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R/S ratio
hey im currently building my motor and am exploring the options for d series. i have a question concerning rod/stroke ratio. is it better to have this ratio as close to 1:1 or is it hte farther away from one another the better?
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Re: R/S ratio
this will start a heated debate for sure, rod stroke ratio, is the relationshp between the connecting rod length and crankshaft stroke. 1.75 is seen as being optimal for small displacement engines. rod/stroke ratio also is effected by deck height. the longer the stroke and the shorter the rod, the lower the ratio will be, and vice versa, the shorter the stroke and the longer the rod, the rod/stroke ratio increases. with a short stroke engine (long rod) the piston will remain at TDC longer and at BDC for a shorter period of time. with a long stroke (short rod) engine the opposite is true, the piston will remain at TDC for a shorter period of time and at BDC for a longer period of time, this is due to the time it takes for each rod to swing through it's arc as the crankshaft spins. longer stroke engines have higher piston speeds as compared to a shorter stroke engine in which the piston speed begins to drop. the D15B maintains a 1.63:1 rod/stroke ratio through the use of a longer than normal rod than any of the 1.5's or 1.6's. the d16z6/y8/y7/a6 have a rod/stroke ratio of 1.52:1, but the D15's maintain a 1.59:1 ratio. i prefer engines with a ratio coming closer to 1.75:1, but the difference in wear between an engine with 1.52 and 1.59 is nill, work with what your budget will allow.
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Re: R/S ratio
can you give an example of a 1.75:1 r/s ratio?...just curious
but anyways, longer r/s ratio for more power. |
Re: R/S ratio
good example is a b16 at 1.73:1 just take your rod length and divide it by your stroke (make sure in same units)
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Re: R/S ratio
This topic started a pretty heated debate between myself and JD on here not too long ago. Although I'm still pretty partial to the 1.75:1 ratio, I have to say I've been leaning more towards JD's take on things, which pretty much boils down to Honda rotating assemblies are so light in design, that the normally accepted 1.75:1 ratio is pretty much meaningless. In the real world it prety much comes down to lower r/s ratio will give you more drivable/streetable torque. And seeing that horsepower is a measurement of torque multiplied by rpm, it pretty much explains why the b16 motor can make such a high horsepower per liter (in n/a form stock) but have a torque number that's lower than your run of the mill d16. You've got 2 choices...either go n/a and end up having to rev to the moon in order to make hp, or turbo it and not give another thought to r/s ratio ;D
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Re: R/S ratio
r/s ratio really only matters for NA cars and not so much for FI.
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Re: R/S ratio
Originally Posted by d112crzy
r/s ratio really only matters for NA cars and not so much for FI.
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Re: R/S ratio
Originally Posted by xstreetfiendx
.............with a short stroke engine (long rod) the piston will remain at TDC longer and at BDC for a shorter period of time. with a long stroke (short rod) engine the opposite is true, the piston will remain at TDC for a shorter period of time and at BDC for a longer period of time.................
TDC dwell is going to be the same as BDC.
Originally Posted by d112crzy
r/s ratio really only matters for NA cars and not so much for FI.
Not even close. Regardless of the setup, you are going to want to maximize the power for your particular operating rpm. That means setting up R/S, turbo, cams, pistons, intake manifold, exhaust, etc, etc. Having a low R/S ratio combined with a huge turbo is not ideal. The turbo's operating range will not match the engine. Matching is the key. Having all your components complement each other is the way to go. Now, this being HMT and trying to make big power cheap we are not going to be bothered with the R/S as changing it is expensive. Getting more torque at higher rpm's can be done with cams and porting. R/S won't do ---- w/o these anyway. |
Re: R/S ratio
Originally Posted by MADMAX
TDC dwell is going to be the same as BDC. Not even close. Regardless of the setup, you are going to want to maximize the power for your particular operating rpm. That means setting up R/S, turbo, cams, pistons, intake manifold, exhaust, etc, etc. Having a low R/S ratio combined with a huge turbo is not ideal. The turbo's operating range will not match the engine. Matching is the key. Having all your components complement each other is the way to go. Now, this being HMT and trying to make big power cheap we are not going to be bothered with the R/S as changing it is expensive. Getting more torque at higher rpm's can be done with cams and porting. R/S won't do ---- w/o these anyway. |
Re: R/S ratio
Originally Posted by slo_crx1
Bottom statement should be more along the lines of "getting more torque at higher rpm's can be done by adjusting cam gears and porting." We're mostly broke-asses here at HMT...last time I bought an aftermarket cam for was my old A20 motor...cost me $159 from Web Cams years ago. :P
"Cams" includes adjusting them with cam gears. Also, cam gears are cheap as ---- these days. |
Re: R/S ratio
R/S ratio is a rich man's N/A game, for turbo cars its a lie.
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Re: R/S ratio
R/S is a unitless number.... and you dont need to put "__:1" you can leave it out when you are talking
BTW a b16 is like 1.74 B18b is like 1.52 for comparisons sake... in general hondas are pretty high.... VW's are normally pretty low..... explaining why VW's always beat us torque wise and we rape them power wise |
Re: R/S ratio
im sittin at 93 bore 89 stroke... want to take it to 94 and 94...
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Re: R/S ratio
94-94, a perfectly square r/s ratio, giving you more linear hp/tq curves?
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Re: R/S ratio
B16B FTW.... 77.4mm stroke 142.3mm rod... 1.84 RS :-*
Rod Stroke Ratio summed up real quick: higher the number the less wear and tear on the motor, also has less torque, but more revablity. the higher the number the more wear on the motor, more torque, peak power at lower RPMs... thats not saying that a low RS motor will not last a long time, the B18A/B motors have a "shitty" RS of 1.54 and still will last a couple hundred thousand miles... and in our game your gonna ---- your motor up before the average life expectancy of the motor. so it doesnt matter... so in closing: you want nice low end power run a low RS, you want all your power at topend and dont mind reving the ---- out of it you need to go with a higher RS. either way if your reading this you will need to rebuild that ---- in a couple years anyway... |
Re: R/S ratio
Stock torque numbers can be fixed with a turbo ;D
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Re: R/S ratio
Originally Posted by wafflesincars
Stock torque numbers can be fixed with a turbo ;D
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Re: R/S ratio
i want more torque than hp numerically, but modeled after stock powerbands.
i can give like a .9 rs ratio if i really want, there is a 97mm stroker crank out there thatll take me to around 2.7l outside of boring to a max of 95.5 which i think is 2850cc? ------ huge for a 4cyl |
Re: R/S ratio
madmax: TDC dwell is going to be the same as BDC.
i'm sorry madmax your wrong on that, with a short rod engine, the dwell time at BDC is longer than at TDC, it's easy to figure, picture the crank swinging at BTDC and ATDC, the crank only has to swing a few degrees to promote a change in rod angle, the opposite is true of a long rod engine, at BDC, the rod is able to swing further for any givin degree at BBDC and ABDC, allowing for a lower dwell time. this relates to piston speeds, long rod engines have higher piston speeds than short rod engines. this kind of debate is all in theory, but it's really pointless, honda engines have superlight rotating assemblies and ya this crap really has no effect, maybe when your talkin an 850+ cube diesel engine for highway use, then for sure it has an effect when you have a piston and rod assembly that weighs over 25 pounds and your max engine speed is 2100, lol. |
Re: R/S ratio
But a perfectly square r/s ratio gives you proportional hp/tq curves?
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Re: R/S ratio
Waffles: There's no such thing as a square r/s ratio. A rod ratio of 1 is, in practice, impossible, since the piston would hit the crankshaft. A "square motor" is one with the same stroke as it has bore, and the only way it makes more torque is with the added displacement if you're moving up in stroke. Hell, if you were to make a square D-series you'd end up with like a 1.3 liter motor.
And there's no disproportion in power curves compared to torque curves, since one is just a function of the other. EDIT: Crap, I was wrong, streetfiend was right. Upon further research, I have found: Another advantage of the increased rod ratio is the amount of dwell time of the piston at TDC. While technically, the pistons are at TDC and BDC for an infinately small amount of time on both engines, the effective amount of time they stay at TDC is increased on a long rod engine. For example, if you consider 5 degrees before and after the actual TDC on the crankshaft to be the effective TDC, then in a long rod engine the piston will move less during this 10 degree sweep than in a short rod engine. This is again because the long rods have a lower change in angle for any given change in angle of the crankshaft. At BDC however, the dwell time is actually decreased with longer rods, though this is not as big of a concern on forced-induction engines. So, there is a "happy medium" for the rod ratio here, which seems to be about 1.80. Increasing dwell time increases the amount of time that the valves can stay open, which increases the volumetric efficiency of the engine (the effectiveness of the engine to move air in and out of the cylinder). See? I can admit when I'm wrong. Still think my joke was funny though. |
Re: R/S ratio
There are reliefs cut into the crankshaft journals to allow for clearance in near square bore-stroke ratios, such as some boxer engines (bmw's flat four bike). But yea a r/s ratio of one is going to give you a more linear hp/tq curve, only in theory, seeing as volumetric efficiency is never 100% in a naturally aspirated motor, which is why most engines have a longer stroke than bore. But when you get into turbo...
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Re: R/S ratio
...Get a rope? How are you going to cut a relief big enough to allow the piston to occupy the same damn space as the journal. Nig nog.
Better yet, how do you cut reliefs into journals that doesn't eat the main/rod bearings? Jesus. |
Re: R/S ratio
Originally Posted by MADMAX
TDC dwell is going to be the same as BDC. |
Re: R/S ratio
Originally Posted by Walter
not..they aren't the same. Wrong Walter, think about it. Unless the crankshaft is not rotating in a circular fashion or the connecting rods magically change length throughout the crankshaft rotation, they are the same. |
Re: R/S ratio
They forge the crank like that that. The piston head is made shorter and the counterweights ------ further. This is a near square bore-stroke. You're right in that a perfectly square one would occupy the same space. But that's why someone thought up a "crankless engine" more of a connecting-rod-less engine, but the use of a bumpstick to displace the piston allows to extremely proportional bore-stroke ratios. Revetec produces it I believe, don't get me wrong- the design is a vibrating nightmare but it allows square bore-stroke ratios.
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Re: R/S ratio
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Re: R/S ratio
After years of building motors there is no perfect R/S. You can change things to make your curves better in some spots and worse in others.
Randy |
Re: R/S ratio
Originally Posted by iceracercrx
After years of building motors there is no perfect R/S. You can change things to make your curves better in some spots and worse in others.
Just build your engine/setup and be a man about cranking on the boost controller. |
Re: R/S ratio
Originally Posted by Joseph Davis
Ding ding ding.
Just build your engine/setup and be a man about cranking on the boost controller. Nah, there is a perfect R/S ratio for each engine. It's perfect when it complements everything else on the motor. But ----, for me, here on HMT it's all about the ------ BOOST. :6 |
Re: R/S ratio
Originally Posted by MADMAX
Wrong Walter, think about it. Unless the crankshaft is not rotating in a circular fashion or the connecting rods magically change length throughout the crankshaft rotation, they are the same. |
Re: R/S ratio
TDC dwell = BDC dwell
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Re: R/S ratio
stroke only decreases the time a piston will spend its time in x amount of length of cyl walls.
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Re: R/S ratio
it's the same to tune cams in both case?
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Re: R/S ratio
Originally Posted by Secondaries
Lmao.. I made the same joke, about JDM VTEC Conrods. But plenty of sources online agree that a lower rod/stroke ratio leads to more TDC dwell and less BDC dwell. I don't particularly agree, but there's too many places agreeing with Walter here for me to say otherwise.
Post some.
Originally Posted by Joseph Davis
TDC dwell = BDC dwell
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Re: R/S ratio
sources agree? or sources prove?
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Re: R/S ratio
Originally Posted by wafflesincars
R/S ratio is a rich man's N/A game, for turbo cars its a lie.
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Re: R/S ratio
One word for those that honestly want to know, but don't have all the cards: trigonometry.
Here, a second: triangles. A third and fourth: common sense. |
Re: R/S ratio
parroted? o rly? I'm sure I read that on D-series.org, but I'm probably wrong. And seeing as the amount of dwell being different between BDC and TDC, this is physically impossible for the engine, seeing as it would neccisitate a variable stroke crankshaft or con-rod. Jah?
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Re: R/S ratio
The difference between you and me is that sometimes I can remember everything I ever read, and everything I ever said, and I know where it's at so it takes me a minute fifteen to find it with HMT search engine broken. :P
https://www.homemadeturbo.com/forum/...9013#msg789013 In the morning I expect I won't remember how to tie my shoes. Again. |
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