I JUST WANTED TO GET SOME FEEDBACK AS TO WHETHER ANYONE ELSE HAS HAD A PROBLEM WITH SHORT LIFE OF THE MAF AFTER THE VANES HAVE BEEN CUT OUT AND FILED DOWN. I DID THAT ON MINE SEPT LAST YEAR AND NOW THE MAF IS DEAD
THANKS GUYS

 

4-Speed,

Excuse me while I get this out to everyone else (not you personally).

<h1>TOLD YOU!</h1>

Thank you.

If you've been following along for the last couple of years, the discussion on the benefits of removing MAF screens and heat sink fins has been extensive. If you have a day or two with nothing to do, search the message archives for "MAF" and "screen" and see what I mean.

The hot wire sensing element in the MAF is extremely fragile. I know, since I have replaced the actual 41-gauge platinum wire in a spare sensor just to see what was involved. I still have a 6" piece of the wire floating around as a souvenier, and at over $30 a foot I couldn't bear just tossing it.

Regardless, the electronics package in the MAF does need some cooling, or the heat sink wouldn't have been there in the first place. The switching transistor can get pretty warm, and needs to reject the heat somewhere. Granted, the heat sink may impede intake flow, but it was using the coolest air in the engine compartment.

Now that your MAF has apparently failed, none of that matters. You'll be needing a replacement, and here is your opportunity to solve a couple of problems. If you could get a MAF that had no fragil hot-wire element, no heat sink, a larger diameter tube, and no real screens in the ends, you would be solving all your problems. Check into a Wells/Conrad SU-145 MAF. It is a clone of the LT1-LS1 (Hitachi) type MAF designed for the replacement of these older Bosch units. It uses a thick film sensor instead of the hot wire, has no heat sink in the air stream, and has a single flat metal honeycomb matrix in the inlet end only instead of the restrictive round wire screens in both ends. The best part is they are new, not remanufactured, and have a lifetime warranty depending on where you purchase them. Mine cost $172.00 in stead of over $180.00 for a remanufactired hot wire unit. Easy call, huh?

Beyond that, even if you use a hot wire MAF, the 544 SCFM air flow through a stock Bosch MAF will easily support a 305 inch engine well beyond 7,200 RPM. If you get your TPI to run that high with any kind of power output, please tell me your secrets. The math is pretty easy, but I'll spare you that for now.

You should be able to get one anywhere that sells the Wells or Conrad parts line, or http://www.wellsmfgcorp.com/ for locations in your area.

 

...and if you want to check my math for supporting the air flow theory, read on. The numbers were using 350 cubic inches of theoretical displacement, so the flow would be theoretically 15% lower for a 305.

Your engine is a four-stroke cycle, naturally aspirated overhead valve piston engine. The theoretical displacement of the eight cylinders of the engine totals almost 350 cubic inches, or about one-fifth of a cubic foot (0.2025463 cu. ft., to be exact). Since your engine is a four-stroke cycle design, it can theoretically flow that amount of air on every two revolutions - one complete cycle for every cylinder. If your crankshaft spins at 5,000 RPMs, that would mean 2,500 complete cycles for every cylinder, or 506.365 cu. ft. every minute. If that same engine achieves 6,000 RPM, the theoretical flow would be 607.639 cu. ft. every minute.

Again, these are theoretical numbers. This assumes that the intake valves actually are removed from the ports, and the valve sizes are 4.000" intake and 4.000" exhaust - the same as the bore. The intake and exhaust ports would have to be the same size to assure no restriction and maximum air flow. The throttle body would have to have two 3" bores to accommodate 1.414 (square root of 2) cylinders on an intake cycle at any given time. If all these things were true, the air flow into the engine would be 607.639 cu. ft. per minute at 6,000 RPM. The stock MAF would then be a restriction, since it can only flow 544 SCFM without creating more than 0.1" W.C. static pressure drop.

This would be in a theoretical world. In the real world, the valves are only as large as the combustion chamber design will allow, and the intake and exhaust ports are significantly smaller than the 4" bore of the engine. This means that there is a significant decrease in theoretical flow right at the heads (more on that later).

Next, the only way the cylinder can fill with a fuel/air mixture is if there is a vacuum in the cylinder to draw the mixture in. The air in the manifold can't "wish" its way into the cylinder. To create a vacuum, even a slight one, the piston must travel down in the cylinder while the intake valve is open. This means that some of the theoretical displacement of the cylinder has been "wasted" to create this vacuum. So even though the cylinder volume may be 43+ cubic inches, you'll be really lucky to get 40 cubic inches of air/fuel mixture in to it, even with the theoretical 4.000" intake valve. And since the intake valves are nowhere near that theoretically ideal size, and the ports offer restriction of their own, the volume entering the chamber on even a well-designed engine is more like 75-85% of the theoretical displacement. If that number sounds very familiar, it should. That is a good number for volumetric efficiency of a top-performing engine. Most "built" engines don't even exceed 70% volumetric efficiency. 85% would be almost miraculous without a blower.

Additionally, these theoretically ideal valves would have to completely open the instant that the piston started on its downward stroke, and instantly close when the piston reached the bottom of its stroke (actually, a few degrees after that, but close enough for argument's sake). Since our small valves open and close very slowly in relation to the piston travel, there is additional loss and restriction. To compensate, the valves are opened a little sooner than the theoretical optimum, and close a little later to ram that last little bit of air/fuel mixture into the cylinder.

Farther upstream, the intake passages in the manifold and/or runners are nowhere near the size they would need to be to feed the engine at its theoretical maximum flow. The same holds true for the throttle body and ductwork. All of these passages would have to accommodate the 4.000" size for a zero-restriction system. This would mean that the MAF would have to be somewhere in the range of 4.756" (120.8mm) in diameter. Since it is only 70mm in diameter, it should be restrictive!

Factor in all the losses on a real-world perfectly designed engine. This engine would have at least 2.20" valves and the cam timing would have to be at least 50°BTDC intake opening to achieve maximum intake flow at that RPM. The exhaust valves would have to be 1.80" minimum, and the individual matched head pipes would have to be tuned and megaphone-belled at about 28-30" to make the most scavenging action at that RPM. (Is this beginning to sound like a top-fuel engine? - It should.) Intake passages would have to be as short as possible to provide the least possible restriction and tuned length, like a mini-ram or LT1 intake or better.

With all these things in place, the 350 cubic inch engine would flow around 534.722 cubic feet per minute at 6,000 RPM. Your stock engine is nowhere near that efficient in flow. By that determination, the 544 SCFM of the stock Bosch MAF should provide enough flow for your engine to achieve 7,000 RPM easily.

Removing the screens and heat sink fins will undoubtedly reduce the restriction and decrease the static pressure drop across the sensor. But the difference will be so minimal that the little gain would probably be overshadowed by the nuisance problems you might experience with a modified MAF.

Just thought you might be interested.