sponaugle

Fellow engine builders:

As some of you may have seen from my previous thread, I'm in the need to rebuild my STIRA GDB engine. I read thru a collection of excellent threads, and have a few comments/questions.

For reference, see these end of this message.

Jorge,et al:
In your pick of parts, you mention that you would be using "2.5 style" pistons, with 2.0 bore (92.5mm). Is the use of the 2.5 style because of the pin offset?

If we do a little math, you can see why I ask. Let's start off with the stock WRX motor. We know it is 92mmx75mm, which calcs out to a displacement of 498.5705 ccs. We also know the motor has a compression ratio of 8:1. When the piston is at BDC, to volume in the piston would be about 498ccs, plus the ccs of the quench area, piston dome, and the head. Using some simple math, we can figure out how much this quench+head+piston has to be. For label purposes, lets call the quench+head+piston volume, also known as the "compressed volume", X.

Since CR=8:1, (498+x)/x = CR. Solving for x: x=498.5705/(cR-1);
Thus x = 71.224ccs.

From several other sources (see sources below), we know that the head volume is 49ccs, and the piston volume is 12ccs. From yet another source, we see that the quench area is 10.1cs, which all adds up pretty close to 71ccs. All looks good.

Further information we need is the rod length. From the JEPiston site, we see the rod length is 5.135in, or 130.43mm for the WRX. (That makes the rod/stroke ratio 1.74).

Ok.. With this in mind, lets assume we drop in the 2.5 crank and leave eveything else the same. Since the 2.5 crank is 79mm stroke, the TDC will be pushed up 2mm. (and BDC 2mm lower). So what will the new compression ratio be? With 79mm stroke, the displacement would be 525.16ccs. Since we are pushing the TDC up 2mms, that would take away 13.3ccs of displacement from the quench+head+piston area, leaving it at 57.93ccs. Do the math, and you would have a CR of 10:1. The is of course assuming the engine would even run, as 2mms of additional height may smack the piston into the valves.

Now what can we do? The 2.5 rods are even longer, at 131.2mms, so that wouldn't work. We could get a custom 2mm shorter rod (128.43mm), which a quick calc would yeild a CR of 8.3:1. The other option, and better option would be to use a piston that his a smaller distance between the pin and the top of piston.

So, from the JE site, they list their 2.5 pistons as having a "Comp height", which I assume is the pin to top height, of 29.5mm (1.162in). The WRX is listed as 32.69mm (1.287in). So using the 2.5 style pistons you would reduce TDC height by 3.19mms as compared to the WRX piston.

With 2.5 Crank, WRX rods, and these pistons, the TDC would be 1.19mm lower, which makes the "Compressed volume" increase by +7.9ccs to 79.124ccs. That makes the CR for this combo 7.63:1.

With 2.5 Crank, 2.5 rods, and these pistons, the TDC would be 0.42mm lower, which makes the "Compressed volume" increase by +2.9ccs to 74.124ccs. That makes the CR for this combo 8.01:1. Perfect!

So, the question in, where did the 8.5:1 CR guess come from in your previous posts? The benefit of doing the math this way is that we don't really need to know the quench, head or piston CCs. We just figure out the total of all three from the stock 8:1 CR. Since we ended up getting very close the 8:1 for the results, even if the factory rating is off, we should be off by the same amount.

There is however another problem. The above calcs assume these "2.5 like" pistons have the same 12ccs of piston volume (actually cutout). They list the 2.5 pistons as having 24.5ccs of piston volume, which would make the compression ratio really really low. I assume from this that you are ordering 2.5 like pistons with standard 12cc piston cutouts, and 92.5mm bore?

Any comments/Ideas? Also, Jorge can you order me the same 4 pistons and rods? Perhaps cheaper in bulk?

Jeff Sponaugle

References/Sources:
http://www.jepiston.com
http://wac.addr.com/auto/obs/turbo/ejcalcs.html
http://forums.nasioc.com/forums/show...h&pagenumber=1
http://forums.nasioc.com/forums/show...5&pagenumber=3
http://forums.nasioc.com/forums/show...hreadid=271215
http://forums.nasioc.com/forums/show...s&pagenumber=3

EJ20K

Stock crank, stock rods, just 97mm pistons and higher compression for better off boost response

http://www.scoobynet.co.uk/bbs/threa...hreadid=153972

EJ20K

97mm x 75mm = 554.2358cc

therefore engine will be ~2217 cc

Imprezer

iTrader: (9)

What is the stroke of the Phase I EJ25? 74mm?

- Alex

DeliciouSpeed

iTrader: (2)

I object to all this mindless stroking
No really. Your forgetting that lovely rod stoke ratio. With those increases in piston speed, you'll also see higher side loads in the cylinders. Higher combustion chamber and oil temps due to the higher piston speeds. Why not re-sleeve the block and overbore? Or use a phase one 2.5 block, WRX or STI(EJ20) crank, with standard WRX rods or as Jeff has noted, the longer STI length rod and corresponding forged piston. And pick your head, AVCS or WRX head. I think these set up's would be much hotter.

DeliciouSpeed

iTrader: (2)

Here is an article on a hybrid build, a 2.5 block w/ WRX pistons. The 2.5 block in the pics is solid deck to. That build up has pretty low goals though, they rework the heads but drop compression and use a small turbo.