Head matching dilemma

jclotz · jclotz's Garage

I am building a 2.7 rs mfi motor and have had a set of 2.4T mfi heads reconditioned with the intake ports opened to 34mm. Two of the 6 heads were in bad shape so we switched in 2 heads from a 2.0MFI assuming they'd be identical. Turned out the 2.0 heads have a chamber volume of 70mL while the 2.4's are 68 mL. Based on my calcs, this would lead to a 0.5 CR difference.
I'd like them to match so my machinist recommended taking off a few thousandths off the head/cylinder mating surface to decrease the chamber volume on the offending 2. However, this would require taking the same amount off the cam housing mating surface of the other 4 heads plus adding thicker cylinder base gaskets, so the chain length isn't altered. Unfortunately this would reduce the CR on all my cylinders by 0.5 points.
Any other suggestions? My inclination is to get two other replacement heads and start over.

Flat6pac · 11-05-2011, 09:53 AM

You want to match the letter, 4 numbers, and a letter in the block on the corner of the head casting. One casting number like for instance w1918d is the number you want to match.
2.0 heads are not matched to anything after 914/6 2.0.
Bruce

HawgRyder · HawgRyder's Garage

I thought the valve angle on the 2.0 heads was different from the 2.2 and later.
I may be incorrect on this....anyone else?
Bob

Walt Fricke · 11-05-2011, 03:48 PM

Get two more heads of the proper type. Messing around with shaving some but not others, futzing with the heads, spacers, other adjustments to try to get everything lined up properly is going to be a nightmare . 2.2 heads can't be that hard to come by.

If you use the two early heads both on one side, you'd only need to reverse deck one other head. You might not have issues with the chain housing lining up so the cam holder piece and O-ring (the 3x6mm bolt collar with O ring and gasket)lining up. If you do, you can shave a little off of the chain housing mating surface, or see if you can get by with no gasket or fabricate a thinner gasket, etc.

Then there is the issue of the chain. You can purchase a larger diameter chain idler wheel if you are running out of room with the chain tensioners. I don't think you'd have to jack the cylinders back up to maintain the right dimensions of everything. And, as you note, that is self defeating in terms of keeping the CR up there.

You will still face potential issues. For instance, will you have enough piston to head clearance? Increasing compression by decking heads reduces this clearance, and you will only be doing that on two heads. Heads made for 80mm cylinders naturally are going to have slightly different dome contours/radii than heads made for 84mm cylinders. Of course, spacing the cylinders out will conrol this, but at the expense of CR.

Anderson points out that the 2.2 heads were recontoured to a shallower, smaller displacement to work with the larger valves.

The 2.0s had an actual head gasket, which has some thickness to it. The 2.2s use the early CE ring in a groove in the cylinder tops. I can't imagine this gasket working with the larger cylinders, but it was part of the dimensional calculations for the 2.0s.

All in all, seems like a whole lot of headaches to avoid acquiring two more heads of the right kind.

cgarr · cgarr's Garage

Do those 2.0 heads have a different chamfer at the sealing surface than the others? That would change the combustion changer cc difference. You really need to make sure that all the heads sealing surfaces are the same measurement from the top. Stock is 84.5mm but you can remove up to .25mm off them but they must remain all the same. I personally don't like jockeying around with shims etc to get the cam carrier true with the heads.

It could be something as simple as the hole around the spark plug not cast the same as the others.

jclotz · jclotz's Garage

Thanks guys,
seems much cleaner to get another 2 matching heads.

Flat6pac · 11-05-2011, 07:06 PM

The measurments of the 2.0 deck height of the cylinders are larger numerically than to 2.2 and later because of the head sealing style.
Bruce

911pcars · 11-06-2011, 12:32 PM

Compare intake/exhaust valve dimensions too. Depending on the model heads, they could be different.

Sherwood

304065 · 11-06-2011, 03:33 PM

Actually the geometry is exactly the same in terms of deck height between the 2,0 and 2,2. It looks like the cylinder heights are different but they aren't, I remember looking this up and there's a post about it somewhere here.

Mixing and matching 2,0 and 2,2 and later heads is impossible. The head sealing surface is different, the combustion chamber volume is different, the combustion chamber shape is different, the valve angles are different, the valve sizes and ports are different too.

Henry Schmidt · Henry Schmidt's Garage

Even Harry Potter and his magic wand couldn't match 2.0 heads with 2.2 or 2.4 heads. Valve sizes, valve angles and chamber shape are all different.

First, find a machinist that understands the dimensioning issue associated with 911 Porsche engines and then find a matching set of heads. It is generally pretty easy to find a set of heads that can be reconditioned to factory specs at a cost far less than the aggravation associated with screwing around with mismatched heads.

Sometimes when you find yourself on a dead end road the best option is to back up and start over.