Boring and Stroking - Subaru WRX Forum
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#1 Old 02-10-2010, 07:59 AM
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Boring and Stroking

This is a pretty random question but I have always wondered. I know what boring out an engine is and I know what stroking it is, but how does stroking work? Is there a difference between the two behaviorally? I know they both alter the displacement but is there any difference between the two between that and how the alteration is achieved?
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#2 Old 02-10-2010, 08:23 AM
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Well a usually when you "stroke" an engine you increase the stroke of the crankshaft. So say your piston moves up and down 5 inches, you stroke it and it moves up and down six inches. You get 20% more volume in the cylinder at that depth and thus more displacement. Boring can be done in conjunction with stroking or just by itself as a means of adding displacement. Generally a stoker kit for the WRX (using a 2.5L motor), say the 2.6L, will just add stroke, where as a larger kit, say the 2.75 will bore and stroke.

Somebody correct me if I'm wrong but I'm pretty sure that's a good explanation.

-Chris
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#3 Old 02-10-2010, 09:12 AM
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if you are thinking of the formula for cylinder volume (the empty space, not the actual cylinder), then stroking increases "h" and boring increases "r"

V = h*pi*(r)^2

-> where h = cylinder height (roughly because top and bottom surfaces are not perfectly flat), r = cylinder radius, and pi = 3.141592653(etc...)


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Last edited by sleepr; 02-13-2010 at 11:18 AM. Reason: "pi" is not squared
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#4 Old 02-11-2010, 03:33 AM
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so they don't really do anything different, but just achieve the same goal using different means? Does stroking make your engine rev faster or boring give you more torque or anything like that? What would you need to stroke your engine? How much do boring and stroking cost? does your ecu automatically adjust the AFR? What is PnP coating? Are there any other kinds of engine strengethening procedures or materials you could use? What about reinforcing the deck? How would you do that? What exactly do they do to perform this process? Is the 2.5L deck is already reinforced? What is the safe limit to bore the 2.0L engine? What about the 2.5? What about the safe limit for stroking? Is stroking ever even dangerous? I know this is a lot of questions, but I was thinking of getting this done to my engine so I wanted to know everything I could about it before i jumped in the ocean with out a paddle. Thanks for all the help
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#5 Old 02-11-2010, 08:35 AM
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Well I'll answer the ones I know the answers to:

To stroke the engine you need a new crank for sure. Most kits come with a crank, connecting rods, pistons, and rings. Kits for stroking with the stock bore run from 3500 or so (Brian Crower) to over 6k (cosworth). You would need to tune for anything like this, but you would probably need to run a piggyback ecu, I doubt the stock ecu is very good at this point.

PnP Coating? Never heard of it, PnP is port and polish though. Usually done to the heads to open up flow. You can also get your cylinders re-honed during the tear down process and that might fall into this category. Coating, unless you're talking thermal coating then I dunno. I just got my exhaust manifolds ported and polished by grimmspeed and had a thermal coat applied to help hold in more heat.

Stroking isn't really dangerous, I've just heard that it can be a lot less reliable than a standard displacement block. If you think about it you're modifying a block to accept more displacement, and it wasn't build to hold higher than 2.0 or 2.5L in terms of structural issues. Now if you stroke it by boring you're removing even more material from the cylinders, weakening them even more. Increasing stroke I don't know how that affects the strength of the block compared to boring, but I would assume it's not as dangerous.

Only thing I know of to really reinforce a deck is to maybe weld in some extra support around the cylinders. Not sure how open or semi-closed the 02 block is but if it's completely open you could possibly add in some support. Semi-closed I don't know what else you can do, I guess go find a closed EJ deck. With a fully closed deck though you have to worry about overheating since there's less area around the cylinder that coolant can touch.

-Chris
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#6 Old 02-11-2010, 01:26 PM
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I used to work in a custom engine shop but I never had the pleasure of building or rebuilding a Subaru motor. That being said, all engines have their pros and cons for boring and stroking and things that you need to look out for.
First off, rcrswetdream is dead on about the boring and stroking. Most stroker kits will come with all of the relevant parts like crank, rods, pistons, main and rod bearings and rings. When doing a stroker kit, you need to make sure that there will be enough valve relief cut out of the piston so that when the piston goes on the up stroke and the valves open, the piston doesn't hit the valve, which you can imagine would be very bad and expensive. The reason you need to check for this is like the kit says, you have a longer stroke so the piston will dip lower in the down stroke and higher on the up stroke. Many people haven't checked this, fired their vehicle up, and destroyed a brand new set of valves. The other thing you need to check for is clearance of the rods and crank on the down stroke meaning, since you have a longer stroke, you have to make sure the rods and crank aren't going to hit the other side of the block when they swing down, or across in our case.
Whenever boring an engine, you always run the risk of boring too much or giving the engine too much boost which creates too much pressure and in EXTREME cases, can blow a hole in the cylinder wall. If you're going to buy a bore and stroke kit, most likely this kit has been tested numerous times and should work great for most applications as long as the block is prepped correctly by professionals. In terms of cost and compared to the parts that you have to buy, boring isn't normally that expensive. If you disassemble the engine and take them the parts, it should only cost a few hundred dollars to properly bore and hone the block to the specified size.
Boring and stroking will not make the engine rev faster. Using lightweight components and having an engine shop lighten and then balance your internals will help your engine rev faster. Using a lightweight flywheel will also help.
It's not true with all applications but stroker kits will normally give you more torque than a stock setup or just boring alone.
Between boring and stroking, stroking is less dangerous in my book, as long as you check the clearances because you don't have to take width out of the cylinder.
Reinforcing your current block can get pretty expensive. If you're wanting to push a lot of boost and power, my suggestion is to buy a block from a company that casts a block that is much stronger because they've had to do the research and development to make that block stronger in the right places while still allowing the engine to work properly with the cooling and such.
Whenever doing a bore or stroker kit or both, you should always have your ecu flashed or a new ecu installed correctly calibrated for the modifications you've done. That's the case with any modification you do to any car.
PnP coating on pistons is a dry lubricant coating that can be added to almost any piston or can come on pistons that come in a kit. Other companies call it different things. For instance SpeedPro calls it DurOSheild.
I think I covered all of your questions but if not, let me know and I'll do what I can to help. Good Luck
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#7 Old 02-13-2010, 11:22 PM
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thanks guys for your help I really appreciate it. Do you know anywhere that sells these custom reinforced blocks? What about bore and stroker kits? Is it cheaper to get them done together or seperate?
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#8 Old 02-14-2010, 12:43 PM
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Personally, if you're going to bore out a 2.0 L block, just buy and build up a 2.5L motor or stroke it from there. It'll probably be cheaper to just do that and you'll get more displacement than a stroked out 2.0.

-Chris
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#9 Old 02-14-2010, 05:47 PM
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Thats actually what im planning on doing. I'm getting a 257 and im planning on boring and stroking it. Would i still need to get it PnPed? What process is actually done when PNPing takes place? Can you PnP an exhaust system or would it make sense to only do the headers? What about the intake? How much does it cost to do a PnP?
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#10 Old 02-14-2010, 06:38 PM
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You would need to get it bored out if the stroker kit calls for it. If it only adds stroke, then you don't. If the pistons are wider, then yes.

I got my manifolds from Grimmspeed for about 440. PnP is basically just making the holes bigger by removing excess material. Polishing the newly opened up area gives you better flow.

The only things you can really PnP are your exhaust manifolds, your heads, and you can port/polish the turbo but if you're going big on that don't bother. If you bore the cylinders that's kindof included in that process of getting them bored and honed.

-Chris
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#11 Old 02-14-2010, 06:46 PM
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You can also PnP your intake manifold and throttle body if you want to go all out.

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#12 Old 04-05-2011, 09:48 PM
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i have an ej20 and had the #4 piston skirt crack and the clip that holds the wrist pin in came out and scratched the wall, it honestly doesnt look that deep but, im not sure how far i can have it bored out or do you guys think i should just sleeve it and then have it bored back to a 2.0 again
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#13 Old 04-05-2011, 10:29 PM
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just sleeve it

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#14 Old 04-05-2011, 10:29 PM
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the walls are so thin already combined with the open deck boring the stock block is just asking for trouble

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#15 Old 04-06-2011, 07:37 PM
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I agree with Matt. Sleeve it. Even Ron the run away engine builder wouldn't bore out used engines. He'd just sleeve and start over.

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