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Discussion Starter · #1 ·
are there any parts of the z6/y8 heads that aren’t interchangeable? cams (with matching gear of course), springs, valves, etc. ik some of this has been covered already but i’ve never found a definitive answer. thanks
 

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91 Civic SI hb/ 01 GSR
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I think they are pretty much the same. I cant think of any differences right off the top of my head.

You could check part numbers, or check aftermarket parts to see what years they cover. Or most times you can tell by looking at the parts.

What did you want to know about. Or if this just a random question that popped into your head?
 

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Discussion Starter · #3 ·
well i found a disassembled y8 with a lot of performance parts, but i’m really not a fan of the y8 because of the oil & air flow issues. i wanted to know if i could move all those parts over to a z6. i know the blocks will work just wondering about the heads
 

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well i found a disassembled y8 with a lot of performance parts, but i’m really not a fan of the y8 because of the oil & air flow issues. i wanted to know if i could move all those parts over to a z6. i know the blocks will work just wondering about the heads
I'm pretty sure the Y8 head flows more air. I could definitely tell side by side that the Y8 had much larger intake runner passages in comparison to the Z6, in both the intake manifold and the cylinder head.

And the Y8 oiling issues should be limited to the bottom end, the cylinder head should be fine. The Z6 head is almost identical to the Y8 in terms of oiling passages, only big visual differences seem to be the design of the rocker shafts and the LMA holder position, and of course camshafts (keep cam gears with the respective camshaft, Z6 gear to Z6 cam, Y8 gear to Y8 cam).

There are lots who put Y8 heads and intake manifolds on Z6 bottom ends for the benefits of the Y8 head. The Z6 has the more robust bottom end. If you've got a Z6 bottom end and a Y8 head with performance parts, throw the Y8 head and intake on there (after getting checked at machine shop).

Springs and rocker arms are interchangable, even though the VTEC arm assembly has a slightly different part number. Physically, they are identical. I'm running Z6 rockers in my Y8 head right now (had to replace due to the cam follower pads being worn out). The only thing you shouldn't swap over would be camshaft and rockershaft caps, since those are line bored to the matching head.
 

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Discussion Starter · #5 ·
the y8 has swirl ports, while the z6 has tumble ports, so the air flows better. the y8 intake has larger runners so it flows better. best flow would be z6 head + y8 intake

The only thing you shouldn't swap over would be camshaft and rockershaft caps, since those are line bored to the matching head.
can you explain this a little more i don’t get it
 

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can you explain this a little more i don’t get it
The caps that support the rockershafts and clamp the camshaft down to the head, the aluminum blocks with half moons cut out of them between the rocker arms, these guys with the 12mm bolts holding the camshaft down into the head:

136335



When they machine the heads at the factory, these caps are installed and torqued to the head and are still not perfectly round/circular/straight. A precision boring tool is spun up and pushed through the camshaft cap holes, effectively cutting a perfect circle straight through each hole. When the caps are loosened, both halves of the circle, the side in the head and the side in the cap, are now matching machined parts.

If you swap the caps from one head to another, the precision of the line bore is now mismatched. Running an engine with imperfect bore circles around the cam journal can eat up cam journals and/or ruin the camshaft journal bores themselves.
 

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the y8 has swirl ports, while the z6 has tumble ports, so the air flows better. the y8 intake has larger runners so it flows better. best flow would be z6 head + y8 intake
Yup, your right. Z6 head flows more!

Went looking for some info and stumbled across this really awesome compare from Bad Guys:



D SERIES CYLINDER HEAD SHOOT OUT!

For this article we put the a6,z6,y7, and y8 castings head to head on the flow bench. We tested them in both stock trim and in DIY porting trim. This level of port work is along the lines of what most guys at home (with lots of patience) would probably come up with. The diy heads were flow tested with OEM valve seat angles and valves.

We also flow tested a few z6 castings from some of the porters in the industry, we featured a head ported by DNR, Port flow, and finally one of our cnc heads. All of these heads were flowed with supertech valves and modified valve seat angles.

First lets take a look at the D16 a6 casting;

In stock form this is the lowest performing casting. It has the smallest intake valves measuring up at 29MM. The casting quality of these heads is actually pretty good. There isn't a lot of core shift in the casting that i have seen, no casting bubbles, decent transition from the bowls to the seats. However the minimum cross sectional areas (and obviously port CC) is much smaller than all of the other D series castings. At a glance the port shape has a port divider that is very far back in the port (Bad for velocity) as well as no valve guide bosses. (Doesn't help the air turn around the guide and valve stem) If you look at the port floor you will notice that the sides of the ports are more of an oval shape, with the port floor being rounded. From what we have seen in testing this is not an optimal shape, if you look at the Z6 casting you will see that the ports make a D shape with the floor being much more flat and widened as well on the short turn. (Much better for flow) The injector boss is also much smaller than all of the other D series castings.

Taking all of this into account I would say that this is why the A6 head is such an under performer.

Our flow results:

@ 28”

Stock Casting
Valve Lift| CFM
0.050 | 33.9
0.100 | 71.31
0.150 | 111.9
0.200 | 143.56
0.250 | 163.43
0.300 | 172.52
0.350 | 175.63
0.400 | 178.71
0.450 | 180.25
0.500 | 183.33
AVG: 141.454 CFM

The D16Y7:

There is not a whole lot to say about this casting other than it is essentially a later model clone of the A6, the only key difference being the 30MM intake valve diameter. From a port stand point the are just about the same design, however even with the larger valves the throat diameter is nearly the same as the a6, this being the key culprit as to why it flows roughly the same CFM although it has larger valves.
Our flow results:

@ 28”

Stock Casting
Valve Lift| CFM
0.050 | 33.66
0.100 | 70.33
0.150 | 111.03
0.200 | 142.41
0.250 | 163.43
0.300 | 172.52
0.350 | 175.63
0.400 | 178.71
0.450 | 180.25
0.500 | 180.25
AVG: 140.822 CFM

DIY Porting
Valve Lift| CFM
0.050 | 34.87
0.100 | 72.79
0.150 | 111.9
0.200 | 144.75
0.250 | 159.92
0.300 | 172.54
0.350 | 178.71
0.400 | 186.41
0.450 | 191.03
0.500 | 194.11
AVG: 144.703 CFM

With this sort of porting we saw gains at about every level of lift there we as few cfm sacrificed at .200-.250 but the gains made from .350+ way more than justify this loss. Resulting in a higher net CFM. We saw an average gain of about 4 CFM across the board. Not exactly what you would write home about but for the average Joe in his garage these are a measurable gain, for nothing more than time hours spent grinding at home.

Moving onto one of the most popular castings, the Y8:

OK so lets look at the port design. This casting features 30mm intake valves and uses a "Swirl" design(rather than a “tumble” design as the other castings do), if you look at the port divider you will see that it is very far back into the port (bad for port velocity as we saw with the non vtec castings) as well as angled, the guide boss's on the head are also off set in size. Now this design is found commonly on a lot of auto manufactures truck cylinder heads. This works Ok for producing low end tq numbers, stuff that would be good for MPG in our application to hondas. The result of this swirl design is a highly turbulent port, not what you want for high HP. The key to power is the port velocity. Now also working in tandem with this is the fact that y8 cams are ground with slightly of set lobes to further exaggerate this swirl concept. Now the chamber does have a smaller cc which is great for compression, but the closed chamber design also makes it very difficult to tune and detonation becomes a major factor. Now lets look at the throat area of the y8, the core quality it self is lacking. The throat area this is a prime example of this, roughly 40% of the throat area is taken up with material and core shift within the casting, this short turn area also is much lower in the port and this kills flow in all aspects. When compared to the non vtec castings such as the A6 and Y7, the net flow is nearly identical.

Our flow results:

@ 28”

Stock Casting
Valve Lift| CFM
0.050 | 35.84
0.100 | 72.79
0.150 | 111.9
0.200 | 142.25
0.250 | 159.92
0.300 | 171
0.350 | 180.25
0.400 | 183.33
0.450 | 183.33
0.500 | 183.33
AVG: 142.394 CFM

DIY Porting
Valve Lift| CFM
0.050 | 37.05
0.100 | 75.74
0.150 | 116.27
0.200 | 144.75
0.250 | 163.43
0.300 | 175.63
0.350 | 184.87
0.400 | 191.03
0.450 | 198.73
0.500 | 201.82
AVG: 148.932 CFM

With this sort of porting we saw gains across the board. With this porting what you should keep in mind, when attacking the port with your grinder is to focus on the bowl and throat area. This is the main area of concern for this casting, because there is so much core shift and there is so much material taking up valuable real estate. By simply opening up this area you can see quite a noticeable gain, Mainly in the upper lift range nearly 20CFM at .500 lift. We saw an average gain of about 7 CFM across the board. This is not expert level territory but you will surely see a gain in the car when all is said and done.

Finally the best choice for D series, the Z6

Now lets look at the Z6 casting. The port design is symmetrical with proper porting you will see much greater velocity and more air into the cylinder as a result(The cam lobes are not off set on z6 cams). Most y8 heads will be down 4-12% from a Z6 in a stock for stock comparison (Depending on the quality of the y8 core it is very hit and miss with core shift Vs the z6 that has much more uniform casting quality). The throat area is much more open in the Z6 with the entire casting quality of the head being better with much less core shift in general. The short turn area is also taller and provides more room for reshaping, the short turn it self is also much wider and in a D shape (factory ITR heads have the throat area opened up further than standard b16 heads and the bowl areas hand ported). Finally the chamber is an open design. If you look at all of hondas VE efficient heads you will see the open chamber design is used. With the open chamber design you will also have larger chamber CC volume, this will make it easier to tune and be far more forgiving when being tuned. H series, B16 and ITR heads use this chamber design. Although you will have less compression this can always be gained with larger dome pistons, milling the head and block deck surface. This is why the z6 head is better than the y8. The compression difference in Stock Y8 engines and Stock Z6 is roughly 4%.


Our flow results:

@ 28”

Stock Casting
Valve Lift| CFM
0.050 | 35.6
0.100 | 73.28
0.150 | 111.9
0.200 | 143.58
0.250 | 168.1
0.300 | 181.79
0.350 | 189.49
0.400 | 192.57
0.450 | 195.65
0.500 | 197.17
AVG: 148.913 CFM

DIY Porting
Valve Lift| CFM
0.050 | 36.81
0.100 | 74.26
0.150 | 112.78
0.200 | 143.58
0.250 | 164.59
0.300 | 178.71
0.350 | 189.49
0.400 | 198.73
0.450 | 206.44
0.500 | 211.06
0.550 | 215.68
AVG: 157.466 CFM

What is key to take away from this is that even in a stock to stock comparison the Z6 casting we tested is up about 5.5% average flow across the board when compared to the stock Y8. When you compare the DIY ported Y8 to the Z6, the stock Z6 still averages more CFM. In the case of the non vtec heads vs the stock z6, the non vtec heads are down an average of about 6% CFM. So for the average guy looking to swap a vtec head onto his non vtec block, you can pick up 6% flow simply by bolting on a Z6 head and call it a day. For the DIY ported head we saw a gain of about 8 CFM through out the lift range, but most noticeably rather than the flow falling off at .500 lift you will see that the CFM continues to climb. Now peak flow is at .550 lift.

Now lets take a look at some of the professionally ported heads we were able to test.

DNR Z6
Valve Lift| CFM
0.050 | 36.1
0.100 | 73.3
0.150 | 111.0
0.200 | 143.6
0.250 | 172.8
0.300 | 191.0
0.350 | 201.8
0.400 | 209.5
0.450 | 214.1
0.500 | 215.7
0.550 | 215.7
AVG: 162.236 CFM

This casting ported by DNR in the bay area, low lift flow was around the same as stock up until about .250 lift, from that point on it states to gain a fair amount of CFM. Picking up about 18 CFM at peak lift. This head Averaged about 13 more CFM or about 9.2% gain throughout the entire lift range.

Port Flow Z6
Valve Lift| CFM
0.050 | 36.3
0.100 | 73.8
0.150 | 110.2
0.200 | 140.1
0.250 | 163.4
0.300 | 178.7
0.350 | 197.2
0.400 | 206.4
0.450 | 214.1
0.500 | 217.2
0.550 | 220.3
AVG: 159.792 CFM

This casting ported by Port Flow in southern CA, was down about 11 CFM vs the stock casting until about .300 lift. After 300 lift this port design starts to come to life. Putting some serious gains in the .450-550 lift area. This head Averaged about 11 more CFM or about 7.9% gain throughout the entire lift range. With a peak gain of 23.13 CFM at max lift.

Bad Guys Z6X
Valve Lift| CFM
0.050 | 38.5
0.100 | 76.23
0.150 | 115.4
0.200 | 149.42
0.250 | 179.77
0.300 | 203.36
0.350 | 221.84
0.400 | 235.71
0.450 | 241.87
0.500 | 244.95
0.550 | 244.95
AVG: 177.454 CFM

This casting was 5 Axis CNC ported By Bad Guys in Santa Clarita CA, shows gains at every level of valve lift. With an average gain of 27.1% or 28.5 CFM throughout the entire lift range. With a massive CFM gains from .200-550 lift. UP 47.75 CFM at peak lift. This head is an absolute animal with a net CFM gain of 217.75 CFM. It is ready for whatever you want to throw at it NA, or forced induction.

To conclude:

The non vtec heads flow about the the same cfm as each other, a stock Y8 flows about 5.5% less air than a stock Z6 head. There are gains to be had if you are patient and brave enough to attempt porting in your garage. However there is considerably more flow on the table if you are willing to shell out the cash to have your head professionally ported. The goal is not to have a giant peak flow number, in the case of the DNR vs Port flow head the DNR head does not move as much air at peak lift but instead has a higher average. Resulting in more net flow into the engine per revolution. The key is averages, averages, averages. Having strong average flow will increase engine acceleration, as well as give you a wider more usable power band. Engines are not static, these Honda engines are not paired to a 15 speed gear box that keeps the engine in a 1000 rpm window. You must have a wide usable power band if you want your car to dynamically accelerate through the rpm range, and through the gears. Another key to consider is what lift camshaft will you have in your engine? All the flow in the world doesn’t matter if you don’t have a camshaft that is paired to your application, and flow numbers that are relative to your lift.

Special thanks to Alaniz Racing heads in Los Angeles for providing a neutral 3rd party flow bench to test all of these cylinder heads on.

-SOHCole
 

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Discussion Starter · #8 ·
If you swap the caps from one head to another, the precision of the line bore is now mismatched. Running an engine with imperfect bore circles around the cam journal can eat up cam journals and/or ruin the camshaft journal bores themselves.
ok i thought you meant you can’t swap the cams, but you were just saying the cam caps. makes much more sense.
 

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The y8 rocker shafts are superior. Anyone who's taken apart enough z6 and y8 heads might have noticed that the oiling holes on the z6 are on top and the y8 are on the bottom. The z6 rocker shafts always get scored and eaten up because of lack of oil where it's needed most, the bottom.
 
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