Short stroke crank question

Saint-Dave · 02-18-2010, 02:28 PM

Is it feasible to use 2.4/2.7 rods on a 66mm short stroke crank, would it work? I have one that has had the rod journals ground down to 2.4/2.7 spec. I bought no. 3 in this add.

standard standard crank blow out, 3 on ebay

Henry Schmidt · Henry Schmidt's Garage

Quote:

Originally Posted by Saint-Dave

Is it feasible to use 2.4/2.7 rods on a 66mm short stroke crank, would it work? I have one that has had the rod journals ground down to 2.4/2.7 spec. I bought no. 3 in this add.

standard standard crank blow out, 3 on ebay

No. The 2.4/2.7 rod is both too short (124.8 vs 130 mm) and too wide (22.8 vs 21.?)
{these numbers are from memory} at the big end unless the journal was widened. You could run a GT3 Pankl titanium rod second generation .
We did it here and it truly is a Bad *ss 2.5.

tom1394racing · 02-18-2010, 05:53 PM

Of course the shorter stroke crank requires a longer rod so that the piston is positioned properly relative to the cylinder/head interface at TDC.

Walt Fricke · 02-18-2010, 06:17 PM

Henry

Do those Pankl's have three instead of only one wrist pin splash oiling holes?

Could one modify a stock or Pauter or other slightly less exotic rod to do this and achieve whatever benefit the Pankl designers saw?

Henry Schmidt · Henry Schmidt's Garage

Quote:

Originally Posted by tom1394racing

Of course the shorter stroke crank requires a longer rod so that the piston is positioned properly relative to the cylinder/head interface at TDC.

Tom
More important than the piston location is the rod angularity. I have posted though out the years and can't emphasize it enough.
When Porsche built the 2.0 (the first 6 cylinder) engines they started with a clean sheet of paper. No previous model to build on. They used engineering principles to achieve the first designs. After the 2.0-2.2 they started to compromise in order to maintain a consistent engine dimension.
Short rods work but they are without doubt a compromise.
Walt
The Pankl ty rod have two oiling holes and I would guess you could change any rod by adding holes. I like the oiling hole on the side of the beam directly under the wrist pin so that you can get pressure feed from the piston squirters.

Saint-Dave · 02-18-2010, 11:43 PM

Thanks for the replies. Can the Pankl rods be used without any modification on my crank, and do you have the part number for them?

Thanks Dave.

tom1394racing · 02-19-2010, 07:20 AM

Quote:

Originally Posted by Henry Schmidt

Tom
More important than the piston location is the rod angularity. I have posted though out the years and can't emphasize it enough.
When Porsche built the 2.0 (the first 6 cylinder) engines they started with a clean sheet of paper. No previous model to build on. They used engineering principles to achieve the first designs. After the 2.0-2.2 they started to compromise in order to maintain a consistent engine dimension.
Short rods work but they are without doubt a compromise.
Walt
The Pankl ty rod have two oiling holes and I would guess you could change any rod by adding holes. I like the oiling hole on the side of the beam directly under the wrist pin so that you can get pressure feed from the piston squirters.

Henry

I understand the force loading diagram and the increased side loading with the shorter rods.

Just to be clear...Are you saying that using the shorter 2.4/2.7 rods on a 66 mm crank (assuming the machining work has been done to make the bottom end fit) is no worse a compromise in side loading that the standard 70.4 mm crank with the the 2.4/2.7 rods as used in the 2.4 and 2.7 engines?

Henry Schmidt · Henry Schmidt's Garage

Yes. The 2.4 rod length is better suited on the 66mm crank than it is on the 70.4 crank.

tom1394racing · 02-19-2010, 12:06 PM

Henry

You've got me thinking about the side loading and our friend Dave who has the undersized 66 mm crank. Does he really need to spring for GT3 rods?

All things being equal, the side loading should be proportional to the half stroke/rod length ratio (the angle of the rod to the centerline at 90 deg from TDC).
The difference in the rod lengths between the 2.0/2.2 and the 2.4/2.7 rods is one half the stroke difference between the cranks or (70.4-66)/2 = 2.2 mm
I don't have my spec book in front of me but if I assume that the 2.0/2.2 rod is 130mm then the 2.4/2.7 rod is 127.8 mm in length.

This gives the following half stroke/rod length ratios:

1. 2.0/2.2 rods on a 66mm crank = 0.253
2. 2.4/2.7 rods on a 70.4mm crank = 0.275
3. 2.4/2.7 rods on a 66mm crank = 0.258

The lower the ratio, the lower the side load, all other things being equal.

This would say that if Dave's crank had been machined to allow the big end of the 2.4/2.7 rods to fit between the counterweights, the side loads would actually be less than on a standard 2.4/2.7 configuration and very close to the 2.0/2.2 configuration. I would guess that the small increased side loading should not be a concern for Dave.

A bigger problem might be the fact that his deck height would increase by about 2.2 mm for this configuration because of the shorter rod. Wouldn't this be a bigger concern and how could he address it?

Your wisdom and insight on this one would be appreciated.

tadd · 02-19-2010, 01:26 PM

I have pondered the same question with my own pile of parts.... Rod ratio improves, smaller journals reduces mass. Henry even sells the easy fix with his spacers that go between the tray and the head. He's mentioned he can do 0.080". All that's left is to remove a bit from the cylinders or skim the case.... Aside from taking a mm of each side of the rod. Iirc, it's 2.2mm difference between the long and short strokes (half the total). The shim should cover that.

Saint-Dave · 02-21-2010, 09:07 AM

There may be some light at the end of the tunnel after all!!

tom1394racing · 02-21-2010, 04:22 PM

Let's see what Henry has to say.

Henry Schmidt · Henry Schmidt's Garage

Hi Tom
Thanks for requesting my advise with regards to this interesting question.
Here's my advise.

Arrogance alert: The following advise may contain some arrogance, please be warned. If you tend to be disturbed by this type if behavior please leave the room.

Options that compromise the functionality of an engine in search of saving money are not generally a good option. Changing cranks, swapping rods and pistons are recommended when no design compromise is achieved.
I.E.: 3.2 piston on a 3.0 crank requires a longer rod but by using a 3.2 piston on a 3.0 crank can save money and produce a better rod/ stroke length benefit which makes this option seem top quality.
By putting a short rod on a crank you gain no design advantage.
My advise for someone with a 2.0 crank that was reground to use a 2.4/2.7 rod would be to buy a rod designed for the crank. Carrello, RR Pauter will all make custom rods. If this option is too pricey perhaps trashing the crank makes sense.

Cheers

Saint-Dave · 02-22-2010, 02:05 PM

Thanks for the advice, unfortunately I was rather naive when buying this crank and thought i was getting a good deal, but thinking about it now why would anyone grind the crank to fit a 2.4/2.7 rod.
The crank owes me about $520 including shipping and i would really like to use it rather than throwing it away.
I have seen GT3 rods on Ebay for $600 second hand, so will have to look out for a set.
Any more advice on a way forward would be appreciated.

Thanks Dave.