rv8 low comp head question

capriandyb1 · Post by **capriandyb1** » Wed Aug 05, 2015 8:20 am

Hi I'm new to this forum so not sure if this is in right section...if not sorry

I'm starting to build a 4.6 twin turbo rv8 and was wondering am I right in thinking that if I put a pair of ERC0216 heads with a composite gasket that will bring my STD compression rate down
The heads will have work don't to enlarge valves and port them to allow more air flow

I'm using that set up until I can build a 5ltr with propper low comp pistons

Thanks for any help

SuperV8 · Post by **SuperV8** » Thu Aug 06, 2015 8:07 am

Yes, if you fit those heads with composite gaskets it will lover the C/R, i'm sure the actual difference in C/R will be on a post somewhere on this forum: From memory I think something like 0.5:1 diff?

What boost are you planning on running?
I'm running 9.5:1 CR and so far 7 psi and absolutely fine. I'm planning on 10psi max with this CR, which others have done with no issues.
Its very important to accurately control the fuel and spark when under boost, what are your plans for fuel and spark?

Unless your going for lots of boost I don't think low comp pistons are necessary?

Tom.

capriandyb1 · Post by **capriandyb1** » Thu Aug 06, 2015 10:39 am

Thanks for the reply I'm looking at around 10/15 psi Max...if you don't think I need low CR pistons that would make my 5ltr block easier to build for sure.....I'm wanting to use 650 holley blow thru carb and either msd or mallory spark I'm leaning more msd.....iv ready a few projects where that set ups been used to good bhp.....

kiwicar · Post by **kiwicar** » Thu Aug 06, 2015 12:10 pm

Hi
with 14psi you will need as much cylinder liner thickness as you can get, most of the 5 litre engines I have heard of use a 96mm bore, that will not be a good thing with that much boost. Also how will you get the extra stroke, chevy 2 inch journal rods and offset grind a rover cast crank? You will find it hard to get a suitable piston that will give you low enough compression and any squish band at all. If this is to be a purely competition engine then you can get away with the combination poor squish relatively high CR and boost by using a suitable fuel or just put the piston 1/4 inch down the bore, but if you do that on a road engine you will have problems with emissions. . . .
You could go for buick 300 heads then you will have a better size combustion chamber so you can use a better piston, but then you will have more power and the cast crank will not withstand that. I would seriously look at a smaller capacity say around 4.2-4.3 litres with that much boost and use buick 300 heads you will get as much power, better reliability and a wider power band that will make the engine better to drive.
Best regards
Mike

SuperV8 · Post by **SuperV8** » Thu Aug 06, 2015 3:02 pm

There's big difference (50%

) between 10 & 15 psi!
10 psi would probably be ok with an intercooler, carefully mapped injection and ignition but 15psi probably won't.

Tom.

sidecar · Post by **sidecar** » Thu Aug 06, 2015 3:58 pm

I think that the OP is asking whether he can just bung on the 36cc chamber heads on a 4.6 along with comp gaskets until he has the time to build a 5 litre lump with low compression pistons.

I can't be bothered doing the maths but the standard heads for the 4.6 are 28cc, the older SD1 14 bolt heads are 36cc. The 4.6 uses comp head gaskets anyway so they are not really a factor. If you know the CR at the moment it is possible to use maths to work out how big the drop in CR will be due to the increase in the chamber size. It may well be around 0.5. Having said that I think that buretteing everything is the only way to be 100% sure.

Also you should know that any of the commonly available head gaskets are not all that great when it comes to high cylinder pressure, the fire ring tends to only be half compressed due to the huge flange at the top of each bore. MLS gaskets sell ones that are around 2mm oversize, they also do various thicknesses, I'd use the 2mm over size jobbies even with a standard bore. The downside is that they are about 80 quid EACH!!!!!

I'd be looking at ARP head studs too!

capriandyb1 · Post by **capriandyb1** » Thu Aug 06, 2015 4:43 pm

Thanks all......with the info you've given me I'll use under 10psi,i was using a intercooler anyway and take it from there I'll have a look at them headgaskets I know they're dear but if they're more reliable I'd rather do that then keep having blown gaskets..
The 4.6 block iv got has already had top hat liners in and I'll be arp top and bottom too..
Ill put the 5ltr on back burner for time being,I want reasonable reliability
its going to be used for shows and strip...iv seen you can use Buick 300 heads but not sure on how excesable they are to get hold of same again it is something I can look at
If I use the 2016 heads they'll be having head work done to fit bigger valves.....and port them out
its a couple of year project so I really appreciate your advice

Post by **Coops** » Thu Aug 06, 2015 5:42 pm

just to throw a spanner in the works,
was running stock 4.6 composite head gaskets on my stg3 4.6 with 14psi of boost for just over 2 years until the piston ring lands collapsed on no2 piston, and was abused regular and hand no gasket blow issues etc on strip down,

sidecar · Post by **sidecar** » Thu Aug 06, 2015 6:00 pm

I was running NOS and the fire ring gave way, when I posted this up at least one other chap on this forum had suffered the same problem, he was not running NOS, just a highly tuned NA engine. NOS or no NOS its the peak cylinder pressure that the fire ring has to withstand no matter how that pressure is generated.

At the end of the day its just a bad design if half of the fire ring is floating about in the combustion chamber uncompressed!

The OP has top hat liners, with any luck they will have a very small chamfer at the top of the bore, not the huge one that Rover machined into their liners so they could just hammer the pistons into the engine!

SimpleSimon · Post by **SimpleSimon** » Fri Aug 07, 2015 10:11 am

Coops wrote:just to throw a spanner in the works,
was running stock 4.6 composite head gaskets on my stg3 4.6 with 14psi of boost for just over 2 years until the piston ring lands collapsed on no2 piston, and was abused regular and hand no gasket blow issues etc on strip down,

Cant beat those Elring gaskets for stockers IMO they was OE to the RV8 from the factory (cheap too) and I know Derek on here with the Chimaera abused these regular till he went MLS recently

capriandyb1 · Post by **capriandyb1** » Fri Aug 07, 2015 11:56 pm

So basically have a go and see how you do lol......do you think the 650cfm blowthru carb will be sufficient or go for the 750 cfm....iv not used holley carbs before so would like your appinian

sidecar · Post by **sidecar** » Sat Aug 08, 2015 7:27 am

capriandyb1 wrote:So basically have a go and see how you do lol......do you think the 650cfm blowthru carb will be sufficient or go for the 750 cfm....iv not used holley carbs before so would like your appinian

I use an Eddy 500 on my stage III 4.6 NA, I tried a 600 carb but I had issues with the transition circuit which I just could not resolve. I would have thought that with forced induction you would not need a huge carb but then again I haven't got a clue about turbo's !

DEVONMAN · Post by **DEVONMAN** » Sat Aug 08, 2015 7:44 am

capriandyb1 wrote:So basically have a go and see how you do lol......do you think the 650cfm blowthru carb will be sufficient or go for the 750 cfm....iv not used holley carbs before so would like your appinian

I had a Holley 600 double pumper on a twin turbo 3.9 Rover at one point and that was not ideal for road use (Too Big) but it gave me my best time up the pod. The problem was getting the mixture right at cruise and medium load, at the point when the turbos are starting to pressurise the almost closed throttles. I had to play around with the air bleeds to improve matters.

capriandyb1 · Post by **capriandyb1** » Sat Aug 08, 2015 2:26 pm

So it might be better using injection with megasquirt? Iv been weary of using it due to me not been very good with electrics.....iv sumizing I could use any set up I want with that?......delete maf etc...could I use it with a 3.9 inlet etc.... and coilpack or would I need to use proper dizzy aswel......iv not bought the fuel and spark set up yet

sidecar · Post by **sidecar** » Sat Aug 08, 2015 3:21 pm

I'm not keen on EFI but I think that with a forced intake system you would probably better off with a decent EFI system (One that is fully tune-able)

With regards to ignition I run a modified Lucas dizzy, all of the guts have been taken out of it, I made a new baseplate and fitted an MSD pickup to it. I run a fully programable MSD system, it made a huge difference to my engine, the system can also retard for boost (or NOS). I reckon that the spark from an MSD system could set light to a damp rag!