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H89 vs gx head
Just can't stay away
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2015/3/22 7:42
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I have always wondered how a gx head can flow more stock vs stock.

I realise the gx has bigger ports then the h89 but the have smaller valves in comparison to the h89.

Wouldnt it the gx head with bugger port but smaller valves tend to flow less or possibly be very similar to the h89?

Hope someone can give some insight on this or even share some flow figures.

Posted on: 5/15 13:34
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Re: H89 vs gx head
No life (a.k.a. DattoMaster)
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Living here for a while now its very surprising that the TS guys
here in Japan prefer to use early A12 round port heads in which
they open it up till it hits the water jackets devcon them,
run 40mm inlets 33mm exhausts etc.
They prefer them over oval h89 and gx

Posted on: 5/15 14:09
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Re: H89 vs gx head
Just can't stay away
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Very interesting i wonder what the reasoning is, the h89 inlet ports look quite well designed in comparison to round ports.

I honestly cannot figure out the theory behind large port whilst retaining smaller valves.

In my mind the h89 will have better flow charecteristics with the larger valves at all valve lifts until port size becomes a restriction at high rpm.

Even then i cant see the gx head outflowing it at higher rpm with the smaller valves if they both where utalize same lift cam.

Posted on: 5/15 22:04
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Re: H89 vs gx head
No life (a.k.a. DattoMaster)
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The GX head was designed for a motor with a 73mm bore whilst the H89 was used on a motor with a 76mm bore. For street use perhaps the smaller valves are optimal for that bore?

By the time the bores were enlarged for the A12A / A14 & A15 Nissan likely wouldn't want to spend the money for the tooling to put the larger valve seats in the head.

The Cherry N10 prep manual specifies 38/32mm valves for racing so again I suspect someone worked out what was optimal for the combo of Bore, Cam & RPM range.

Naturally this is all pure speculation on my part.

To further muddy the waters; for a brief time I used the so called H-production round port head on my car. It had been expertly ported and was set up for a Weber DGV downdraft. The motor pulled every bit as well as it did with the standard GX twin carb set up.

Posted on: 5/16 16:31
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Re: H89 vs gx head
Not too shy to talk
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I to wish to add mud as well, I wanted a Datsun A series cylinder head ported and 42mm intake and 33mm exhaust installed, so I gathered up a GX, H72 and H89 heads and took them all to a machinist/engine builder, who used a flow bench for head porting. I wanted a cylinder head that would deliver, good flow between 3,000 to 7,000 for a slightly modified A12 engine- slightly larger pistons, 74mm and a mild camshaft.

He flowed all the heads and liked the H89 due to what I wanted and the short side radius of the head, coming in second the H72 head and the GX head last. However the he like the GX head for rpm over 6,000 with the larger valves, as the air/fuel mixture dumped straight onto the intake valve.

Point of personal ponder... he said the flow characteristics were VERY similar to first gen small block chevy heads.

Posted on: 5/17 2:38
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Re: H89 vs gx head
No life (a.k.a. DattoMaster)
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H89 is the perfect head no need to spend money to get it to work
but it surprised me even that a rally crew 15 odd years ago wanted
to give me money a common round port head after coming over to buy
a set of 40mm dellortos saying the round port heads work best for
rally conditions or where all round (:>) performance was needed.
I gave it to the because they need bigger valves, careful porting
and basically alot more money than a H89 head off the bat.

Application was going to be around 1570cc A15 etc
At least some choices are available but for custom work you
need to know what a round port needs to get them to buggie.

Posted on: 5/17 8:49
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Re: H89 vs gx head
No life (a.k.a. DattoMaster)
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I use 38 + 33mm valves in mine
Don't recall the head casting
Worked OK for me

Ports are 36 x 28 inlet and 33 x 30 exhaust

Posted on: 5/17 11:39
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Re: H89 vs gx head
No life (a.k.a. DattoMaster)
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When I was taking to Simon about a turbo motor he mentioned he preferred the H89 head.

Again I bet it comes down to a particular set up; cam & RPM range etc.

Posted on: 5/17 16:16
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Re: H89 vs gx head
No life (a.k.a. DattoMaster)
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He had it stock then had the H89 worked on but said
he got no big improvement overall to stock.
There could be a limit for boost with wedge design
and probably an e15et hemi heads would be more rewarding
when it comes to 2 valves per chamber.
Wedge is very healthy for NA though probably even more so
if they are welded up and reshaped to late chev design
like TK used for an L series. Also many makes prefer
square inlet ports over rounded in many applications.
We would prob never see an cnc a series head with these
features though but would be interesting if some kid or
oldboy in the future makes a billet version :)
However plenty of 16v heads fit the A series as well.
https://www.onallcylinders.com/wp-cont ... 1/tfs-51410002-m58_xl.jpg
https://www.onallcylinders.com/wp-cont ... 2016/01/Presentation1.jpg

https://www.onallcylinders.com/2016/01 ... -choosing-cylinder-heads/

Posted on: 5/18 2:04
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Re: H89 vs gx head
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GX has very much larger ports than H89
GX valves are just a touch smaller than H89

But I was wondering ... do the GX actually flow more? The H89-derived H92 made more power than the GX stock for stock. Both engines used the same cam specs

Posted on: 5/18 23:42
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