I know the b18c doesn't have the best rod/stroke ratio compared to the b16a or b18c5(i think) and i was wondering if there is anything you can do to help that out. From what i understand, a good rod/stroke ratio would help for FI right? Thanks for the help. Corey

 

If i just had it sleeved, would i not have to worry about it?

 

Bore does not affect the rod/stroke ratio in any way.

You could get the same RS ratio as the B16 motor from a B18C, but you'd have to destroke it and end up with 1.6L of displacement...not much point in that.

An ideal RS ratio is more of an perfect scenario than reality, and not necessary to make the motor produce good power or high rpm capability.

 

ok, sounds good. thanks

 

Well, one way the B18C deals withs it poor R/S ratio is by using a block girdle.

 

yes, which keeps the crank in place, but the rod can still throw the piston through the wall. you can just bore the B16 block and get the same 1.8L with a pretty good R/S ratio that the OG B16 has. that would be my plan if go B series, or destroke a B20 block.
that will work REALLY well with FI application.
or an all motor race setup.
stan

 

There are lotsa GSR's running around that have lasted years without putting a piston through a wall. Just food for thought, I'm not arguing with you... As a matter of fact I went with the B16A block the second time around because it has a good R/S ratio, but you'd have to bore the B16 alot to get 1.8 Liters, my block is being built with an 84 mm bore and that takes it up to 1707 cc's.


[Modified by xThexHeadx, 8:08 AM 9/14/2002]

 

There are kits coming out with relocated piston pins to fit a slightly longer rod, then there is also the new Dart blocks with the extended desk height and the deck plates that Golden Eagle sells, gotta go custom rods with either of those two though.

 

just so that I have a grasp on this, what is the actual variables when calculating rod/stroke ratio? could someone post the formula?

 

Rod length / Stroke

IE 137mm / 87.2 = 1.57

 

Actually you'd have a better R/S on a B18C with a B16 crank. You'd have a CTR R/S ratio.

OH, and BTW, the GSR and ITR have the exact same R/S ratio.

 

you can not bore a stock B16 cylinder that far to yield a 1.8L displacement

this would entail 86mm bores for 1789cc or 86.5mm bores for 1810cc

The ideal setup would be the B20 with a B17 crank.
this would yield the 1.75 r/s ratio you desire for FI

 

Yeah, that would be ideal. But how much are B17 cranks? They are pretty rare.

 

btw r/s is bullshit and so are the payn/dart blocks r/s isn't the magic engine building answer. anything over 1.50 is fine if you want to waste your money and destoke your b18 or b20 give me a call i will gladly sell you a crank... r/s by itself has little to do w/ power output...there are ways to design an engine to use a less than ideal r/s and still make power and more importantly a hell of a lot more tq...destroking is the biggest waste of money...if you want a tq-less wonder just buy a b16..i've seen a couple stock length rod gsr's pull 11k rpm no sweat...w/ thier "shitty" 1.59 r/s. compression and porting (cc design) makes these monsters breath very well considering their piston speeds and limited dwell time

 

Finally, someone agrees with me. R/S ratio is not THE performance factor. It all depends what you want the motor to do and how the rest of the engine was designed/modified.

 

Preach on brother......

AMEN!

 

I understand that, but it doesnt hurt anything to have a good R/S ratio.

People be overreacting

 

Dont worry man, near ideal R/S ratio is best for FWD.
Im into bigger displacement B18's and B20's, but I think Too much torque in the beginning is bad for FWD launches.

The best of all compromises and cheapest way to go big NA power is 1.54:1 found in B18a/b or B20b/z 's.
It takes more $money$ and mad skillz (if youre doing it yourself) to get it near 1.75:1 with large displacement, but you'll get less bearing stress, and it makes linear power. I havent seen any B18a/b's or B20b/z's making torque past 8000rpm ..that is hard to acheive.

Btw, someone mentioned a block girdle. It's not there to keep the crank in place, all it does is distribute bearing load for even wear. It is not needed.


[Modified by Quick 200k Mile Motor, 7:19 PM 9/14/2002]

 

the better R/S ration is not a performance modification. no, but is it a building block for one. and more importantly it is the main aspect for reliability. and i don't know about you, but i don't feel like rebuilding my engine every 30-40K miles. yes longer stroke gives more umph down low. hell, i had a stroked L28 on my Zcar yielding 3.1L, but i prefer my honda motor high revving, and loving it.
yes b20 with b17 or even a B16 crank is better than boring B16 block, but boring the smaller block can be and is done. 84mm is not s problem with stock sleeves. and if you resleeve, you can go 86mm.
get an engine that can live in 8-9K RPMS with a turbo and you will love it.
that is if you drive the car and want to for a while daily.
my own, very biased thought.
stan

 

I think you've misunderstood the point.

No-one said that the so called ideal r/s ratio relates directly to power output.
The benefit of this ratio is that the side load on the cylinder bores it at a minimum/optimal level in compromise with torque levels.

the larger the crank stroke, the smaller the rod length, hence the smaller the r/s ratio. We should all know by now that a smaller r/s ratio provides more torque earlier. The 1.75 figure is a figure which has been arrived at whilst considering side load & broad rpm torque production (ie where it makes the torque, not how much it makes)

I'd like to see those gsr's pull 11k rpm for sustained periods, especially on stock rod internals.

 

Although Im only an engineer for an aluminum fabrication comp., I think about this stuff everyday.nerd Sometimes I can't get anything done at work..
Anyhow, I would like to add that there are compromises when going boost.
The best to have, and the cheapest to build going for NA & Boost later on, would be a 1.68:1 (which is nearly in-between 1.54:1 & 1.74:1, and can be built with off-the-shelf OEM parts and retain a displacement of 1800cc or higher).

For boost, anytime you stray away from 1.75:1 it will not produce linear power. That kind of power which is soo desirable in a FWD car will make you have traction in the beginning, plus the more you rev, the more you will get.

Look at the RB26 Skyline! The short crank (short stroke) + longer connecting rod combo offers better midrange to topend. Piston dwells longer at TDC, allowing the engine to build up pressure (Booost!) and push the piston down harder.

Also, a rod > stroke (long rod + short crank) will lower the squish velocity. A short rod + longer stroke crank will raise that velocity. So its better to stay below 1.75:1 for rod/stroke if youre going NA.

If you have rod < stroke (short rod + longer stroke), basically a NA built up and you decide to boost,
you can get some highend output by having the head & combustion chamber reworked ..for more volume, squish thickness or squish area. You want boost to build up at TDC. Oh and did I mention BIG midrange Torque!! (like the KA20's)


[Modified by Quick 200k Mile Motor, 5:20 PM 9/15/2002]

 

R/S ratio doen't determine power...but generally a better R/S ratio will help the engine to carry torque longer...creating more power. However, many other things are also required to take advantage of this.

 

-HXman

Youre right, R/S ratio doesn't determine power. It determines the kind of power:

"generally a better R/S ratio will help the engine to carry torque longer ...creating more power."

exactly, torque stays up.

 

OK guys lets get onto bore/stroke ratios...

a bore stroke ratio("BSR") of 1:1 is called square,

a BSR of 1:1.xx is called "over-square" with the bore being larger than the stroke.

a BSR of 1:0.xx is called "under-square" with the bore being smaller than the stroke.

all B series are under-square with only the B16A being over-square,

the S2000 is also over-square.

over-square designs improve high RPM power production and under-square improves torque production.

as an engine becomes less square the piston speed increases, as do the loads placed on hte engine components... and vica versa

of my particular interest is the comparison of hte B18 and the B20s BSRs...

B18 BSR = 1:0.91
B20 BSR = 1:0.94

this begs the question:

does the B20 therefore have a better higher RPMs characteristics when compared to a B18?

theory indicates yes...

t..

 

I also say yes, but not that much.
Dynos might lie on the output numbers, but with the same cyl head (vtec) given, and comparing characteristics of torque curve from both the B18a/b & B20b/z, the B20b/z likes to keep the torque up a little better than the B18a/b.

As your bore size gets nearer to your stroke length, you extend the torque to the top. F1 and Superbikes make their torque up high because they're oversquare. For example, a Yamaha YZF R1 makes peak hp @ 10,000rpm; peak torque @ 8,500rpm, and has a boreXstroke of 74mmX58mm.

bore/stroke ratio (examples)
Yamaha R1: tq@8,500 & hp@10,000rpm
F1 engine: tq@10,000rpm & hp@17,000 (insane!! damn pneumatic valves)

The Yamaha R1 has a b/s ratio of 1.28. F1 engines run with 2.0 or so, with 3 litre displacement.


[Modified by Quick 200k Mile Motor, 12:43 AM 9/16/2002]