brenx

I've been looking into rod ratios on 383/393-6/406-8 strokers. I've also been reading up on acceptable rod ratios. I'm curious to see what you guys think.

From what I've seen. Most are using either a 6" rod or 6.125" rod. My thoughts are doesn't the 6.125" rod push the limits too far and also push the gudgeon pin too far up into the oil rings.

What do you guys think is an acceptable rod ratio for something that rev's to around 7000rpm?

Here's an example

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XB Fairmont (street car) 11.07@123.02mph.

xdclevo

The 3.85 stroke would be a 393.
If you were after street life as well, i would opt for the 6 inch rod, as i've never been keen on the idea of the pin intersecting the ring grooves.

tony xa

Most 3.850 cranks are set up for 6.125 rods and there are pistons avail that are designed to be used with the 3.850 crank using 6.125 chev rods so the pins shouldn't intersect the ring grooves.
The above is not from a expert so don't treat as 100% correct

I did notice that most americains run 6 inch rods on their 393s
I just think if the cranks are advertising 6.125 then it should be easier and correct also easier to get the correct pistons to.

brenx

I'm no expert in those regards but I'm interested in finding out how it effects an engine. I know the 393/396 has bugger all difference piston wise to a 406/8. With a 383 there isn't much difference either. The 6" rods don't look too far off intersecting but 6.125" one definately do.

It's a whole new ball game to me. I was thinkign about buying a kit this week but the bills have killed that idea. Maybe a blessing in disquise though as I'm still sussing out parts. It's damn confusing.

I want to have it all figured out rather than pay a machinist stupid amounts of money when they hit me for hidden costs.

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XB Fairmont (street car) 11.07@123.02mph.

tony xa

This was the part that got to me when I first started looking into the 393 .
I just went with the 6.125 because thats what the crank that I brought said to use .
and also the pistons that were avail also suited this set up

davis

My 393 has 6" rods and the pins intersect the oil ring.



...I can't imagine a 6-1/8th" rod not moving the pin higher up in a 9.2 deck. My deck clearance was .008".

This is about the easiest thing to sort out...just measure your deck and do the math...gents! Let's say that your block measures to: 9.204" before machining. Figure that you're going to clean it up .004"-.006"

9.200 (possibly 9.198)

1.925 (2.00 for a 4" stroker)
6.125
------
8.050

9.200
8.050
------
1.150" - 1.148"

You don't want to be under 1" for the pin height. Also, if you're intending to run reverse dome pistons, you're probably better off going with a 6" rod to gain a bit more room under the dome.


Also, no problem spelling Americans as americains. I've seen far too many americains spell Australian as austrailian. It must be why so many Aussies call us Yanks?


:davis:

davis

Brenx,

It affects the engine performance to practically no degree worth fussing over.

The difference between a 393 and a 408 piston is as follows:

9.200 (nominal)

1.925
6.125
------
8.050

9.200
8.050
------
1.150" pin height for a 383

2.000
6.125
------
8.125

9.200
8.125
------
1.075" pin height for a 408

1.150
1.075
------
0.075" difference between a 408 and a 393 pin heights using a 6.125" rod.

That extra 1/8" of rod length doesn't matter very much, use what fits the best for your block dimensions. I thought that we sorted through all of that while "Brahma-yo-mamma" was still with us rather than being out on some supernatural plain way above us. The way to go is to ensure that you have adequate ring spacing and pushing too high up into deck won't give you any more performance.


:davis:

tufLTD

Hi Brenx,
I'm building a 383 stroker but I'm using 6" rods. This increases the rod ratio up to 1.6, reducing side loading on the pistons. A lower rod ratio increases rod angularity, placing a lot more side loads on the pistons. I'm definitely no expert but it seems logical to me.

brenx

hehe Davis. Sometimes my memory is like a sieve. Seems to me if the block supports it a 6.125" rod will do the job. I'm more leaning toward 6" rods but we'll see what is offered in other kits. I still have to find a virgin block. That way I can be sure I have the deck height.

I know where there is a bare block but it's been sitting in the weather. It'd be no issues but I perfer to get a known running engine.

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XB Fairmont (street car) 11.07@123.02mph.

Racer

Its not just a matter of chopping and changing between rod lengths. The reason stroker rods are longer, is to allow the crank counterweight to clear the piston skirt.

Windsor guys have it easier. 393 = 351 rods and 302 pistons - its that easy!

PS:
There is nothing in rod lengths in a street or street/strip car. Just use what fits and balances the easiest.