[Gary H]

Starting in 1973 the Trans Am hood scoops were closed. Yet, you could still get the ram air air cleaner on Formula's through 74'. Also, the 1974 GTO hood scoop was functional.
Why would Pontiac have fresh air intake on the GTO and Formula's but not on the Trans AM? I had read somewhere that it was due to some EPA noise rule. If that is true, how did the other's pass the rule?

[bird72]

Good question. Formula scoops open, maybe quieter. GTO shaker with 350 under also quieter? Guesses. If I had a 73 TA, I would use 70-72 opening shaker and keep original parts in storage. But that's me.

[formula kid]

I have long thought about this very question. What I have concluded through looking at what engine combinations were available in 1973-1974. On the engine and carburetors that were sold on Pontiacs. All Formula 400 4 bbl and 455 cid with 4bbl engines except the SD 455 were available with RamAir option. All of those engines were fitted with 750 cfm carbs. The 73-74 Trans am cars were all sold with the closed shaker. Yes, they also included the 750 cfm carb All SD455 cars came with the 800 cfm carb, the only hood and air cleaner you could get was that of the Trans am. California would not allow due to noise the SD. So what do you do if you are Pontiac? Are you not going to sell the top engine after all development cost incurred to bring the engine to market. Also, the exhaust system of the SD was much louder that the standard Pontiac engine. Yes, the 74 GTO had the functioning shaker standard exhaust and 750 cfm carburetor.

[Keith Seymore]

Noise pass by is very platform specific. Exhaust tip location (rear facing vs side facing), air induction system (not just the ambient air opening location and orientation), engine calibration and horsepower rating (both amount and target RPM), tire noise, wheelbase, track width, etc, all play a part.

These days with computer controlled engines: some manufacturers have installed "cheaters" that sense an approaching pass-by test and change the calibration/HP for the duration of that test.

Here's another example, a personal favorite. If you look at an original 2001-up GMC Denali it has a styled tailpipe tip - but - the exhaust pipe sticks through almost to the end.



Why is that, do you suppose?

Well - it's because the addition of the tip alone created a megaphone type effect and caused us to fail noise pass by testing. Testing with just the bare pipe resulted in a pass. Not having the tip was a "theme buster" for the styling, marketing and planning guys. Dropping the engine horsepower rating was out of bounds.

So the solution was to leave the bare pipe for the full length, essentially installing the styled tip over the top. Best of both worlds (sorta).

Your tax dollars at work....



K

[keith k]

Originally Posted by Keith Seymore

Noise pass by is very platform specific. Exhaust tip location (rear facing vs side facing), air induction system (not just the ambient air opening location and orientation), engine calibration and horsepower rating (both amount and target RPM), tire noise, wheelbase, track width, etc, all play a part.


K

Yes, this.

And a lot more induction noise made it out of the Trans Am's shaker scoop (sitting directly over the carb...) than worked its way forward through all the zigs and zags of the Formula's system. Thus the Formula could maintain the open scoops while the T/A shaker with similar engines/exhaust systems couldn't.

Pass by noise regulations have changed in recent years, too. This is why there are some raucously loud production cars these days (and those with active valve systems obviously push it even further).

[giles]

Pass-by noise standards were introduced for ‘73 and are a federal law, so all 50 states must comply. I’m am unfamiliar with the recent changes, but the prior rules required that the subject car drive on a single lane road with microphones placed on either side 50 ft from the road. The car drives between and accelerates at WOT from 20-50 mph and the sound level is measured. The noise can’t exceed a specified dBa level (don’t recall the threshold). The noise limit is a total level from the vehicle, so induction, exhaust and mechanical noises all contribute to the total. I’ve seen all manner of fixes investigated to lower noise levels ranging from induction inlet baffles, added muffler volume, even forged cranks, which make a surprisingly good, if expensive solution. Apparently the shaker was the significant contributor on the TA and 3 rivets was a very inexpensive and expedient solution.