The V8 Owners Forum -

Hi I've got the option on a 4.6 block, but it has been over bored to 94.5mm

could I use standard pistons with oversize rings or would I need bigger pistons.

If I need bigger pistons, could I get 10:1 pistons. I'll be running a 4.0 crank. and pistons as want to keep it to 4.0 or as good as. if it goes to 4.05 then that's fine.

Ideally I don't want to spend a fortune, but have a upper limit of about £300 inc rings.

Leigh,

You will need oversize pistons and rings, I know that 4,6 pistons are available in +20thou but not sure if 4.0 ones are available yet.

Unless you are getting the 4.6 block very cheap and are sure it's ok, it must be more economical to rebore the one you have, providing you can get the oversize pistons.

As you are building a limited horsespower engine you could use a high torque cam and limit the RPM to max. 5500. Just concentrate on making torque and you will have a very quick car in your racing class, without exceeding the BHP limit and, the engine should last a long time.

It will be important to maximise the compression and keep the squish area as tight as possible as this will help build torque.

Kevin

Leigh,

Re the pistons, you could use Ford 255 pistons which only have a small dish in the crown, they would come to approx 20thou below the deck height and would yield a static compression ratio of 11.3/1 or if you closed up the squish area by machining the decks you would get 11.7/1 which would be great for the track but not much use if your going to use the car on the road.
I am only going from memory re the piston bowl volume, I was going to use them in ny new engine but had a change of plans and am now using chevy 305 pistons so threw most of the old data away,

There was a set for sale new in the for sale section but it's dropped off the bottom, I think the seller was Rossco and the price was £80.00 He posts quite regularly so find an old post of his and send him a PM, He should be able to tell you the bowl volume.

The only problem with using this piston is the Gudgeon pin size os smaller than the 4.0 small end so you will need to get them bushed with a thin wall bronze bush, I would have thought well within the capability of Thurstons.

The 4.0 gudgeon pin is .966" and the Ford is .911" + .055"

Kevin.

Leigh,

Sorry just found some more data and the Ford pistons compression height would put it approx 2mm out the top of the bore, which is far too much to machine so disregard the above.

I suggest you find out who manufacture pistons for the OE Rovers and see if they have a +20thou piston and then use tin gaskets for a compression ratio of around 9.8/1

Kevin

CastleMGBV8 wrote:Leigh,

You will need oversize pistons and rings, I know that 4,6 pistons are available in +20thou but not sure if 4.0 ones are available yet.

Unless you are getting the 4.6 block very cheap and are sure it's ok, it must be more economical to rebore the one you have, providing you can get the oversize pistons.

As you are building a limited horsespower engine you could use a high torque cam and limit the RPM to max. 5500. Just concentrate on making torque and you will have a very quick car in your racing class, without exceeding the BHP limit and, the engine should last a long time.

It will be important to maximise the compression and keep the squish area as tight as possible as this will help build torque.

Kevin

Well not 100% sure if they can bore my liners, they recomended new ones. so £600-£800 is going rate for tophats.

I am limited to 230BHP and 4.0 ltr. so can't run a 4.6 with a RPM limit.

Leigh,

You missunderstood what I said. if you were using another block ie 4.6 it would still be a 4.0 with your crank rods and pistons. What I was suggesting is that you build a torque optimised engine, peaking at no more than 5500 RPM this will easily make 230BHP and give you a strong low and midrange which will make for good overall performance and be very easy to drive on track, as you will have very smooth power delivery
and it will accelerate strongly out of the corners.

Don't be tempted to go for to wild a cam as this will probably return slower lap times, all you need is to clean the heads up behind the valve seats and fit bulleted valve guides which is good for 30+BHP and your already at approx 220+BHP even on a standard cam, so all you can do is increase torque at lower RPM by careful cam selection and this should give you an advantage and you should still be within the class limit of 230BHP

Kevin.

Kevin.

Hi thank you for the reply.

I have decided that even tho its a bit more expensive I'm going to stick with the 4.0 block and get it top hatted. as this will be less complex.

I think the new style head are allready a better design. do you think you can still get an extra 30bhp from them, with minimal work.

thanks
Leigh