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Discussion Starter #1
I've never really looked into the honda R/S ratio, and assumed it would be higher than it is since these motors are typically (before we get ahold of them) reliable for 100's of thousands of miles. I was messing around with the zeal compression calc and noticed for my d16y8 it said 1.52 r/s....I thought that I had for sure had a typo, but nope, all was correct.
A lower r/s means more sidewall pressure which I would take to mean shorter engine life...one article I was reading (honda tuning) said anything below 1.6 needed to have the strongest rods possible and possibly a sleeve job. Granted the writer was probably referring to higher than stock power output, but the same r/s ratio applies, which means my d16y8 is putting a significant amount of pressure on the sidewalls vs the supposed 'ideal' 1.75:1 r/s ratio.

All that to say, I'm just curious how these two seemingly opposing facts (short r/s ratio but long engine life) coexist so perfectly in our honda motors...
then my question is, where the hell can I find 138-138.70mm conrods for P2P pistons! Theoretically that extra mm or two would decrease stress on the bottom end, create more power, and the piston would still be sitting below deck. I can't see the negatives, but there has to be some since there isn't such a thing unless you go custom
 

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Formerly weebeastie
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Info

So I asked this question when I first came here and got flamed a little by those who didn't know. But I did find out some great info. In the world of V8's Rod/stroke ratio is super important because there is so much going on. The reason the D series engine stays together so well is because of the low reciprocating weight. Have you looked at D series pistons? They're tiny. The rods are super light and so is the crankshaft. (Compared to a V8) Also, the close tolerances that went into each D from the factory are another reason that these engines have lasted so well.


Honestly I wouldn't worry about the custom rods. Stock geometry engines with factory length H beams have well exceeded 300 horsepower with a turbo. Some have knocked on/over the 400 door.

What are your plans for your D?
 

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Discussion Starter #3
I am well acquainted with the flame...lol
In the midst of the build, everything torn down....decision making before the money starts to get spent...I know there are many great rod/piston combo's out there that have been proven to work...I just want to make sure I consider everything without reinventing the wheel...
 

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Rod to stroke ratio isn't as significant of a number as some people make it out to be.

There does come a point where reliability becomes unacceptable with a paticular crank-rod combo, but that condition becomes known quickly by destroyed bearings. D16 crank-rod has proven reliable. While there may be a more ideal rod length for cylinder wear, aspiration, etc. the differences in those aspects are very minimal.

Really the primary aspect that an engine builder should be concerned about in engine building is the bore to stroke ratio, and that aspect is determined by intended use. Street engine? Make it as undersquare as possible within physical and reliability limitations. Race engine? Make it as oversquare as possible within physical and reliability limitations.
 

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Discussion Starter #8
Does anyone know the rod to stroke on the fjt i beams and vitaras? Should be a little better then stock.
Careful man! That kinda talk will get you flamed in a hurry! rods are 138.65 IIRC... just go to zealautoworks.com (if that doesn't work, just google zeal autoworks compression calc)
 

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then my question is, where the hell can I find 138-138.70mm conrods for P2P pistons! Theoretically that extra mm or two would decrease stress on the bottom end, create more power, and the piston would still be sitting below deck
Our custom length D16 rods for vitara pistons are very close to that as they are 138.65mm long... but they are meant for boost.
 

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Discussion Starter #11
Hey fjt...I was trying to access your site earlier today but whether it was on my end or yours, I couldn't get there. I know you say they are 'meant' for boost, but would they fit stock d16y8 p2p pistons? If they do fit, any reason why not use them?
 

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because your pistons will be sticking out of the deck, and because as mentioned the d15 comes stock with a higher r/s than the d16. not to mention, this is all pointless unless you're building an engine that's going to spend the majority of it's life over 7500 rpm (probably higher but i reckon stock redline is a good place to mark) which is not even something the B engines capable of exceeding 10,000 rpm do on a regular basis.

the D has a better torque curve anyway, so the B can eat a dick for lunch with it's 1.75 which btw is only available on ONE B crank and the rest are pretty much where the d15 is hah

not to mention, making any extra power up there would cost you an additional 3g in head work. for the combined cost of all the shit you'll need you could get a K swap with the truly ideal ratio of 2:1
 

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because your pistons will be sticking out of the deck, and because as mentioned the d15 comes stock with a higher r/s than the d16. not to mention, this is all pointless unless you're building an engine that's going to spend the majority of it's life over 7500 rpm (probably higher but i reckon stock redline is a good place to mark) which is not even something the B engines capable of exceeding 10,000 rpm do on a regular basis.

the D has a better torque curve anyway, so the B can eat a dick for lunch with it's 1.75 which btw is only available on ONE B crank and the rest are pretty much where the d15 is hah

not to mention, making any extra power up there would cost you an additional 3g in head work. for the combined cost of all the shit you'll need you could get a K swap with the truly ideal ratio of 2:1
what the hell are you talking about, 3g in head work? A top of the line RLZ head will cost about 1500-1800, thats no where near 3g that and a stock k20 only has a R/S or 1.62, only .1 more than the D15B with the FJT rods
 

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Discussion Starter #14
because your pistons will be sticking out of the deck, and because as mentioned the d15 comes stock with a higher r/s than the d16. not to mention, this is all pointless unless you're building an engine that's going to spend the majority of it's life over 7500 rpm (probably higher but i reckon stock redline is a good place to mark) which is not even something the B engines capable of exceeding 10,000 rpm do on a regular basis.

the D has a better torque curve anyway, so the B can eat a dick for lunch with it's 1.75 which btw is only available on ONE B crank and the rest are pretty much where the d15 is hah

not to mention, making any extra power up there would cost you an additional 3g in head work. for the combined cost of all the shit you'll need you could get a K swap with the truly ideal ratio of 2:1
I'm fairly confident that on the d16y8 that a rod length of 138-138.65 would still sit flush or most likely under the deck with stock p2p pistons. As previosuly stated, I don't know if that is even worth considering, just checking all my options...
As far as any of those numbers that were barfed up, i'm not sure they are relevant to this post, seeing that the OP (that being me) was simply questioning the relationship between a low r/s ratio and the honda dependability that we have all come to know and love...as far as the B series and K series...they are great....FOR ME TO POOP ON!!! Thats right, I wipe my ass with k series motors on a daily basis.
 

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I'm fairly confident that on the d16y8 that a rod length of 138-138.65 would still sit flush or most likely under the deck with stock p2p pistons. As previosuly stated, I don't know if that is even worth considering, just checking all my options...
As far as any of those numbers that were barfed up, i'm not sure they are relevant to this post, seeing that the OP (that being me) was simply questioning the relationship between a low r/s ratio and the honda dependability that we have all come to know and love...as far as the B series and K series...they are great....FOR ME TO POOP ON!!! Thats right, I wipe my ass with k series motors on a daily basis.
may i have that k-series that you wiped your ass on? 190whp from the factory is nice n/a :p
 

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And again I say........

Do not think that Rod/Stroke ratio is never important. That makes you ignorant. In a V8 application that makes more than 400 HP its very important or you'll have rods hanging out the side of the block!! Low Reciprocating weight is the reason it doesn't really matter on a Honda. Take a big block Ford FE crank that weighs 50 lbs and then we'll talk about rod/stroke ratio.


But don't totally write it off in engine building. The greats pay attention to it!
 

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A lower r/s means more sidewall pressure which I would take to mean shorter engine life...one article I was reading (honda tuning) said anything below 1.6 needed to have the strongest rods possible and possibly a sleeve job. Granted the writer was probably referring to higher than stock power output, but the same r/s ratio applies, which means my d16y8 is putting a significant amount of pressure on the sidewalls vs the supposed 'ideal' 1.75:1 r/s ratio.

All that to say, I'm just curious how these two seemingly opposing facts (short r/s ratio but long engine life) coexist so perfectly in our honda motors...
Yes a lower R/S means more side loading but the big question is HOW MUCH MORE? And how much more can the cylinder sustain? And most pertinantly - how much more can a d-series sustain?

Those aren't opposing facts - just opposing trade-offs that can be addresed based on the application of your engine. Obviously Honda engineers felt that the R/S design of the D16 or even the D17 has merit for a reliable street car.
 

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Discussion Starter #18
Right, but as you say, what are the trade offs? They obviously did something right because look at all the friggin cars on the road. I was just simply looking at the 'typical' numbers, and our honda's carry a very low value from what is considered ideal. Perhaps that is a variable as to why they only recommend overbore to 75.5mm...
 

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I can add that the lower r/s ratio makes these engines have better milage. Those engines are designed for that, more low end torque that allow it to cruise at constants speeds at low rpm's.
Larger r/s ratio are better for high end power, also the engine will rev faster.

The fact is in the accelaration of the piston relative to the crank speed. Larger r/s ratios means lower piston speed and accelaration. If the piston accel is lower it yields that the combustion gases will be more time exerting pressure over them while the crank is at constant speeds or accelerating faster.
Then with less piston accel and the same mass you will have less force (side loading); less stress in the rotating aasembly, bearings etc.

Just for comparisson, the b18C and b18B are nearly the same motor but with different r/s ratio.
 

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R/S ratio has to do with the rod angle. As far as reliability goes, it's not directly related to reciprocating weight but stress on the cylinders and rods.

Most D's don't rev too high(IIRC highest redline is like 7200rpm) which is why they can last long with a relatively low R/S. B18s have low R/S ratios too but utilize stronger parts than D-series parts. The S2000s and bikes have R/S ratios of over 1.8, and rev pretty high.


The difference between the B18C and B18B is more than just R/S ratio. The B18C's have thinner rod BE's, shorter strokes that make the engine legal in the 1,8l race class, thinner PE widths, better rods, etc.
 
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