This tech article is a "work in progress". Information will be added as it becomes available.

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.

The US cast iron heads are commonly referred to as "log heads" due to the integral cast iron intake manifold, which looks like a "log". Hence the nickname. They were produced in two basic versions, a small log and a large log. While we won't go into the differences in this article (manifold volume, valve size, etc), we will note that the small log manifold has a 1-1/2" carb bore, while the large log has a 1-3/4" carb bore. The importance of knowing the difference will become clear as we go along.

As we stated previously, we already had flow numbers from an Australian 250-2V cylinder head, as well as our aluminum head, both in stock and modified configurations. Therefore all that was required was to flow test the log heads, in roughly the same configurations, if we wanted a good heads-up comparison. Unfortunately the only small log cylinder head in our possession, had already been modified and converted to a 2BBRL, so we were only able to flow test the exhaust ports for comparison. As such, we had to estimate the numbers for the intake ports, for a stock small log cylinder head.

Porting
We considered porting all four cylinder heads for maximum airflow, however we decided it would be best if we kept the port work within reason, porting them in the same manor as the average customer, which is in a street/strip configuration. Since we already had flow numbers for the Australian and Aluminum heads in a street/strip configuration, all we needed to do was to port both log heads. We also needed to install the largest valve sizes possible to keep all four heads on an even keel. Keep in mind, all four cylinder heads still have room for improvement and additional gains in air flow, and that the numbers posted below do not represent the maximum air flow capabilities of the cylinder heads. However we should also note that any additional port work is time consuming and may be very expensive. This is especially true of the log heads, since the intake ports are hard to reach and very difficult to port. As such, additional porting may not be worth what it cost, at least for the average builder.

2V Conversion
We should also note that there are two methods for installing a 2BBRL carb. The most popular method, and the easiest, uses a 2-1 carb adaptor that funnels the air flow into the existing one barrel carb bore. While this method is quick, easy, and cheap, it does absolutely nothing to relieve the asthmatic conditions of the log style manifold. The other method, commonly called a "2V-Conversion", may cost more, however it does a great job of relieving the asthmatic conditions associated with the log manifold. This method also uses a carb adaptor that is bolted to the manifold in much the same manor. However prior to mounting the conversion adaptor, the manifold is built up with a filler putty or furnace brazing, milled flat, then the carb opening is hogged out, making it a true 2V intake. As such, the air flow is no longer funneled through the restrictive one barrel carb opening, which greatly improves the air flow capabilities of a log style manifold. The "2V Conversion" is the method we will be using on our modified log heads. Along with a little port work and larger valves, we should see some impressive gains in air flow.

Photo of a completed 2V Conversion. While this head is set up to use a Weber DGV, we also have conversion adaptors for Holley-Autolite 2BBL carbs.

We thought about flow testing a head with only the 2V Conversion completed (without bigger valves and porting), however we thought this would just be a waste of time. Our reasoning was pretty simple.... we figured if someone went to the expense and trouble to converting the log manifold to a 2V carb, they would probably install the larger valves and do a little port work at the same time. However this was probably a mistake. Therefore we plan to do some more testing in the near future. We also want to test the installation of larger valves and/or porting, since there might be a few guys that want to keep the 1BBRL carb to maintain a stock appearance, or for those who plan to use the new Vaporizer carb.

Exhaust Ports
First lets look at the exhaust numbers of the small and large log heads, and the Australian head, in a stock configuration. These are pretty easy to compare, since all three heads share identical exhaust port designs. The only variance that might effect the results, would be the smaller 1.288" exhaust valves (which came in early versions of the small log cylinder head), however all three of our cylinder heads had 1.380 exhaust valves. As such we felt it was reasonable to assume that the peak flow numbers for the stock small log head, in a stock configuration would be 105cfm, even though we never tested it on the flow bench.

When you plot the CFM gains on a graph, you can clearly see how porting, and the installation of larger valves, vastly improves air flow on the cast iron heads. Remember the graph only shows the gains, not total air flow. The small log, large log, and the Australian heads were all pretty equal, but this was easy to understand since they all share identical exhaust port designs. The Aluminum head doesn't fair quite as well, however this was also expected. The exhaust ports were designed to maximize air flow (cfm) using computer simulations, and unlike the cast iron heads, we didn't need to install larger valves since the aluminum heads already come with them. Therefore the only modification performed, was a small amount of porting to clean up the valve pockets and guides. Hence the lower gains.

Intake Ports
If we look at the intake numbers we see that all three heads start off slow at low lift, but once they reach the 300 mark the gains improve. The log head continues to gain up to 500 lift, where it starts to drop off. The gains in the Australian head are not quite as impressive, but unlike the log head, the gains continue to improve as valve lift increases. On the other hand, the aluminum head has the advantages of both the log and Australian heads. Not only are the gains impressive (like the log head), they continue to improve as valve lift is increased (like the OZ head). When plotted on a graph, the numbers look something like this (see the graph below). Remember the graph only shows the gains in air flow over their stock counterparts, not total air flow, thus showing how well the heads respond to porting.

When you look at the numbers, it's clear to see that the small six cylinder heads suffer not only from a restrictive log manifold and intake ports, they also suffer from their relatively small intake valves. While the aluminum head clearly has an advantage with 1.84" intakes, verses the large log and Australian heads 1.75" intakes, the valve sizes are limited by the shape of the combustion chamber and the size of the cylinder bores. The small bores make it impossible to install larger valves, like those that are commonly used in engines with bigger bores. As such, we can only dream of the horse power that could made with 2.02 intake valves. This also explains why porting, back cutting the valves, and three angle valve jobs are so beneficial. Anything you can do to increase flow, or offset the smaller valve sizes, becomes paramount for the small six cylinder heads if your building for power.

This is also why high ratio rockers are so beneficial, especially for the log style cylinder heads. High ratio rockers open the valves quicker and higher, and closes them a little later. This translates into more air flow, especially at low valve lifts where it's needed most. Swapping out those stock 1.5 ratio rockers for a set of 1.65 high ratio rockers, increases the valve lift by 10%, which in-turn increase air flow. A 250ci with a mild cam, headers, and a 2BBL carb on a reworked head, saw a peak gain was only 2 HP, with an average gain of 10 HP with a set of 1.6 rockers. Now lets try to understand exactly what's being said here. The reason for the low gain at peak is simply, the air flow is already max'd out. The cylinder heads simply won't flow more air, no matter how far you open the valves, due to the restrictive log intake. However if the peak gain was only 2 HP, while the average gain was 10 HP, then the low and mid range gains had to be considerably better to average out. This means the valves are flowing considerably more air at low lift, and the rockers are doing exactly what you want them to do, adding ponies on the low end of the power band.

In Summary
While it is common knowledge that the small six suffers from an extremely poor cylinder head design, which incorporates an asthmatic log manifold, many enthusiast still put money into their small log heads in an effort to improve air flow and performance. And up until we completed our flow test, we would have agreed that this was reasonable option. However flow testing showed us that this simply isn't a viable, or reasonable solution. Even when you spend hundreds of dollars on a 2V conversion, extensive port work, and installing larger stainless steel valves, the small log cylinder head still doesn't flow as well as a stock large log head. Therefore it makes more sense to purchase a late model, large log cylinder head, and do a a standard valve job on it, as it will flow more air and make more power than the reworked small log head. As such, the only time Classic Inlines recommends reworking a small log head, is when the owner is doing a contours restoration, and is required to keep the small log with matching numbers. Otherwise we'll always recommend purchasing a large log head from your local salvage yard, off E-Bay, or from your local classified. Sometimes you can get them for free, if you are lucky enough to find someone who just completed a V8 swap. They simply don't want to waste their time hauling it off, so more often than not, they're just happy to get rid of it. If not for free, $100-150 bucks is the normal asking price for a good used core, plus shipping if applicable.

If your not worried about maintaining a stock appearance or matching numbers, you have several options for the cylinder head. Your decision will probably be based on the desired level of performance, carb selection, and/or your budget. And if your like most people, your budget will be the deciding factor, over anything else. As such, we decided to cover the cost of the various cylinder heads, in stock and/or modified configurations, so you can make an informed decision when the time comes.

The amounts listed below do not include labor to R&R the cylinder head. We should also note that machine shop prices may vary considerably, not only from shop to shop, but in different parts of the country as well.

Stock Small Log: $250 to $550 (plus carb)
About the only expense you'll have is a good valve job, if you already have the cylinder head. However it's generally a good idea to replace the valves, valve guides, valve springs, and the valve seals, whenever you rebuild a cylinder head. If you use unleaded gas, you may need to install hardened valve seats, unless they were installed during a previous rebuild. Finally, the head should be resurfaced appropriately. This is also good opportunity to increase the compression ratio for improved throttle response and performance. You may want to add the cost of a rebuilt or new carb as well. For example, adding a new Vaporizer would increase the total cost by another $400 dollars.

Stock Large Log: $250 to $850 (plus carb)
Basically you'll have the same expenses as a stock small log head (see above), however you may need to add the cost of a good core (used cylinder head) if your up-grading from a small log head. A good core will cost anywhere from $100 to $350 dollars, unless your lucky enough to pick one up for free, which is possible. Most of the salvage yards in my area get $150 for a good used core. On the other hand, shops that specialize in rebuilt heads add $300-350 for a core charge.

Modified Small Log: $750 to $1150 (plus carb)
under construction - add cost for conversion ($150-200) larger valves and seats ($300) and optional port work.

Modified Large Log: $750 to $1450 (plus carb)
under construction - add cost for conversion ($150-200) larger valves and seats ($300) and optional port work.

Stock OZ-250: $000 to $000 (plus carb)
under construction - estimate cost with shipping

Modified OZ-250: $000 to $000 (plus carb)
under construction - add cost for larger valves and seats, and optional port work.

Stock Aluminum: $000 to $000 (plus carb)
under construction - estimate cost with carb

Modified Aluminum: $000 to $000 (plus carb)
under construction - add cost for optional port work.

Compare cost....... possibly a graph
Compare cost vrs total air flow.... possibly a graph

Final comments.....

Written By: AzCoupe