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3.2 Rods & Pistons in a 3.9?

4K views 28 replies 10 participants last post by  NZ 
#1 ·
Hey guys.

An idea I had toyed with in the past was to set up an I6, using a 3.9L Crank, 3.2L Rods and Pistons. Assuming the 3.2&3.9L heads are the same, use a 3.9 head, or if the 3.2 has a smaller chamber, the 3.2 head

The purpose of this would be to create a higher comp stroke, just like the V8 boys do with there mixture of 302 and 351 goodies.

Any opinions/Info on the topic?

Cyaaa
 
#2 ·
The idea is good as you end up with a better rod/angle ratio, and less pistong speed, both of which are a good thing.
Engine is able rev higher with ease.

However the problem I see is the pistons would stick up out of the block because of the extra length of the rod.
You would need a special piston made with the pin set higher.

That's what also needs to be done when using 302c rods in a 351c, although pistons are available off the shelf for this mod.

Something worth looking into though is AU 4L pisons as I believe Ford increased the rod length for these engines.

Rick.
 
G
#3 ·
I'm pretty sure the only difference between the 3.9L and the 3.2L is the physical block height(bore stroke isn't as long) and I'm guessing the piston rod lengths.

Therefore I can't really see using any 3.2L components is gonna help. If anything it will reduce compression(but hey... thats good if you wanna turbocharge it) ;)
 
#5 ·
Pistons are the same and the block is the same.

The only things which are different are the crankshaft, conrods, and head chamber volume.

3.2L specs.
Crankshaft has a 20mm shorter stroke.
Conrods are 10mm longer.
Head volume is 3cc less.

The decreased head volume is because of the decreased stroke, so as to maintain the same CR as the 3.9.

The conrods are 10mm longer so as to keep the same piston height in the bore at TDC, because of the decreased stroke.

As I said in my other post, it would be a good thing to do, however pistons with a 10mm higher pin height would be needed.

CR would stay the same if the new pistons had the same shape as the old ones.

Rick.
 
#7 ·
NZinAUS said:
Use the 3.2 pistons on the 3.9 rods would be the easiest way. Would be very hard to get a 3.2 head if they are in fact different.
Pistons are the same.
In any case, this would only change CR (if the pistons were different), I think the OP wants to increase revability in the same way it can be done with 351 & 302 Clevelands.
IE, better rod/angle ratio.

Rick.
 
#10 ·
Hi Karl,
I would have to look up the spec's for the rod length, and the Crankshafts were also different between the two engines. From memory if you fit a set of 3.2 pistons to a 3.9 crank (and 3.9 rods?) then you end up with a compression ratio around the 10:1 using the standard bore. The standard 3.2 and 3.9 were both 8.8:1 compression ratio and the 3.2 heads were different to the 3.9. From memory the combustion chambers had a different volume between the two heads (but i would need to look at the spec's to confirm this), there is also different port sizes through the various levels of MPEFI heads as you move through the models too. I would need to have a look and a measure up to refresh my memory on this conversion, but we have done this type of thing in the past with the 't-drive' and it worked really well. Talk with you tonight, bye

That is an Email from Scott Hart. The 3.2 pistons are flat topped.
 
#12 ·
NZinAUS said:
Hi Karl,
I would have to look up the spec's for the rod length, and the Crankshafts were also different between the two engines. From memory if you fit a set of 3.2 pistons to a 3.9 crank (and 3.9 rods?) then you end up with a compression ratio around the 10:1 using the standard bore. The standard 3.2 and 3.9 were both 8.8:1 compression ratio and the 3.2 heads were different to the 3.9. From memory the combustion chambers had a different volume between the two heads (but i would need to look at the spec's to confirm this), there is also different port sizes through the various levels of MPEFI heads as you move through the models too. I would need to have a look and a measure up to refresh my memory on this conversion, but we have done this type of thing in the past with the 't-drive' and it worked really well. Talk with you tonight, bye
That is an Email from Scott Hart. The 3.2 pistons are flat topped.
Also the 3.2 has much smaller valves.
Intake 42.03mm, Exhaust 35mm (3.2)
Intake 47mm, Exhaust 39mm (3.9, 4.0)
I stand corrected.
If all you wanted to achieve was increased CR, it would be easier and cheaper to have the head skimmed and fit a thinner head gasket.
Fitting the longer rods (3.2) with an appropriate piston will improve the rod/angle ratio, which will achieve better revability.
If it were me, I'd look for rods out of an AU engine, as I'm pretty confident they went to a longer rod, though I have a feeling I read somewhere they also changed the big end size, so it may be easier to use a 3.2 rod, and perhaps an AU piston??

I forgot about the different valve sizes.

Rick.
 
#13 ·
hrmmmm, very inresting.......looks like there may be a few methods for easly increasing compression in the I6's by using various parts from various engines.

I would think using 3.2pistons with 3.9crank+rods+head would result in comp around 10:1, and with head shaving compression could be braught up arround 11:1. perfect for LPG :)

The only problem i can see using flatops in the 3.9/4.0l is with high lift cams the valve may touch the piston crowns,

It would be intresting to get a rundown of the dimensions of

EA 3.2 crank, pistons and rods
EA 3.9 crank, pistons and rods
EBII/EL pistons and rods
AU pistons and rods

such as pin height, dish volume, rod length and bore diameter
 
G
#14 ·
3.2L specs.
Crankshaft has a 20mm shorter stroke.
Conrods are 10mm longer.
Head volume is 3cc less.
Hmm hang on, if the Rods are 10mm longer it'd smash into the head and the piston rings would pop out of the top of the bore if you put it on a 3.9L crank wouldn't it? There is definately not 10mm of extra height available for the pistons can go... maybe 3mm max
 
#15 ·
donuts2001 said:
Hmm hang on, if the Rods are 10mm longer it'd smash into the head and the piston rings would pop out of the top of the bore if you put it on a 3.9L crank wouldn't it? There is definately not 10mm of extra height available for the pistons can go... maybe 3mm max
That's why you would need pistons made with a higher pin height.
It's quite common to fit 302c rods to the 351c.

Rick.
 
#19 ·
EF bottom end is the same apart from different designed crank etc.
EF gained increased compression from using less of a dish in the pistons.

EF vs EA pistons - note the dish is around half the depth in the EF


Side veiw. Much more refined wouldnt you agree?


EF 12 counterweight VS EA 6 counterweight
 
#20 ·
there is only a 1.15cc difference in the dish volume between the two. also the ef pistons have a higher compression hight which will increase the compression as well.
does anyone know what the deck hight is ? there may be some room to shave some metal off the top off the block too.
 
#24 ·
Can somebody please explain, maybe I'm just dumb, but if the block is the same then shouldn't the 3.2 pistons, rods, and crank bolt straight in and run?
After all, the deck height is the same, therefore the crank must allow for the extra length of the rods, i.e. 20mm for the crank and 10mm for the piston.
I don't know how the crank stroke is measured, but looking at that one would assume it is thru the entire revolution. Therefore, 20mm shorter means an allowance of 10mm at TDC and BDC for the 10mm longer rods.
 
#25 ·
Yes it'll bolt in. But then youd have yourself a 3.2 l motor with 8.8:1 compression assuming you also use a 3.2 head. Im presuming a 3.9 head will up the compression.

So i guess a mod for 3.2 owners would be in theory to use a 3.9 head for increased compression and bigger valves
 
#26 ·
yeah thats sorta what I was thinking.
I wonder if anybody does stroker kits for these motors?
Heh....not that I can afford it right now anyway
 
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