PDA

View Full Version : getting a 1.75 rod/stroke ratio



**DONOTDELETE**
02-11-2003, 02:56 AM
how much of a bore would give the KA24DE a 1.75 R/S ratio...it already has 1.72...can anyone help

Irie_eyes
02-11-2003, 03:09 AM
You have to decrease the stroke and increase the rod length.
Since you have a set deck height, decreasing stroke length mean increasing rod length to conserve original piston to deck distance. Shortening the stroke means a decrease in displacement, so increasing bore size can increase displacement.
Altering the bore will not alter rod/stroke ratio.
You can alter rod/stroke by increasing rod length by getting new pistons that have a wrist pin higher up the piston, or decreasing the diameter of the rod journals on the crank, then getting custom rod made.
The other way is getting a new crank and accepting a loss in displacement.
Altering rod/stroke isn't easy. It's mostly by inheirent design.

iHATEstupidity
02-11-2003, 05:31 AM
do you know why you want a 1.75 r:s ratio?

THINK

fooddude
02-11-2003, 06:27 AM
you can have a custom deck spacer and taller sleeves made to go with longer rods. I dont think anyone has done it on a KA though.

iHATEstupidity
02-11-2003, 06:50 AM
that's because you don't need it with a KA24... and it also doesn't use sleeves because it's already an iron block.
having an ideal r:s ratio for high-revability, and actually having a motor that can rev high, are two totally separate things. just having a good ratio doesn't necessarily make for a rev-happy motor. that's where people like this dude are getting confused...(esp. evident since he didn't realize that a change in bore will not affect the r:s at all)

THINK

d240t2
02-11-2003, 03:18 PM
You don't have to get custom anything to get a higher rod/stroke ratio. Carillo/Nissan Motorsports sells rods that are longer than stock, and JWT stocks pistons for those rods for turbo KA applications.

Dennis

Soccerbum21
02-11-2003, 04:36 PM
Have you looked at a KA block? They look very siamesed and sleeved from the factory. If i had pics of my blown motor i would post them but since i dont i will describe. There is a noticeable difference in the metals used around the cylinder compared to the rest of the block. It looks like a sleeve to me. Someone please correct me if i'm wrong on this one.

Brad

'97 S14 SE Turbo
02-11-2003, 05:05 PM
Key word: "Looks"... It's not sleeved. You are seeing the pattern left by the metal/graphite headgasket.

**DONOTDELETE**
02-12-2003, 04:12 AM
thanks Dennis, you are the only one who responded to my inquiry with any kind of courtesy.

Yes, I want my motor to be just a little happier revving to go along with the other future modifications...is that SO WRONG!?!?!?! I just happen to know that a 1.75 rod\stroke ratio is pretty favorable when it comes to building a motor for performance and the KA24DE is not that far off...

Its like some people get on these boards just to harass others when they might not know every LITTLE detail about different mods to motors.

I mean, I'm NO noob to tuning at all, I just know what is tried and true and I want that with MY setup built with MY money and time.

Again, thanks Dennis and anyone else who was not hating.

BTW, do you have AIM, I need to chat with you about your KA-T setup! http://forums.freshalloy.com/images/graemlins/grin.gif

rextreme
02-12-2003, 06:16 AM
Have you looked at a KA block? They look very siamesed and sleeved from the factory.



I don't think so.

http://www.racingextreme.com/turbo240/phase3/3.jpg

asad
02-12-2003, 10:20 AM
Yes, I want my motor to be just a little happier revving to go along with the other future modifications...is that SO WRONG!?!?!?! I just happen to know that a 1.75 rod\stroke ratio is pretty favorable when it comes to building a motor for performance and the KA24DE is not that far off...



The problem is that you don't seem to realize the importance (or lack thereof) of rod ratio.

In getting a motor to be "happy" at higher RPM, a few factors come into play: Bore/stroke, cylinder head design, intake manifold design, exhaust manifold/header design, cam grind, and rod ratio, to name a few. Of these, rod ratio is probably the LEAST important. Witness motors like the SR20 (1.58 rod ratio), B18C5 (also 1.58), which are happy at high revs despite the relatively "poor" rod/stroke ratio.

Asad

**DONOTDELETE**
02-12-2003, 03:05 PM
oh [censored]...that's all the stuff i didnot consider. When I first asked the question, I was asking about bore, but most people were telling me that it had NOTHING to do with bore. I knew I was on to something.

I don't necesarily want higher revs cus I know very well that in my car, revs is not what this motor is all about, I just want the motor not to be working as hard when at a higher RPM, do you understand what I am trying to say...I cant really articulate the exact thoughts in my mind, but if you (Asad&Dennis)keep talking and explaining, I think it will become more clear...

it is very true that I don't fully understand how to accomplish what i'm doing, that is why i'm asking. I am off to tech school in about a month and word is, we will be tearing down and rebuilding a KA24DE. I'm gonna talke this opportunity to refreshen MY motor for turbo duties, so I was hoping that there was a semi-easy way to achieve this seemingly elusive 1.75 since the KA is so close as is...

thanks,
Mike

'97 S14 SE Turbo
02-12-2003, 03:42 PM
Stock rod is 165mm in length or ~6.5". The Nissan Motorsport/carillo rods are 6.7" in length. Do the math...

...having going down this path, don't worry about the rod to stroke ratio. Worry more about the cams, head, valves, valvesprings, etc... That would effect more on high rpm happiness of the KA (under the 7,200rpm redline).

If you plan to do more than that, get a custom full counterweight crank. The KA crank is half counterweighted, leading to interesting dynamics at a certain rpm range between 7,200 and 7,600 (IIRC) rpm. That's the death roll of the KA, causing it to fall apart from the inside out. Never dwell in that rpm range for the KA. it's okay passing through it to 8,000rpm if you want to, but don't stay there.

asad
02-12-2003, 03:47 PM
When I first asked the question, I was asking about bore, but most people were telling me that it had NOTHING to do with bore. I knew I was on to something.



Well, the problem is that you asked the wrong question. Like everyone already said, cylinder bore doesn't have anything to do with rod/stroke ratio -- rod/stroke ratio is just that -- the connecting rod length divided by the crankshaft stroke, and that's it.

Since there are only 2 numbers that go into calculating the rod/stroke ratio, the only thing that can change it is by changing one or both of the 2 numbers. Longer rod and/or shorter stroke all give a better rod ratio. Neither can be achieved "easily", requiring different (read $$$) parts to use a longer rod, or welding/offset grinding the crankshaft to get a shorter stroke.




I don't necesarily want higher revs cus I know very well that in my car, revs is not what this motor is all about, I just want the motor not to be working as hard when at a higher RPM




I suggest you forget about rod ratio, as it's already "pretty good" as it is, and concentrate on the other, easier aspects which will likely have a larger effect. Things like more aggressive cams, a header (if you're non-turbo), cylinder head porting, short-runner intake manifold, etc, will all serve to make the engine happier at high RPM, and it won't feel as "strained".

Asad

**DONOTDELETE**
02-14-2003, 03:40 AM
<-------=SKOOL'D http://forums.freshalloy.com/images/graemlins/blush.gif

that's the kinda answer I was looking for...it's NOT easy (read:inexpensive) to change rod/stroke ratio...so my time in building this engine will be spent on "the basics".

thanks again,
Mike

Irie_eyes
02-14-2003, 11:36 PM
Higher rod/stroke ratio have less piston travel per crank angle degree between certain crank angles while lower rod/stroke ratio have greater piston travel per crank angle degree between those same angles. Therefore high rod/stroke has lower piston speeds compared to low rod/stroke. Piston travel per crank degree travels in a sine wave.
Mechanical rpm limit is dictated by these speeds. The piston can only take so much ft/sec of reciprocation before they start failing. Rods can only take so much of reciprocation and piston inertia before they stretch and yield, and the crank can only handle so much of the piston/rod inertia and friction created by increased piston speed. Increasing rod/stroke ratio can lower piston speed which can lead to increases in rpm while having safe piston speeds.

Low rod/stroke ratios have a broader variance in airflow demand. As rpm rises and piston speeds increase, vacuum increases, which increases intake velocity, which means at lower rpms these motors tend to build more torque. As rpm increases, vacuum is much greater, and soon airflow becomes restricted and a loss in power. Since mechanical rev limit is limited, intake manifold, cam and intake ports are tuned to work within that shorter range, which complements low, mid, and peak rpm.

High rod/stroke ratios have a lower variance in airflow demand. As rpm rises and piston speed increase, vacuum increase. As rpm continues to rise, piston speed change is not that great, meaning the vacuum/intake velocity rises steadily instead of rapidly, so there is sufficient airflow throughout the rpm range. But since mechanical rev limit is higher, the intake manifold, cam and intake ports are tuned for increased airflow at higher rpms. Unfortunately, larger intake manifold and ports do not compliment velocity at low rpms, combined with the fact that it does not have high piston speed, low rpm torque is lost. Then there is also the valvetrain issue...

Another issue is combution rate and pressure. As piston speeds increase, the pressure acting on the piston diminishes since the piston is moving faster away from it. Higher rod/stroke ratios can broaden the powerband, but combustion rate and pressure is dependant on volumetric efficiency, whatever rpm the intake, cam and intake ports is tuned for, combustion chamber design, and fuel. High rod/stroke ratio can have torque drop less, while low rod/stroke ratio torque drops off faster.

Typically, high rod/stroke ratio engines do not like turbos or nitrous. During nitrous activation or between shifts, the shock of increased BMEPs at low piston speeds can damage the engine. You may make more power, but internals go through more stress. Low rod/stroke ratio engines are happy with nitrous or turbos because higher piston speeds won't see a spike in BMEP.

Around 1.7 is a good number. Increasing it a bit won't hurt if you are going to change the rods and pistons which are available and do not require costly modifications.
With rod/stroke ratio, it's all in the design and pretty much everything else compliments it. The only reasons where I would mess with rod/stroke ratio is if I was competing where the rules say only n/a or n/a of a limited displacement, or if there was a block with a large bore tHat would fit my car and destroke it and have a valvetrain that can handle high rpms.