4.0 litre pistons in a 4.6 block.
Discussion
My old 4.6 ran a set of 4.0 pistons.
These did not have the valve cutouts machined into them, coupled with a typhoon camshaft, standard RR heads and a 72 mm plenum it gave a credible 275 bhp in the early days.
I did not think that any machining was done to the pistons.
These did not have the valve cutouts machined into them, coupled with a typhoon camshaft, standard RR heads and a 72 mm plenum it gave a credible 275 bhp in the early days.
I did not think that any machining was done to the pistons.
Edited by phazed on Monday 21st January 09:34
For clarification purposes, mines a 2000 year serp engine cross bolted etc.
So if I was in the market for a compression hike to 10.1 using different pistons which ones would I be looking for and from what year to what year of manufacture.
I understand shaving a bit off the tops and cut out etc
Does this effect piston strength if staying N/A and sticking below say 340 hp remembering i’m using trigger wheel and coil packs so utilising extra timing advance to support more boom
especially low to mid range which is where I lose out against 5.0 equipped cars.
I also lose out on hp but that’s more heads and induction capacity at higher revs until I have blown heads.
It’s all academic really as this won’t be happening overnight but it’s good to work the basics out so I know what I’m looking to do.
Dom would answer all this in a sentence or two but I like to share this for others to consider such madness,,,,
Only just read above,,, hmmmm like it.
So if I was in the market for a compression hike to 10.1 using different pistons which ones would I be looking for and from what year to what year of manufacture.
I understand shaving a bit off the tops and cut out etc
Does this effect piston strength if staying N/A and sticking below say 340 hp remembering i’m using trigger wheel and coil packs so utilising extra timing advance to support more boom
especially low to mid range which is where I lose out against 5.0 equipped cars. I also lose out on hp but that’s more heads and induction capacity at higher revs until I have blown heads.
It’s all academic really as this won’t be happening overnight but it’s good to work the basics out so I know what I’m looking to do.
Dom would answer all this in a sentence or two but I like to share this for others to consider such madness,,,,
Only just read above,,, hmmmm like it.
phazed said:
My old 4.6 ran a set of 4.0 pistons.
These did not have the valve cutouts machined into them, coupled with a typhoon camshaft, standard RR heads and a 72 mm plenum it gave a credible 275 bhp in the early days.
I did not think that any machining was down to the pistons.
Hmmm, that implies LR used shorter rods in the 4.6 (slightly compromising rod length/stroke ratio) as compression heights (position of the wrist pin in relation to piston top) would be the same between 4.0 and 4.6 pistons, but the 4.0 would have a shallower bowl (less cc) to get higher compression when used in a 4.6? These did not have the valve cutouts machined into them, coupled with a typhoon camshaft, standard RR heads and a 72 mm plenum it gave a credible 275 bhp in the early days.
I did not think that any machining was down to the pistons.
In that case, I'd be inclined to use the longer 4.0 rod and machine the pistons down instead...
[edited to say: probably impractical given the 5.5 mm (ish) height difference ((82 mm -71.1 mm):2) ]
Edited by 900T-R on Monday 21st January 09:33
900T-R said:
Hmmm, that implies LR used shorter rods in the 4.6 (slightly compromising rod length/stroke ratio) as compression heights (position of the wrist pin in relation to piston top) would be the same between 4.0 and 4.6 pistons, but the 4.0 would have a shallower bowl (less cc) to get higher compression when used in a 4.6?
In that case, I'd be inclined to use the longer 4.0 rod and machine the pistons down instead...
[edited to say: probably impractical given the 5.5 mm (ish) height difference ((82 mm -71.1 mm):2) ]
3.9 and 4.0(late) pistons are not interchangeable smaller wrist/gudgeon pin and piston pin height , when fitting 4.0(late) pistons to a 4.6 no rod mods necessary although a balance would be advised IMO , on a side note people also often get the 4.6 and the 4.5 (earlier) bespoke TVR engines mixed up a completely different animal ...... In that case, I'd be inclined to use the longer 4.0 rod and machine the pistons down instead...
[edited to say: probably impractical given the 5.5 mm (ish) height difference ((82 mm -71.1 mm):2) ]
Edited by 900T-R on Monday 21st January 09:33
3.9 Piston .....
4.0 (late) Piston 
No question which is the stronger piston assy 
the 4.0/4.6 piston is lighter also.Edited by Sardonicus on Monday 21st January 11:13
900T-R said:
Um yes but we were comparing 4.0 and 4.6 pistons, the 3.9 doesn't come into it (at least not starting from a genuine 4.6 block)...
Why did you mention it then ? 
900T-R said:
Difference between 4.0 (3.9) and 4.6 pistons; compression height:
Aim: by machining down the piston top (crown) of the 4.6 piston, you'll reduce the capacity of the bowl from liek 22 cc to 12 cc (or so) and the static compression ratio goes up. This is pretty much identical to the factory 4.3 engine (well at least most of them) which uses standard 9.35:1 3.9 pistons, machined down to give a 10+:1 comrpession ratio and we all know how well they go. smile
Note that you will need to have valve reliefs cut into the piston tops or you'll be very restricted in your choice of camshaft.
'4.0' and '4.6' heads are for all intents and purposes identical. However there's an appreciable difference in cc between 14-bolt (older) and 10-bolt (later) heads.
spitfire4v8 said:
Why did you mention it then ?
I was under the impression we had progressed from there somewhat. In short: if 4.0 pistons are a direct replacement for 4.6 ones in a 4.6 engine, and deliver a higher static c.r. in said 4.6, it follows that the 4.6 engine uses shorter rods than the 4.0 (by half de difference in stroke) and that the 4.0 pistons have the same compression height but a shallower bowl. Which is different from what I had assumed originally, soz for that.
So to build from a late 4.6 RR cross bolted block
New top hat liners
A set of late 4.0 pistons ( high comp) new rings balanced with rods and crank.
Machining of bores and crank
New shells.
BV heads ported to a mm of their lives, and induction mods to suit.
Depending on Cam choice possibly cut outs in piston crown.
Vernier timing gear
Lightened flywheel and a re balance of pulley wheel.
Is this the basics of it.
I’m starting to see why people go for forced induction.
That lot above is a small fortune for what it will actually achieve.
Under 12 seconds,,,,,,well worth it
New top hat liners
A set of late 4.0 pistons ( high comp) new rings balanced with rods and crank.
Machining of bores and crank
New shells.
BV heads ported to a mm of their lives, and induction mods to suit.
Depending on Cam choice possibly cut outs in piston crown.
Vernier timing gear
Lightened flywheel and a re balance of pulley wheel.
Is this the basics of it.
I’m starting to see why people go for forced induction.
That lot above is a small fortune for what it will actually achieve.
Under 12 seconds,,,,,,well worth it
Classic Chim said:
So to build from a late 4.6 RR cross bolted block
New top hat liners
A set of late 4.0 pistons ( high comp) new rings balanced with rods and crank.
Machining of bores and crank
New shells.
BV heads ported to a mm of their lives, and induction mods to suit.
Depending on Cam choice possibly cut outs in piston crown.
Vernier timing gear
Lightened flywheel and a re balance of pulley wheel.
Is this the basics of it.
I’m starting to see why people go for forced induction.
That lot above is a small fortune for what it will actually achieve.
Under 12 seconds,,,,,,well worth it
Yeah, but you'll have a box fresh engine to start with and the extra cost compared to a std rebuild is not huge as you're still building from 'standard' parts. With forced induction you are at best deferring those costs to a later point, at best. New top hat liners
A set of late 4.0 pistons ( high comp) new rings balanced with rods and crank.
Machining of bores and crank
New shells.
BV heads ported to a mm of their lives, and induction mods to suit.
Depending on Cam choice possibly cut outs in piston crown.
Vernier timing gear
Lightened flywheel and a re balance of pulley wheel.
Is this the basics of it.
I’m starting to see why people go for forced induction.
That lot above is a small fortune for what it will actually achieve.
Under 12 seconds,,,,,,well worth it
Personally I'd stick with intermediate large valves rather than the TVR 'Big Valves'(also known as 'ultra lage valves' outside the TVR world on a road engine up to 4.6 litres capacity.
Where things might get expensive (depending on your starting point) is the induction side of things and how to control it all (some form of mapped ignition at a minimum IMHO).
Classic Chim said:
That’s a very kind gesture and not the first from many great Tvr friends. 
What are they actually worth though.
Ok you have made them somewhat second hand lol, I’m sure they have more than enough life left in them for my little old car but just out of interest what could you get for them on the open market.
They’d end up going hard by the time I get to use them.
What size wheels do they need?
16 x 255 x 50What are they actually worth though.
Ok you have made them somewhat second hand lol, I’m sure they have more than enough life left in them for my little old car but just out of interest what could you get for them on the open market.
They’d end up going hard by the time I get to use them.
What size wheels do they need?
spitfire4v8 said:
Why did you mention it then ?
900T-R said:
Difference between 4.0 (3.9) and 4.6 pistons; compression height:
Aim: by machining down the piston top (crown) of the 4.6 piston, you'll reduce the capacity of the bowl from liek 22 cc to 12 cc (or so) and the static compression ratio goes up. This is pretty much identical to the factory 4.3 engine (well at least most of them) which uses standard 9.35:1 3.9 pistons, machined down to give a 10+:1 comrpession ratio and we all know how well they go. smile
Note that you will need to have valve reliefs cut into the piston tops or you'll be very restricted in your choice of camshaft.
'4.0' and '4.6' heads are for all intents and purposes identical. However there's an appreciable difference in cc between 14-bolt (older) and 10-bolt (later) heads.
Thanks for that ^ 900T-R said:
Difference between 4.0 (3.9) and 4.6 pistons; compression height:
Aim: by machining down the piston top (crown) of the 4.6 piston, you'll reduce the capacity of the bowl from liek 22 cc to 12 cc (or so) and the static compression ratio goes up. This is pretty much identical to the factory 4.3 engine (well at least most of them) which uses standard 9.35:1 3.9 pistons, machined down to give a 10+:1 comrpession ratio and we all know how well they go. smile
Note that you will need to have valve reliefs cut into the piston tops or you'll be very restricted in your choice of camshaft.
'4.0' and '4.6' heads are for all intents and purposes identical. However there's an appreciable difference in cc between 14-bolt (older) and 10-bolt (later) heads.
thought I was going loco for a moment 
900T its not just the C/rods length that maketh the 4.6 but crank throw too Sardonicus said:
hanks for that ^ thought I was going loco for a moment 900T its not just the C/rods length that maketh the 4.6 but crank throw too
That's a given, if the stroke wasn't longer you wouldn't need shorter rods to end up at the same deck height if the pistons have the same compression height. thought I was going loco for a moment 900T its not just the C/rods length that maketh the 4.6 but crank throw too 
t did I just say that out loud) 
900T-R said:
That's a given, if the stroke wasn't longer you wouldn't need shorter rods to end up at the same deck height if the pistons have the same compression height.
Wasn't being funny 
but seeing as you run off track initially I thought I would just add that bit, its not a pissing contest after all 
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