This letter was written to me by a GT-40 Buddy. Joe
/ubbthreads/images/graemlins/laugh.gif /ubbthreads/images/graemlins/laugh.gif /ubbthreads/images/graemlins/laugh.gif
Noise level is a big problem with a GT40, stereo or not. Most GT40 cars are in the high 90db range but I have measured one street car with 103 db in 5th at 45mph. A sustained level of 85db is routinely used by OSHA as the cut off for safe and anything above that is classified as producing some degree of hearing damage. Virtually all the new super high performance street cars are at 80 db or below in routine operation.
Fixing the cockpit sound level is a little like Sears with good, better, and best. You don't have to do everything, just a few items will make a big difference. I could only come up with 5 reasonable (practical) ways to improve the sound level in the car. (1) Muffler, (2) air intake, (3) insulation, (4) tire noise, and (5) engine rpm. Obviously a quieter muffler is an option but I will only do that as my last option. After all I still want the car to sound mean. Carb air intake is very noisy on cars with velocity stacks and that is mainly why I opted for a singe 4 barrel. If necessary I will use an air box and a duct to draw air to the carb from the back end of the car like almost all new mid-engine car designs. But I actually do not what to have to do this either because I like the look of the carb under the rear window glass.
My next option is insulation. Before my engine is installed I am going to add a product called Dynamat Extreme to the engine side of the bulkhead and to the entire inside of the cockpit. It is a 1/8" thick material with a heavy aluminum foil layer on the outside and a elastomeric butyl barrier+adhesive on the applied side. This product has no petrolum smell like "aluminum backed tar roof matting" and is highly resistant to heat. Under the engine bonnet I am also going to put in a layer of Dynamat Extreme and a heat shield called "Hoodliner" from the same company. All the seams and edges will be covered with aluminum tape. I have talked with the Dynamat engineers for specific recommendations a couple of times. They claim 18 db reduction for a quality application, 15 db very routine, and 12 db easy to achieve even if not all areas are done. The sound insulation and thermal shield will add less than 50 pounds to the car. Their website is "dynamat.com" and is full of information and examples. The part numbers and US$ costs are: Dynamat Xtreme Automotive Bulk Pak, #20455, $250.00, and Hoodliner, #11905, $70.00. PDF spec sheets for each product are attached here.
Tire noise is also a big factor. As a test I changed my Corvette tires from the factory Goodyear run flat tires to Goodyear Eagle F1 GS-D3 285-40-YR-17. Sound level went down 5-6 db which was a huge noticeable improvement. Most car magazine tire tests list these with a group of 10 or 12 other performance tires as # 2 for performance and # 1 for low road noise. "tirerack.com" gives a good comparison chart of all the tires in the maximum summer performance category. Tire rack lists the standardized roadholding performance and road noise scores on all its tires which are in line with the published car magazine tests.
Engine rpm at highway cruise speeds of 60-80 mph can also be lowered to drop cockpit noise. I went with a ZF transaxle so that I could pick the gear ratios. There is more details to the story than I will mention here but I picked a final drive ratio and a 5th gear ratio so that at 75 mph engine rpm would be about 1900 rpm. Try this yourself. Drive at 75 in 5th and notice the sound level then lower your speed down to 1900 rpm and stay in 5th. Again a very significant change.
Also, a minor engine point, I have "under drive pulleys" on my engine so the alternator and the air conditioning compressor will not be spinning as fast as on a standard pully size engine. Most car buffs would do this to up the available hp but for me it address sound. Double bonus.
If I do all of these things I hope to get the cockpit sound level down to new car standards with the only significant sound being the exhaust.
Even though you already have your car done, and do not want to change mechanical things, you could add the cockpit insulation for about $400. Most of the carpet is just held in place with Velcro. Enough of the Dynamat Extreme material for the inside of the car only costs about $250. Each 3db reduction is a 50% improvement so if you got only a very conservative 6db change that would be very noticeable. After I do my car I will send sequential pictures of the job so you can see if you think it would be worth the effort.
