GMMG Impressions
Thread Starter
Joined: Nov 2003
Posts: 6,216
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From: Wichita KS / Rancho San Diego
GMMG Impressions Well, I installed my GMMG with a friend who has a shop that does exhaust work. With the two of us and his torch and tools, it took us about an hour and a half to install. The only problem was the left side tailpipe was canted inwards and we had to heat and bend it out. I had had some kind of crappy turbo muffler that looked like a Flowmaster on my car that the previous owner had installed. The thing actually sounded good, but the chunks of rust that fell out of it when we took it off indicated a sick puppy. I was surprised at how quiet the GMMG is at idle; the old system was louder. Its tone is deep, but there’s no mistaking it for my 428 GTO. Cruising at steady throttle is also quiet; very little drone or resonance – certainly no more than the old exhaust. As the throttle opens, the noise level increases exponentially compared to the old system. At mid to wide-open throttle it sounds awesome – very deep and no rasp. I am pleased with the sound, but surprised at how quiet it is. SOTP feels good; the old system was like having a cork up the rear. Note: my car is basically stock with manifolds and cats.
TECH Fanatic
Joined: Nov 2001
Posts: 1,098
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From: I-L-L-I-N-O-I-S
Yeah, the GMMG is fairly mellow at idle on an otherwise stock LS1 after the engine is fully-warmed up. Although I wouldn't say part throttle is necessarily quiet with it. Have someone accelerate steadily down the road in your car as you stand behind it and you'll hear how much louder the rumble is (as compared to stock). It just sounds kinda quiet from inside the car... 
All I can say is though after having the GMMG on two LS1's now (both otherwise stock), the Kooks LT headers w/ highflow cats that I put on REALLY woke up this system. Maybe a bit louder at idle (although more clear), but it just sounds so much more deeper and meaner at part throttle and is crazy loud at WOT (yet maintains a deep, clean muscular rumble all the way to redline). I honestly still cannot believe how much better the headers/highflow cats improved the sound quality of this set-up...
Mike
All I can say is though after having the GMMG on two LS1's now (both otherwise stock), the Kooks LT headers w/ highflow cats that I put on REALLY woke up this system. Maybe a bit louder at idle (although more clear), but it just sounds so much more deeper and meaner at part throttle and is crazy loud at WOT (yet maintains a deep, clean muscular rumble all the way to redline). I honestly still cannot believe how much better the headers/highflow cats improved the sound quality of this set-up...
Mike
Yep, with stock manifolds and pipes it was pretty quiet at everything but WOT (although a nice deep tone), but with FLP's it sounds really nice and the low resonance carries pretty far. The only thing I don't like is the sound without cats (too raspy for my taste). I think this exhaust was truly made for a headers setup although it sounds good without.
