4.6 with 4.0 pistons

KyleT

Well-known member
Mar 28, 2007
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Fort Worth, TEXAS
whats the advantage or reason to run 4.6 rods and crank with 4.0 pistons? are they the same? i have seen a few references to this lately....
 

no694terry

Well-known member
Sep 29, 2009
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pittsburgh, pa
I thought I read somewhere 4.0 pistons will increase compression in a 4.6. And I imagine you would need a 4.6 crank to run 4.6 rods because I assume stroke is longer. I could be wrong. I don't think raising compression in these motors will benefit anything but the head gasket manufactures
 

FB111

Well-known member
Jun 7, 2004
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If it raises the compression a point you might get another 10 hp and torque.
 

no694terry

Well-known member
Sep 29, 2009
983
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pittsburgh, pa
i thought a 4.6 was just a longer stroked 4.0, so im my thinking if you used 4.0 rods on a 4.6 crank with 4.6 pistons you would have like 4:1 compression
 
Apr 20, 2004
6,575
45
Lexington, Va
rovercanus said:
Yes. It will compress your pistons into your heads.

You are correct sir!

I have a 4.6 with 4.0 pistons...and a 4.6 without.. Both were installed in d90 trucks..both engines completely redone... 4.6 without or with you are not going to notice a huge increase. Going from a 3.9 to a the 4.6 with 4.0 pistons allowed my d90 5spd to run through the hills in 5th at 65-70 without dropping back into 4th.

Is it a Ferrari ?? Hell no.. is it more powerful in the hills...absolutely.
 
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mbrummal

Well-known member
Jan 23, 2009
2,851
2
Montgomery, AL
no694terry said:
i thought a 4.6 was just a longer stroked 4.0, so im my thinking if you used 4.0 rods on a 4.6 crank with 4.6 pistons you would have like 4:1 compression
he 4.0 rods are longer. that combined with the longer stroke of the 4.6 crank would result in:

rovercanus said:
[compressing] your pistons into your heads.
 

FB111

Well-known member
Jun 7, 2004
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You can't/shouldn't run a piston on a rod that doesn't near the top of the cylinder. You screw up the squish area and can end up having detonation even though you have lowered the compression. In this case the piston is going to be about 3/10 of an inch below the top of the cylinder. I might run but it would certainly be a dog.
 

FB111

Well-known member
Jun 7, 2004
475
0
You don't lower the compression by running a shorter rod. You do it by enlarging the dish in the piston.
 

Ppaulred96

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Jan 18, 2007
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0
Virgina
KyleT said:
whats the advantage or reason to run 4.6 rods and crank with 4.0 pistons? are they the same? i have seen a few references to this lately....
I currently am running this set-up (4.6 crank & rods with 4.0 pistons) and have done so for the last few years. I have a 96 GEMs disco running 4.6 crank 4.6 rods with 4.0 pistons installed on the 4.6 rods. You gain compression up to about 10.9 : 1. I am also running a 4.6 cam and using my old 4.0 heads, intake etc.

I noticed a nice gain in both torque and power. Enough that I really like this set-up. I've been told by others that if I were to put a chip in the GEMs computer I'd see even more gains but frankly I like what I have. It is much better than a 4.0 - a very nice upgrade.

I will share as to not over sell this, good friend has a stock 4.6 Bosh in a similarly outfitted disco II and I think the stock Bosh 4.6 still might have an edge on what I have (maybe the bosh injector system or computer?) BUT the change was such an improvement from the old 4.0 I feel good about telling folks it is worth doing - - I am not sure I would have needed the 4:10's if I had this motor first.

I DO think a chip would give more power to the truck but I do not have any knocking or pinging problems and it performs well much better than it did prior without a chip added. Looking into a chip it looked expensive so I have not gotten there yet. The engine has also held together nicely - - even though the first weekend it was in the limiter multiple times.

Others have it correct you cannot nun 4.0 rods on a 4.6 crank as you will hit the heads - the 4.0 & 4.6 pistons are the same height from the wrist pin to the top of the piston the difference is in the dish size of the pistons - the 4.6 has a larger dish thus why lesser of a dish of the 4.0 piston gives you more compression.
 

DOHC Holliday

Well-known member
Jul 20, 2006
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Western CT
Now does anyone know if this works on a serpentine 3.9 using the 3.9 block and pistons w/ maybe a Gems 4.6 crank and rods?

Edit - Answer found. Run a 4.0 gems block and it will work according to my searches...
 
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