My concern is that while you are definitely splitting hairs with respect to rod ratio effects at that point, you may not be splitting them when it comes to the structural and thermal compromises that come with reducing the compression height of the piston to that degree. Everything is a tradeoff, and in this instance, I feel that further reducing the compression height to accommodate a minimally longer rod may be treading in a negative direction with respect to the heat and stress that piston will see. The 159mm rod is where I'd draw the line, but definitely consult Marco and Aaron to solicit their opinions.

 

bah. I typed a big response and I accidently deleted it.

Basically, you're right Ted. Aaron and I were talking about the thermal mass requirements of a piston last night as well as the structural integrity of that light/small CH piston and came to a similar conclusion.

I'd like to talk to Ross and have them answer a few questions as well as see what they can teach me in a phone call.

Thanks again Ted.

 

great thread! Also your sig says, in colorado till the 17th. Are you here now? if so, are you in Denver?

 

Yeah I'm still in Denver. we're driving back and forth on Colfax. Staying at the Denver West Marriott another night.

We need to kick it tonight. I've got a co-worker here that wants to go out on the town tonight. lmk

541 980 6022.

 

On the 2.4l long rod stroker, would a 156mm rod have a differnt compression then 150mm rods? Or do you have to have pistons made for the 156mm rod?

 

If you notice on the spreadsheet, the lr 2.4L has a shorter compression height piston. It's also lighter! The compression ratio can be the same with a longer rod and a shorter piston.

 

So pistons for 150mm rods wont work with 156mm rods? Or it would just be higher comp?

 

Ah, ok. Don't confuse compression height with compression ratio. When we speak about compression height we are speaking of the distance between the center of the piston pin and the top of the piston.

The compression height has to match the rod length so that the top of the piston is basically flush with the block deck at the top of its travel. If you change the rod length, you must change the compression height of the piston (which requires a different piston) to match the change in rod length. Even a 3mm change in rod length works out to 0.120" change in deck height, which is quite a lot. Changes in compression ratio are made by changing the contours on the top of the piston, so that is a different matter.

 

ok. Thanks

 

How much RPM difference is there from 1.69 rod ratio vs 1.65? this is a 4g64 block, 94mm crank, and rod ratio depending on either 156 or 159mm.

im trying to determine if its worth the custom rod or not.

i understand both combinations require a custom part no matter what though. the 156mm rod is off the shelf, then requires the custom piston.
the 159mm rod is custom, but can utilize the stroker piston (off the shelf but in my case most likely custom built anyways for strength).

 

No, the functional difference between those two rod ratios is about half of what's needed to observe a measurable difference in even the most clinical research settings.

In this context, the greater piston compression height allowed by the 156mm rod carries greater value in that it provides for increased durability.

 

ok well that answers that. Thanks Ted.

now since you said what you did, i am curious on this. what is your outlook on just sticking with a unmodded piston and rod, for example a standard 2.4L

vs what i am building a 2.2L with the custom crank and pistons. do you feel it has enough benefits to be a good combination, or do you feel anytime you modify a piston pin position (3mm up in this case) you risk failure at the piston? im focusing my main issue being a piston failure since i recently had one fail on me with the 2.3. the 2.3 has a 6mm raised pin, so that would make the 3mm reduction a 50% increase in durability? and the not modified piston 100% durability?

how much impact is 3mm raised do to the strength of the piston?

Thanks.

 

I followed the route I did because I was going with a top tier set of custom billet rods, and it was convenient to have made a 156mm rod that gives a 1.77 RS ratio (2.0L), which leaves just enough compression height (~28mm) for a piston with enough crown thickness and skirt mass to handle the heat from that specific output, while keeping the ring package effective and giving good long term durability.

That fit the criteria for what I wanted from my setup, and represents a threshold whereby I determined that reducing the compression height further creates more problems than it solves.

 

ok well thats interesting. your setup is a 6mm stroker piston then in terms of wristpin location since its a 156mm rod. you just told me to avoid a 6mm wristpin.

so if you're running a 6mm setup and recommended against it, then you must feel there is a benefit to having 3mm vs 6. that is why i asked in regards to just shelfed products vs custom.

money isnt an issue. i just want to make sure what im using will work and not give me any abnormal problems that i could avoid by going other routes.

i plan on producing somewhere around 800whp with my next combination in parts.

the car only sees e85 and nothing else. the reason i went 2.2 L route was to have more rev range since the car will see a lot of 1/4 time + 4g64 blocks come by easy.

 

Yes, you are correct. I just did this too quickly in my head. To get compression height, first take the deck height, and subtract 1/2 stroke length and rod length. I do not advise anything <28mm in this application.

Factory Compression Height Calculation:
4G63 Deck Height - 228.9mm
1/2 Stroke - 88/2 = 44mm
Rod Length - 150mm
Compression Height - 34.93mm

My LR 2.0:
4G63 Deck Height - 228.9mm
1/2 Stroke - 88/2 = 44mm
Rod Length - 156mm
Compression Height - 28.9mm

For you to end up where I did:
4G64 Deck Height - 234.9mm
1/2 Stroke - 94/2 = 47mm
Rod Length - 159mm
Compression Height - 28.9mm

So yes, a 159mm rod should be fine for your setup.