Inliners International Forums General Discussion Hi Performance 250 Chevy mods 5.7 rods?

I have been buried in work for the last 3 months and am now returning to the site.
I just read a little about guys modifying 250 GM engines with 5.7 or 6.0 rods.

What gives here? What are the advantages, modifications necessary and costs to do want to do this? Is this useful for race or could it be applied to street too?

James

There are a couple of schools of thoughts on this. One is a longer rod slows down the piston speed, allowing the piston to spend more time at top dead center, building combustion pressure. Another is short rods improve pumping work and blowdown during EVO, which increases power. My engine builder friend in NC is trending toward short rod motors. He has built record holding engines in Stock Eliminator and various tire classes. NASCAR engines still have long rods. I'm still a long rod guy; I'm putting 7.263" rods in my 292. I think forced induction engines benefit from shorter rods. Basically you can pick whichever and there's a theory backing it up.

In case you didn't know, your 250 already has 5.7 inch rods. They are very similar to small journal (283 v8) rods. Identical actually as far as I can tell with less trimming on the side of the big end.

The 6" allows a shorter (lighter) piston. Whether that saves any total weight (i.e., substituting .30" of steel beam for .30" of aluminum skirt), or reciprocating weight is a different question.
It should reduce shaking force very slightly. You can away with a tiny bit shorter skirt since the side-thrust vector is reduced.
These are all very small changes but all positive.
The negative is that it also increases knock sensitivity, and reduces DCR for the same IVC and static CR.
What I'm still trying to make sense of is the effect on breathing. Popular opinion is that with a small/restricted port it helps by extending the period of vacuum over more degrees of rotation (and won't help if the port is generous, like BBC). What makes it complicated is the fact that for the Gen-3 both the port and bowl are not bad, but "busy" (siamese).

<side-thrust vector is reduced>

Just as an example, if you took the short rod to the extreme, you could have the rod journal pushing sideways on the piston pin. Everything is a trade off but this one is something to consider. As small block Chevy rods are too narrow on the big end, where are these 6 inch rods FOR the inline 6 listed?

Crower has billet or street forged units/Sportmans rods


MBHD

I only have time for a short reply now:

Off the shelf 6" connecting rods for the Chevrolet 250:
Crower Sportsman (I beam) SP91211B-6 and SP9311B-6
Scat (H beam) 2-250-600-2100

When GM re-introduced the Chevrolet 250 in Brazil from 1995 to 1998, they redesigned the engine to use 6" connecting rods.

The rod/stroke ratio has been discussed/debated for years. Some say not must practical difference, especially for street engines.

Google "rod to stroke ratio" will produce numerous articles.
A couple of articles:
http://www.hotrodders.com/forum/stroke-rod-ratio-32680.html, see post by 'deuce 454'.
http://www.strokerengine.com/RodStroke.html

Once the engine deck height has been established, 9.150" for the Chevrolet 250 unless the engine is redecked, then the following is jiggled to make it up:

Connecting Rod Length 5.700"
Piston Compression Height 1.655"
Quench Height (piston top to cylinder head) 0.030" in the hole
Stroke/2 3.53"/2 = 1.765"

In the case of Chevrolet 250, increasing the rod length to 6" allows a reduction of the piston compression height to 1.355". The new rods will gain about 40 grams and the new piston will easily loose 100 grams. The over all piston height will be reduced also for less friction. Basically one is going from a early 1960's pistion design to a 1970's or later design. The rod/stroke ratio increases from 1.61 to 1.70.

Making a grand generalization here, but pistons with a compression height from about 1.200" to 1.400" allow comfortable dimensions for good performance, wear, weight, and stability for the street. Ask the custom piston manufacturer technical reps.

For a visual comparision, note the link to Keith Black Chevrolet 327 pistons for 5.7" and 6.0" rods, with 4" bore instead of 3.875" bore of the Chevrolet 250.

327 Piston with 1.385" Compression Height 6.0" Rod (KB154):
http://www.kb-silvolite.com/kb_car/performance.php?action=details&P_id=46

327 Piston with 1.678" Compression Height 5.7" Rod (KB156):
http://www.kb-silvolite.com/kb_car/performance.php?action=details&P_id=44

Never mind - he removed it when I requested.