The V8 Owners Forum -

I've been measuring my engine and I would like someone to check that my maths are OK before I do the deed

Engine bottom end is a 4.8 with 94mm bore and 86mm stroke. (measured)
Volume above the pisiton is 19ml, volume in the head is 29ml. (measured) I make that a CR of 12.2:1. (calculated)
In order to get down to around 10.5:1 I need a volume increase of around 10ml. (calculated)
The diameter of the piston bowl is 68mm, therefore r is 3.4cm and the amount of metal I need to machine from the piston bowl is 2.7mm. (calculated)

If someone can confirm this or tell me I'm crap I'd be very grateful

I don't know where these pistons came from, there are markings on the crown RA 4105 X and MID .001 OC Anyone know anything about these, I got them from Wildcat.

thanks
Kev

By volume above the piston I take it you mean allowance for the head gasket thickness plus the distance the pistons are below the the deck at TDC. are there cut outs for the valves?
I think you will be unable to take 2.7 mm off the tops of the pistons, the top ring will be too near the top of the piston and the piston land will fall off! also I don't think most pistons have thick enough crowns at the centre to take that much off. What pistons do you have?
Best regards
Mike
PS I use CR= 1+ (swept volume) / ( chamber volume +piston dish volume+ (Pi x R x(distance below deck +gasket thickness))

Sorry just read your bit about not knowing the make of piston. cant you put the piston further down the bore with a different con rod? are these rover rods or modified chevy? what is the gudgion pin size?
Mike

sounds to me like you haven't included the head gasket or you have flat top pistons. If you have included gasket which one, tin or comp? Going to comp would get you down a bit if you worked out on tin. Failing all that probably the cheapest option would be to get copper head gaskets made the thickness that suits.

Volume above the piston is to the deck. I've made an allowance of 1mm for the gasket.

I'm assuming that I would take metal from the bowl of the piston, not the suish area. The diameter of 68mm is what I measure for the piston bowl inside the piston so It wouldn;t remove any metal directly above the ring.

Is 2.7mm too much metal to remove?

Can you give me a ballpark cost for copper gaskets and where would I get them made?

Thansk for the replies chaps. I must say I like the idea of a thicker gasket but wouldn't that lose all the squish?

cheers
Kev

I wouldn't remove material from the piston tops I would rather remove it from the head combustion chamber. No idea how much copper gaskets would cost but there is a guy on here does water jet cutting so might could do you a pair at a reasonable cost.

Hi
Did this all come as a kit of crank, rods and pistons? just it all seems a bit high as normally the person selling the kit would choose a piston with a deck height to match cam and rods? I think it is worth a recheck.
I think you are going to have to sacrifice squish if you are going to get the compression in the right region. Marki is right your easiest way is going to be a thick head hasket, then get the grinder out and open up the combustion chamber if you have to, I wouldn't take more than about 1/2 to 3/4 mm off the top of the pistons at the centre if they are cast pistons, if they are forged maybe 1mm but I think it is asking for trouble.
What cam are you planning on using? just although 12.2:1 is rather high you may get away with 11:1 or even 11.5:1 with the right cam, somthing in the region of 235 deg at .05" and LSA of 106 to 108 deg, this with a 4.8 litre engine would be a fairly warm cam but road drivable.

It's along story how this came about, but I was going to use Wildcat heads with 36ml volume and changed my mind.
The block had to be machined as it was out of true. The block is now decked square and true to the crank with the top of the piston now only 0.2mm from the deck at TDC. Not sure what the pistons are but they are better than standard and the rods are "H" beam, so it should now be a good botom end.

I'm using a serpentine front end so if I can't get an intermediate front cover I'm stuck with using a Piper 285 which I already have. I want to increase the capacity of the engine from my 4.0 to the 4.8 in order to get a more driveable car so I wouldn't want to soften the CR by using a hot cam. I think the 285 is as warm as I want to go.

The objective in this is that I need to reduce the CR.

It appears from the replies here that machining the piston is not recommended.
Using a thicker copper gasket is an option but will sacrifice some squish but as I've only got 0.2mm now then this may not be too bad if I end up with a 2mm gasket.
Removing metal from the head is an option but I don't know how much to remove and where to take it from.

My ammended calculations show that if I allow 2mm for a compressed gasket the CR will be 10.8:1.

Can anyone confirm that the composite gasket has a compressed thickness of 1mm? I've read it somewhere but can't remember where.

Real steel have the gaskets listed as 1.2mm. If your putting a thicker gasket in to compensate for decking the block then you aren't loosing any squish as in theory anyway your just returning it to stock. As for removing material from the heads, you could get all scientific and re-shape the chamber if you know what your doing. If not I would go for just blending all the way round. Your not changing any shapes then other than making it slightly deeper which is no worse than original 36cc heads.

There are loads of cams you can use on the serp front cover now by the way so your definitely not stuck with a 285.

Have these heads been modified in any way or are the stock 10 bolt heads?

I've dug out the heads I'm intending to use and they look bog standrd 10 bolt heads with the 29ml volume.
However, when I went to the shed to get them I found a pair of 14 bolt Vitesse heads that I'd forgotten about I reckon if I clean these up and do the usual, new bullet valve guides, 3 angle seats and blend the ports and match them then that should flow enough to keep me happy.

From memory there isn't much difference between the casting from the Vitesse 0216 casting and the later castings so unless anyone says they're crap I reckon the way to go is with the Vitesse heads and composite gasket.

Regarding camshafts, which would you recommend for this setup to go in my Cobra to give good road manners and a nice kick up the arse higher up the range. I've been looking on the internet and the only supplier I can see who lists for the serp cams is V8tuner.

thanks a lot for the guidance
Kev

Yes that was going to be my suggestion, just use 14 bolt heads and your problem is solved. If you use wasted stem valves (3.5/3.9 efi) they will flow just abut the same as 10 bolt heads in standard trim. They have slightly larger ports but they all come with none wasted valves. Main advantage I can see for them is they are meant to be a slightly thicker casting so can be taken out more apparently. Cam wise your really limiting your self with having fairly standard heads. Your plans will give you better flowing heads but not by a large amount and especially on a 4.8. Heads are the key to a rover engine if HP is what your after, unfortunately it also means £££££. If this is the way you want to go then I would go for a torque monster engine and forget any high rev cams. Absolutely nothing wrong in that as people get hung up on the HP figure where its the torque figure that gives you the push in your back and fires you up the road. As to what cam, I have no idea but I am sure if you give Paul @ V8 Tuner or Rob @ V8 Development a call they will put you onto the right lines.

DJE do Custom Serp Cam's

I'm using one on the Race Car engine.