RV8 4.6 tuning a build advice required

[sidecar]

Hi Guys,

I'm now the proud owner of a brand new 4.6 short engine.

I want to build this engine to the best spec that I can within reason hence a few questions:-

I want to run 9.75:1 compression ratio but this will prove a bit tricky with the SD1 heads as the volume is normally around 36cc. My heads have been skimmed and the volume is about 30cc. They have also been stage III'd which is why I want to stick with them. The problem is that with a composite gasket the compression is a low 9:1
(This is despite the high comp pistons that are fitted).

I could yet even more skimmed off the heads but I've checked the deck height and it's a fairly large 45 thou so I was thinking of getting 20-25 thou taken of the decks. This will get the compression up and also close the squish up to 55-60 thou. (Not great but better than 80!).

Skimming the heads won't close the squish as the heads are already flat in that area, what's more, more has to come off the heads than has to come off the decks to get the compression up. The pushrods will need to be shortened but that's not a problem.

What I don't know is can the big end bolts be re-used (They've never been run) or will I need new ones?

The other thing that I'm a bit concerned about is the pistons hitting the valves, as in the end they are going to be about 1.5mm closer by the time all the skimming has been taken into account. (The cam lifts the valves 12mm which is also more than standard).

Does anyone have experience of this sort of work, I'm I going to end up with problems?

Does my plan sound OK to you guys?

Cheers,

Pete

[katanaman]

the only way to be sure is to measure everything. Its not the lift that makes valves hit pistons as by the time the valve is at full lift the piston is long gone. its duration and overlap that makes them hit. You can always not bother and while the engine is apart get the pistons pocketed then your good to go whatever cam you use. Another alternative is forget all the machining and fit 4.0 pistons this will get you closer to where you want to be. Not sure what the machining ver pistons cost would be. If you want to to all the prep though on the machining route I can send you a PDF on how to go about it.

[RoverP6B]

All of the bolts, big ends, mains, are all high tensile steel, so reusing is not a problem. The only bolts which cannot be reused on the 4.0 and 4.6 litre engines are the stretch bolts used for attaching the heads to the block. New ones must always be fitted once the heads have been removed.

Are you going to use studs and nuts for head retention?

Ron.

[ppyvabw]

All of the bolts, big ends, mains, are all high tensile steel, so reusing is not a problem. The only bolts which cannot be reused on the 4.0 and 4.6 litre engines are the stretch bolts used for attaching the heads to the block. New ones must always be fitted once the heads have been removed.

Are you going to use studs and nuts for head retention?

Ron.

Personally I would agree with you there, but let it be said that some people say to replace mains/conrod/head bolts. In particular, RPI say this. Apart from replacing mains bolts for studs on my last v8 build, neither me, or my dad have replaced bolts as religiously as some would have you believe you should on any engine build. Never had any problems.

Stretch bolts are different though.

I'm probably being really thick, but am I right in saying there isn't an OE piston for the 4.6 with 9.75 compression. Expenses permitting, would it not be simpler to just get some after market cast pistons with higher compressions to save the pissing about? By the time you have had the maching done, shortened the pushrods, erm...machined the inlet manifold,...what ever else, it's not gonna be much more expensive to buy some higher compression after market pistons.

As per valve lift. Is that really an expensive thing? Didn't tony recently cut his own valve cut outs?

[sidecar]

Hi Guys,

Thanks for the help.

I will probably use studs for the heads, V8Dev do some with an allen key hole in the end so that you can get them out if needs be. (Its a tight fit in my cobra so they should help).

I've no experience of the 4.6 lump, only the 3.5 so I don't know that the standard pistons look like. The ones in the block have 3.9 stamped on them, are they therefore the same as the 4.0 lump as the capacity was actually the same? (RPI told me that the high CR version of the 4.6 was 9.35:1 but that is obviously with 28cc heads).

They do have small cut outs on the squish band, is this normal for the standard 3.9 pistons?

I can not really afford to replace the pistons with other ones and the ones in the block are brand new. (They have H39 M6 stamped on them)

I do want to stick with comp head gaskets as I've pulled apart two 3.5 lumps that had tin gaskets, both showed leakage into the valley.

Cheers,

Pete

[sidecar]

the only way to be sure is to measure everything. Its not the lift that makes valves hit pistons as by the time the valve is at full lift the piston is long gone. its duration and overlap that makes them hit. You can always not bother and while the engine is apart get the pistons pocketed then your good to go whatever cam you use. Another alternative is forget all the machining and fit 4.0 pistons this will get you closer to where you want to be. Not sure what the machining ver pistons cost would be. If you want to to all the prep though on the machining route I can send you a PDF on how to go about it.

Hi katanaman,

Please can you send me the PDF.

Cheers,

Pete

[katanaman]

you will get leakage into the valley with tin gaskets if the outer row of bolts are used on the heads. This will happen often even with the comp gaskets because they force the head to tilt which is why your engine doesn't have the hole for them in the block. Personally I would and do use the tin gaskets and never have a problem but each to their own. Remember as well your 3.5 engines were probably together for 20 years.
At 9.35:1 you have the proper 4.6 pistons assuming the quoted is correct, they cant be 3.9 pistons as they don't fit 4.6 rods.

Send me a PM with your mail addy for the PDF

[sidecar]

you will get leakage into the valley with tin gaskets if the outer row of bolts are used on the heads. This will happen often even with the comp gaskets because they force the head to tilt which is why your engine doesn't have the hole for them in the block. Personally I would and do use the tin gaskets and never have a problem but each to their own. Remember as well your 3.5 engines were probably together for 20 years.
At 9.35:1 you have the proper 4.6 pistons assuming the quoted is correct, they cant be 3.9 pistons as they don't fit 4.6 rods.

Send me a PM with your mail addy for the PDF

I'll send you a PM!

The pistons state H39 on them....it better be a 4.6! I better check the stroke!

Now worried,

Pete

[RoverP6B]

Pistons for 3.5, 3.9 and 4.2 litre engines have gudgeon pins of 22.215 to 22.220mm in diameter.

All 3.5, 3.9 and 4.2 litre engines use connecting rods of the same length, ie..143.71 to 143.81mm, as measured between the centres.

4.0 and 4.6 litre engines are quite different!

Gudgeon pins for both 4.0 and 4.6 litre engines are 23.995 to 24.000mm in diameter.

Connecting rods between the two engines are different, as the pistons used in both engines are also different.

For the 4.0 litre engine, connecting rods are 155.12 to 155.22mm in length.
For the 4.6 litre engine, connecting rods are 149.68 to 149.78mm in length.
Measured between the centres.

Information obtained from Land Rover engine overhaul manuals.

Ron.

[ChrisJC]

That's useful data - should be reformatted into a sticky for piston / rod identification?

Chris.

[sidecar]

Pistons for 3.5, 3.9 and 4.2 litre engines have gudgeon pins of 22.215 to 22.220mm in diameter.

All 3.5, 3.9 and 4.2 litre engines use connecting rods of the same length, ie..143.71 to 143.81mm, as measured between the centres.

4.0 and 4.6 litre engines are quite different!

Gudgeon pins for both 4.0 and 4.6 litre engines are 23.995 to 24.000mm in diameter.

Connecting rods between the two engines are different, as the pistons used in both engines are also different.

For the 4.0 litre engine, connecting rods are 155.12 to 155.22mm in length.
For the 4.6 litre engine, connecting rods are 149.68 to 149.78mm in length.
Measured between the centres.

Information obtained from Land Rover engine overhaul manuals.

Ron.

Hi Ron,

Thanks for the info.

I don't know whats going on with my pistons then as they do have 3.9 stamped on them! The stroke is 82mm (measured with a ruler!) so the capacity has got to be 4.6 litres. I think that the short engine orginally came from V8 Tuner so I'll ask them.

Regards,

Pete

[CastleMGBV8]

Pete,

Nobody seems to have asked what the engine No. on the block is, and whether its a four bolt mains engine. If the No. is still on the block what is it?

If it's a four bolt then it could be an interim 3.9 with a stroker kit to get it to 4.5 or possibly 4.6 which is a little unusual but would still be a good basis for a performance engine albeit with the smaller journal sizes.

Kevin.

[RoverP6B]

That is a very good point Kevin!

The interim 3.9 and 4.2 litre engines manufactured after April 1994 had the same main bearing caps as the later 4.0 and 4.6 litre engines, although they were not drilled and tapped to accomodate the cross bolts, but the casting in the block was there, so they could thus be drilled and tapped to become fully cross bolted blocks.

They retained however, the short nose, small journal crankshafts.

I would imagine though that in order to increase the capacity, the crankshaft big end journals would need machining, and different connecting rods may well have to be employed.

Ron.

[sidecar]

Hi Guys,

There are no numbers on the block as it is brand new.

The guy that I bought it from has had it for about 4 years, he bought it from V8Tuner as a brandnew short engine.

It is cross drilled and is a red grade block. I'm not sure of the journal size and would rather not remove the mains or big ends if I can avoid it.

The pistons had 4.6 stamped on them in red ink.

Paul at V8 states that he has never sold a block with pistons that have the cut outs but the guy I bought it from swears that it was bought like that!

Paul reckons that there is no problem with having cut outs but someone on the cobra forum has said that they cause the ring land to break!

This is starting to do my head in!!

At the end of the day I guess it does not matter waht the pistons are as long as it is a 4.6 lump!

I've worked out what machining needs doing to get the compression that I want and it is possible to achieve.

I guess your comments on the breaking of pistons with cut outs would be nice to know!

Thanks for all your help.

Pete

[katanaman]

Lots of people have valve pockets cut in the piston and I have never seen the crown break into the rings due to them. Tony has a pic of his engine with the pockets and you will see there is still plenty left. In my garage there is an old full race engine that competed It belonged Scots of Eastfield garage Duton for those in the area). Instead of cutting pockets someone ran the mill straight across the pistons. This engine ran for years like that and never broke a piston even at high revs.