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BBC exhaust ?

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GRAYTEXK5

1/2 ton status
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hey guys, can anyone tell me what would be the right size exhaust to run with a mildly built 454 BBC? I swapped the motor in, and just took it today to have all new pipes ran, true duals all the way from the headers back, with flowmaster 40's, no cats. after I got the exhaust on, it ran like total CRAP! as soon as you step on it...it just falls dead on it's face, almost dies. it didn't do this at all on the way up there, with open headers. he put 2 1/4" pipe, and I told him I didn't think that would be big enough before he did it, but he assured me it would be plenty, so I took his word for it, assuming he knows more about it than I do. but it seems to me like it's just totally choking it down. the pipe is smaller than the collector, and also smaller than the inlet/outlet tubes on the mufflers. I just wanted to hear some other opinions before I take it back and try to get him to redo it. do you think 2 1/4" is way too small, or is something else causing the problem? oh, and as an added benefit...I think it also sounds like crap now too....way too quiet, IMO. could this also be de to the smaller pipes? because I really thought it would be much louder with the flows...you can barely even hear it. my Z71 is louder than that, with a stock 5.3, dual cats, and a single flowmaster! anyway, sorry for the long paragraph, just need some extra input on this. thanks in advance.

John
 
ftwk5 said:
hey guys, can anyone tell me what would be the right size exhaust to run with a mildly built 454 BBC? I swapped the motor in, and just took it today to have all new pipes ran, true duals all the way from the headers back, with flowmaster 40's, no cats. after I got the exhaust on, it ran like total CRAP! as soon as you step on it...it just falls dead on it's face, almost dies. it didn't do this at all on the way up there, with open headers. he put 2 1/4" pipe, and I told him I didn't think that would be big enough before he did it, but he assured me it would be plenty, so I took his word for it, assuming he knows more about it than I do. but it seems to me like it's just totally choking it down. the pipe is smaller than the collector, and also smaller than the inlet/outlet tubes on the mufflers. I just wanted to hear some other opinions before I take it back and try to get him to redo it. do you think 2 1/4" is way too small, or is something else causing the problem? oh, and as an added benefit...I think it also sounds like crap now too....way too quiet, IMO. could this also be de to the smaller pipes? because I really thought it would be much louder with the flows...you can barely even hear it. my Z71 is louder than that, with a stock 5.3, dual cats, and a single flowmaster! anyway, sorry for the long paragraph, just need some extra input on this. thanks in advance.

John
Click to expand...

If you do go bigger, dont go bigger than 3". That's what I'm running on my big block and I think it sounds pretty darn good and don't notice any "bogging" down. I ran 2" with my mild small block, so I think you should have deffinitaly gone bigger in the first place.
 
yeah, I was thinking it should probably be at least 2 3/4", if not 3". but I know there is also such thing as going to big, so...I didn't really argue with him too much, since I wasn't sure. but when I gt home, I checked my old exhaust, from when I still had a small block...and it was 2 1/4", and it always sounded good, and ran great. it also had glasspacks. I'm starting to think maybe I should have gone that route this time too, cuz I'm not really digging the sound of the flowmasters. the glasspacks sounded kinda "raspy" with the small block...but maybe they'd be different with a big block?
 
No smaller than dual 2.5, no bigger than dual 3". 2.5 is fine if your looking for low end power/torque, 3" if you want the power/ torque slightly higher in the rpm range.
 
In my opinion 2 1/4" is probably too small. I am running 2 1/2" straight pipes with headers and no cats and it runs great. I also have a somewhat built 383, so my guess would be that you need at least a 2 1/2" exhuast pipe. You say they put 40 series flowmaster mufflers on your vehicle, however there are a few versions of the 40 series. Depending on which one they installed, that could determine why your exhaust is not loud enough. Because the original 40 series flowmaster muffler should be plenty loud.
 
My BBC sub has dual 2.25's, and it runs fine, but I still think it is too small. I'm not sure what brand the mufflers are.
You can hear the thing inhaling better than you can hear it exhaling! If you are standing next to the fender, you can hear the engine itself, but not the exhaust note. I will probably go with dual 3"'s and flow 50's when I re-do it. Its the wifes truck really, and she doesn't want it to "sound like some stupid race-car". I am even considering the 70 series SUV mufflers to keep her happy.
 
2.25 probably on the small side ESPECIALLY if its not mandrel bent tubing. If it was just bent up at the shop, it's probably not and in the bends the tube diameter gets alot smaller. I'll be running dual 3" with an X pipe on my monster 496 with 40's all mandrel bent. If your 454 mild built probably 2.5 mandrel.

There's a really good article on exhaust that's been posted a few times. Search and ye shall find what you seek. You can figure your optimum tubing size based on CFM of your carb and the volumetric efficiency of your motor.
 
i would say that your exhaust is smaller than i would have chosen, but i really doubt that the exhaust diameter is what is causing your engine to run that poorly.

i would check to make sure the exhaust shop didn't pinch or mash a fuel line, or something to that nature. how long did the truck sit between engines? are your sure the fuel in it is good? what about the fuel filter?
 
I would definately go with the 3" I had a mildly built 468 BBC with full headers, cam, heads, intake and rollers in an 81 GMC 1/2 ton 4x4 and it sounded good but not overly loud. Check your timing and Air fuel mixture maybe? I had a 350 with way too big of a carb and it would bog during hard throttle and ran fine normally.
 
I'm 3" from my headers to my the x-pipe, the exist side of the x-pipe necks down to 2 1/2" - seems a nice way to do it. I went 3" before the x-pipe as the turn to go above the transfer case is pretty tight and as the pipe is not mandrel bent so I new the 3" with the bends would be closer to 2 1/2". My bbc is stock on the bottom end with a good low end torque cam and new intake and carb.

I too would be surprised if the 2 1/4" is the reason for the big difference though.
 
yeah, that's another thing...it's not mandrel bent tubing. so it does kinda "crimp down" a bit at the bends. as far as a crossover pipe...no, they didn't install one. I also asked him about that before he did it, and he said they don't really make much difference...just like he told me the 2 1/4" would be moe than enough. and these are supposed to be the original 40 series flowmasters. just for a little nfo on the motor...it's a 454, bored .040 over, 781 heads with 2.19/1.88 valves, edelbrock performer rpm intake, 850cfm edelbrock performer q-jet carb, and an aftermarket hi-po cam...I'm not sure what the specs on it are, as this is how I got it from the previos owner...and it was originally spposed to be going in the chevelle that I got it with, so I'm pretty sure it's not exactly a 4x4 cam, to say the least. I'm not sure what's causing it to run like this....but all I know is, it didn't do it before I got the exhaust put on...so I can only assume that it had something to do with it. if the exhaust isn't the problem...I can deal with sounding too quiet for my taste....as long as I can get it running like it should. any other ideas what it could be?
 
Make sure he did not mount a muffler in the reverse direction. Flowmasters have a specific inlet and outlet, and should be marked on the muffler end cap.

When you add an exhaust system it will change the amount of air that the carb uses at a given rpm. Possibly your carb is "dumping" more air/fuel than the engine can use or exhale. That problem could be made worse with mechanical secondaries on a quadrajet carb. Probably would not notice with a larger free flowing exhaust as much.
 
I have a pretty well built BBC and im running Dual 3" all the way back with 80 series flowmasters (The 40's got the previous owner a nice noise ticket) and it sounds fricken sweet with my cam!
 
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