4.6 with 4.0 pistons

terryjm1

Well-known member
Jan 23, 2011
465
59
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.
Assume you have to use premium high octane fuel? Really old post so if anyone else wants to chime in, please do.
 

robertf

Well-known member
Jan 22, 2006
4,193
78
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Youre already supposed to be running premium with factory pistons


Mine is extremely picky about fuel. It wasnt a problem when it was just a daily driver in a big city, but high octane fuel has been unavailable on some of the trips Ive take it on and the engine doesnt like that
 
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terryjm1

Well-known member
Jan 23, 2011
465
59
Perusing piston options listed for the 4.6, there are low compression pistons 8.13 and high compression 9.35.

Are the 8.13 pistons 87 octane compatible?
 

robertf

Well-known member
Jan 22, 2006
4,193
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Perusing piston options listed for the 4.6, there are low compression pistons 8.13 and high compression 9.35.

Are the 8.13 pistons 87 octane compatible?
they werent ever sold in the US and the rest of the world has different ways of rating detonation prone fuel, so who knows

Since whatever octane fuel you buy these days is diluted with lower btu ethanol it doesn’t really matter what they said it needed in 1995
 

terryjm1

Well-known member
Jan 23, 2011
465
59
Maybe try a couple or gallons of 87. if it knocks at all, fill it with 93. Next fill up, try a few gallons of 89, if it knocks, just accept I have to go with 93? I haven’t yet had a V8 Discovery but am in the process of deciding how to rebuild the engine in a new to me D1 I purchased not running.

A coworker has an infinity that is supposed to need 93 octane and with over 200,000 miles he has always put 89 in with no issues. I dunno...
 

Discomania

Active member
Feb 10, 2017
25
4
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Engines have knock sensors that retard timing when detonation is detected, so running 89 in a 93 vehicle isn't bad, but you're not realizing all the power the vehicle could be making with timing at the right point.
 

robertf

Well-known member
Jan 22, 2006
4,193
78
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Engines have knock sensors that retard timing when detonation is detected, so running 89 in a 93 vehicle isn't bad, but you're not realizing all the power the vehicle could be making with timing at the right point.
if the cyl head temps are too high retarding ignition wont do it
 

jastutte

Well-known member
Nov 10, 2009
283
16
Maybe try a couple or gallons of 87. if it knocks at all, fill it with 93. Next fill up, try a few gallons of 89, if it knocks, just accept I have to go with 93? I haven’t yet had a V8 Discovery but am in the process of deciding how to rebuild the engine in a new to me D1 I purchased not running.

A coworker has an infinity that is supposed to need 93 octane and with over 200,000 miles he has always put 89 in with no issues. I dunno...
drives a fucking Infinity but won't penny up for premium?