[Unknown User]

Has anyone run these heads from the 81-83 ZX Turbo block on a non-turbo P79 block with flattop pistons? (the turbo motors have domed pistons). I heard that this is the ultimate combo because these are the best flowing heads, but that to get the right compression with a P79 block, the P90 or 90a head would have to be shaved.

 

[Nathan]

> Has anyone run these heads from the 81-83 ZX
> Turbo block on a non-turbo P79 block with
> flattop pistons? (the turbo motors have
> domed pistons). I heard that this is the
> ultimate combo because these are the best
> flowing heads, but that to get the right
> compression with a P79 block, the P90 or 90a
> head would have to be shaved.
First of all the turbo motors and dished pistons.
I wouldn't really call it the untimate combo as the differance in flow between the P79 and P90 is not really worth changing your head and exhaust manifold for.

 

[Ian]

First, let me clarify something. The pistons on a stock L28ET (aka turbo motor) are not domed, they are dished. With the P90 and P90A heads, the C/R comes out to 7.4:1 on the L28ET.

Now, the P79 and P90 heads have the same size combustion chambers, so swapping from P79 to P90 would only change the exhaust port design. Yes, the P90 exhaust ports do flow better than the P79, but this is not a good enough reason to do the swap. The increase would be negligable, if noticed at all. I warn everyone not to get involved w/a P90A head. A rebuild on this thing is rediculous because of the cost of the hydraulic lifters as they are only available from Nissan. However, if cost is no object, you wouldn't have to adjust your valves anymore....

> Has anyone run these heads from the 81-83 ZX
> Turbo block on a non-turbo P79 block with
> flattop pistons? (the turbo motors have
> domed pistons). I heard that this is the
> ultimate combo because these are the best
> flowing heads, but that to get the right
> compression with a P79 block, the P90 or 90a
> head would have to be shaved.

 

[Bob H]

> Now, the P79 and P90 heads have the same
> size combustion chambers, so swapping from
> P79 to P90 would only change the exhaust
> port design. Yes, the P90 exhaust ports do
> flow better than the P79, but this is not a
> good enough reason to do the swap. The
> increase would be negligable, if noticed at
> all. I warn everyone not to get involved w/a
> P90A head. A rebuild on this thing is
> rediculous because of the cost of the
> hydraulic lifters as they are only available
> from Nissan. However, if cost is no object,
> you wouldn't have to adjust your valves
> anymore....

Being the P-90 advocate I am, I just wanted to add my 2 cents. First, a rebuild on a P-90A head is only rediculous if the lifters are bad. They will cost about ~42 each from courtesy nissan with a IZCC discount. That does add up really fast for all 12. But if you have a low mile head, whose oil was changed regularly, they will be fine. They basicly have a check ball that locks in when the lifter starts to see pressure from the cam, and keeps the oil that is inside the lifter there. If oil has not been changed regularly, deposits form, and inhibit the check balls funcition, as well as the overall lifter. If it is high mileage, the lifter may be close to being worn out, because, like other cam parts, it wears out over time.
But all that said, they can be used, and can be used with a performance camshaft. I am running a 460 lift, 280 duration cam on mine. You just have to insure the spring pressures remain reasonable, otherwise you overpower the check ball, causing lifter bleed down.
Again, just my 2 cents. There are many good things about the hydraulic head, but you must chose to use it only if you really understand what makes it work, and what doesn't.
And as was stated above, the improvement in exhaust flow over the P-79 head is not worth it if that is all you are looking to gain. Now, if you want a higher rpm breather, big exhaust, etc.. the P-90 is a better choice. But you can make a car really run with the P-79. Check out the head mod page on the datsun garage. He talks about the P-79. The link is below.
-Bob Hanvey