# #670 or 6x heads



## bobby326 (Dec 13, 2009)

i am building a 400 for my 67 lemans and i would like to get some advice on a head choice. with haeds would make more hp and tourqe 670 or 6x. i will have them machined and ported and dressed with all new aftermarket spings and rockers. i have a mild cam and an edlebrook performer rpm intake that will go on the motor also. would like to achive 400-450 hp. any info would be appriciated thanks guys


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## 66tempestGT (Nov 28, 2009)

theres a lot of sharp pontiac guys on here but i would call butler and run that question by them.


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## 68greengoat (Sep 15, 2005)

I don't claim to be one of those "sharp pontiac guys" out there, but from what I can tell, the 670 would be the better head. They both have 2.11 intake and the 670 has 1.77 exhaust where as the 6x have 1.66's. The 670 is a higher compression head at 10.5 at 72cc. And it would be period correct if that is an issue...


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## jetstang (Nov 5, 2008)

:agree. The small CC heads are going to put your compression ratio high, unless you get dished pistons, race fuel. The 6X heads are also really good. High compression will def make more HP, but you'll pay at the pump.


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## Too Many Projects (Nov 15, 2008)

The 670's will breath a little better than the 6X because of the larger exhaust valve. You will need to run a LOT more than just a mild cam with the RPM manifold BUT as said, the pistons need to be matched to the heads to keep compression livable on pump gas or E85. Quite a few of the guys around here have changed their carbs to work with the E85 and keep their late 60's, early 70's high compression engines original.

Speaking of the cam, you need to match it to the Performer RPM manifold. The RPM manifold is designed for mid-range to top end performance and will sacrifice low end torque. It isn't a particularly street friendly part. In my opinion, you would be better off building the engine for 450-500 Lbs ft of torque for the street and let the hp fall wherever it may. Torque is what gets the car moving, not HP, and is usually found in the lower to mid-range rpm band.


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## geeteeohguy (Feb 2, 2008)

All good advice. The 670 is a '67 GTO head, among other things, and is a good head. It is also unique in that it is of the old "closed chamber" design, unlike '68 and up HP heads. Because of its chamber design, you will have an engine that is more prone to detonation than if you ran an open chamber head. A lot of the cylinder head gurus' open up the 670 chamber a bit, unshrouding the exhaust valve. Seems to help, but you'll still need dished pistons. Get on the websites, and learn all you can. YOu will also want to build your motor with zero deck height to minimize detonation. Many have been successful at treading the tightrope of high compression and today's fuel. I haven't had very good luck with that one!


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## bobby326 (Dec 13, 2009)

thanks 4 the advice. i already have the 6x heads. i bought them used. didnt get them machined yet. just wanted to know what would be the better choice. sounds like the 670 would take a little more work. prob just stay with the 6x.


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## geeteeohguy (Feb 2, 2008)

If you don't already have the 670 heads, there are a lot better choices for iron heads out there. You can have bigger valves/seats installed in your 6x heads by a competent machinist. I somehow thought you already had the 670's.....


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## jetstang (Nov 5, 2008)

Are your 6X heads -4 or -8? Big dif in combustion chamber size.


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## bobby326 (Dec 13, 2009)

:confused sorry jetstang i dont know what -4 or -8 means can u explain that to me


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## jetstang (Nov 5, 2008)

bobby326 said:


> :confused sorry jetstang i dont know what -4 or -8 means can u explain that to me


On the middle of the head it says 6x, on the outer exhaust it says -4 or -8. 
-4 is smaller chamber, -8 is larger chamber.
1975-79 350/400 6X-4 2.11/1.66 98 CC Screw in studs- dual valve springs- D port 
1975-79 400/455 6X-8 2.11/1.66 101 CC Screw in studs- dual valve springs- D port 
I don't think the CC size is correct, I just pasted it from a website.


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## bullet20cc (Apr 4, 2009)

*6x Heads*

6x heads came out in 1975 through 1979. All very low compression.
6x-4 are 8.0 compression ratio.
6x-8 are 7.6 compression ratio.
All made for regular gas.
#48 or 64 heads are 87cc and would be much better horsepower on premium gas and not to much compression to cause detonation.


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