bridfi

ive been reading around, and different people have different opinions..

if i increase the length of the connecting rod, will i be able to rev higher? (as opposed to a smaller rod)

do increased rod lengths also increase the compression ratio?

(for those of you that are curious, im trying to build a high compression [12:1 or higher?], fast revving engine [8000 is about my goal] that can run on 91 octane pump gas, very reliably, and put out about 250 hp at the crank)

thanks in advance, guys

hatrickstu

iTrader: (1)

by increasing the rod length you are typically increasing the "stroke," this however usually means you will not rev as high. you can not just increase the rod length whenever you want to though. typically high revving motors have a super short stroke, but dont make as much torque as a low revving long stroke engine.

Witt

Actually stroke is determined through the crank, not connecting rod length.

Its a fine balance between piston speed and connecting rod leverage.

Supercharged06

if you want higher revs you are gonna wanna shorten the connecting rod (stroke) you will be able to rev higher but your tourqe will suffer. thats why hondas rev to like 9-10k and make good hp #s but there TQ is non existant

hatrickstu

iTrader: (1)

k, im not an expert (or even close too in this department) and still tryign to learn myself, but wouldnt keeping the same crank and just adding a longer rod increase the overall stroke?

bridfi

well yeah.. i was referring to keeping the factory stroke at 94.6 mm, and get rods that are a little longer than factory.

or would i have to get a crank that has a larger stroke than factory?

USMCFieldMP

iTrader: (1)

No... that would cause your Piston to punch your Cylinder head...

hatrickstu

iTrader: (1)

i think that would actually hurt you as far as revving higher bro. not to mention you cant just add a longer rod without doing some other things

Witt

Again, its not rod length.
It would seem that way but changing the rod length actually just moves the piston higher up in the bore. It also reduces the height at bottom dead center by an equal amount. It has the same effect as changing piston pin height. iirc con rod length is a function of deck height and piston pin height.

I'm no expert either but some members here can probably explain it in a little better detail than me as a few design engines for a living.

bridfi

so what about a smaller stroke/larger rod length? or vice versa? whats better for an n/a build?

USMCFieldMP

iTrader: (1)

Have to get crank. Not really worth in unless you are building the motor and putting in new Pistons, Rods, and Crank and planning on making a lot of power.

For an N/A build, I would suggest a Stroker Kit (I don't know of anyone off hand that makes them for us), High Compression Pistons, and NAWZ.

bridfi

i didnt plan on changing out the crank, unless it helps out alot.

im pretty sure i can get to 250 chp without changing out the crank, right?

LilHaile

Even with longer rods your stroke is going to be the same... the piston will still move the same distance up and down... to change the stroke you will need a new crank. You would also have to make sure when you get new rods that they won't push your piston up to far and cause some serious issues.

bridfi

so keep the same rod length, get a crank with higher stroke and taller pistons?

Witt

Yep. If you stroke the engine you have to compensate with shorter connecting rods or a higher piston pin height. Opposite if you destroke it.

Example, the LSJ has longer rods than the L61, the L61 which is stroked further via the crank.

RooTBeeR

iTrader: (4)

like head/valve damage

hatrickstu

iTrader: (1)

ok i have a question then. speaking in radical terms, if you were to use a CR that is say 1.5 inches shorter than the previous, you are not decreasing the stroke?

bridfi

right, im not talking about chaning the distance.

i want to rev high. im asking whats easier on the engine, so i can rev higher. im not talking about changing the rod length to change the piston's traveling distance. but i would think that the taller rods would increase C/R slightly?

no.

stroke = how much the crank changes the rod's position, from TDC to BDC

right?

RooTBeeR

iTrader: (4)

the only way to change your stroke is to change your crank.

Witt

All that will do is move the piston travel area further down the bore and significantly reduce static compression ratio.

RooTBeeR

iTrader: (4)

if you want to rev higher, then you will need a smaller stroke crank

bridfi

is the LSJ's setup better for F/I?

i would think so, thats why they changed it. so i can increase my gains with an N/A build by taking the L61's setup a step further, by shortening the rods and increasing the stroke?

USMCFieldMP

iTrader: (1)

No... you will be decreasing COMPRESSION. With the shorter rod... the piston will still move the same distance (hence, the same amount of STROKE)... but it just won't be doing it in the same spot.



If you want Higher Revs though... you don't Stroke... you DESTROKE. And you could do that by installing the LSJ Crank & Rods... but then you need a new Flywheel, etc... it gets complicated.

bridfi

so, in contrast, i will be able to increase C/R by going with a taller rod?

hatrickstu

iTrader: (1)

so=

increase rod length, typically increase compression, but has no effect on stroke. also, does this not allow you turn as many revs right?

stroke is purely determined through the crank?