What should I do about my exhaust?

[JON70CST]

Hey guys,

I finally got my 454 dropped into my 1970 CST/10 this past weekend, got it fired up and it runs, sounds, and looks great! The truck was a factory BBC 402 truck so that made things a LITTLE bit easier.

Now that you know a bit of the background here's my question:

The 454 I put into the truck is built up a bit with a 280H Comp Cam, roller rockers, domed pistons RPM Airgap Intake, Holley Street HP 750 carb, Flowtech longtube headers just to mention some of the important stuff. I am debating whether I should have 2.5" or 3" dual exhaust installed on the truck. Either way I am planning on going with DynoMax Welded Ultra Flo mufflers. I have been given prices of about $700 for the 2.5" and $950 for the 3". My goal is to have this thing on the street within the next 3 weeks.. as it is a big car weekend coming up here in Lethbridge (Street Machine Weekend) and I would love to be able to take the truck for a few laps around town that weekend. There are a few things left that I need to take care of before I get the insurance safety inspection done including the exhaust, hence why the price of the 2.5" would be nice, but at the same time I would much rather do it right the first time if 3" is going to be more applicable.

I have one muffler shop recommending the 2.5", saying the 3" won't truly give me that much more flow, and another shop strongly suggesting the 3"

What do I do?!?!? and sorry for the novel of an explanation.

CHEERS!

[70cst]

My vote is 2.5" which is what I have for my 402BB but lets get some other opinions.

[GASoline71]

I'd run 3"... but that's me. 2.5" will be just fine. Probably cost less too.

Gary

[JON70CST]

thanks for the quick input guys! haha uh oh, 1 for each side so far. 2.5" is definitely less costly

[ChevLoRay]

I listened to Click and Clack last Saturday. On one of the calls, they determined that the problem that had doomed her car was due to running it without a muffler, reducing back pressure and that lead to the valves become overheated and ruined the heads.

With that said, how much of a performance handicap do you foresee with the smaller exhaust vs. the larger one? I'm guessing you have enough cubic inches and go-fast goodies to make up for any shortcomings that the exhaust might bring.

I'd go with the 2.5 inch system, and use the change you get to buy some gas and go cruising.

[19seventeeCustom]

Originally Posted by ChevLoRay

I listened to Click and Clack last Saturday. On one of the calls, they determined that the problem that had doomed her car was due to running it without a muffler, reducing back pressure and that lead to the valves become overheated and ruined the heads.

With that said, how much of a performance handicap do you foresee with the smaller exhaust vs. the larger one? I'm guessing you have enough cubic inches and go-fast goodies to make up for any shortcomings that the exhaust might bring.

I'd go with the 2.5 inch system, and use the change you get to buy some gas and go cruising.

^ that theory has been debunked numerous times. There is no such thing as "no back pressure" causing burned valves. I ran my camaro with open headers for 25-30 miles before I ran full exhaust, really damn loud but no burnt valves. When I had exhaust for the LT's fabbed it was only a 3" y-pipe into a single 3" axle back, (no muffler). Ran that setup for at least a year.

To the OP, 3" header back exhaust has enough CFM to flow 800hp. Unless your searching for louder exhaust, 2.5" would work great too (cheaper)!!

[JON70CST]

Very interesting perspective! thanks for the input! and with gas prices the way they are more money for gas is always a great thing! haha

[BureauMan]

The TV show GEARS says that unless you are putting down some real horsepower like 450+ 2.5" would be find.

[fastwillie 696969]

Originally Posted by BureauMan

The TV show GEARS says that unless you are putting down some real horsepower like 450+ 2.5" would be find.

i saw that they used horsepower to pipe size ratio 500 + 3 inch .so 2 1/2 inch should be good

[JON70CST]

The truck is going to be dedicated to the street for the most part, maybe a trip here and there to the strip just to see what she's capable of. Should this affect my decision at all?

[JON70CST]

I'm hoping I'll hit 450 hp but I doubt it'll be making any more than that. Hard to tell what its at for HP without runnin on the dyno i suppose eh?

[brad_man_72]

I'd start by buying some decent headers. I had a set of fLowtechs ONCE. If that's not an option I'd go w the 2.5"
I missed the big cruise weekend because I wanted headers got some flowtechs, they didn't fit sent them back new set didn't fit. I spent ruise weekend moving my engine back 2" and clearancing my frame and suspension for "cheap" ceramic coated headers that were rusty in 6 months.

[QKENUF4U]

from what ive read in a HOT ROD/CAR CRAFT mag. they did a study on this..... get the CORRECT SIZE for the engine needs BEFORE THE MUFFLER and then run any size tailpipe you want AFTER THE MUFFLER.... so do your 2.5 in front if thats what this motor needs and run a 3 tailpipe...or run all 2.5 or all 3... all comes down to the engine needs.... i know my 68 camaro 327 didnt need 3" but man it would have sounded so awesome if i had....

[djgebert]

You are not running a real aggressive cam. In my opinion you will not notice a difference whatsoever between the 2.5" and the 3". I have had numerous big blocks from 300hp to well over 600 hp and until you get some serious HP the 3" does nothing noticable except sound SLIGHTLY more throaty. IMHO I would save the money and get the 2.5" stuff. I would also shop around, I was getting quoted from $400-600 just to bend and install my exhaust and I stopped into a small town shop when driving by one day and got the whole thing done for less than $150 (I bought the stainless pipe and mufflers I wanted) and the workmanship is perfect. There is a huge price difference between shops sometimes.
Another thing to think about is all the old muscle cars all had 2.5" .

[70cst]

Originally Posted by djgebert

You are not running a real aggressive cam. In my opinion you will not notice a difference whatsoever between the 2.5" and the 3". I have had numerous big blocks from 300hp to well over 600 hp and until you get some serious HP the 3" does nothing noticable except sound SLIGHTLY more throaty. IMHO I would save the money and get the 2.5" stuff. I would also shop around, I was getting quoted from $400-600 just to bend and install my exhaust and I stopped into a small town shop when driving by one day and got the whole thing done for less than $150 (I bought the stainless pipe and mufflers I wanted) and the workmanship is perfect. There is a huge price difference between shops sometimes.
Another thing to think about is all the old muscle cars all had 2.5" .

Good points

[JON70CST]

I checked out all of the local shops, each being very close in price.
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[ChevLoRay]

Wanna have some fun? Take the savings money, and buy some electrically-operated cut-outs....then, well....you'll know what to do.

[Ole Greenie]

I'd like to chime in if I could...

I have an LS6 motor that came in the 70 Chevelle SS. It has a conservative estimate, from the factory, of 450 HP. I too have a 750 CFM Carb. It goes real good with the 750 and an X-pipe with 2.5" piping through flowmasters. Your limiting factor may be the carb. If you want to see what it will really do something over 800CFM will be needed. It is much easier to compress air through pressure (Exhaust) than it is through vacuum (Carburator). I don't have headers, rather stock manifolds but it can't keep the tires under it either.

Just a short note for your driveline...make sure you don't break a driveshaft when you take it to "see what it will do." There were zero C10s made with 450 HP!

Happy father's day!

[69sixpackbee]

I have a 440-six pack (446 Cu/in) in my 69 1/2 A12 Bee. I have just a tad over 545 HP at the flywheel. I am running 2-1/2" all the way back through series 40 Flowmasters. When I had a 3" system it dynoed at 537. That was enough proof for me
Sometimes smaller IS better

[Longhorn Man]

the click and clack thing on burning valves due to no muffler is bull... kinda amazed no one has jumped on that yet in this thread.
if it is streetbound, then you'll hardly ever see the RPMs to need the larger pipes. The smaller pipes may even build more low end torque which is what you need anyways.

[70cst]

I think the jury is in and the best bang for the buck is the 2.5" pies ...

[ChevLoRay]

Originally Posted by Longhorn Man

the click and clack thing on burning valves due to no muffler is bull... kinda amazed no one has jumped on that yet in this thread.
if it is streetbound, then you'll hardly ever see the RPMs to need the larger pipes. The smaller pipes may even build more low end torque which is what you need anyways.

I thought it was interesting, too. I've heard of burning valves because the exhaust was being dumped right out of the manifold, but never thought about the other viewpoint. They were saying that the back pressure lets the exhaust gasses stay around long enough to cool the valves and that with less backpressure, the exhaust gas leaves too quickly to carry away the heat.

You'd think that two MIT engineers who run an automotive shop.....well, they know that it's easier to baffle with BS, than dazzle with brilliance....at times.

I guess I posted that to see if anyone had any thoughts and you did. Thanks.

[JON70CST]

Thanks for all the input everyone! I dropped off the truck tonight and she's getting 3" in te morning , maybe overkill but at the same time it gives me room to grow. And I say I won't be hitting the strip in her, but I know that'll change after I start driving it
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[crazy longhorn]

Do the cross over(H or X).....if you are racing, the X may show you more crazyL

[C20-67_N_MO]

You can install whichever size pipes you want, but everyone here agrees that your best bet if the 2.5" ones.

Good luck.