2.0 Long Rod questions

[Kapok6]

I have searched, and racked my brain and the brains of others and can't seem to figure this out. On a 2.0 LR motor, you use stock 88mm crank, and 156mm rods and stroker pistons. My question is, does this change the advertised compression ration of the pistons.

Lets say I used 88mm crank, 156mm rods, and 9:1 CR stroker pistons. Is the compression still going to be 9:1?

 

[RvlutionMtrsprt]

No, stroker piston compression ratio's are based off a 100mm stroke. You will need to calculate the actual compression with an 88mm stroke. What compression are you shooting for? What head gasket are you going with, what deck height? All of those things are going to come into play and more. You'll likely need to order stroker pistons with a 5-10cc dish to get close to 9:1. You'll need to crunch some numbers.

 

[Kapok6]

http://www.gtsparkplugs.com/CompRatioCalc.html

Based on this, a 10.5 stroker piston would yield a CR of 9.32:1 on an 88mm crank.

A 9:1 CR stroker with a -21 dish would yield 8.05CR on an 88mm crank.

I am going off the wiseco specs posted on extremepsi.

Though in order to get the CR's to match whats advertised by wiseco while using their dish measurements, I had to use an HG thickness of .025, about half as thick as an oem composite which I believe is .051".

If I use .051" on a 9:1 stroker piston and 100mm crank, its calculates a CR of 8.6:1.

 

[RvlutionMtrsprt]

You don't modify head gasket thickness to get the desired compression, you order pistons with the desired dish. If you don't want the wait time for a custom piston then you just get the closest thing available off the shelf.

 

[Kapok6]

I modified the HG thickness to get the baseline calculations to match what the mfg says the comp ratio is, the used reverse mathematics to see what "approximately" 10.5:1's would approximately be on an 88mm stroke.

So lets run through an example. Punch in the info for these pistons using 85mm bore:

http://www.extremepsi.com/store/product.php?productid=21528&cat=1586&page=1

Bore 3.346"
Stroke 3.937"
Chamber Volume 47cc
HG Thickness .051"
HG Bore 3.346
PtD Height 0
Piston Type Dished
Dome Volume dished 21
It gives you a comp ratio of 8.53:1 Far off from the 9:1 it is supposed to be.

So from here, I modify the HG thickness until the CR matches what the MFG says it is. In this case, taking the HG thickness from .051" to .020" gives us 9:1.

Now take that 100mm stroke, and change it to 88mm/3.465", you get a comp ratio of 8.04:1. If you then change the HG thickness to OEM .051", you get a comp ratio of 7.63:1.

So if you ran the piston in the link above in a 2.0 LR setup, the CR, according to this, should be somewhere in the ballpark of 7.63:1.

Is this incorrect? If so, where is the flaw in the calculation?

 

[RvlutionMtrsprt]

You are not going to run a .020 head gasket in the real world when you build the motor so why bother skewing the numbers via that method? Makes no sense. If you take an off the shelf Wiseco stroker piston with an advertised compression of 8.5:1 and run it with an 88mm crank it'll be roughly 7.5:1. If you want to change that to 9:1, just change the dish volume from 21cc to 7cc, BAM!! This is what you will do in the real world, order pistons with a smaller dish to bring the compression back up to where you want it. You're not going to go searching for a .020 head gasket. That would mess with the piston to head clearance and the valve train geometry, if you could even find one.

EDIT: Also, just ran your #'s and yes, you come up with 8.53 however you're using #'s that Wiseco advertises as 8.5:1 so it's pretty close. You're also using a 0 deck height and Wiseco's 8.5 is based off .012 for the stroker piston. They also call for .042 on the HG thickness, change the .051 to that and you get 8.49:1 on the calculator you're using.

 

[Kapok6]

I wasnt really going to run a .020 HG. I simply had to change it to that thickness in the calc to get the manufacturers advertised CR. Once I was able to meet that CR, I could put a different dish volume in for a higher advertised CR piston to try to see what it would actually be in my application.

As for changing the dish volume, that is fine and dandy if you are going to pay for custom pistons, which i'm not doing at this point. If I was doing that, I would just have them make me a custom set of HD 9:1's and be done with it. Instead, I was trying to see if there is another way using a combo of off the shelf parts to achieve my desired approximate CR, which it seems there is not.

So I am back at square one, with 8.5:1 or 10.5:1, and since 10.5 is too aggressive for my liking right now, it looks like 8.5 it will be.

Thanks for the explanation of everything above.

 

[RvlutionMtrsprt]

Honestly, well worth the $740...
http://www.forceengineered.com/ProductDetails.asp?ProductCode=CWDSM
Worlds better than an off the shelf piston!! Sure, it's a couple hundred more, but that's a drop in the bucket when it's your motor we're talking about. You'll be happy you did it. My off the shelf Wiseco's cracked at the skirts and one broke a wrist pin. Running 10.5:1 HD's now in my 2.3.

 

[Kapok6]

HD wisecos broke a pin and cracked at the skirts? Or the regular ones? I haven't heard of the HD's having those issues, hence why that's what i'm leaning towards.

 

[RvlutionMtrsprt]

No no no, off the shelf pistons did that. The HD pistons are rock solid. Went with 22mm pin and 10.5:1 for my 2.3. You won't be disappointed.

 

[Kapok6]

OK, that's what I thought.

These are what I will most likely be rolling with:

http://www.extremepsi.com/store/product.php?productid=21468&cat=1586&page=1

K625M855

Mainly because of the pin. They are the only mfg I have found yet to make an h13 tool steel pin with a .225" wall. Being that it was a wrist pin that snapped on my last motor, i'd like to avoid that moving forward.

Manley makes an off the shelf "9:1" piston, but to get the pins I want, I would have to buy pins separately from a place like Trend, again, increasing the build cost. So right now, the Wiseco HD's seem to be the best way to go for me at this time.

 

[RvlutionMtrsprt]

Make sure you find the part number for 100mm pistons. That link is showing 88mm. Before you order anything I think it's worth putting a call in to john at force engineering to see what he can do on the exact pistons you need. 22mm pin, 7'ish cc dish, HD. It might be $100 more but that's really not much money and you'll probably regret it if you don't.

 

[Kapok6]

I think i'm not being clear. I have a 2.0 and am going to be doing a standard 2.0 88mm stroke motor.

The only reason I had ever entertained the LR motor (100mm) was in hope that in doing so, I couple potentially use the high compression stroker pistons and 156mm rods and have a lower than advertised compression due to 6mm less stroke on the down turn.

The part # I referenced is correct for the direction I will go with this particular motor.

Standard Rod, 21mm pins (since I already have eagle H beams), 8.5:1 6 bolt Wiseco HD's with 21mm pin to fit the rods I already have.

 

[RvlutionMtrsprt]

Oh, so you're building a straight up 2.0, not a 2.0LR, got it.

 

[Kapok6]

Exactly. I was only entertaining LR if it allowed me to achieve the comp ratio I was seeking, which it appears, will not.