The V8 Owners Forum -

Hi
I want to up the compresion on my 4.2 v8 I have some 4.6 heads that will do the job with steel type gaskets. but how much do I skim of the intake manifold to mathch the heads.

Mick

You have to skim the manifold faces by the same amount as the heads. From memory it's about 0.5mm you need to take off each side, which is the same amount that the 4.6 heads are thinner by compared to the 4.2 heads (assuming the 4.2 was tin head gasket).

You'll also need shims for the rocker shafts, and the valley gasket may need some fettling.

Chris.

I heard that it was .707 of the total amount taken off the heads, it's something to do with trigonometry. As I paid very little attention in maths I can't be sure.

My heads were skimmed to up the compression and my Idelbrock manifold fitted without any mods. (I port matched it at the same time as it was miles off, much more than caused by skimming the heads).

The rockers may well need shimming up.

HTH,

Pete

the difference is 1mm from new between the different heads so about .7mm off each side of the inlet will bring it back in line. As chris said you might have to mess about with the inlet gasket. I made mine using two gaskets cut and stuck together.

Thanks for the replys guys. I have sorted the rockers will machine the inlet manifold .7 sounds about right to me the gasket I will look at when I get the manifold back.

Cheers Mick

Sorry to disagree on the trigonometry, but it's a fact that whatever you skim off the heads you have to skim off the manifolds too. No scaling factors required.

Chris.

Chris is correct.

It is easier to think about if you can visualise the block, heads and manifold to form a square resting on one corner (the crank centerline) at the bottom. The block banks/head form two sides, the heads/manifold form the other two sides.

If you shorten two sides by skimming the heads (or decking) then you need to shorten the other two sides (the head to manifold) by the same amount else the square won't fit together anymore.

I'll do a picture to explain of you still need it.

Only works for a 90deg V where the manifold base is not used to seal the cam plenum. Then you do need also to machine the manifold base by the appropriate factor.

Russ

Yep, chris is correct as the banks are at 90 degrees to each other. Just an intuitive though as a mathematics student, if the banks were at 45 degrees, then there would be a 1/sqrt(2) scaling factor, as you say.

Might be a better idea to skim it off the heads though, because then the manifold can be re used on another engine or if you ever change the heads again in the future you wont need to buy a new manifold.

The problem with skimming the inlet side of the heads is that they need to line up with the ends of the valley so the gasket has a smooth transition from the curved end walls onto the side of the heads.

If you skim the heads there'll be a step which might be quite hard to seal against.

Chris.

russell_ram wrote:Chris is correct.

It is easier to think about if you can visualise the block, heads and manifold to form a square resting on one corner (the crank centerline) at the bottom. The block banks/head form two sides, the heads/manifold form the other two sides.

If you shorten two sides by skimming the heads (or decking) then you need to shorten the other two sides (the head to manifold) by the same amount else the square won't fit together anymore.

I'll do a picture to explain of you still need it.

Only works for a 90deg V where the manifold base is not used to seal the cam plenum. Then you do need also to machine the manifold base by the appropriate factor.

Russ

Hi Russ,

You explained that very well, I can see what you mean now. I don't know where I got the .707 from. Just proves that it's best to ignore 99% of everything I say!

Regards,

Pete

.707 is the sine/cosine of 45 deg right number wrong application . ever wondered what else they implanted in your brain when you were at school that could imerge at any time....
Mike

kiwicar wrote:.707 is the sine/cosine of 45 deg right number wrong application . ever wondered what else they implanted in your brain when you were at school that could imerge at any time....
Mike

Hi Mike,

I knew that it was the cosine of 45 deg but could not see how that related to a 90 deg V engine so did not mention it! Maybe I read it somewhere where a 45 deg V was the topic. (A Harley or something!)

.707 is also the number that you multiply a peak AC voltage by to get the RMS value. My Dad always said that EVERYTHING in the whole wide world could be boiled down to maths!.....I guess he is right.

All good stuff,

Cheers,

Pete

Hi All
Just for reference I had 1mm machined off bot sides and it fits spot on.

Thanks for the help Mick