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Great FE Intake Comparo, Report #4, Part 1

December 3 2006 at 12:20 AM

Jay Brown  (Login jaybnve)

Report #4 is going to consist of all the multiple carb intakes tested on my 428CJ. This is part 1, and I hope to have part 2 completed and posted by Monday night. Please allow me the following brief introduction, to explain a couple of technical details that have colored the test procedures and results.

This series of the tests is going to be a little different than the previous tests, because of the differences in the carbs. On all the single four intakes I have standardized on a 750 Holley double pumper, with a specific set of jets. So, these tests are true manifold only comparison tests; the only variable has been the intake. However, for the multiple carb setups, it is of course not possible to use a single 750 Holley, so there is obviously more variation. Further, we are comparing a tripower intake in the mix of the 2X4 manifolds. So, this testing, while useful, can't really be grouped with the testing done on the single 4 manifolds. I think that a comparison of power numbers is probably valid, but the variation in the carburetors has to be considered when interpreting the results.

In addition to this, the learning process that I have gone through over the past 9 months or so while doing this testing has led to a few changes in test procedures, and one change in the correction factor I used for the dyno. Regarding the correction factor, I discovered this summer that the torque link on the dyno has a minor temperature dependence, drifting up about 10 foot pounds with 500 foot pounds on the link, over a temperature range of 50F to 110F. This is a pretty small drift, and in most cases may impact previous results by 2-3 ft-lbs. However, starting with these manifold tests, I have installed a correction factor in the dyno software to compensate for the torque link drift. The correction factor is based on the data I collected on the torque link. So, I think that the results of the multiple carb intake tests are slightly more accurate than the results of the previous single 4 intake tests.

The other key observation I have made is with respect to the Air/Fuel ratio calculated by the dyno. Since I first set the dyno up last March, the A/F data has appeared to me to be rich. At the beginning of the summer, I purchased an Innovate Motorsports wideband oxygen sensor to dedicate to the dyno, and I have been tracking the performance of the A/F ratio from the dyno in comparison to the A/F ratio of the oxygen sensor. It turns out that the dyno is reading a little rich as compared to the oxygen sensor, but not as much as I thought it was.

The point of this observation is that when I settled on my 750 cfm carb parameters back in March and April, I didn't have the wideband sensor, so on my first set of tests on my Blue Thunder intake, I jetted down until the engine made peak power, and left the jets there. Well, I'm pretty sure at this point that I did not jet far enough down at that time, despite what the power numbers were saying. For most of the single carb intake tests, the dyno is saying the A/F is between 11.5:1 and 11.8:1 at WOT, and the Innovate wideband is saying 11.7:1 to 12:1. Peak power should be occuring with an A/F in the 12.5: to 13:1 range. So, all the single 4 intake test have been run a little on the rich side.

The clincher for the idea that I didn't jet down enough orignally on the BT intake came today during testing of the MR 2X4 intake. We had started out very rich, and jetted down to a certain point, and ran a test. We then jetted down another two steps, and the test showed that the power was down a few HP from the previous run. However, the Innovate wideband and the dyno A/F numbers were both still in the 12.0:1 range. Previously, with just the dyno A/F numbers available, and my mistrust in them, I would have probably gone back to the previous jetting and stayed there. However, with the A/F numbers between the two instruments in agreement, and appearing still rather rich, we decided to jet down two more steps despite the loss in HP on the last test, and shoot for a leaner A/F. Good thing we did that, because the engine gained nearly 10 HP on that test, and A/F came up to about 12.4:1. We jetted down one more time, saw the A/F at 12.5-12.7, and called it quits on the jetting. I think that I probably did not take that extra step in jetting down with the BT intake last April. So, it seems pretty clear now that the jetting on my 750 Holley is too rich, and there is probably more power to be had with many of the single 4 manifolds if we lean the carb out a little bit.

In any case, with the multiple carb intakes I decided at the start of this test to optimize the A/F in search of peak HP, so the testing on these intakes should reflect an accurate peak HP number.

Looking ahead, I have about another 5-6 single 4 intakes to test, and I am going to test them with the 750 Holley jetted as is, to provide a basis of comparison between these tests and all the previous ones. Then, in order to bring the single 4 tests in line with the multiple carb tests with respect to A/F ratio, I'm going to pick the top 10 single 4 intakes, and test them again, with the jetting changed to provide more optimum A/F numbers. We're going to call the baseline single 4 tests the "regular season", and the top 10 manifolds will make the "playoffs", where the A/F on the carb will be adjusted to provide better power numbers. I expect to have the "playoffs" sometime between now and the end of this month.

I started the multiple carb test session yesterday with my 406 tripower intake. This is a factory 406 intake, port matched to the 428CJ heads, with a set of new reproduction tripower carbs on the manifold. Also, a reproduction of the factory linkage was used on the setup. The intake and carb combo peaked at about 385 HP, which is consistent with a good low riser type intake, but not up to the performance of some of the better dual plane single 4 intakes I have previously tested. There are more details, and photos of the induction system, in my post from yesterday.

Last night I swapped on the factory Medium Riser 2X4 intake, and the Holley 450 cfm carbs. I tried to work out the throttle linkage so that I would be ready to go early this morning with the testing. It took me until 2:00 in the morning to get it right; I had to scavenge the bellcrank assembly from my factory 2X4 linkage set, and also manufacture some of my own linkage parts in order to make everything work. But this morning I was ready to go at 9:00 AM. Here's a picture of the 450s and the MR intake on the engine:

The first pull of the day showed the carbs to be really, really rich. A/F on the dyno was reading only about 10:1, and the engine stuttered and coughed throughout the pull, making only about 380 HP. The Holley 450s have a single metering block in the primaries, plus a metering plate (as do the other 2X4 carbs), so we were stuck with making jet changes to the primaries only; I didn't want to start messing with the metering plates. We pulled the primary float bowls and found 58 jets in the carb. 58!! My Holley jet box only goes down to 64. I didn't have any jets that I could jet the carbs down with.

My friend Kurt had shown up for the test session today, and he had brought with him a set of very, very small reamers. So, we tried the old trick of soldering the jets closed, drilling a small hole, and reaming them out to a different size. I'd never seen this done before, and was curious if it was going to work or not. Kurt reamed out the soldered jets to .055", and we stuck them back in the carbs and tried again. The power was better at 388 HP this time, but A/F did not appear to be improved. At this point I gave Shoe a call; he was planning on coming over a little later. He lives somewhat close to a speed shop in St. Paul, so I asked him to run over there before he came here, and get whatever jets they had in the 52-58 range, up to four of each type. He said he'd be happy to do that.

As long as we were going to wait for Shoe, we decided to put on the 660 center squirter carbs, and test those. Here are a couple of pictures of the 660 carbs installed on the engine:

Removal of the primary float bowls showed that the 660s did not have power valves (the 450s did), so they were jetted at 76. We took a guess that this would be too rich, and jetted down to 72s before installing the 660 carbs. These carbs were immediately better, yielding 395 HP, but again they appeared rich via the A/F measurements. So, we started slowly reducing the jet sizes. Shoe showed up in the middle of this, and we all watched as the A/F would improve very slightly with the jet size reduction on subsequent pulls, but nothing too dramatic in terms of HP improvement. Finally, we went with 64/65 jets in the primaries, and saw 407 HP and an A/F of about 12.4:1. We really needed 63 jets and smaller to take the next step, but I didn't have any; again we were out of range. Shoe volunteered to go get jets again! What a guy. While he was gone (this time to a more local speed shop), we took another stab at reaming jets, this time foregoing the solder, and just reaming a set of 57s up to .063". But it didn't work out too well, showing no improvement in A/F and a decrease in HP. When Shoe returned, we popped in 63s,and ran one more time. We got 410 HP with this combination, and the A/F was reading in the 12.5 - 12.7 range. We decided to call that tune good for the 660 Holleys. One note on these carbs is that the manifold vacuum during these pulls was ZERO. Normally with the single 4 setups, the manfold vacuum starts at 0.2 inches or so, and increases to around 0.9 inches at the top engine speed. Obviously, the extra throttle body area was resulting in less pressure drop through the carbs.

Back on went the 450 Holleys, this time with 52 jets. The engine would hardly even run, it was so lean down low, but once the main circuit kicked in it ran OK, so we made a pull anyway. Again HP was disappointing, but the A/F was still very rich. This seemed very odd, so we made a cruise test with these carbs, which is where you slowly added load to the engine at a certain speed, and look at the dyno output parameters. This showed that with a small load on the engine, the A/F ratio was around 14:1 or 15:1; very lean. But as soon as it hit WOT, it was running very rich. We concluded at this point that without a lot of messing around with the 450 Holley fuel curve, we weren't going to get them to run well on this setup. Also a note on the vacuum; these carbs showed a peak manifold vacuum of around 0.5 inches at WOT and 5800 RPM.

Now we decided to try the third set of carbs, courtesy of Royce Peterson, a running pair of original BJ/BK carbs. Installing these carbs was easy as compared to the other two sets, because we were able to use the factory style linkage that I had on hand. It made the throttle system much more friendly, and I was pretty impressed with these carburetors from an ease of use standpoint.

For the first pull on this set of carbs, we elected to leave the secondaries alone. My experience with this has been that vacuum secondary carbs will usually not open the secondaries properly on the dyno, without changing to the very lightweight springs in the secondary diaphram. However, we tried it anyway, and got predictably poor results. Our next step was to take some small tie wraps, and wrap them around the carb linkage in such a way as to make the secondaries "mechanical"; they would now be forced to open with the primaries. Results were much better, and the engine made 398 HP with these carbs. Here is a comparison of the HP production with the secondaries left alone vs. set up to operate mechanically:

I was looking forward to starting some jetting changes to optimize the A/F with these carbs, but when I looked at the data I was surprised to see that the A/F was nearly perfect; it was hanging right around 12.5 to 13.0. Royce mentioned to me that he has tuned the carbs with the aid of a chassis dyno, and it certainly shows; they were spot on. Also, manifold vacuum throughout the pull was 0, just like the 660s. After standing around scratching our heads about this for a while, and running one more pull to back up the results (they were nearly identical), we decided that it would be foolish to try to mess with the calibration on these carbs. They were right on, just about the same as our modified 660s, so there wasn't too much more that we could do with them.

So, at this point we decided to standardize on the 660 carbs for the remainder of the 2X4 manifold tests. They made the best peak HP, although they are probably not the best choice for a streetable engine. Here is the comparison of the three different carb types, showing the best run of each:

By the way, 410 HP for this intake is the best HP numbers for any of the dual plane intakes I have tested so far. Keep in mind the prior A/F discussion, though...

We had burned most of the day on carburetor testing and tweaking, and now it was nearly five o'clock. We decided to put on one more intake before we quit, and this was the Low Riser 427 intake. Again we installed the 660 carbs, and made two pulls. This intake, while down on power as compared to the MR intake, still made excellent power; peak HP was right about 400, and A/F with this jetting was slightly higher on this intake than it was on the MR intake, running right about 13:1.

Here are the results for the three intakes tested so far:

I am far behind schedule on this testing, but I have a bunch of people coming over to help tomorrow, so I'm hopeful we can get another five intakes tested. This will finish off the multiple carb testing, because I don't have any Carter carbs to use on my Edelbrock F266 intake, and the factory tunnel wedge intake appears to be identical to the Dove tunnel wedge intake, so I will only be testing one of them. Stay tuned...

Jay Brown
1968 Shelby GT 500 Convertible, all aluminum 489" 1030 HP Supercharged FE
1969 XL Convertible, 460
1969 R code Mach 1, 706 HP 511" all aluminum FE, 10.457 @ 127.47, 2005 Drag Week Winner, Naturally Aspirated Big Block
2005 Ford GT, 2006 Drag Week Winner, 12.0 Daily Driver

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