4.0 Pistons in 4.6
Discussion
Having completed my engine strip I am about to commence the rebuild of my 4.6 engine. One thing I have read about is the use of 4.0l pistons to increase the compression ratio from 9.35:1 to something in the region of 10.5:1 (with valve cutouts).
Has anyone had any practical experience of this? The engine will be running a sequential injection & electronic ignition with knock control and I'm happy to run it on 99ron so the compression ratio doesn't worry me too much, but are there any other practical issues to overcome?
Lastly, does anyone have any good 4.0l pistons with valve cutouts laying around?
Cheers, Adam
Has anyone had any practical experience of this? The engine will be running a sequential injection & electronic ignition with knock control and I'm happy to run it on 99ron so the compression ratio doesn't worry me too much, but are there any other practical issues to overcome?
Lastly, does anyone have any good 4.0l pistons with valve cutouts laying around?
Cheers, Adam
Boosted LS1 said:
You'll probably have to make your own cut outs or pay for the servce. I doubt these will be found 'lying around'. What compression height do you need?
My understanding was that they were fitted in 4.0l cars with the TVR 435 cam, and therefore shouldn't be too hard to come by? Again, my understanding is that the compression height will be the same as a standard 4.0l, with the compression ratio increase coming from the additional stroke.Happy to be corrected!
4.0 and 4.6 engines are the later crossbolted items which have different pistons to the earlier non cross bolted engines. I've a set of pistons here but can't recall if 4.0 or 4.6. Did TVR use the crossbolted blocks?
You'll find stock pistons but I reckon cut out versions will be scarce unless you find somebody who swopped in forged items.
You'll find stock pistons but I reckon cut out versions will be scarce unless you find somebody who swopped in forged items.
I doubt very much that your 4.6 has cr of 9.35:1 despite what's stamped on the block. It's nearer 9.0:1. When I stripped and rebuilt mine for turbo install I measured and checked, then checked again it it was actually 8.9:1. If you want higher cr why not skim the heads or/and thinner 'cometic' MLS head gaskets. ARP studs too whilst your at it.
Adam205 said:
My understanding was that they were fitted in 4.0l cars with the TVR 435 cam, and therefore shouldn't be too hard to come by? Again, my understanding is that the compression height will be the same as a standard 4.0l, with the compression ratio increase coming from the additional stroke.
Happy to be corrected!
The stroke is determined by the crank, not the piston.Happy to be corrected!
DickyC said:
Adam205 said:
My understanding was that they were fitted in 4.0l cars with the TVR 435 cam, and therefore shouldn't be too hard to come by? Again, my understanding is that the compression height will be the same as a standard 4.0l, with the compression ratio increase coming from the additional stroke.
Happy to be corrected!
The stroke is determined by the crank, not the piston.Happy to be corrected!
Boosted LS1 said:
4.0 and 4.6 engines are the later crossbolted items which have different pistons to the earlier non cross bolted engines. I've a set of pistons here but can't recall if 4.0 or 4.6. Did TVR use the crossbolted blocks?
You'll find stock pistons but I reckon cut out versions will be scarce unless you find somebody who swopped in forged items.
I believe all serpentine engines used the cross bolted block, but that's a good point. My 450 engine definitely is. Anyone able to confirm whether the 4.0l engine was the crossbolted block or earlier 3.9?You'll find stock pistons but I reckon cut out versions will be scarce unless you find somebody who swopped in forged items.
macdeb said:
I doubt very much that your 4.6 has cr of 9.35:1 despite what's stamped on the block. It's nearer 9.0:1. When I stripped and rebuilt mine for turbo install I measured and checked, then checked again it it was actually 8.9:1. If you want higher cr why not skim the heads or/and thinner 'cometic' MLS head gaskets. ARP studs too whilst your at it.
Interesting point. Mainly because swapping in pistons is reasonably straightforward and doesn't affect the lifters etc. Given that everything else is in surprisingly good nick it seems like the path of least resistance (I may be incorrect here).I was considering ARP studs, although most places seem to be listing them at nearly £300 a kit. Any decent sources?
Adam205 said:
DickyC said:
Adam205 said:
My understanding was that they were fitted in 4.0l cars with the TVR 435 cam, and therefore shouldn't be too hard to come by? Again, my understanding is that the compression height will be the same as a standard 4.0l, with the compression ratio increase coming from the additional stroke.
Happy to be corrected!
The stroke is determined by the crank, not the piston.Happy to be corrected!
Twas a terminological inexactitude.
DickyC said:
Adam205 said:
DickyC said:
Adam205 said:
My understanding was that they were fitted in 4.0l cars with the TVR 435 cam, and therefore shouldn't be too hard to come by? Again, my understanding is that the compression height will be the same as a standard 4.0l, with the compression ratio increase coming from the additional stroke.
Happy to be corrected!
The stroke is determined by the crank, not the piston.Happy to be corrected!
Twas a terminological inexactitude.
Adam205 said:
I believe all serpentine engines used the cross bolted block, but that's a good point. My 450 engine definitely is. Anyone able to confirm whether the 4.0l engine was the crossbolted block or earlier 3.9?
All serpentine engines are not cross-bolted. Most 450's are and a few 500's but not many.Adam205 said:
You change the compression ratio by changing pistons. The 4.0l has a smaller dish in the piston to create the same theoretical compression ratio as the 4.6 at the same compression height. When you fit the 4.0l pistons with 4.6 crank the compression ratio is increased by the factor of the stroke change. This is my understanding of how the engine was designed, however I am looking for useful advice on the practicalities of the change. Does that clear things up?
No points towards my Pedant Badge? Drat. I only need a few more.Best of luck with the rebuild. I'll be following developments.
Adam205 said:
DickyC said:
Adam205 said:
DickyC said:
Adam205 said:
My understanding was that they were fitted in 4.0l cars with the TVR 435 cam, and therefore shouldn't be too hard to come by? Again, my understanding is that the compression height will be the same as a standard 4.0l, with the compression ratio increase coming from the additional stroke.
Happy to be corrected!
The stroke is determined by the crank, not the piston.Happy to be corrected!
Twas a terminological inexactitude.
Sorry if I wasn't clear. The engine is a crossbolted 4.6 from a 97 450, so the only change would be the 4.0l pistons with smaller dish.
If the 4.0l blocks were actually "3.9" non cb blocks then that scuppers my plan for buying TVR pistons with the valve clearance already in but does present a new question... Do the 4.0l "cross bolt" pistons need additional valve clearance?
If the 4.0l blocks were actually "3.9" non cb blocks then that scuppers my plan for buying TVR pistons with the valve clearance already in but does present a new question... Do the 4.0l "cross bolt" pistons need additional valve clearance?
Never any harm in doing that if the pistons are at deck height.
On most rovers the pistons sit 1 mm down the bore and have about .040/.050" of gasket thickness depending on manufacturer. If the pistons don't sit above the deck then there won't be issues. The TVR own brand gaskets are the thickest of the lot iirc.
On most rovers the pistons sit 1 mm down the bore and have about .040/.050" of gasket thickness depending on manufacturer. If the pistons don't sit above the deck then there won't be issues. The TVR own brand gaskets are the thickest of the lot iirc.
Adam205 said:
Sorry if I wasn't clear. The engine is a crossbolted 4.6 from a 97 450, so the only change would be the 4.0l pistons with smaller dish.
If the 4.0l blocks were actually "3.9" non cb blocks then that scuppers my plan for buying TVR pistons with the valve clearance already in but does present a new question... Do the 4.0l "cross bolt" pistons need additional valve clearance?
There is no off the shelf OE/LR 4.0/4.6 pistons with cut-outs/eyebrows already present, and to make things clear there are two 4.0 RV8 varients with the serpentine belt drive one is crossbolted same as you have and the other is the interim serp/small crank journal non X bolted (although the block bosses are present) I like to refer to this as the 3.9 (although both are the same displacement) you cant use the 3.9 piston in your build but you can use the 4.0 X bolted one like you intend to do If the 4.0l blocks were actually "3.9" non cb blocks then that scuppers my plan for buying TVR pistons with the valve clearance already in but does present a new question... Do the 4.0l "cross bolt" pistons need additional valve clearance?
good luck its a worthwhile mod for a sensible hike in CR 
and like Macdeb said the stock 4.6 are seldom 9.0 CR in actuality regardless of the 9.35 stamped this includes the smaller displacement RV8's too Gassing Station | Chimaera | Top of Page | What's New | My Stuff