All Small Six Flow testing the "log" heads

This relates to all small sixes
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Over the past few years we've flow tested several Australian 250-2V cylinder heads, as well as our own Aluminum cylinder heads. However we've always wondered how the cast iron "log heads" stacked up, not only to the Australian and Aluminum heads, but to each other, both in stock and modified configurations. Therefore we decided to do a little work on the flow bench.

Since we already had flow numbers from an Australian 250-2V cylinder head and our aluminum head, both in stock and modified configurations, all we had to do was flow test both log heads in roughly the same configurations for a good heads-up comparison. Unfortunately the only small log cylinder head we had in our possession had already been modified and converted to a 2V intake, so we had to guesstimate the stock numbers.

All four of the modified heads included over-sized valves, and were ported in a street/strip configuration. Both the OZ and Aluminum head were tested with the 2V manifolds installed, while the modified log heads were tested with the 2V conversion adaptors installed. I should also note that we tested the #2 ports on all four cylinder heads. All heads were tested at 28" H2O.

The one thing I found interesting, is that with our experience modifying and porting log heads, we were able to get a modified large log to flow nearly as well as a stock OZ250-2V, which I always assumed was impossible.

Summary: We pretty much concluded that it's not worth putting money into a small log head. Even with larger valves and port work, the small log won't flow as well as a stock large log head. So if your going to use a log style head, and you want to increase performance, buy a used large log head and put your money into it. You'll be better off in the long run, and a lot happier with the end results.

For those who'd like more information, I'm working on a tech article that will cover everything in detail, it's just not done yet. However, I'd thought I'd share the peak flow numbers in the mean time. The next step is to spend some time on an engine dyno, which may happen next month if I can afford it.

Here's the peak numbers in a bar graph.


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this is AWESOME!!!

i've alway's wanted to get the performance of the cooler heads!! but with a port job on the larger log i'll get close to it, THANKS MIKE!!!


Edit: since this post I now have a 78 head flow tested after porting at 144cfm
FalconSedanDelivery":10bys2k2 said:
Was the flow tested on a single carb setup or a tri carb , or a 2100 2 brl ?
Stock head was obviously with a single 1BBL carb, the modified log heads were converted to a single 2BBL (Weber, Holley, or Autolite).

The reason we flow tested the log heads, and posted the results, was to benefit those who are staying with a log head. I only posted the flow numbers for the OZ250 and Aluminum head as a comparison. If anyone else has flow figures on the log heads, please post them, or you can send them to me via PM or email. Thanks

Please try to keep your comments on topic, which is "US LOG CYLINDER HEADS". Thanks
AzCoupe":1kknwtzv said:
The reason we flow tested the log heads, and posted the results, was to benefit those who are staying with a log head. I only posted the flow numbers for the OZ250 and Aluminum head as a comparison.

Please try to keep your comments on topic, which is "US LOG CYLINDER HEADS". Thanks

If you want info on the XF head, search the forum, or Google.
Out of curiosity, how much HP does a stock 250 Aussie head have on it? I'm sure it's more than the US log head but this would be a great idea for anyone who just wants to look around for a large log head. And I'm probably gonna go with this idea since I've found a 77 Maverick parts car that has a 250 in it. hopefully it's the same year 250 in it and I can come away with a good 77 head.
The 250 as fitted to XY XA models was 155hp @ 4000 rpm. There was also a low compression engine that was rated at 138 hp @ 4000 rpm
Exactly what years would you say are the best suited to be called large log heads? Mid 70s and up? And does large log head apply to 250 heads only? If not, how much of a difference is there between a large log 200 vs. a large log 250?

Reason I ask is because there is a seller nearby who has a 75 250 motor he is parting out and am wondering if I should go for that head or keep trying to find a later 250 or 200 head.
Ford started offering the large log heads in '69 on the 200ci and 250ci, however the intake volumes changed over the years. Unfortunately I don't have specs for the 75-76 heads, but I think they were 1240cc's.

69-74 - 1240cc
77-80 - 1345cc
250 ci
69-74 - 1240cc
77-80 - 1345cc
80-up - 1420cc
I don't know the first thing about small sixes, but just looking at the specs it would appear that about the same year that the log volume went from 1100 to 1240, the head volume went up. Does that mean that to get the factory head that can be modified to flow good numbers, you'll still be stuck with an open-chamber smogger head with no squish? I ask in pure ignorance.
Well then, would a good alternative be to pick the head (presumably an early one) looking only for the best chamber shape . . . and then cut off the restrictive log, leaving short stubs to which you could braze flanges, and weld up whatever kind of manifold you want from exhaust U-bends? You could open up the stubs somewhat. Mike's findings are interesting and surprising, but why bother with the log at all, when you could delete it with a sawzall, get the stubs milled flat, and if you don't weld, take the head to your nearest tech school and have one of the more advanced welding students braze flanges to the stubs? Make one more flattening cut to the faces of the flanges, and you have a closed chamber head all ready for a home-brewed I.R. intake. Still nothing like as nice as the aluminum head, but at least it's low-buck, and doesn't look "factory-stock". :thumbdown:

I was thinking about doing a small-six head in this way, just for fun and maybe to sell, a while back. My pal with a machine shop "gave" me a head (year unknown) that was lying around. Unfortunately, I fooled around and the scrap-man saw it and thought it was for him, and "poof". So what year heads have the "best" combustion chambers (small chamber, big squishband, fixable ports)?

You didn't make that run with street tires, did you?
Seattle Smitty, you just described what is possible with a 250 2V Australian head, which the CI alloy head is based on.
So do you think it would be worth my time to take the d5 head i and have it milled down. Would it be that much better than the c8 that's on my maverick now
Porting the head, milling log for a Holley 2bbl adapter, and milling head to raise compression will all give noticeable improvements. Bigger valves depend on how much you want to spend. From reading on the forum instead of the engine falling flat on its face at 4500rpm it should now rev to 6000rpm.

The later head with the larger log is a improvement in itself.

My head is a D8 with the larger log. Top is milled for Holley 2300 series 350cfm 2bbl, I have exhaust port divider installed, head was surfaced IIRC .020" to true it. I will be running turbo so didnt want to raise compression. My head is not ported but does have the larger valves but I dont remember now if I went 1.75" or 1.80" on intakes. I did go 1.50" on exhaust.
We recently added 1.46" exhaust valves to our inventory. Not only are they easier to install, recent flow testing showed that the 1.46" valves actually flowed better than the 1.50" valves, but only if the shrouding issues are not addressed. Deshrouding the 1.50" valves also increases the CC's just a tad, so you need to keep that in mind if your shooting for a specific C/R. Basically, if you have a tight budget, the 1.75/1.46" combo is the best way to go.
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