Why pre-ignition ater installing new exhaust system?

[Dr. Diesel]

Last month I had one of those rare opportunities... found and bought a "new" 1977 triple black Pontiac Grand Prix SJ with 8,035 original miles in 28 years!... Stored in a garage in Illinois... driven only occasionally...

I drove it 2,000 miles home to Washington State... engine ran perfectly...now has over 10,000 total miles...

The engine is a Pontiac big block 400, 4bbl. Rochester, 7.6:1 compression ratio... had original 1977 mechanical (non-electronic) factory emissions controls intact (including single exhaust, 1977 style flat catalyctic converter, EGR valve and the typical web of little vacuum hoses conecting air cleaner temp sensor-EGR valve-etc...

Yesterday, in a custom exhaust shop, we dropped the original choke-to-death 1977 single system to the floor, and installed a true dual exhaust system, no crossover, individual connections at each manifold, 2' pipes, no catalyst(s), a top name brand performance muffler (daylight passed thru each) on each side... obviously offering the engine considerable back-pressure relief... Prior experience doing the same to a 1978 Corvette produced excellent results... dramatic performance and fuel economy improvement...

OOPS!!!... Big-time PRE-IGNITION, where there nad been NONE prior to this exhaust change, was the result in this Pontiac... sounds like a pop-corn machine at operating temp with the slightest acceleration... again... the ONLY CHANGE was to install a wonderfull low back-pressure exhaust system... Altho' we feel pretty savvy on engines... this cause and effect remains a mystery... perhaps the lower BP has some negative effect with the EGR valve???... HELP will be very welcome!!!... Feel free to email direct tedbets@msn.com as well as posting... Thanx...

[RonfromCanada]

Mr Diesel, only thing I can think of is better scavenging of the engine. This might allow a leaner air fuel ratio to enter the cylinder due to the carb not being able to provide the proper air fuel ratio. I see two options, retard the timing or richen up the fuel ratio. Changing the ratio will realy let that engine get up and go. Fuel economy will be, well you don't realy want to know. But it should give you some feel it in the butt power. You may have to fiddle with a combination of timing and ratio's to get it just right though. There are othe more learned guys that may have a better answer so check back often.

[Dr. Diesel]

Thanx for the response, Ron... You make sense... But Hooray!!!... PROBLEM SOLVED!!!... I found the answer to my own problem earlier today... I only arrived home in Vancouver, Washington, just a few days after buying the car in Illinois... really hadn't had much opportunity to rub, feel, smell, & touch everything under the hood... but looking at a manual... I noted (and remembered) the myriad of 1/8th inch vacuum hoses, like a plate of spaghetti, connecting multi-port temp sensors (at the 'stat outlet, air cleaner), EGR valve, intake manifold, carburetor, vacuum advance, etc. ... read the description on how these components are all inter-related, modifying timing, and EGR as ambient and operating temps vary... pretty crude controls compared to today's precise electronics...

Any way... I found two of these little hoses (one t'eed at the EGR, other at distributor) disconnected... plugged 'em back on their outlets, drove up the road... NO MORE PINGING!!!... SMOOOOTH AND POWERFUL...

Again, there was no pinging prior to the exhaust system change... even with these hoses not connected... but obviously combined with the newly applied back pressure reduction, the loose EGR and distributor connections were a recipe for pre-ignition...

Thanx again... stay in touch... ...Ted