[mrmark1957]

How much performance difference, if any, would there be between a set of 670's compared to a set of 13's,on a mild 400 build ?. 068, logs, Q-Jet. Both sets being stock, unmodified or both sets being lightly cleaned up. Both sets cc'ing to the same size.

[Dragncar]

13s all the way.

[b-man]

Probably the 670 heads.

Maybe by not too much over the #13 heads if anything at all.

I know when Pete McCarthy did his testing the 670 heads were the superior low-lift (up to .400”) head, the 068 cam could be just the right match for them.

It’s a toss up, just use whichever one suits your fancy or best matches the year of the car if that’s a factor.

[25stevem]

The 670s have fatter Intake flow numbers between .200" and .350" lift then any other D port Pontiac head, even the 421 980 casting.

If your not going to run a bit over .500" lift then this is what you want out of a head.

If a proper valve job is done the takes the 30 degree Intake seat OD out to 2.05" and then a wide 60 degree is put in that narrows that 30 degree back to .050" then flow numbers a talk about above will increase even more.

The stock Exh flow numbers between both heads are very much the same because the arc of the short turns are very close .

With either head you want to add a 30 degree back cut to the 45 on the rear of the Exh valves .

This back cut should narrow down the stock .100"wide seat on a new valve down to .075".

Note that due to the big Intake valve size and the 30 degree seat used on most of the iron heads that I consider setting up the Intake springs for 130 to 135 psi the way to go.
The Exh valves can do with 125 psi

[mrmark1957]

Note that due to the big Intake valve size and the 30 degree seat used on most of the iron heads that I consider setting up the Intake springs for 130 to 135 psi the way to go.
The Exh valves can do with 125 psi[/QUOTE] This would be spring pressure on the seat, correct ?

[25stevem]

Correct, in terms of new springs, not ones already broken in.

The real limit for a hydro flat tappet is the 320 psi open number unless your running lifters with a EDM hole on there bottom.
That’s my opinion.

[Formulajones]

I run the 68404 springs at 130 on the seats with the hydraulic flat tappet, with a set of the stock replacement Johnson lifters, it's lived for years as a daily and raced regularly

Of course after run in and some miles they aren't 130 anymore, as typical with most.

As far as head comparison between 670's and 13's, On a 400 with basically no camshaft (068), same compression etc..... I don't think you'll see a damn bit of difference in performance. You might be able to see a slight difference on a dyno, talking single digits here and there, some of that would be variances in the dyno from one pull to the next, but in the car and on a track, you won't find anything. They are both very small heads with minimal flow.

If ya want to make a change, you'll need to make a bigger one than that.

[25stevem]

The problem with the 670 heads is that once you get passed about 218 Intake duration @ .050" they need very rich jetting conditions.

In 67 the engineers experimented with the chamber shape like the 670s, and then the extremely open shape of the 061 casting.
The engineers then took the in between path a went with the chamber shape used from 68 to 78.

Go with the #13 castings you have even if they are stock.

[Skip Fix]

I flowed some 670s after hearing Pete talk about them-I guess mine were a bad set flowed a click less at all ranges than my other D ports. Sold them to a guy with a 67.

[b-man]

Originally Posted by Formulajones

I run the 68404 springs at 130 on the seats with the hydraulic flat tappet, with a set of the stock replacement Johnson lifters, it's lived for years as a daily and raced regularly

Of course after run in and some miles they aren't 130 anymore, as typical with most.

As far as head comparison between 670's and 13's, On a 400 with basically no camshaft (068), same compression etc..... I don't think you'll see a damn bit of difference in performance. You might be able to see a slight difference on a dyno, talking single digits here and there, some of that would be variances in the dyno from one pull to the next, but in the car and on a track, you won't find anything. They are both very small heads with minimal flow.

If ya want to make a change, you'll need to make a bigger one than that.

Agree 100%.

In the end choose whichever set of heads that require the least amount of money to put back into shape if keeping with the 068 cam, Quadrajet/iron intake and log exhaust manifolds.

[chiphead]

I ran 670s on a 350 and a 400. Ditto what these guys are saying. The 670 is often cc larger than 72 cc's. 75-77 is common. I also noticed that they were a little more sensitive to timing and a little more thirsty than the 68-later heads. The 670 is the camel hump head of the Pontiac world. Great for it's time but tech has improved a lot since then.

[25stevem]

It's the unburned fuel pockets that there ( 670s ) shape chamber produces that makes them more prone to detonation when you are comparing a apples to apples compression situation.

After the main burn starts to place then the gas temp in the cylinder is hot enough to light off any unburned fuel .

The issue is then all of that fuel goes off at once and is not a burn, but a minor explosion and that flame front slames into the original burn / flame front and now your down on power and hammering the clearance out of the rod bearings.

[Kenth]

According to Pontiac the #13´s has 75.70 cc combustion chambers.

[Dragncar]

These are numbers taken by Dan Whitmore when he was a dealership mechanic from factory un millied D port heads.
16s 74 cc
12s 67cc
48s 65.5 cc (must have been 350 HO heads)
13s 77cc
62 75 cc
64 90cc


13s are the head of choice IMO for a pump gas 400 with flat tops.