Exhaust Manifold Performance Stock vs HO
 

Does anyone know of what advantages the "HO" exhaust manifolds have over "Stock" exhaust manifolds?

I have the 9777646 (left) HO manifolds on my '66 GTO, which has a 428 in it.

I am curious if there is much gain over "stock"?

Thanks in advance for your input & knowledge.

Just an ol man trying to learn:old:

Read HPP article, it’s a 2 parter, they do manifolds and exhaust at the track to see what’s best

Sorry, I'm not familiar with the "HPP" article. What are you referring to?
Thanks,

Allan

Here’s a good read to get your started:

https://www.sdperformance.com/moreTech.php?newsID=39

On a stock or near stock mild engine you gain maybe 8 horsepower switching from standard passenger car log style exhaust manifolds to the streamlined HO / Ram Air exhaust manifolds.

All of the early GTO 389 4-barrel 325 hp (1964) and 335 hp (65-66) engines as well as the Tri-Power GTO 389s (348 hp 1964, 360 hp 65-66) had log exhaust manifolds. It’s not like they didn’t hold their own on the street in factory trim.

So of course the high performance manifolds do offer a performance advantage but on a mostly stock engine you have to be the judge of whether or not spending in the neighborhood of $600 for the manifolds and down pipes to match is worth it to you for 8 horsepower. Of course they look neat but again with some people’s budgets they’re not worth it.

Hey Joe,

A great read, and intersting numbers in the comparision.
I wish I knew what cam is in this engine, but it runs so well, & I'm not pulling it apart to find out. I am, however going to enjoy it for what it is, and it "ain't no slug"!:D

Thanks for the information.

Allan

“Sorry, I'm not familiar with the "HPP" article. What are you referring to?”
Thanks,

Allan

Allan,
To answer your question, HPP is High Performance Pontiac magazine. I thought it was a great rag. Plenty of tech and performance articles that were how to and informational. Unfortunately it died a suddden and painful death a few years back.

HPP also does a strip day article swapping of a bunch of headers to see increases.

1973 GTO , 406" automatic 3.23 posi setup with stock #13 heads and Summit 2801 cam. The stock exhaust manifolds with 2" full exhaust and AP turbo mufflers ran 15.2 sec @90 mph. Engine done by 4500 rpm. I changed the exhaust over to Ram Air reproduction manifolds with 2.5'' openings and 2.5 inch full exhaust with Dynomax Super Turbo mufflers. Car ran 14.7 @94 mph and pulled right to 5000 rpm shift point. Power speed calculator said engine went from 250 to 285 net horsepower. Driving it has much more throttle response and a nicer sound.

Great information! Thanks for posting!