Another weird idea-2.2 pistons, 2.5 crank-shorter rods?

[turbovanman²]

So, what would the ultimate effect be, using a 2.5 engine, 2.2 TIII pistons and shorter rods with a TIII head. Would this be a good idea or not. I am not sure on how much custom rods cost but I seem to remember they should be the same as hybrid pistons?

[ShelGame]

It would save you having to buy custom pistons (but, still buy custom rods), and give you a crappy rod ratio to boot!

[turbovanman²]

It would save you having to buy custom pistons (but, still buy custom rods), and give you a crappy rod ratio to boot!

Thats what I was wondering, the rod ratio? why would the rod ratio be crappy? what would you expect?

[ShelGame]

Well, lets see, the 2.2 stroke is 92mm, the 2.5 is 104mm. So the difference is 12mm. To use a 2.2 piston in a 2.5, take half that off the rod. So, to use a 2.2 piston with a 2.5 crank in CB, you'd need a rod 6mm shorter, or 145mm long.

That would give you a rod ratio of 145/104=1.394

By comparison, the 2.5 rod ratio is 151/104=1.452

And, the 2.2 is 151/92=1.641

And, the "long rod" 2.2 would be 157/92=1.706

1.7 is supposed to be a "magic" number - don't ask me why.

The higher the rod ratio, the lower the piston side skirt loading, and the lower the friction (among other things, like piston velocity and acceleration). It's a relatively minor effect, but can be significant in a high HP engine. I guess I shouldn't have said the rod ratio would be "crappy" - I don't think there's a point at which it's "bad" vs. "good". It's just the wrong direction...

Plus, wouldn't you have to remove part of the skirt on the 2.2 piston? I thought the skirts were too long and would hit the 2.5 crank.

[altered7151]

Rob pretty much summed it up. You'd have a hard time getting it to rev, and would be putting alot of stress on the rods with that high of velocities.

[turbovanman²]

Thanks Rob, great info. Guess I am stuck buying new pistons or building a 2.2L TIII.

[GLHS60]

This was Dave Zelkowski's take on rod ratio. I copied this from Gary Donovans Dodge Garage. I hope I haven't broken some rule.

On "long rod, tall deck" 2.5 engines...


I've been following this thread and I think it's a poor allocation of resources to concentrate on rod ratio. The amount that it contributes to output is miniscule, if it does contribute at all, compared to the gains to be had in the cylinder head/induction system. In the reference to the V8 gain of 8 HP, what was that as a percentage of total output? If it's less than 2% or so it's probably not even real. Most dyno's won't even repeat run-to-run within that range. It's also likely that the runs were not done on the same day. Therefore, a correction factor would have been applied and believe me they don't correct that closely. I've got three performance dyno cells running at CTC now and this is from experience. To be sure that a change is effective I do a minimum of three runs for a baseline, change the hardware make three runs, return to the original baseline hardware and make three more runs. Then the data has to make sense. There better be good correlation in output, BSFC and BMEP or I'll do it all over again. I know in the old Stratus Super Touring Engine we placed zero importance on rod ratio. We talked about it but we couldn't find anyone with good credentials to give any credence to the theory, like Geoff Goddard from Walkinshaw Racing. We ran different rod lengths over the course of the three years we worked on the engine from 136mm to 146mm but these changes were only to accommodate different piston designs. In fact JanSpeed Engineering of the UK, who ran the Nissan super tourers, suggested SHORTER rods. There maybe something to the idea of longer piston dwell time at TDC but it was felt that it would only be worth something if we spun the engine to 10,000 rpm and we were dictated by rules to run no higher than 8500rpm. Don't try revving your 2.5 to 8500rpm. The 2.5 turbo is definitely not a revver. Piston speeds are through the roof! Will placing all importance on having a perfect 1.8:1 rod ratio make all the difference? I doubt it. I am convinced from the Super Touring project that the intake side of the engine is where the power comes from. Of course that engine was normally aspirated and should be more sensitive to hardware choices than a turbo engine. On the Super Touring engine a 10mm change in intake length could change output by 30HP making no other changes!



So, what would the ultimate effect be, using a 2.5 engine, 2.2 TIII pistons and shorter rods with a TIII head. Would this be a good idea or not. I am not sure on how much custom rods cost but I seem to remember they should be the same as hybrid pistons?

[turbovanman²]

Ok, so whats that in english, :lol:

[852jamaica]

Ok, so whats that in english, :lol:

Well as you can guess the implication is if you plan on using an 8v head, don't even worry about rod/ratio cause you won't reap much/any benefit vs. cost as head is rpm limited. Spend money on the head and intake improvement.

Yes, when comparing engine dyno results the induced error is probably a couple percent, so you need to see differences better than that to indicate real change. And forget about using chassis dyno numbers for small increases, but thats another thread.

As for rod length, its hard to dechper from the comments without data but the implication is that the change in rod lengths provided little/no effect from their testing. Interesting that the rod length changes were over 10%, because that should have provide some effect. But, since piston designs were changed also and who knows what else from test to test its hard to garner much from this excerpt. With all due respect to Dave Zelkowski, and that this excerpt is probably been taken out context, I suspect without more info. that the rod length effects were treated by the engineers more as DOE noise as opposed to a testing variable.

As for the shorter rods suggestion, in the same engine you would need to change either piston compression height (move pin) or stroke, so can't comment on this without more detail or making assumptions on what would become two different engines. Believe they might be impling to use short rods and de-stoke for higher rpm of over-square engine advantage or move pin down and increase piston stability and sealing?

As for the increased piston dwell with longer rods this provides for an increase state of compression by keeping the chamber volume small which is good particularly at high rpm, but with lots of other considerations.

Now to try and answer your orginal question. TIII piston in a CB with 4.09" stroke would yield a poor rod ratio, so in a CB could really only use the 2.2 crank. But in a 2.5 TB with a 4.09" stroke you could yield a 1.5 ratio by putting the extra deck ht. into the rod, in fact assumming the TIII has a 1.6" piston CH the stock rod length gets you close enough for consideration, and more importantly so does the FM Oliver rod.

[turbovanman²]

Now to try and answer your orginal question. TIII piston in a CB with 4.09" stroke would yield a poor rod ratio, so in a CB could really only use the 2.2 crank. But in a 2.5 TB with a 4.09" stroke you could yield a 1.5 ratio by putting the extra deck ht. into the rod, in fact assumming the TIII has a 1.6" piston CH the stock rod length gets you close enough for consideration, and more importantly so does the FM Oliver rod.

So what your saying is, a 2.5L CB with a 2.5L crank with stock 2.2 TIII pistons and custom rods is doable?
I compared the 2.2 piston to the 2.5 piston and the 2.2 is aprox 1/4 inch higher.

[89acclaim]

Though if you did manage to have the pistons clear the counter weights with short rods(around 145 mm) in a CB. The dish volume would be the wrong size for the added engine displacement.

Daniel Merrill

[turbovanman²]

Though if you did manage to have the pistons clear the counter weights with short rods(around 145 mm) in a CB. The dish volume would be the wrong size for the added engine displacement.

Daniel Merrill


Why would it be wrong? TIII head, TIII pistons at TDC gives same CC's regardless of rod length or crank stroke!

[89acclaim]

Yes the CC's above in the area above the piston are the same on tIII headed 2.5 with tIII pistons, but the cc's from BDC to TDC is larger. Same reason a 2.5 with flattops has a higher Cr than a 2.2 with flattops if they have the same head.

Daniel Merrill

[turbovanman²]

Ahhhhhhh, I see. Thanks.

[GLHS60]

I'm not sure what you are trying to acomplish here, perhaps you have some T-III pistons and want to use them?? Although as Dave Z stated rod ratio is of little importance, there is the option of using a tall deck 2.5 block, crank, rods and 2.2 pistons, giving the best possible rod ratio using stock parts. The block is not quite up to common block standards and the long rods would be considered LW but it would be easilly do able. Also I think T-III pistons have a bigger wrist pin and much shallower dish.

Thanks
Randy

[turbovanman²]

Throwing out idea's, I have TIII pistons, so trying to figure out how to use them. If I can't, then I will just buy some Hybrid ones. We are TD'ers, trying to mix it up, :thumb:

[852jamaica]

So what your saying is, a 2.5L CB with a 2.5L crank with stock 2.2 TIII pistons and custom rods is doable?
I compared the 2.2 piston to the 2.5 piston and the 2.2 is aprox 1/4 inch higher.

No, I am saying a 2.5 TB (Tall Block) with the 2.2 TII pistons and custom rods is doable. In a CB stock 2.2 pieces work not stock 2.5. Also, I believe that the stock compression hieght of the 2.5 TB (Tall Block) piston is also around 1.6" as is the TIII, but not sure on dome/dish design of TIII


Though if you did manage to have the pistons clear the counter weights with short rods(around 145 mm) in a CB. The dish volume would be the wrong size for the added engine displacement.

Yes the CC's above in the area above the piston are the same on tIII headed 2.5 with tIII pistons, but the cc's from BDC to TDC is larger. Same reason a 2.5 with flattops has a higher Cr than a 2.2 with flattops if they have the same head.

Yes this is true as no matter CB or TB (Tall Block) when going 2.2 to 2.5 using a 2.2 designed piston BUT the 0.3L increase is about 13.6% so 13.6% additional CR over stockish 8.3 gets you about 9.4. Not ideal for 20+psi but doable for lower boost. And with custom rods and judicial machining, offsets,undersize, tweak reliefs, drop deck hieght, inc. gasket thick, max. quench, concern for interference fit, etc. you might shave a few tenths if your block&head surfaces require minimal machining, relatively speaking.


I'm not sure what you are trying to acomplish here, perhaps you have some T-III pistons and want to use them?? Although as Dave Z stated rod ratio is of little importance, there is the option of using a tall deck 2.5 block, crank, rods and 2.2 pistons, giving the best possible rod ratio using stock parts. The block is not quite up to common block standards and the long rods would be considered LW but it would be easilly do able. Also I think T-III pistons have a bigger wrist pin and much shallower dish.

Yes the TB (Tall Block) makes it doable with custom rods, didn't mean to imply LW rods for buildup only to illustrate that their length yeilds a more approachable rod ratio then in the proposed short rod CB scenerio. AS for TB block strength I don't think that its been shown to be conclusive that the CB is stronger than the TB ( I wish folks who had failures with the TB would post their expereinces). Pin wouldn't be a concern with custom rods and dish is discussed above.


Throwing out idea's, I have TIII pistons, so trying to figure out how to use them. If I can't, then I will just buy some Hybrid ones. We are TD'ers, trying to mix it up,

Well the TIII piston would take some work to run in 2.5 configuration in TB only and probabaly more than their worth so its either use them to build up in a 2.2 CB short block or buy 2.5 hybird pistons and use in CB with the 2.5 crank, T2 rods and send the TIII pistons my way.:eyebrows: Besides, you'll have to deal with other TIII issues like timing belt, ignition(distributor or not) etc.

[89acclaim]

P.S. I already did the math, here is some things if you want to have some fun on your own.
http://members.uia.net/pkelley2/DynamicCR.html

81-82 2.2 flat tops -.0104" deck clearance
83-85 2.2 flat tops -.003" deck clearance
84-85 2.2 T1 pistons 9 cc -.008 deck clearance
86-89 2.2T pistons 15cc -.008 deck clearance
86+ TBI/carb 2.2 swirl pistons 4.8cc -.003 deck clearance unless shelby.
+.018 deck clearance for shelby carb 2.2 engines
89+TBI CB 2.5 pistons 16.4cc -.008 deck clearance
89+ T1 2.5 pistons 24cc -.084 deck clearance
TIII pistons 6ccs with 0 deck clearance. .068" head gasket

swirl head is 49-50cc
G-head is 55-56cc
TIII head 57cc's
2.4/2.0 dohc head 50cc

Some cam info if you want to use with the Dynamic Compression Ratio Calculator.

http://www.fwdmopar.com/sites/dennis/camshaft.html
http://www.fwdmopar.com/sites/dennis/factsrumorslies.html

Let me know if some info is wrong.

Daniel Merrill

[turbovanman²]

Wow, good job guys, now I need to sit down and read this another 10 times over, :p

[BadAssPerformance]

Someone PLEEZE summarize this data into a FAQ table!

[89acclaim]

If you want me to I will work on a faq of the data I have. I will try to get it done for monday.

Daniel Merrill

[turbovanman²]

If you want me to I will work on a faq of the data I have. I will try to get it done for monday.

Daniel Merrill


Sure, :thumb:

[BadAssPerformance]

If you want me to I will work on a faq of the data I have. I will try to get it done for monday.

Daniel Merrill

Sweeeeet! :thumb:

[852jamaica]

You may want to add the chart from this link. You might be able to use it to extrapolate boosted DCR.

http://www.kb-silvolite.com/article.php?action=read&A_id=57