Why is the exhaust grounded?

[Mark E.]

Why does each side of the exhaust on my '70 have ground straps to the cross member? Does this reduce radio interference? Or better ground the engine? Is the exhaust grounded only on radio cars?

[Dan B.]

Re: Why is the exhaust grounded?

Some newer metal cars have "ground" straps from the exhaust to the frame as well. The reasoning is to dissipate any static charge built up. On these cars it would help with static suppression on the radio and that's about it. It's not meant to be an actual electrical grounding point though.

[Michael J.]

Re: Why is the exhaust grounded?

I'm pretty sure that is the reason, radio interference, and my non-radio equipped '63 has no ground straps anywhere, nor any shielding around the ignition.

[Michael J.]

Re: Why is the exhaust grounded?

AntUprGrnd640.jpgAntFrmGrnd640.jpgActually the C2 does have a ground plate around the antenna base at the rear, grounded to the frame on radio equipped cars.....

[Lawrence M.]

Re: Why is the exhaust grounded?

Mark,
In the 1970 AIM, UPC U69 Sheet A8 is titled Radio FM-Ground Straps. It shows the exhaust pipe ground straps and a ground strap at the L.H. front body mount. The sheet is marked canceled under the revision record with a date of 1-9-70. I wonder if later production cars do not have radio ground straps ? I only have AIM's for 69 & 70, so I can not check later AIM's to see if the ground straps were dropped.

[James G.]

Re: Why is the exhaust grounded?

By late 71 they are no longer on the cars.

[Mark E.]

Re: Why is the exhaust grounded?

The 6th edition of the 70-72 JG says '70 with a radio have straps; '71 and '72 do not.

I wonder why straps were discontinued? Did the slightly improved radio reception not justify the cost? Or maybe an improved radio design negated the need?

[Lawrence M.]

Re: Why is the exhaust grounded?

Possibly the dropping of the ground straps was a result of the RFI shield that was added to the distributor allowing the reduction in the size and number of pieces of the radio ignition shielding.

[Terry M.]

Re: Why is the exhaust grounded?

[ATTACH=CONFIG]101707[/ATTACH][ATTACH=CONFIG]101708[/ATTACH]Actually the C2 does have a ground plate around the antenna base at the rear, grounded to the frame on radio equipped cars.....

As does the C3, at least to 1972. The ground plane at the base of the antenna improves radio reception. I can't speak to those Corvettes newer than 1972, nor can I account for those Chevrolet cars with the radio antenna imbedded in the windshield.

[Terry M.]

Re: Why is the exhaust grounded?

Note that the connecting strap in Michael's pictures is a flat copper strap, not a "wire". Flat straps have more surface area, RF travels on the surface of the conductor, flat surfaces (including the braided straps used on the exhaust) are more efficient at conducting RF.

Terry: I thought about the windshield-embedded antennas, but I don't believe they were ever offered on a non-metallic bodied car. My 70 El Camino had one and it seemed to work just fine for the 250,000 miles I drove it as my regular car. It had a metal body, of course.

The new information regarding the limited-surface ground planes in the C2/C3 (which I did not know about this morning , but now I do, thanks!) and that the exhaust grounds were discontinued in the 70-71 time-frame, is further reinforcement that the exhaust grounds were for reception.

AM and FM have different characteristics, and "antenna quality" is particularly important with AM. (If you unplug the antenna from an AM/FM receiver, it will still pick up strong FM stations. But it will not pick up any AM stations. When working on a receiver problem where the antenna or cable is suspect, this is a quick check for antenna integrity. If there is NO AM reception but there are some FM stations, the antenna, cable, or connections are faulty.) The ground plane is an intregal part of the antenna system which is so important for AM reception.

I have a thought as to why "reception" was an issue in C2 and early C3, but by 1971 may not have been as big a deal. Through the 60's, there weren't many FM stations on the air. AM was more built-out and used. I graduated high school in 1971; I still remember the stations I listened to as a teenager, all on the AM dial. By the early 70's there were more FM stations, and with FM's higher fidelity and lack of susceptibility to interference became popular and utilized. AM usage declined; customer satisfaction with AM reception wasn't as important as it once was because fewer and fewer users cared about AM. "Parts left out cost nothing and don't cause service issues". Hence the exhaust grounds were eliminated.

Dave et al,
Of course I am aware that the windshield antennae were used only on metal bodied cars. I should have mentioned that in my post.

It may be a coincidence, but the exhaust grounding provisions on Corvettes ended about the time that the distributor RFI shield was installed.

Now I'll throw another brick into the discussion. I am a fan of the last real Chevrolet police cruisers (Caprice 9C1). Those cars have more ground straps than you could ever imagine. The exhaust system was just the beginning. The hood, trunk lid and engine all have the flat style woven ground straps. There may be some others that I haven't found in my 20 years of owning these cars as daily drivers. I am sure this is for the communications and computer systems that the police agencies installed, and has little to do with he commercial AM/FM radios. Of course these are metal bodied cars.

[Michael J.]

Re: Why is the exhaust grounded?

I think you are correct Terry. When my son was first learning to drive, I purchased a used 1996 former Texas State Trooper, Ford Crown Vic Police Interceptor as his first car, figured it would be very safe and reasonably peppy. It also had so many ground straps I could hardly count them.

[Tom M.]

Re: Why is the exhaust grounded?

Bonding is for radio interference suppression. Panels rubbing together that are not at the same voltage potential can cause static especially on early radio systems operating low power in the field with marginal antennas. During WWII every jeep left the Willys and Ford lines with bonding straps throughout the body and driveline. Before release to the govt, the Jeep was put into a radio interference lab chamber and checked for RFI emission. If the emissions were suppressed properly an "S" was stenciled onto the cowl - S=Suppression.
Sorry the pic got flipped but my 1942 has bonding from firewall to hood, engine, throttle and choke cables, speedo cable, water temp capillary tube and oil gauge tube. There are straps on the exhaust and transmission/transfer case as well.
20 years later the shielding concept was still valid especially since the Corvette fiberglass body didn't bond or shield.


firewall bond.jpg