question about 4.6 pistons....
Discussion
Someone better informed will probably confirm or refute this but I think you will find that the con rods are different lengths and this may mean that the compression heights of the pistons are not be the same, unless the con rod length is exactly compensating for the change of stroke which isnt necessarily the case. The pistons on my 4.6 have valve reliefs also and i dont know if this is the case on the 4.0 / 3.9. So in short , I dont know. that wasnt very helpful really was it.... sorry.
brummiewedge said:
I think, the 4.6 pistons will give a lower comp ratio. When i built my 4.6 i used late 4.0 pistons on 4.6 crank and rods. This gave me a comp ratio of 10.25:1 if i remember rightly. You can see the differance in the tops of the pistons plain as day.
I was wondering about this today, I have a got 4.0 pistons for my 4.6.
How did you arrive at 10.25?
A
neal1980 said:
Will 4.6 pistons from a large journal range rover engine fit my 1999 3.9 serp engine?? I have a set and have heard they are stronger than the 3.9 ones.
They are the same diameter, if so will they go on my 3.9 standard rods?
Cheers
Neal
They are the same diameter, if so will they go on my 3.9 standard rods?
Cheers
Neal
If the serp engine you have also has the large journal crank then things like rods and pistons are interchangeable to some extent. Basically the 4.0 and 4.6 pistons have different compression heights. Can't see there being any real difference in strength between the 4.0/4.6 pistons though.
If you have the older style crank, rods and pistons then the crossbolted engine stuff is way better but none of it will fit
Boosted.
Other fellow RV8 builders in the area ( off roaders etc.) have built motors with this spec, so i built the motor using their experiance. I was told by the others that this was the comp ratio that it would be at, so i confess i have never worked it out for myself. This is using composite gaskets. If you use the tin gaskets you can go even higher, but pinking might be a problem then. Again others have done it already! Mine pinks now if you use crummy fuel, and the timing isnt at silly settings.
Just in case we are creating confusion for the anyone not so well up technically we are now talking about compression height and compression ratio as well. I thought I'd point out that compression height is: the distance from the centre line of the gudgeon pin to the top of the crown, as distinct from CR which is: the ratio of the volume above the piston at BDC to that at TDC.
Clearly the compression height will affect CR, but the point is that the CH determines the relationship between the crown and the block deck and if it is too great then the piston crown will be above the deck at TDC and possibly physically come into contact with the head
So for a given rod a lower CH will be safe but reduce CR (and power) but a greater CH is potentailly a big problem (unless remachining the pistons to suit).
Basically CH is varied to allow the use of different con rod lengths but maintain the crown at or just below the deck height at TDC. Con rod length will affect the torque curve and piston speed ( shorter rods giving peak torque at lower revs but higher piston speeds and acceleration - and so more stress at a given rpm - and longer rods the opposite).
Clearly the compression height will affect CR, but the point is that the CH determines the relationship between the crown and the block deck and if it is too great then the piston crown will be above the deck at TDC and possibly physically come into contact with the head
So for a given rod a lower CH will be safe but reduce CR (and power) but a greater CH is potentailly a big problem (unless remachining the pistons to suit).
Basically CH is varied to allow the use of different con rod lengths but maintain the crown at or just below the deck height at TDC. Con rod length will affect the torque curve and piston speed ( shorter rods giving peak torque at lower revs but higher piston speeds and acceleration - and so more stress at a given rpm - and longer rods the opposite).
GreenV8S said:
tahiti450 said:
Basically CH is varied to allow the use of different con rod lengths but maintain the crown at or just below the deck height at TDC.
... or just above, in some cases. Mine ran proud of the block before I put the [whisper]decompression plates[/whisper] in.
Ah but we're talking rovers here so the squish is piss poor and pistons are below the deck, usually by a huge amount
Boosted.
neal1980 said:
my engine is a 1999 3.9 small journal. The pistons are 4.6 from a range rover large journal engine. So will they fit to my 3.9 rods and will they lower or increase my CR
cant put any question marks in cause im on a foreign keyboard and i cant find it..
cheers
cant put any question marks in cause im on a foreign keyboard and i cant find it..
cheers
Not a chance. Pin diameters are different as are the rod lengths. They won't fit, sorry.
Boosted.
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