bit all over the place


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## infamous_t (Jul 9, 2007)

ok, heres the maths of it....

MOTOR	CONROD LENGTH STROKE R/S RATIO
------------------------------------------------------------------
RB26 121.5 mm (4.78 in)	/	73.7 mm 1.65
RB30 152.5 mm (6.00 in)	/	85 mm 1.79
OS30 138.35 mm (5.45 in)	/	86 mm 1.61

Maybe Rob can clarify on rb30 stroke length, as some sources say 85mm stroke, others 86mm.


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## Sub Boy (Jan 28, 2008)

Thanks Infamous T

Supprised to see the OS30 being one of the worst concidering the engine has a deck plate to increase its displacement.

What about the downside to longer rods? 

more chance of bending them at the same revs as shorter ones? 

More pressure on the bearings or less....Is this another reason why the RB26 is hard on Bottom end bearings?

Whats the rod angle with the RD29 crank in a RB30 block?


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## infamous_t (Jul 9, 2007)

Sub Boy said:


> Supprised to see the OS30 being one of the worst concidering the engine has a deck plate to increase its displacement.


I assume they didn't want to deal with customers complaining that they had to cut of bonnets etc like you need to with rb30 sometimes. So the spacer + rb26 block height is less than rb30 block height, iirc os giken uses a ~20mm spacer, whereas the rb30 is 38mm higher.


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## Sub Boy (Jan 28, 2008)

Thanks again "T", 
Thought this would have had more interest with lots of people looking at stroker kits and RB30 (And bigger) builds on the go.

Also...Higher compression with longer rods?

....Anybody? 

RIPS?
TR?

I just want to make sure I know what I'm better to get built for what I want to do with it.


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## arnout (Oct 29, 2003)

infamous_t said:


> with 1.80-2.00 being closer to most engine builders preferred rod stroke ratio.


1.75 is considered 'the best'. the higher the rod stroke ratio the longer the piston is at the dead centres and that can result in detonation as well.
There is no 'real' 'always' the best in engines. If you would make 'the best' engine you would have besided variable valve timing and intake manifold (what most engines have these days), variable compression, variable stroke and variable rods!


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## R.I.P.S NZ (May 7, 2004)

I don't have to much to add except going against what I was told would work, years ago, I just got stuck in and started with Nissan RB30s and I'm so glad I did.

I think the only guys who knock a 30 or genuinley prefere a 26 for a strong street car obviously have not been in a good RB30 GTR, there is just no comparison, we've proven 9000-10,000 is not only possible, but to date, has been 100% reliable (in a street/drag situation at least) the response is better, the drivablility is better, the slow cruze is better, the top end can be just as good, if not better, there really is no down side as far as I have found.

As far as rod/stroke ratio's go, it seems from the little I have looked into it, that the 26 is actually considered pretty poor and the Nissan RB30 is considered ideal.

Each to their own and I don't think there will ever be a 100% right or wrong, its just personal choice and you can't really blame a guy who has only ever been in big single turbo 26 GTRs for thinking its the "nuts", one day hopefully he'll experience a good all round RB and move up in cubes, one thing is for sure, I don't know of anyone who has had or been in a good 30 car and then said they prefered a similar spec 26.

Rob


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## Sub Boy (Jan 28, 2008)

The more I look into it, There doesn't seem to be a lot of downsides to going to the RB30 (this was really always my plan) But I was just wanting to get a idea of downsides etc.

Have you ever looked into the use of RD28 cranks Rob? I know you have had no reliablity problems, but with the bigger bearings (so I'm told) would this not be a plus?


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## godzirra (Sep 16, 2009)

infamous_t said:


> MOTOR	CONROD LENGTH STROKE R/S RATIO
> ------------------------------------------------------------------
> RB26 121.5 mm (4.78 in)	/	73.7 mm 1.65
> RB30 152.5 mm (6.00 in)	/	85 mm 1.79
> OS30 138.35 mm (5.45 in)	/	86 mm 1.61





arnout said:


> 1.75 is considered 'the best'.


in a nutshell


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## Cris (Sep 17, 2007)

If you were desparate to improve the rod/stroke ratio could you not more the gudgeon pin higher up and then run a longer rod?

It would mean custom rods and pistons though.


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## Cris (Sep 17, 2007)

Sub Boy said:


> The more I look into it, There doesn't seem to be a lot of downsides to going to the RB30 (this was really always my plan) But I was just wanting to get a idea of downsides etc.
> 
> Have you ever looked into the use of RD28 cranks Rob? I know you have had no reliablity problems, but with the bigger bearings (so I'm told) would this not be a plus?


From what I know of RD cranks they have a 83mm stroke vs the RBs 85mm stroke. So you'd loose 100cc. I guess that you'd need a lower gudgeon pin too (or longer rods).

Whilst I'm at it my RD28 crank destroked in a RB26 idea comes up with a rod/stroke ratio of 1.5. Given that some of the Japanese stroker kits are near to this ratio would this actually present a problem?


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## bigkev (Mar 2, 2008)

this is very interesting stuff, not something iv ever looked into really.

kev


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## souroull (Jul 21, 2008)

Cris said:


> If you were desparate to improve the rod/stroke ratio could you not more the gudgeon pin higher up and then run a longer rod?
> 
> It would mean custom rods and pistons though.



and the downsides of this would be...?


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## Cris (Sep 17, 2007)

souroull said:


> and the downsides of this would be...?


Money spent to gain no tangible benefit perhaps?

Tomei quote shorter rods (119.5) for their 2.8 kit. How important is the ideal ratio?


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## Piggaz (Sep 5, 2002)

Connecting Rod vs. Stroke Analysis: panic Tech Paper No. 1
In summary:
Short rods increase stress on the pin, more side loading, quicker to stop and accelerate from TDC BUT you can run a larger cam and will make more low end grunt then a long rod motor. Also it it less prone to detonation because of how quickly it moves away from TDC.


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## Cris (Sep 17, 2007)

Piggaz said:


> Connecting Rod vs. Stroke Analysis: panic Tech Paper No. 1
> In summary:
> Short rods increase stress on the pin, more side loading, quicker to stop and accelerate from TDC BUT you can run a larger cam and will make more low end grunt then a long rod motor. Also it it less prone to detonation because of how quickly it moves away from TDC.


Cheers!


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