4.0 litre pistons in a 4.6 block.

[Classic Chim]

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

Edited by Classic Chim on Sunday 20th January 13:41

[Classic Chim]

LongBaz said:

Hmmmmm. You worried;)

Not really, MT’s and ride height would have got me low 12’s I believe,,,,, but just to be sure

[Classic Chim]

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.

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

[Classic Chim]

jojackson4 said:

Sneaky
I will have to deploy the blowers

[Classic Chim]

LongBaz said:

Just bought some new MT's you can have the old set if you want......last used got a 11.06

Thanks Baz, one new Dif is enough for me,,,,,,,, for now
Let’s see how PJ gets on first

What are they worth ?

[Classic Chim]

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

I’d have to go top hat so never a cheap option.

[Classic Chim]

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.

[Classic Chim]

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.

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.

[Classic Chim]

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.

[Classic Chim]

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?

[Classic Chim]

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.

[Classic Chim]

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

[Classic Chim]

jojackson4 said:

Just drop a crate LS in it less hassle and probably a lot cheaper (st did I just say that out loud)

What sort of race would we have if I did that.

Got any old 4.0 pistons Pete I need em bad,,,, all in jest