Will destroking an engine change anything with the cams?

[toki]

If someone were to destroke a KA24 to 1.8L or something, would the cam need to be changed at all? And would after market cams still work? I dont know much of anything about timing chains (excuse me if this isn't what controls cam timing), but I have a feeling this somehow connects to the crank, which would be different now due to the stoke being significantly reduced?

[cosmo]

not to thread jack, but no one has ever explained the destroking process to me. Now we can kill 2 birds......

[Eswift]

used in order to maintain stock CR and reduce risk of detonation.

problem: weakens pistons, which is especially not good if you are searching for power in a force-inducted application.

http://www.hastingsmfg.com/Ser...s.htm

you see this being done mostly for domestics.

I dont think you could easily change a 2.4 L into a 1.8, not enough piston material to remove.

sure, you could use different rods, but then your balancing will be WAY off.

cams would be the least of your worries, as stock ones would work fine, (although their profiles wouldnt be optimal, they would still be operating in synch with the rest of the engine)

[Eswift]

the question is, why do you want to rev higher? at some point you will have to watch out for valve float too.

[Stoneage_Turbo]

i thought changeing the pistons dome heght only reduced or raised compression ratio?

destrokeing i thought was always changeing the crakshaft to a shorter throw which would involve locateing a short stroke crank,pistons and rods to work with that ?

where do you plan on getting a new crank made to take it down to 1.8 litres?

id imagine the cam would need to be done up also as to make good power at the revband were looking at , your cam was onyl designed to go to the stock redline and make emissions freindly hp while doing it , destrokeing it would drop alot of rotating weight in the engine allowing it to rev higher and faster , F1 cars typicaly have a very short stroke,lightweight internals and a penumatic valvetrain to allow the engine to see 21000 rpms , i also think thats why honda runs a variable valves system so they can make decnt hp at the wicked rpms they redline at stock ,

i cant recall the cam specs on a 327 small block vs the 1968-69 camaro 302 but that would give you a idea , GM destroked the 327 by placeing a 283 crank instaed of useing the 327s crank (makeing it 302 cubic inches /scca mandated 5.0 litres) , turned close to 9k rpms did it relibly and in the trans am series made close to 600 hp reliably , if i rember right teams went thru on avg 2 engines a year

itd be easier to get lighter internals in the engine and keep the same bore and stroke , if detonation from wicked boost is the concern then maybe a thicker headgasket and dished pistons would be a awnser (unless the KA already has dished pistons )if it isnt a turbo and it can be de stroked decking the head/taller pistons on it would bring a intersting engine to the track

[Nismo_Freak]

i thought changeing the pistons dome heght only reduced or raised compression ratio?

Dome height/design also plays a big role in the combustion and a/f mixing cycles. This is another wrench in the cogs so to speak.

Bear in mind that the V8 will need much less stroke reduction per cylinder, which makes it quite a bit easier to do.

[s13sr20chris]

get a crank from a jdm ka 20. dont want to rev too high on stock ka valvetrain as the valves are big and heavy. they breath ok but are gonna float something awfull.

[PROJECTRB240SX]

TO DESTROKE YOU EITHER CHANGE JUST THE RODS AND RE BALANCE THE SYSTEM OR YOU GET A CRANK AND ROD SET THAT IS A SHORTER TRAVEL PATH. CHANGING PISTON DOMES CHANGES COMPRESSION, AS DOES ADDING BELOW DECK CLEARENCES OR THICKER GASKETS, AND MILLING THE HEAD DOME. ANYTHING THAT INCREASES THE DISTANCE FROM THE TOP OF THE PISTON TO THE TOP OF THE CYLINDER HEAD LOWERS COMPRESSION. VICE VERSA FOR THE OPPOSITE. THE EASIEST WAY TO REDUCE DISPLACEMENT IS TO SLEEVE THE BLOCK AND USE SMALLER PISTONS.

DISPLACEMENT = (PI) (BORE / 2) (BORE / 2) (STROKE) (# OF CYL)COMPRESSION = (DISPLACMENT + HEAD CC) / HEAD CC

[s13sr20chris]

when you think about it, just shortening the rods is not an option. i am not 100% sure but i dont think its a good thing to have the pistons not come all the way up to the deck.

[PROJECTRB240SX]

some engines are designed to not reach deck height. and shortening the rods by about 1mm will not be bad if it is spec'd and balanced properly.

[Eswift]

sure, its possible to shorten the stroke, but you are already eating away at your obtainable torque.

[PROJECTRB240SX]

but increasing revability.

[toki]

revability, is that a technical term?

[PROJECTRB240SX]

OK, IT WILL INCREASE MAXIMUM REVOLUTIONS OF THE ENGINE. HAPPY?

[s13sr20chris]

i like high revving motors and all, but the race teams all stretch their engines to the maximum allowable displacement. it really just depends on what you are looking for.

[IvoryJ30t]

even if you just shorten the rod length, your doing nothing to change the displacement because the crank is still causing the shortened length rods to travel the same distance. all you would do is lower the compression ratio. you need to modify the crank to stroke shorter, which involves offset cutting the rod journals and getting rods with a smaller big end [not good idea] or a custom crank, or a crank from a smaller displacement similar engine [already said]

[s13sr20chris]

nope, shortening the rod will lower the displacement. of course it will lower the compression ratio as well.

[IvoryJ30t]

how do you figure shortening the rod will lower displacement, the crankshaft is still moving the piston/rod assembly the same distance, your just moving the center of motion farther down the cylinder bore

[IvoryJ30t]

oh, yeah, and hitting the piston skirt on the cranks counterweight in the process.

[s13sr20chris]

yeah, at tdc. at bdc you are moving it farther up the bore. tdc does not matter too much for disp. bdc volume is what you want to know for the most part.

[Eswift]

I dont see how changing the rod length would alter the displacement.

displacement is swept volume.

if you consider the bore to be a constant, that elimates two variables from the equation, reducing it to swept length.

swept length= max rod distance - min rod distance (from crank)

max rod distance = crank throw + rod length

min rod distance = rod length - crank throw

thus, your swept length is 2 times the crank throw, and not a fuction of rod length at all.

[IvoryJ30t]

i understand that, if you just shorten the rod, at tdc the piston will sit farther away from the deck, and at bdc, the piston will be an identical distance from where it was at the bottom before you shortened the rod.

if you take 4 mm off of the rod, at tdc the piston will be 4mm deeper in the bore, and at bdc the piston will be an additional 4mm from where it was at bdc with the original rod. your not changing the stroke. i personally built a 4g63/4g64 dsm hybrid motor, so dont tell me im wrong, because if all that was needed was a set of rods, every dsm stroker motor wouldnt need a new crankshaft from a 4g64:rolleyes

[s13sr20chris]

i have to admit that i have not looked it up, but i thought combustion chamber volume and head gasket thickness had to be added to swept volume. if im mistaken then i withdraw.

[IvoryJ30t]

that method is just increasing the static volume in the head, and not changing displacement, but just lowering compression

[IvoryJ30t]

im not getting mad or anything, im just telling you that you can not increase or decrease displacement by playing with the rod length, just so you were not misinformed

[s13sr20chris]

ok, what is the formula for disp then? i really should write down all this stuff eg. formulas and other stuff i get into arguments about.

[s13sr20chris]

i could have sworn it was (vol at bdc)*(number of cyls)

[IvoryJ30t]

no, your correct, its just that if you shorten the rod length, the piston sits lower in the bore, increasing the percieved displacement, but when that comes back up to the top, the additional amount of travel you have at the bottom is lost when the piston doesnt come all the way back. ie if the piston gains, say 5 mm on the down stroke, it will lose that 5mm on the upstrokethink of it this way, if you hold a 1 foot stick in you hand and move it up and down 1 foot, the distance traveled by the stick is one foot. if you hold a two foot stick in you hand and move it up and down 1 foot, the tip of the stick is still travelling one foot.

[IvoryJ30t]

the stroke distance is governed by the crankshaft, not the rod.

[IvoryJ30t]

ok, simplest terms, lets say at tdc, said engine has a piston depth of 4mm from the deck with the stock rods. lets say there is a 85 mm stroke. that said, at bdc the piston will sit 89mm from the deck.

ok, lets shorten the rod by 5mm.

at tdc, the piston will have a depth of 9mm from the deck of the block. at bdc the piston will be 94mm from the deck of the block.

94-9=85mm89-4=85mmstroke stays constant due to the fact that the crankshaft is moving the piston via the rod. all you would be accomplishing is increasing the rod angle which is detrimental to engine life due to the lateral forces it exerts on the piston [ovaling of the bore].the smaller the angle of the rod throughout the range of motion of the crankshaft in relation to vertical the better. [aka longer the rod, the lesser the angles incured by, the better off it is]