Connecting Rod Length

Building a new engine and finalizing the parts list - can't decide what length rods to go with.
5.4 or 5.5?
Any suggestions will be appreciated.

Here are the specs on my engine:

Type 1 - 2110cc dynamically balanced engine

engine case - CB Performance aluminum case, 10 mm stud inserts, full flowed

crankshaft - CB Performance Super Race, 82 mm, VW journals, Aircooled.Net crank damper

connecting rods - CB Performance ? Super Race w/ARP bolts

flywheel - CB Performance lightweight 12.5 lbs, 8 dowel

gland nut - CB Performance chromoly

pistons & barrels - Mahle 90.5mm

head studs - CB Performance 10 mm chromoly

cylinder heads - CB Performance 044 Super Mag 42/37mm ported/polished heads

carburetion - dual 44 mm IDF Weber carburetors, CB Performance hex bar linkage and CB match/ported big beef intake manifolds

exhaust - A-1 merged 1-5/8"

rocker shafts - Scat forged 4340 chromoly rockers 1:4:1, Gene Berg swivel feet adjusting screws

camshaft - Webcam 86b

pushrods - lightweight chromoly

clutch - stage 1 Kennedy w/200 mm disc

oil system - full flow - Gene Berg 26mm pump, Gene Berg 1-1/2qt aluminum oil sump, external oil cooler/fan with thermostatically controlled switch

cooling - Jake Raby DTM T-1 fiberglass fan shroud with type 4 oil cooler

ignition system - Bosch 009, Compu-Fire ignition module

David,

I'm no hot-rod air-cooled engine mechanic (Type 1 OR 4), just a guy who reads too much during the winter, so take this for what it is- some guy you don't know, offering advise on the web.

That motor looks VERY similar to the one I just had built. My engine is a 2110, with heads and carburation by AJ Sims. My philosophy with the build is centered around torque, and the use of AJs Kadrons with his custom 46mm throttle bodies. Kads are plenumn based carbs and have a single plenumn in the intake manifolds, rather than individual runners like your 44s- I built around this with my cam and valve selection, this is the main difference in our builds.

I've got ported heads with 40x37 valves- the exhausts seemed a bit oversized, but AJ recommended them, so I went with them. I've got an FK43 cam with Scat solid 1.4 rockers. I'm running a vacuum advance distributor and 9:1 compression.

The crank, pistons, and flywheel are the same as yours. I'm using H-rods in the stock length to increase piston speed. There is a lot of unburned mixture hanging around in the intake manifolds with my set-up, so any advantage I can get in accelleration is what I was after (short rods, vacuum advance, higher compression). Your needs may be different, but I don't think you are going to get way off with either rod-length selection.

I don't have the engine in the car yet, but AJ dynoed it- it made 141 lb/ft torque from 3000 to 6000 rpm- that's a lot of power right where I want it. I'll be interested to see how the throttle response is.

Good luck.

David, as per Gene Berg's gospel, he recommends using the Porsche width rod when using 82 & 84 mm strokes to add strength. Regarding length he recommends using 136 mm (5.354") for best torque or the stock 137 mm (5.4") as the longest.

Longer the Better!

Jack

A while back on the cal look forum some one dynoed the same motor I think 3 times with 3 different lengths of rods. Each time it made the same power and torque. The rod length made no difference. You should be able to search for the thread. I ended up with the Scat stock length rods. They came with the good bolts (That

Bruce,

That's the thing- bigger isn't always better. I got 041 heads with a port job and moderate valve sizes. I'd be happy to give up 10 hp on top, for more torque, sooner, down low.

David,

Why don't you just ask CB? You are buying the case, crank, heads and probably the pistons/barrels from them. And, rods from them too!They know exactly the correct rod length for you.

Plus, if you decide what you want your compression ratio to be, they should be able to predict the correct barrel spacer shims as well.

Why not 94 mm pistons, and Stage 2 clutch?
Mark

With smaller VW stroker engines rod length of 5.4 is OK (like 1,915cc), I would use 5.5 or 5.6 on an 82mm stroke 2,110cc engine, and 5.6 or 5.7 on an 84mm or 86mm stroker. Longer rods mean power at higher RPM and less piston speed, but the engine gets wider which can cause problems fitting the cooling tin or getting the engine into a car.

Depends on whether or not you want more power in the upper rev range, or you want to minimize piston/bore/ring wear.

A rod ratio of 1.65 to 1 is normally a pretty fair compromise.

Ditto, go with the longer rod with shims to compensate!