Ok... so I just bought this... (for my V6 Fiero) (Page 2/2)
82-T/A [At Work] FEB 24, 08:56 PM

quote
Originally posted by lateFormula:

There are formulas that can be found online to calculate the proper exhaust tubing diameter. Doing the long math is really only critical if you are going to make a custom set of headers. If you use those calculations for the rest of the exhaust system, you're gonna find that even the long math is pretty much always going to come up with the same tubing diameter as the old general rule of "one inch of tubing diameter for every 100 horsepower". This simple rule applies to single exhaust systems only, I don't know if there is a similar rule of thumb for true dual exhaust systems.




Thanks LateFormula... so... it's kind of as I thought then. I didn't know the 1" per 100 horsepower rule, but back when I did the "complex math" I figured if 1-7/8ths is optimal for a totally stock Fiero, then

1.875" / 2.8L = 0.6696

So... if we do the inverse with some basic algebra, 3.4L x 0.6696 = 2.27"

Thus... 2.25" would be optimal for a stock 3.4 V6/60.


Really... my goal is 200hp from the 3.4 V6/60... I've got ported heads, a hot cam with longer duration, and now the absurdly huge headers... so I think 2.5"s is probably a bit too much if I'm using the 1" per 100hp rule you mention. Certainly the car isn't producing 250hp. I think after the DAWG mod, I'm probably reasonably looking at 210hp from the 3.4 V6/60 (knowing full well that it has 160hp stock, but with all the stuff I'll have done to it... 50hp isn't unreasonable).

Question... if I've got a down-pipe that's 2.5"s... and I get all the way to the muffler, and I dump 2.5"s into the muffler... can I then reduce it there through the dual outlets? Or better yet... if I have a 2.5 to 2.25" reducer at some point... and the rest of the exhaust after that point is 2.5"s... can that properly restrict the flow so that it'll be optimal? Or am I basically ruining the flow due to the fact that the whole rest of the exhaust before it will be 2.5"s?


Thanks!

[This message has been edited by 82-T/A [At Work] (edited 02-24-2024).]

lateFormula FEB 24, 11:34 PM

quote
Originally posted by 82-T/A [At Work]:
Really... my goal is 200hp from the 3.4 V6/60... I've got ported heads, a hot cam with longer duration, and now the absurdly huge headers... so I think 2.5"s is probably a bit too much if I'm using the 1" per 100hp rule you mention. Certainly the car isn't producing 250hp. I think after the DAWG mod, I'm probably reasonably looking at 210hp from the 3.4 V6/60 (knowing full well that it has 160hp stock, but with all the stuff I'll have done to it... 50hp isn't unreasonable).



Being a little bit larger will not have any negative affect on the exhaust flow. Exhaust diameter size affects engine performance more when you are using tubing that is smaller than it should be to support the HP number that the engine can produce.


quote
Originally posted by 82-T/A [At Work]:
Question... if I've got a down-pipe that's 2.5"s... and I get all the way to the muffler, and I dump 2.5"s into the muffler... can I then reduce it there through the dual outlets? Or better yet... if I have a 2.5 to 2.25" reducer at some point... and the rest of the exhaust after that point is 2.5"s... can that properly restrict the flow so that it'll be optimal? Or am I basically ruining the flow due to the fact that the whole rest of the exhaust before it will be 2.5"s?

Thanks!



Yes you can. Most vehicles you see on the road today that have dual muffler exhaust systems really have a single pipe running under the length of the car/truck. Those systems always have smaller diameter tubing for the legs to the mufflers. To be sure that the smaller diameter muffler outlets/tailpipes will not have a negative impact on exhaust flow you just need to calculate the cross sectional area of the primary tubing, then make sure the muffler outlet/tailpipes have a combined larger cross sectional area than the primary pipe.

So if your primary tubing diameter is 2.5" the area formula is A= π r². Radius is half of the diameter so 1.25 squared is 1.5625 times Pi (3.14159) which equals 4.91 square inches. So then working the math for the two muffler outlets, you would need a minimum pipe diameter of ~1.8" for both of the outlets so as to not choke up the exhaust flow. So dual 2" outlets, or 2.25" outlets would allow the exhaust to flow without restriction.
richard in nc FEB 25, 05:47 AM

quote
Originally posted by 82-T/A [At Work]:
LOL... hahah... yeah, I can't even imagine. It's going to be a while before I can even get to it... but I'm wondering if I can make use of the end (modify it), or at the very least... I can use everything UP TO the point where the last part bolts up. I have the Ocelot system already on there. I think I mispoke... it's "aluminized stainless steel" (whatever that is). Basically the 2" exhaust they sell from the Fiero Store. At a minimum, I can replace all the crap I already have in there, and then where the very last piece bolts into the rest of the exhaust system, I can simply cut the exhaust on my Ocelot and weld a flange to the Ocelot side so I can simply bolt it in.

I'm having all kinds of dreams about what I want to do with that engine, and La Fiera's post didn't help me. Haha... I want my engine and engine bay to look VERY stock... but I'm seriously considering spending an inordinate amount of money balancing the **** out of that motor and going the extra mile so that I can squeeze every hp out of that stock looking engine. I just don't want to go nuts because at the end of the day, I am an old-ass man at this point... and not the 18 year old that used to drive it around.



no popping or anything.its stock.id like more power but the stock engine runs so well.
82-T/A [At Work] FEB 25, 04:01 PM

quote
Originally posted by lateFormula:
Yes you can. Most vehicles you see on the road today that have dual muffler exhaust systems really have a single pipe running under the length of the car/truck. Those systems always have smaller diameter tubing for the legs to the mufflers. To be sure that the smaller diameter muffler outlets/tailpipes will not have a negative impact on exhaust flow you just need to calculate the cross sectional area of the primary tubing, then make sure the muffler outlet/tailpipes have a combined larger cross sectional area than the primary pipe.

So if your primary tubing diameter is 2.5" the area formula is A= π r². Radius is half of the diameter so 1.25 squared is 1.5625 times Pi (3.14159) which equals 4.91 square inches. So then working the math for the two muffler outlets, you would need a minimum pipe diameter of ~1.8" for both of the outlets so as to not choke up the exhaust flow. So dual 2" outlets, or 2.25" outlets would allow the exhaust to flow without restriction.



Thanks Formula! I found a video with a Fiero that has a VERY cool looking exhaust, and it may be what I end up deciding to do. The only thing is, I have to temper myself because I don't want to do too much and disrupt the originality of the car. I want it to look stock, just perform much better than when originally stock. Either way, what I'll probably do initially is just continue to use the existing Ocelot exhaust system I have... but use it only at the "last mile" part. So looking at the image above of the exhaust system, I'll simply cut the pipe on my Ocelot system and weld in a fitting that will allow me to bolt it up to the fancy exhaust:



I wouldn't cut the pipe that I just ordered... but I'd cut the pipe on my Ocelot system to install a flange so that it could replicate this "section" of the exhaust system. (if that makes sense).


Anyway, this is what I happened to see on YouTube. It's actually pretty nice...





quote
Originally posted by richard in nc:

no popping or anything.its stock.id like more power but the stock engine runs so well.



Nice!!!
1985 Fiero GT FEB 25, 04:52 PM

quote
Originally posted by 82-T/A [At Work]:





Looks good, but I would imagine having those pipes stuck down like that would negatively effect aerodynamics (already not great on the fastback), and potentially hit things, which wouldn't be good.
fliphone FEB 25, 05:31 PM
My exhaust has pretty similar if not more "out there" routing. Because you're so close to the rear wheels, it's actually not too bad when it comes to ground clearance. I think my cars are about 2" lower(?) and I'm yet to smack anything. And while I probably do get some negative aero effects, I'm yet to really notice anything at any speed I've managed to hit to date.






I'm not sure if this is where you got that picture from, but there's actually a video of that grey car starting with a 3.4DOHC

https://www.youtube.com/watch?v=21KNSoHSMGM

------------------
Black 88 GT, a 19 year old here about 20 years to late
- Running(?) LQ1 Swap-

BingB FEB 26, 11:17 AM

quote
Originally posted by 82-T/A [At Work]:
Bonus Question: without a muffler... but WITH a correctly-sized catalytic converter... will the exhaust be absurdly loud and obnoxious? Last time I drove my Fiero (like, it was actually running, and not pushing it on a trailer) I was 24 years old. I'm almost 46 years old.





No muffler and a cat will be pretty raucous. But if you don't have to pass smog you can put a glass pack where the cat is supposed to go and it will be just right with no other muffler.

And the 2.5" should not hamper performance at all. I have heard guys talk about how too big of an exhaust with reduce the "scavenging effect", but it is not going to make any difference on a 200-250 HP engine. Plus it is not really that much oversized anyway.

And everytime you have to spend more money on your car just remind yourself that you have actually chosen a pretty affordable car to restore. Nothing is cheap, but it is not like you are buying parts for a 67 Mustang or a 65 Vette.
82-T/A [At Work] FEB 27, 06:29 PM

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Originally posted by 82-T/A [At Work]:

I've been eyeing it for MONTHS...



... and noticed the seller added a "Make Offer" button, so I offered him $425... which, honestly... I know it's worth more than that. but he took it.




Ok... hahah... apparently, it's not the last one, and I was a sucker for the (LAST ONE!!!) eBay thing. The auction is still up, and he's selling more of them.

https://www.ebay.com/itm/285304660965

I got it in the mail today, and it's actually really, really nice... like, super high-quality all around. I'm assuming he has a few...
Patrick FEB 27, 09:18 PM

quote
Originally posted by 82-T/A [At Work]:

Ok... hahah... apparently, it's not the last one, and I was a sucker for the (LAST ONE!!!) eBay thing. The auction is still up, and he's selling more of them.



Yep, oldest trick in the book by sleazebag sellers everywhere.
82-T/A [At Work] FEB 28, 07:42 AM

quote
Originally posted by Patrick:

Yep, oldest trick in the book by sleazebag sellers everywhere.




I totally fell for it... but I don't feel so bad. It's really, REALLY good quality. I mean... it's definitely worth the $425 I paid for it (which is about $125 less than he's listing it for).

And the way it's designed, I can buy an aftermarket catalytic converter that's 2.5"s and build an intermediary pipe that I can simply bolt in to replace this cherry-bomb thing that's currently in there... without having to cut it.