Long stroke & short rods

[teekoo]

Hi,

My first post to the Lounge!

I'm building 2161cc engine (84x90.5). I have set of 5.325" Scat H-profile rods.
That makes the rod ratio quite low, 1.61. What do you guys think, is is gonna last or should I go with 5.5" rods?
What's going to wear quickest, P&C or rod bearings?

Thanks
Toni

[Torben Alstrup]

It will work, but be alert to skirt/ crank clearances. There is basis for a stomp puller. Cylinders and rings will wear out faster.

I like 5,5" rods with the 84 stroke for most engine set tups. Personall I would´nt use such short rods above 82 mm stroke. But that´s me.

T

[Udo]

5.5 will be better but it works with the shorter also - we used original Porsche rods with 86mm crank on type 4 engines in the past without problems

Udo

[dragvw2180]

You can make most anything work but like these other gentlemen have said a 5.5 connecting rod will save you alot of grief clearance wise. Mike McCarthy

[teekoo]

I have checked the clearance already and there's no problem.
Closest place is between piston bottom side and opposite side rod big end. There is about 2mm clearance there.

My car is kind of daily driver, even thought the driving season in northern Finland is quite short.
Do you think that with 5.325" rods it will still last for decent mileage?

It seems to be really hard to find 5.5 Chevy rods right now. At least from here...



-Toni

[Zach Gomulka]

By the time the rings wear out, you'll be ready to tear it down for upgrades anyway

My next engine will be an 84 with 5.325's.

[Jeff68]

I have Porsche length rods (Carillos) on a 82mm crank. The piston skirt and pin boss had to be clearanced. The crank is a Porsche journal forged Berg. The pistons are "B" 90.5s also from Berg. You can run them just check these places closely. What kind of crank will you be using?

[teekoo]

Crank is Scat 84mm stroke with Chevy journal.

[dragvw2180]

I have run an 84 crank/ 5.35 rods  in a race car and used A stroke pistons for clearance but it is not my first choice , but like others have said it can be done and has been before. 

[teekoo]

So is the clearance the main concern or the faster wear out?

Here's "nice" picture of the difference between 5.325" and 5.5" rods.
Difference in rod angle is only 0.5°. Piston travels 4.5mm closer to the crank.
In the picture the difference doesn't look too much.
Any thoughts?

-Toni

[modnrod]

Difference in rod angle is only 0.5°. Piston travels 4.5mm closer to the crank.
In the picture the difference doesn't look too much.
Any thoughts?

-Toni

Thoughts?

Optimising everything to suit a long rod engine at high revs will gain slightly (just barely measurable) at the top. If you're running a 600+HP superstocker index sedan. then an extra 10-20HP may be worth 1/2 a tenth, which means not getting holeshot.

Using a long rod design on a 100-150HP ACVW streeter?
It's fluff! 
Make the piston clear the crankshaft, make it all fit inside the case and the fan housing, then pick a rod length which joins the dots.

[Martin S.]

This is where you get different opinions depending on who you ask. My builder told me to get the longest rods I could so I order 5.7" rods for my chevy journal 84 crank engine. But he had to do the fitting including modding all the tin to seal with the wider engine to run cool on the street in any weather. It was a ton of extra work, but since he was doing the work it was ok with me! He said, 'look at a BMW engine' as an example. It's an interesting subject, filled with engine stories and lore.

[fish]

Tony, which heads, cam and what brand P&C, ring pack, crank and bearings will you be running with your Scat Chevy 5.325" rods

[teekoo]

Here's the specs:
Scat 84 crank, Silverline bearings
Scat 5.325" rods, Clevite rod bearings
DRD L6 heads
DRD 298 cam, Mahle bearings
Mahle 90.5x82 P&C, Mahle rings
44 IDF's
and so on...

I just checked the clearance and rod big ends go really close to opposite side piston.
So I have decided to forget these short rods. Just need to quickly find 5.5" rods somewhere.

Thanks to all who replied!

-Toni

[fish]

With 5.5" and a healthy dose of compression that thing will boogie, are they the early hand ported DRD or cnc? If later they may need fingering with Emery paper as some of the ones I've seen are pretty rough.

[teekoo]

Compression will be around 10.2. Heads are CNC ported and they are quite nice. I'm actually still waiting their match ported manifolds, but that's another story. There is only couple of places to be handled with the heads.

[fish]

I bet the match ported manifolds were way off. Hand finishing the heads will produce better results IMO.

[teekoo]

Good bet. ☺

[TexasTom]

I'd go with the shorter rods in a heart beat.
TxT

[Zach Gomulka]

So is the clearance the main concern or the faster wear out?

Here's "nice" picture of the difference between 5.325" and 5.5" rods.
Difference in rod angle is only 0.5°. Piston travels 4.5mm closer to the crank.
In the picture the difference doesn't look too much.
Any thoughts?

-Toni

Thank you for posting that. Hard to imagine there would be measurable wear from an extra .5°