Keep Manifolds or get shorties?

[The_Full_Monte]

I'd stick with manifolds. Shorty's aren't really a performance or sound upgrade. If you are really wanting more performance (especially at mid and higher RPMs) and sound, long headers would be worth the investment. As @Das Hatt pointed out, long headers will require ECM remapping/reprogramming for the engine to run properly and avoid serious damage.

 

[L31MaxExpress]

Manifolds are garbage, shorties are noticeably better. I have never seen long tubes cause a condition that would hurt an engine and I have tuned dozens of engines with them.

Even these performance 2.5" outlet manifolds ended up coming off my latest build. Headers are better everywhere on power and MPG, plus sound a lot smoother in exhaust tone. I gained over 1 in/hg manifold vacuum at idle after putting the headers on due to the increased scavenging effect even at idle. The engine even idles on less fuel now.

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In my case this is what replaced those manifolds. Even putting cats on it, the engine makes much more torque everywhere and at every amount of throttle opening from idle on. My rather basic L31 responded to headers and the 2.5" duals like a small shot of nitrous.

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I full exhaust system from under meath while fabricating the tail pipe section. It is all 2.5" mandrel bent all the way through. Fabricated from inexpensive components.

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Cruising 60-65 mph down the road pulling a box into the wind, the engine is making 16-18 in/hg manifold vacuum. Rolling ~60 mph, I rolled on it to pass and was at 85+ in no time.

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[Road Trip]

That's impressive. I have always liked the 305, good engines but they get the same respect the 307 sbc got in the day lol

Moofus02,

I've always been a bit of a big bore bigot. All that blah blah blah about smaller bores leading to
valve shrouding between cylinder wall and valve head, especially with inline valve layouts.

So when Power Nation did a max reasonable effort 305 SBC build, I was expecting to be underwhelmed.
But check out what they managed to generate with 2 different versions of a '305': (Power Nation 305 - big torque and big HP)

That was certainly an eye-opener for yours truly. Just like whenever you are working to improve the
throughput of any complex system, it's best to focus on the bottleneck(s). So if you ever wondered what
a deep breathing set of cylinder heads (that fit a smaller bore) would do when paired with the lowly 305, go
make a big bowl of popcorn and check that video out. (Think 1st-Gen Mighty Mouse makes LS kinda power.)

Enjoy --

 

[0xDEADBEEF]

I've never seen an A vs B test showing shorties do much of anything. If you are lucky you might net single digits of HP, but I've seen a lot of tests that show nothing. Not worth it to me if they aren't leaking or something.

 

[Road Trip]

For anyone new to the engine hobby, in ascending order from adequate to ultimate,
exhaust plumbing stacks up as follows:

* Stock cast iron manifolds / shorty headers
* Long tube 4 primary into 1 collector (4 into 1) headers (better ones actually tuned to help extract exhaust past stock head bottleneck in desired powerband.)
* Tri-Y long tube headers. 2 sets of 2 primaries paired with each other, and then both pairs combined a third time. (These pull harder, you can feel the difference.)
* Doug Thorley Tri-Y headers. (This guy was the real deal: Doug Thorley NHRA bio)
* Custom 180° headers chasing ultimate power for $$$$ but also giving you that 'flat crank' sonic signature.
(Think Bundle of Snakes exhaust)

****

The best overall perspective is that the factory cylinder heads are the VE (Volumetric Efficiency) bottleneck
on the engines made in the GMT400 era. And furthermore, the exhaust is the bottleneck of the cylinder
heads. And although the most affordable headers are a nice visual engine bay upgrade, they aren't tuned/timed to
use the negative pressure created behind the passage of the previous cylinder's exhaust pulse to help extract the exhaust
from the current cylinder on the exhaust stroke. There's no real perceptible performance improvement, but they
are admittedly easier on the eyes than the cast iron manifolds.

In terms of actual performance improvement that you can feel, the long tube Tri-Y headers are the best you can
implement for the street. Their design leads to an extended torque curve improvement. And of course I've included
the 180° headers just to cover the 'there's always someone faster thanks to their cubic dollars' aspect of this hobby. :0)

This seems to be a popular question, especially for those new to the old truck hobby.

Hope this brief header pecking order explanation helps to sort through all the claims & counterclaims in the land
of exhaust system marketeering.

Enjoy your ride!

 

[L31MaxExpress]

Tri-Ys make a definite difference. My first experience with them was adding a set of SLP Tri-Ys to a 1990 Camaro LB9/5spd. HUGE difference in useable torque with that car. It went from running 13.90s @ 101 mph to 13.30s @ 104-105 mph. Running tri-ys with a single exhaust using a nice merge Y seemed to really wake that 305 up. Prior to the tri-ys, that car had a Random Technology catalytic converter and a Hooker cat back exhaust as the only changes, replacing damaged/failed OE parts.

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[L31MaxExpress]

There are two things that need to come back around for street performance for SBC and LS engines. A dual plane MPFI intake manifold and tri-y headers. Now that I have combined both on an otherwise fairly mild L31 350, it absolutely gets it. The Mercruiser SBC Vortec dual plane MPFI manifold makes me wish I had grabbed one of the Dart dual planes when they were available for the 8.1L and added injector bungs to it.

 

[Sean Buick 76]

I've never seen an A vs B test showing shorties do much of anything. If you are lucky you might net single digits of HP, but I've seen a lot of tests that show nothing. Not worth it to me if they aren't leaking or something.

Agreed

 

[L31MaxExpress]

Agreed

I never did an A to B dyno test, but removing the manifolds and putting direct fit shorty headers improved my fuel mileage and added enough torque to keep my van in overdrive up grades it previously downshifted to 3rd gear going up. With the 4L60E and 3.42s at the time it was only turning 1,950 rpm @ 70 mph. Shorties added a very noticeable amount of torque even at that rpm. Mine never had the tiny 1-7/8" pipes off the manifolds. I feel those tiny pipes are choking the engine worse than the manifolds on a stock L31. Then there was also the fact the stock ~65K mile manifolds practically fell into 3 pieces per side when removed. I do not think I have owned a SBC that did not have atleast 1 stock cast iron exhaust manifold crack before it had 100K on it. The 1994 G10 305 I had, I put headers on because of a cracked manifold as well, it had close to 200K on it though.

 

[PlayingWithTBI]

IMHO, shorty headers alone may add some TQ/HP but, in conjunction with other free flowing components you'll see a marked improvement. Things like a mandrel bent Y pipe, which helps in scavenging, to a 3" High Flow CAT, especially as apposed to the OE pellet ones, and a 3" straight through muffler, like the Borla Pro XS. YMMV