Rod ratio explanation

[unichi]

I understand that it has to do with piston speed and the engine's tolerance to high rpm, but what is the actual ratio that we're talking about here is it rod-stroke or stroke-rod. Additionally, are large or small numbers better. Thanks.

[unichi]

Guess this isn't very interesting to people. Sorry, but I do have another question. Is there a way to theoretically calculate redline from a rod ratio or are there too many different variables?

[PalmerWMD]

Uni: Dont feel bad you dont have a reply yet.this isnt one of our most active sub-froums but we make up quantity of posts with qulaity..

Fred..

[gobstopper]

I'm certainly no expert, but here goes.

'Rod ratio' is acutally the ratio of rod length to stroke length. For example, and from memory, the QR25DE has a 165mm connecting rod and a 100mm stroke, for a rod ratio of 1.65:1 (165:100, reduced down to lowest common denominator).

Rod ratio tends to indicate an engine's willingness to rev, and it's general character. Low rod ratio engines (probably 1.65:1 and below) tend to be torquier, since the piston spends more time at/near both TDC and BDC. The lower rev ceiling is due to side-loading of the piston in the bore, which enhances wear of the cylinder and stresses the conrod, so it's kept lower on purpose. A higher rod ratio engine (maybe 1.75:1 and higher) doesn't make gobs of torque, but is willing to spin fast because the friction of side-loading the piston is much less. Honda engines like the B16 tend to be about 1.74:1 (pulling from memory), and motorcycle and F1 engines tend to be over 2:1. The 2.4 liter V8s that will be used in the future in F1 will likely be over 2.5:1, as they'll be spun to about 21000rpm : to make the ~950 hp the current V10s make at 18500rpm.

There isn't really a 'better' direction for rod ratio, but for a hot but streetable engine, something in the 1.7 neighborhood might give the best of both worlds regarding torque and willingness to spin.

[unichi]

Thanks! So, I assume that you're talking about the center distance on the connecting rod and not the actual length. In that case the length of the rod in a KA engine is about 165 mm and the stroke is 96 mm giving a rod ratio of 1.72. I guess that's pretty good. Does anyone know why the KA24DE doesn't have a higher redline then?
Modified by unichi at 3:58 PM 6/21/2005

[Nismo_Freak]

Thanks! So, I assume that you're talking about the center distance on the connecting rod and not the actual length. In that case the length of the rod in a KA engine is about 165 mm and the stroke is 96 mm giving a rod ratio of 1.72. I guess that's pretty good. Does anyone know why the KA24DE doesn't have a higher redline then?

An excellent rod ratio must be paired with an excellent engine layout.

In the KA the higher ratio eliminates some of the sheer loading on the bearings but (compared to say the SR) the engine is a low RPM engine. This extends from the heavier piston, heavier crank, longer stroke (read piston speed), and heavier rod. Physics is what is limiting the KA, however given the length and size of the components in the KA, excellent gains in response can be gained by moving to aftermarket forged components.

Unlike the SR the KA has a head design that allows it to safely rev out it's mechanical bottom end limit. Whereas on the SR, the rocker arms limit the effective reliable RPM range.

[MacBg007]

there is an equation for figuring out the rpm the motor will make peak hp at but not for max rpm, that depends on the balancing of the rotating assembly and the quality of the crank. I cant remember it now but it will get back on this, it has to do with piston speed (4000 fpm is a hot street motor) as well as stroke and a constant (3456 i believe). Ill get back with that shortly...

[unichi]

In the KA the higher ratio eliminates some of the sheer loading on the bearings but (compared to say the SR) the engine is a low RPM engine. This extends from the heavier piston, heavier crank, longer stroke (read piston speed), and heavier rod. Physics is what is limiting the KA, however given the length and size of the components in the KA, excellent gains in response can be gained by moving to aftermarket forged components.

Unlike the SR the KA has a head design that allows it to safely rev out it's mechanical bottom end limit. Whereas on the SR, the rocker arms limit the effective reliable RPM range.

Interesting, so really, it is only the strength of the bottom end that is limiting the rpm. If you rebuild with lighter components, and do everything to lower reciprocating mass the head can handle it. Does anyone sell forged cranks for the KA? is the KA becoming more of a popular tuner engine?

[Nismo_Freak]

Interesting, so really, it is only the strength of the bottom end that is limiting the rpm. If you rebuild with lighter components, and do everything to lower reciprocating mass the head can handle it. Does anyone sell forged cranks for the KA? is the KA becoming more of a popular tuner engine?

The stock crank is forged, it however has 4 counterweights to the SR's 8. The SR was built with balance in mind (mainly due to quiet engine operation), which explains the reason behind difference.

The crank isn't a big problem as it's relative acceleration is linear. The piston and rod however have to endure extremely high acceleration and deacceleration forces. Thats where the most stress is placed, where rods snap, bearings flatten, rod bolts fail, wrist pins escape, etc.

The KA has a 96mm stroke which makes peak piston acceleration alot higher than a 86mm stroke engine. This means more emphasis should be placed on piston gram weights, and secondly on rod weights. Piston rings, clips, and pins are all a factor of piston weight.

Naturally you want to run a tight overbore (0.020") to avoid increasing weight, and opt for a forged piston with lower compression (less mass). If you are building a low powered engine you can have the piston skimmed via a custom build. Typically they can shave a few grams off of each.

One day I will build my race engine and it will look like this:

SR20VET

Nissan SR16VE Head (SR20VE Cams - Tomei Cam Gears)Tomei 1.0mm MLS Headgasket86.5mm Arias Custom Lightened Pistons (9:1)Custom Titanium Wrist Pins (H22 fitment milled to smaller bore)Crower Forged Titanium Rods w/ Narrowed Crank Journal (Honda B18C Spec Diam.)ARP2000 Rod BoltsReworked Crank - Lightened / Balanced / Knife-EdgedDry-Sumped w/ Cog BeltElectric Water PumpBoost Logic SS321 Ex. ManifoldCustom Built GTiR Int. Manifold w/ PlenumGT3076 0.86 A/R Hot - 40mm Tial W/G3.5" Downpipe (dumped)

Nikasil cylinder coatings on steel sleevesCarbon coated on Titanium wear surfaces

With the SR20VE cams I should have a solid 3500 - 8500 powerband, however if I kept the SR16VE cams I would see good VE until 9200.

The engine setup should be capable of 9500 RPM

[S13Coupe]

Does anyone happen to know what the stock pistons weigh in the KA? I want to compare to aftermarket forgings.

[s13sr20chris]

alan,any particular reason for the honda sourced components?