what should i do? r/s ratio 1.57 vs 1.54?


qais_vtec

qais_vtec

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Jul 15, 2012
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Finnally getting round to building my b20v, i dont know if i should use the b20 crank (89mm) or my itr crank (87.2mm) in the b20 block, i will be using ycp 84.5mm pistons and revving to itr redline with sk2 s2 cams (old skool ones).
If i use the itr crank im losing 40cc but gaining on the r/s ratio. Is it worth sacrificing 40cc's for an itr r/s ratio? So its 1996cc or 1956cc with it having less sideload on the b20 sleeves or am i sacrificing potential power and worriying about the low r/s that b20s have? Note: i will be using the b20 rods in either case due to shorter b20 block.
 
I know this is no use to you but what does r/s ratio stand for? Something to do with the stroke?
 
qais_vtec said:
Finnally getting round to building my b20v, i dont know if i should use the b20 crank (89mm) or my itr crank (87.2mm) in the b20 block, i will be using ycp 84.5mm pistons and revving to itr redline with sk2 s2 cams (old skool ones).
If i use the itr crank im losing 40cc but gaining on the r/s ratio. Is it worth sacrificing 40cc's for an itr r/s ratio? So its 1996cc or 1956cc with it having less sideload on the b20 sleeves or am i sacrificing potential power and worriying about the low r/s that b20s have? Note: i will be using the b20 rods in either case due to shorter b20 block.
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The difference will be negligible. In fact, a lot of this R/S ratio stuff you read of forums isn't realistic. Just flip a coin over it.....
 
jesse888 said:
I know this is no use to you but what does r/s ratio stand for? Something to do with the stroke?
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Connecting rod legth/stroke length ratio, the lower this is the more the piston wants to go sideways instead of up or down generating more stress on sleeves, increasing friction, increasing stress on con rods and raises piston speeds so not good for high engine speeds.
 
Rich777 said:
The difference will be negligible. In fact, a lot of this R/S ratio stuff you read of forums isn't realistic. Just flip a coin over it.....
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How do you mean negligible? as in feel the difference in power from either cranks or the insignificant increase of r/s on thinner b20 sleeves?
 
Generaly depends where you prefer your torque range.

The b20 89mm crank throw is what gives it the lower wider torque range. But in turn is what makes them less rev happy up in the higher ranges. Think of your crank as a lever the longer the lever the higher turning force but as a result you have a longer heavier crank that wont be as balanced as the 87.2mm Type R one.

If you run the ITR one then your motor will expect to be more reliable and give higher peak hp as it will be capable of maintaining power higher in the rev range. The B20 one won't carry power up as high in the rev range giving lower top hp figures but will have produced is torque from a lower RPM and generaly have a higher torque output so won't really need to be reved as high anyway, so its kind of 50/50.

So its up to you and what you want the engine for. If its going to be thrashed at 8k around the track all time then the 87.2mm crank would probably be better. If its mainly for street and a bit of track then you may aswell stick with the 89mm as it will make the engine more usable.

Been looking into this myself lately as I picked up a B20 with light crank damage and have a spare B18C in the shed. Take a look over at Lee's ek9 1.9 doing just that. Waiting to see his results.

http://www.ek9.org/forum/members-rides/52289-my-1-9l-b20-ek9.html

Either way I don't think the difference will be massive. Not sure how much work is needed to fit the B18 crank and rods.
 
abunai888 said:
89mm more torque .
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I know its more torque but will it be noticible because its only an extra 40cc?, so if the r/s difference is even slighley significant then id scarifice the few potential HP for reliablity, the block is unsleeved and the b20 sleeve design is weaker than b18's.
 
qais_vtec said:
How do you mean negligible? as in feel the difference in power from either cranks or the insignificant increase of r/s on thinner b20 sleeves?
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You won't notice the difference. Look at the figures that you're comparing...
 
Thanks to everyone who replied. I will be doing some more research about the pros/cons of either setup.
 
That's the problem though. You won't notice the pro's /con's in the REAL world going by those figures.
 
Picking between the cranks is worthwhile, fussing over R/S ratio is not.

Always pick the longest rod you can realistically use, but do not let it become a primary design factor, certainly never compromise something as important as stroke for it!

I would use the ITR crank personally, with custom pistons and B16B rods... ;)
 
Kozy said:
Picking between the cranks is worthwhile, fussing over R/S ratio is not.

Always pick the longest rod you can realistically use, but do not let it become a primary design factor, certainly never compromise something as important as stroke for it!

I would use the ITR crank personally, with custom pistons and B16B rods... ;)
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How did I know you'd try to confuses the situation :p
 
Rich777 said:
How did I know you'd try to confuses the situation :p
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X2 on that..

Im going to go with the b20 crank seen as its only a marginal decrease in the rod ratio over an itr, it will be my everyday car so wont be raced and the sleeves posted with aluminum rods as peace of mind.
 
Sounds like you're over thinking now... Make sure you use decent quality aluminum rods .

I was only joking with Kozy. He knows it...
 
Rich777 said:
Sounds like you're over thinking now... Make sure you use decent quality aluminum rods .

I was only joking with Kozy. He knows it...
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I got the engine block aluminium grade number from endyn and could not source it anywhere in the uk, i kept getting told that its an aircraft grade and is not used in the automotive industry as its a costly overkill so had to settle for what i could get.
 
It will be block guarded properley (installed and then cylinders re-bored), girdled and balanced, i am thinking too much about it because i want reliability, iv had enough of stripping/rebuilding and havent really the time anymore.
 
You don't need a block guard. But, if you insist on installing one then there's a lot more involved than just honing to ensure the cylinders are not out of round. You'll need a machine shop to precisely machine the block guard so it has full contact with the sleeves and block. If you aren't prepared to do this then forget about it.

If you're really sick of tearing it down and rebuilding it then I highly recommend that you let D&R build it for you. That way you know it's been done properly. A lot of it comes down to experience and you can't just fall into it. Most of the time it's gained the hard well (I can tell you that).

Are you sure you have purchased aluminium rods ? I would never recommend them personally to 9/10 people (Rs250nut is a acceptation as I know his build).
 
I cant afford to let anybody else build it and iv built a few honda engines and iv learnt something new each time
Iv got me verniers, stretch gauges, torque wrenches, dti, timing discs etc on hand and all torque settings/sequences for every fastener involved in the build...
I cant wait to have it remapped aswell ;-)
 
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