4.0 litre pistons in a 4.6 block.
Discussion
Just thinking ahead, maybe quite some way ahead.
What’s the difference between the 4.0 and 4.6 pistons.
If I wanted to build a 4.6 block with 4.0 pistons what should I be aware of.
Are they the same length etc
Other than a slight raise in compression what other benefit does this mod offer.
Will 4.6 heads be ok with said pistons.
Thanks
What’s the difference between the 4.0 and 4.6 pistons.
If I wanted to build a 4.6 block with 4.0 pistons what should I be aware of.
Are they the same length etc
Other than a slight raise in compression what other benefit does this mod offer.
Will 4.6 heads be ok with said pistons.
Thanks
Edited by Classic Chim on Sunday 20th January 13:41
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.
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.
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.
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.
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.
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 Eric.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.
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.
I better thrash out the idea with Dom and decide if I can even be bothered with it but 10.1 comp, flowed heads etc etc does sound nice.
It all revolves around a job that might be coming my way so I’m just thinking aloud until then anyway.
Thanks again
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.
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.
Maybe I am confused here but best not confuse the 3.9 or interim pre/serp serp common in the Griffith & Chimaera with the 4.0 big journal version I know displacement is the same but 3.9 Vs 4.0 makes things easier in my book 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.
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.
machining any material from the piston crown will always lower CR unless you bring things back by decking the block or using a longer con-rod the 4.3 rotating assy has little in common to the 4.0/4.6 engine 
different rods,small journal crank,and different pistons inc smaller pins and different pin heights (non interchangeable) 4.0 pistons fit straight into the 4.6 no machining necessary for an easy CR hike ...... however with your engine Alun I would check that deck height 1st your builder may of punched up the CR by decking the block already and 4.0 pistons may push things a little to far for your purpose/future plans ordinarily its wont like I say maybe I misunderstood post 
Edited by Sardonicus on Sunday 20th January 21:29
Sardonicus said:
Maybe I am confused here but best not confuse the 3.9 or interim pre/serp serp common in the Griffith & Chimaera with the 4.0 big journal version I know displacement is the same but 3.9 Vs 4.0 makes things easier in my book machining any material from the piston crown will always lower CR unless you bring things back by decking the block or using a longer con-rod the 4.3 rotating assy has little in common to the 4.0/4.6 engine different rods,small journal crank,and different pistons inc smaller pins and different pin heights (non interchangeable) 4.0 pistons fit straight into the 4.6 no machining necessary for an easy CR hike ...... however with your engine Alun I would check that deck height 1st your builder may of punched up the CR by decking the block already and 4.0 pistons may push things a little to far for your purpose/future plans ordinarily its wont
Thanks Simon, I did wonder, Eric’s not to know I’ve had the block decked as thats a secret machining any material from the piston crown will always lower CR unless you bring things back by decking the block or using a longer con-rod the 4.3 rotating assy has little in common to the 4.0/4.6 engine different rods,small journal crank,and different pistons inc smaller pins and different pin heights (non interchangeable) 4.0 pistons fit straight into the 4.6 no machining necessary for an easy CR hike ...... however with your engine Alun I would check that deck height 1st your builder may of punched up the CR by decking the block already and 4.0 pistons may push things a little to far for your purpose/future plans ordinarily its wont 
I’d have to go top hat so never a cheap option.
Classic Chim said:
Thanks Simon, I did wonder, Eric’s not to know I’ve had the block decked as thats a secret
I’d have to go top hat so never a cheap option.
Depends how much I’d have to go top hat so never a cheap option.
still do-able on your engine IMO you dont need to top-hat to deck I removed 1.6mm from mine 
no chamfer on my liners anymore due to this only a positive deck height , i.e pop out pistons 
Edited by Sardonicus on Sunday 20th January 21:41
Classic Chim said:
That’s a sexy pic that is
I was referring to future reliability more than anything.
I still think deep down I’d be mad to change anything as it’s just perfect for the road.
I wouldnt change your motor unless you need more , that 4.0 piston mod will give you a bit more low down punch and more scope for a bigger cam to do its thing but the costs just not justifiable TBH if you went proper maxed out BV heads that CR/4.0 piston mod and spikier cam then that may change .... but the costs .......I was referring to future reliability more than anything.
I still think deep down I’d be mad to change anything as it’s just perfect for the road.
for not a great deal more in the real world road etc Sardonicus said:
I wouldnt change your motor unless you need more , that 4.0 piston mod will give you a bit more low down punch and more scope for a bigger cam to do its thing but the costs just not justifiable TBH if you went proper maxed out BV heads that CR/4.0 piston mod and spikier cam then that may change .... but the costs ....... for not a great deal more in the real world road etc
Yeah I think your right, sorry I know your right. for not a great deal more in the real world road etc As it is now the cars fast, reliable and smooth as a babies,,,,
Id just enjoy doing it, lots of advice etc and just systematically do it. I loved building little bike engines as a boy so just an extension of my youth with precision thrown in.
I have huge admiration for yours/ Peters/ Daz and all the boys engines that have had time and thought put into them, my dream motor would be a 4.8 most likely. If I could just turn things around a bit it’s not the most expensive thing in the world.
While I’m on the subject Eric mentioned the heads cc
Are the 10 bolt ( late ) heads 28 cc and the 12 bolt 32cc or the other way round. Sure I can look it up but this is more fun and defo more worrying for the competition
My heads are skimmed too which reduces that figure very slightly.
Are the 10 bolt ( late ) heads 28 cc and the 12 bolt 32cc or the other way round. Sure I can look it up but this is more fun and defo more worrying for the competition
My heads are skimmed too which reduces that figure very slightly.
LongBaz said:
Free to you
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?
Sardonicus said:
Maybe I am confused here but best not confuse the 3.9 or interim pre/serp serp common in the Griffith & Chimaera with the 4.0 big journal version I know displacement is the same but 3.9 Vs 4.0 makes things easier in my book
That's irrelevant to the issue. I am assuming you're not going to mix parts between the two.Sardonicus said:
machining any material from the piston crown will always lower CR unless you bring things back by decking the block or using a longer con-rod like I say maybe I misunderstood post
I think you did. The 3.9 pistons (early) or 4.0 (late) would stick out of the block deck if you put them on the end of a 4.6 crank. So you machine a couple of mm off the tops to make them line up at TDC again and end up with shallower bowls. Less cc = higher compression. like I say maybe I misunderstood post TVR themselves did the very same thing with the 4.3 engine, using 3.9 pistons on a 4.2 crank with a couple of mm machined off the piston tops to match the block deck height with the same rod length (and also valve reliefs cut in). The fact that this is an earlier engine with smaller journals etc. is neither here nor there, the same principle applies.
Edited by 900T-R on Monday 21st January 08:49
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