Wet flow testing has moved in and out of the realms of mainstream power topics many times during the last 40 years. Like many aspects of race engine development that are a step aside of normally debated subjects, wet flow testing has both staunch supporters and detractors. That leaves a huge majority middle group who are just waiting to see the results support the rhetoric. If you're one of many waiting for the touted advantages of wet flow testing to actually show test results that equate to an undisputed power advantage, your patience has just been rewarded. But before we look at any test results, we need to look more deeply into what wet flow testing is, how it needs to be done to actually achieve positive results and, lastly, what qualifies me to write about the subject.
Back in the mid-'80s the late Ken Sperling, then boss of AFR, got on a roll with wet flow testing his then-new aluminum Chevy heads and the NASCAR heads he was doing. And the next thing you know, a mutual friend, Roger "Dr. Air" Helgesen turns up at my California shop with an armful of gear to do wet flow testing and, as usual, is hot to trot. This, along with some of Smokey Yunick's preaching, was, so to speak, my baptism into the world of how fuel/air mixes react in the induction tract of a single four-barrel carbureted high performance V-8 engine. Since the Ken Sperling wet flow era of the mid- and late-'80s, wet flow seemed dormant on the editorial front until about 2002. This is when Joe Mondello's innovative wet flow bench hit the headlines.
Wet flow testing is far from complicated, but, like most things, if not done right it can be mostly meaningless. First and most obvious is that without some really good fire/explosion precautions, you cannot wet flow with gasoline. This means that whatever liquid replaces the fuel must have some or all of its primary characteristics modified so that in all relevant respects it reacts like gasoline other than the fact it won't burn. If water or alcohol/water mixes are used, the surface tension of the mixture needs to be similar to gasoline.
Also, the air-to-liquid ratio is important. The most meaningful results are when the air-to-test-liquid ratio is about 15 percent less (leaner) by volume than the air/fuel ratio the engine normally runs. The entire system being tested needs to be run at a temperature a little hotter than the mixture temperature normally seen in the intake tract of a high performance engine (about 60 degrees Fahrenheit). Running the test liquid at 90 or even 100 degrees F seems to offset the fact that the intake valve will be a lot cooler in the test bench mode than in a running engine.
Components such as heads, intakes, and carbs can be analyzed separately. However, the final analysis is best done as a system, because there is a domino effect by each component as the charge progresses through the system. Any time a carb is involved, the booster action of the model needs to reasonably replicate the one in usage, as this can affect results. And one last point: To get really meaningful results, it's best to flow with a big pressure drop across the intake at low lift (up to 100 inches H2O) and low pressure at high valve lift (about 15-18 inches H2O).
Now, just so we are all clear on this: Wet flow testing does not necessarily deliver results in cfm. We are not measuring the flow but the state of the mixture and its ability to have minimum rivulets and maximum dispersion as it enters the cylinder. In short, it's all about the combustible quality of the mixture. Few people are likely to know this better than those whose efforts have aspired to the development and building of championship winning Pro Stock engines, and this is where Dart comes into the picture.
If you ever meet Dart's boss, Dick Maskin, and penetrate his leather-like personality, it will become very clear right at the get-go that his whole goal in life is going faster than the next guy-at almost any cost or effort needed. As a Pro Stock champ, Dick understands the value of wet flow testing, but a Pro Stock engine with its two four-barrel carbs, a tunnel ram intake, and a highly developed head along with a 7000-10,000 rpm power band is an easier animal to deal with by far than a single four-barrel on either a single or two plane intake. Add to this the high performance engines that most of us deal with that have to operate satisfactorily or better, from idle to anywhere from 6,000 to 8,000 rpm, and a more complex situation by far emerges.
Dick decided that he didn't want to take just a simple step forward in an attempt to beat out the competition, but to take a quantum leap forward. With this in mind, Dick put in a call to Mondello and discussed the possibility of building a high-tech King Kong version of Joe's regular wet flow bench. A design was decided on, and a deal was struck. Six months later that bench was delivered and Tony McAfee, Dart's chief head development engineer, went to work.
For a back-to-back test like this to produce realistic and representative results, the engine used must already be at the pinnacle of development within the spec/price range it's intended to compete in. This is where Lloyd McCleary, the boss at T&L Engines came in handy. He knows engine combinations and he knows how to put that to good effect on what he concentrates on these days-affordable custom crate motors with a race-winning pedigree.
The test engine you see nearby is one of T&L's 383s just as Lloyd sells them out of the door-other than the fact we've changed the front cover to make cam changes faster. Note: Our test engine has Dart heads in the first place. Why? Because Lloyd's considerable previous dyno testing has shown that our baseline Dart heads already represented one of the best buys on the market in terms of horsepower per dollar.
And-courtesy of Maskin's foresight, Mondello's wet flow bench building, McAfee's patience toward developing the head and McCleary's engine building expertise-here are the definitive results of before and after wet flow testing.
(First let's take a quick look at the heads we're dealing with. In step #6 you can see an assembled 200cc port Pro 1 and the Platinum Pro 1 that replaces it. At first they barely look any different, but closer inspection shows otherwise.) Starting at the intake manifold face, the port entrance has been made to closer tolerances to bring about a better port match with a typical high performance intake. The other aspect that's relatively plain to see, and this applies in total to the new Platinum heads, is the smoother casting finish. But the trued up entrance to the intake port-though significant in terms of high valve lift flow when coupled to an intake manifold-are not where the real advantages of the Platinum Pro 1 heads lie. For this we have to delve much deeper.
Looking into the combustion chamber reveals most of the changes made to produce the Platinum Pro 1 head. First take a look at the original Pro 1 head then check out the differences depicted by the arrows in the adjacent photo. Going through these in order we first have the introduction of a long guide vane in front of the guide boss. This serves a number of purposes. First it provides better streamlining of the guide boss, which proves beneficial both in terms of a small increase in high-lift flow, plus a small increase in velocity throughout the lift range if the guide boss reduced port volume.
In addition to this, the pre-guide boss vane can be used to change the path of wet fuel flow within the port and the swirl characteristics. The trailing guide vane (arrow #2) also serves in much the same way. The area indicated by arrow #3 is scalloped out slightly deeper than is seen on the original Pro 1 head. This has the effect of turning the charge more as it enters the cylinder, thus generating a better swirl action. The small area indicated by arrow #4 looks seemingly unchanged, but as little as 0.020 of metal added to this point can benefit the swirl greatly, especially in the 0.400-inch lift range on up. It can also eliminate the intake valve airflow tip-over so often seen at about the 0.600-inch lift point on 23-degree SB Chevy heads. The area indicated by arrow #5 has been scalloped away more than the original Pro 1 head, and the bump to the right evident on the original has been eliminated. In addition to this, the plug has been repositioned.
Up until now what we've looked at has been directly related to improving the wet flow characteristics of the head. Some additional refinements that appear to have been done to the exhaust are also worth talking about. One of the criteria for an exhaust port is to have the exhaust flow out freely, but with the least tendency to reverse flow. By paying attention to this aspect, the heads will not only make more low speed torque, but they will also tolerate a bigger cam before the low speed output becomes unacceptable.
By rounding off the corners (as indicated by arrow #6), the flow is increased without the need for a bigger port. In addition to this, attention was given to the short side turn in an effort to get the flow to stick to the surface longer as it goes around the turn into the main body of the port. The significance here is that the short side is always the first site for reversion. By having the flow stick to the short side turn more effectively, the tendency to reverse flow at low rpm when the piston is around TDC is cut. Result: more bottom end torque.
Not only are the exhaust ports different, but so is the header flange surface. Changes were made here for installation convenience reasons. First off (and probably most importantly) was the new wider flange that allows the drilling of the Stahl-style header bolt pattern used for those bigger tube headers. Another factor not so easily seen is the thicker deck to accommodate the high stresses seen with heavy nitrous loads and big boost numbers.
Visually, we now know what we are dealing with. Now it's time to look at how such differences affect flow bench results. To establish this, a calibration test was first made on T&L's flow bench and then an example of both the original and new Dart Pro 1 heads were flow tested. The chart shows the results (step #15, see next page). As can be seen, there was very little difference in actual flow between the original Pro 1 (dark blue curves) and the Platinum Pro 1 (red curves). What we do see here is the elimination of the tip-over that so often occurs in the 0.550- to 0.600-lift range. This is due to the minor chamber reshaping at the point indicated by arrow #4 (step #9). Other than that, a conventional flow test reveals very little difference between these heads.
Where the difference did show up is in the swirl and mixture distribution as revealed by Dart's wet flow bench. At the time this feature was done, Dart's setup was not really geared to photographing exactly what goes on, as testing is done in very low light conditions. Essentially this involves a marker dye in the test fluid. This, when irradiated by an ultra violet light, causes the dye to become luminescent. This makes it easy to see what's going on with the naked eye, but it's not easy to capture on film.
It's now time to actually test the concept of wet flow testing. Our 383 T&L test engine, equipped with a Comp Cams single pattern 288 Xtreme Energy flat-tappet hydraulic cam and an Edelbrock Performer Air Gap intake produced the before and after results shown in step #14. Even with the original Dart Pro 1s, the T&L 383 made more low speed torque than the dyno could handle in the range below 3000 rpm-hence the graph curves starting at 3000. Although the Platinum heads produced a few lb-ft more at the bottom end of the range tested, the true gains showed up from about the 3700-rpm point. From there on up the gains were truly outstanding.
Peak torque with the original heads was already an impressive number, but the Platinum Pros bumped the figure up by 14 lb-ft. Likewise, peak power rose by 22 hp and the biggest power gain, seen at 5800 rpm, was over 26 hp. Remember that the flow difference between these two head styles is, within the lift range used (0.536), non-existent. This means that all the extra output we see here is the result of fuel/air mixture management within the intake tract.
So, is wet flow testing is worthwhile? The results speak for themselves.
This view of the old exhaust port clearly shows the relatively sharp edges of the guide boss. These are very much rounded on the Platinum Pro 1 heads.
Checking out the exhaust short side turn on the Platinum Pro 1 heads reveals a finish and form about what you would expect of a fully reworked race port.
The original Dart Pro 1 exhaust flange sported a bolt pattern that only accommodated stock-style header flanges.
The Platinum Pro 1 below has the capability of accepting big tube headers without an adaptor plate.
Where the wet flow is going is shown to great effect when the dyed fluid is irradiated with ultra violet light. The brighter the blue, the more fuel the air locally contains. The trick is to even out the coloration as much as possible.
The new Dart heads benefited greatly from the extensive wet flow development program they were subjected to. Peak torque was up by 14 lb-ft and peak power was up by 22 hp. The biggest gain was at 5800 rpm, where over 26 extra horses were produced.
Conventional flow testing (such as seen here) reveals little difference between the original Pro 1 (dark blue curves) and the new Platinum Pro 1 (red curves).
Dart's Tony McAfee studies the results from a test. A key factor toward getting meaningful results is keeping a close eye on the air/fluid ratio so that it closely mimics what happens in a running engine.