C Pistons + Long Rods?

[Bret Young]

Hey guys. Been doing a fair share of research looking at engine combo's for a build I am preparing for. Been doing some reading up on rod length - some people prefer shorter rods and some prefer longer rods. Lots of talk about rod angles and piston side loads, but the consensus for the right length rod for an engine combo is the one that makes the engine easiest to assemble, allows the motor to successfully fit in your engine compartment (bug) and keep your deck height in the reasonable range to minimize the amount of shims used during assembly.

But if the parameter is to make the engine easy to assemble and keep it stock width, has anyone ever run longer rods with C pin height pistons? I know they are not super common, but I am going to purchase a set of Super Squish 94mm pistons, which come in A/B/C configurations. The C pin height pistons got the gears turning in my head and made me wonder what would make them work.

I did some calculations to determine two different combo's that should technically create a very close to stock width combo with off the shelf parts. Since piston pin height and rod lengths are in inches, I will convert them to millimeters so all units of measure are equal. Give me your thoughts.

So some quick calculations:

Rod Length's
5.394" - 137mm
5.4" - 137.16mm
5.5" - 139.7mm
5.7" - 144.78

Pin Height's
A - 1.558" - 39.57mm
B - 1.380" - 35.05mm
C - 1.055" - 26.8mm

So let's compare two engine combinations that are very commonly used, highly suggested and known for being stock width, easy to assemble, and put your deck height pretty close to where it needs to be.

First combo - 78mm crank/5.5" rods/B stroker pistons
78mm + 139.7mm + 35.05mm = 252.75mm total stroke length
Rod ratio = 1.79

Second combo - 82mm crank/5.4" rods/B stroker pistons
82mm + 137.16mm + 35.05mm = 254.21mm total stroke length
Rod ratio = 1.67

Same displacement as above, but with C pistons + long rods instead - 82mm crank/5.7" rods/C stroker pistons
82mm + 144.78 + 26.8mm = 253.58mm total stroke length
Rod ratio = 1.76

So the combination is smack dab in between the two common combos - off of each by 1 millimeter. But it is an uncommon engine combo, just because it measures out the same as the other combo's doesn't always mean its a wise decision. Thats why I am extending it to all of you. I don't see why it would be any more difficult than assembling an 82/5.4/B combo.

Let me know your thoughts.

[GTV]

One small correction, 1.005" = 25.527mm

Not being a long rod/high for ratio fan, these interest me more for the fact that you could build a 90mm stroke engine (pent roof aluminium case) with 5.6" rods that is only slightly wider than stock That would put you right at 2500cc.
Too bad they aren't available in 4"- 2919cc!
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EMPI Power Rules!

[Bret Young]

[GTV]

Your "total stroke length" way of thinking has me a little confused. If you ad up a stock engine you get 69+137+39.57= 245.57

I think of one side of the engine and compare it to stock, makes it easier for me to visualize. An 84 stroke crank pushes the piston up an additional 7.5mm/.295" over stock. A 5.325" rod is about .069" shorter than stock. A stroker B piston pulls it down another .220". So, .295"-.220"-.069= +.006". +.006" is where the top of your piston will be compared to stock.

I've always thought the difference between A and B pistons were .220", are the SS pistons different?
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EMPI Power Rules!

[58 Plastic Tub]

[GTV]

[KROC]

I build 86 X 94 with 5.7 Rods and JE "C" Pistons fairly often.
Using Cima Regular length 94 Barrels the Width is very close to stock

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[Eaallred]

I'm switching my dragster over to C pin pistons currently.

The frame of my dragster won't let me widen the engine at all (without modifying frame and body panels) and with adding a turbo to the mix, I had to lower the compression. Too much money in the heads, i'm not changing those, so C-pin pistons it is. Should work fine for what i'm trying to accomplish.

They do however, look like a PITA to assemble compare to an A or B piston.
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Eric Allred

You have to remember something: Everybody pities the weak; Jealousy you have to earn.

[Bret Young]

[Type 5 Joe]

I'm sure he means that installing the wrist pin / clips, then the oil control ring, then try to compress the rings (without overlapping the oil scraper) and install the piston in a barrel with sealer on it... Then removing the ring compressor... Then pushing the cylinder into place.

Will require a bit more effort and technique than the traditional way of installing loaded pistons in the jugs.

[Eaallred]

Yes, i've been spoiled with this:

Now i'm going to have to work for it.
_________________
Eric Allred

You have to remember something: Everybody pities the weak; Jealousy you have to earn.

[Type 5 Joe]

The trick is to end up with a piston installed that you are 100% sure is done right... Without an overlapped oil scraper and the rings indexed approximately how you want them.... Without disturbing the nice, small bead of sealer you have on the jug sealing area.

It is tricky to not end up with sealer on your hands... And everywhere you don't want it.... Oil on surfaces that you wanted to remain clean..... Cussing like a sailor.

What is needed is that tapered ring compressor.... That would split in half after you use it, for easy removal. I don't think anyone makes them, but that is what is needed.

[modok]

For oddball ring compressing jobs I take a strap of good quality springy metal, such as from a large hose clamp, and just bend the ends 90 degree at the right spot to fit the size needed, clamp together with vise-grips.

[Bret Young]

[modok]

Simple answer is because they are wrong
78+stock rod(137)+b(35) is stock width.

80+stock(137)+b(34) is stock width

80's vintage piston specs
https://www.thesamba.com/vw/gallery/pix/714659.jpg
there you go, no charge

[mark tucker]

you lost me with the stroke length,rods dont change the stroke length. allread cold knotch a cylinder's for pin incertion with no issues.load cylinder as usual and stick it togeather. there are many diferent pistons for the basic 94 bore with 866 pin made by manley,dss,diamond and others with many diferent dish shapes and sizes up to about 24 cc dish. pinsizes range around 1.175-1.22. and if you got the 24 cc dish and only needed12cc there is more that can be milled off the top witch efectivly rases the pin. I have the mahle motorsports 1.175 pin hight pistons for the ford 4.6/5.4 modular engine(big bore) to go with my 86 stroke&5.6 rod.I may use a 5.7rod and stick the 5.6 in my 2332 with the same pistons. for me I like to use the longest practicle rod possiable....for the most part. but using strange stuff can also throw in to the mix,but thats not for the advreage diy guy. And stacking 3 .240" shims so you can use a 6.124 rod isant practicle. as shortening cylinders to use a 76 or 78 crank isant practicle for most diy guys. if you need to stay with a narrow engine then use the highest pin hight & longest rod you can to get what you need. remember the piston pivots on the pin,the lower the pin the more levrage the top of the piston has to try to wedge in the bore and colapse skirts.thus higher pins are another nice thing to have from that stand point.

[GTV]

[GTV]

[KROC]

The "C" piston I use has a .945" compression height so it's even closer to stock width.

Cheers

[GTV]