[disturbed1]

just wondering...I thought that stroker engines meant that they have a new crankshaft, pistons, and rods to create a higher compression. Am i correct? higher compression means you have back out on the timing right?

I always thought its better to run lower comp. pistons if your gonna boost the engine. so why would you stroke out a subie engine. i am highly confused on this topic please answer.i want to learn more stuff. i was thinking of running 8.75 comp. in my 502 Nova so that i may run a supercharger. i was told the lower the comp. the more boost you can run without the chance of detenation.so if you raise the comp. on a STi block doesn't that mean you have to run less boost?i know top fuel dragsters run about 6.75 comp. so they can run high boost.

[slorob]

No stroker means extending the strock the piston travels up its displacement.

[MartinSTi05]

Your both wrong, kind of.
In it's purest for a stroker is simply extending the stroke of the crankshaft, either with a new longer throw crank, or through offset grinding of your existing crank.
Generally though new rods and/or pistons are supplied with a stroker kit. If you were to simply increase the stroke and keep the standard rod/piston package you would push the piston higher up the bore/pull it further back down the bore a distance equal to half the additional stroke. On most engines including Subaru this would not only raise compression beyond acceptable limits it would also crash the piston into the combustion chamber of the head.
So most people end up using a piston with a shorter compression height (distance between wrist pin centerline and piston top) to avoid crashing into the head, and keep compression in check. In some instances it is also necessary/ possible to shorten the rods in addition to fitting different pistons.
It is also common for the block to require clearancing due to the effective diameter increase of the crank.
Hope this helps...

[disturbed1]

so what is the purpose of stroking out a subie engine? what are the performance gains out of this. why would you stroke it out and then get a smaller piston. i am highly confused about this topic.i thought you should drop the comp. and run more boost.

[slorob]

very nice

[slorob]

displacement stroking give your more displacement like 2.5-2.7 L

[WRB Kid]

Originally Posted by disturbed1

so what is the purpose of stroking out a subie engine? what are the performance gains out of this. why would you stroke it out and then get a smaller piston. i am highly confused about this topic.i thought you should drop the comp. and run more boost.

To get more displacement. The bigger the combustion chamber means the greater amount molecules of air and fuel that can enter that chamber. More air and fuel, at the right air to fuel ratios, usually means more power.

[Turbineguy]

More torque.

[MartinSTi05]

Originally Posted by disturbed1

so what is the purpose of stroking out a subie engine? what are the performance gains out of this. why would you stroke it out and then get a smaller piston. i am highly confused about this topic.i thought you should drop the comp. and run more boost.

If you increase the stroke of ANY engine while keeping the bore constant you gain displacement. Since the piston is physically moving further up and down the bore every stroke you are displacing more volume.
Generally performance gains with a stroker amount to gains in low/mid range torque along with an increased horsepower potential at a given RPM barring any other restrictions.
When you say smaller piston we are not reffering to bore reduction which would lower displacement. We are reffering to a reduction in compression height. The bore remains the same, and it is pulled lower/pushed higher in the bore yielding more displacement. As I stated earlier, the reduction in compression height is solely to keep compression in check, and avoid piston/cylinder head interference. Compression height like it sounds does not alter displacement, only compression.
The primary disadvantages of stroking an engine revolve around increased piston speed (more component stress at any given RPM). And depending on rod selection an alteration in rod ratio, and possible encroachment on your oil rings (again, depending on the kit).

Originally Posted by WRB Kid

To get more displacement. The bigger the combustion chamber means the greater amount molecules of air and fuel that can enter that chamber. More air and fuel, at the right air to fuel ratios, usually means more power.

Wrong.
Stroking an engine gives you gains in displacement. The combustion chamber volume is not increased by stroking an engine, and plays little to no part in how much air the engine ingests; it is simply a factor in determining compression ratio. Displacement has to do with the swept volume of the piston. You can increase this by stroke or bore, and thats it.

[Turbineguy]

^^^Yeah that. Stroke and compression can be altered relatively independant of each other to a point.

[MartinSTi05]

Example: I have a 2.71L stroker engine. 102mm bore x 83mm stroke. The compression height of the pistons had to be reduced in order to avoid piston to cylinder head contact. However I can vary things such as piston dish/dome, head gasket and combustion chamber volume to reach my desired compression of ~8.5:1.
Ex 2: Take a stroked ej engine and a non stroked ej engine. Assume that they both have the same volume piston dish, combustion chamber volume, bore and both have their pistons sitting flush with the engine block deck at Top dead center (this is possible due to the reduced compression height pistons in the stroker engine). The stroker would have a higher compression, because it is squeezing more volume into the same space. BUT, as I stated earlier you can easily compensate through things like piston dish volume, head gasket thickness, combustion chamber volume etc.

[wrxcelration]

Not to hijack but just a question that may add to this thread. How far can you bore out the cyl walls to increase displacement before you must add stroke?

[MartinSTi05]

Originally Posted by wrxcelration

Not to hijack but just a question that may add to this thread. How far can you bore out the cyl walls to increase displacement before you must add stroke?

You can't go far at all on a standard EJ257 block before sleeving. And even on a sleeved motor the largest bore that is considered safe is ~102mm (stock is 99.5mm). The reasons mostly revolve around center to center distance of the bores in the block. The walls of the block get too thin for safety, and the distance between bores also gets thinned out to the point where a gasket and/or o-ring will have a hard time sealing.

[VanishingPoint]

Wow

Stroking an engine does increase displacement in effect creating a bigger engine.
Alot of gain is from a longer lever on the crank. Think of the crank
as a lever that turns the rear wheels a longer lever more leverage. This will change the rod angle that may cause a loss and clearance problems. I believe Cobb (not sure) was playing with longer rods to change the rod angles.

Pistons/rods need to be changed to compensate for the added stroke, so that the compression is not raised, and or clearance problems (cylinder) .

Other than a race engine I dont believe it would be worth the money to stoke a sti engine.
Alot of engineering goes into a stoker. A small block Chevy, or a Ford 4.6 there is enough of them being done to justify the cost VS the gain.

[MartinSTi05]

Originally Posted by VanishingPoint

Wow

Stroking an engine does increase displacement in effect creating a bigger engine.
Alot of gain is from a longer lever on the crank. Think of the crank
as a lever that turns the rear wheels a longer lever more leverage. This will change the rod angle that may cause a loss and clearance problems. I believe Cobb (not sure) was playing with longer rods to change the rod angles.

Pistons/rods need to be changed to compensate for the added stroke, so that the compression is not raised, and or clearance problems (cylinder) .

Other than a race engine I dont believe it would be worth the money to stoke a sti engine.
Alot of engineering goes into a stoker. A small block Chevy, or a Ford 4.6 there is enough of them being done to justify the cost VS the gain.

\
Rod ratio is simply a ratio of stroke length to rod length. Rod ratio affects piston height in bore Vs. crank angle. using a rod ratio that places the crank closest to 90 deg while at peak cylinder pressure will produce mucho torque. additionally the rod ratio affects peak piston speeds on the upstroke or down stroke of the engine.
Too short a rod ratio creates more and more side loading between the piston and wall. And by nature of stroking it is hard to add rod length to compensate for an increase in stroke because the deck height becomes a limiting factor.

Also, another issue not confined to but prevalent on a Subie stroker is piston skirt length. In order for the piston skirt to clear the crank at BDC it must have it's skirt shortened which affects how the piston rocks in the bore.