I am trying to gauge how much running factory exhaust manifolds is going to hold my 360 back.

Tomorrow i'll kick off making the following modifications to a 1976 2bbl 360:

Comp Hyd Cam [last line shown below]
Edelbrock Performer RPM intake
2.02 J-Heads - milled, 63cc chambers
650cfm Thunder AVS carb


This setup on with long tube headers on my 323 [318 +0.030"] made about 270whp with a 904.

I am thinking the bigger bore 360 will help with potential valve shrouding, perhaps have a decent ~9:1 compression ratio but it will be running through factory exhaust manifolds.

Any estimate for what I may be able to get out of this 360/727?

Well, that is not a particularly big cam, neither the lift nor the duration @ 0.050" are huge, but on the flip side that will make for a decent cruising setup...and that setup got you a 270 WHEEL hp? That is pretty good!

I would think in a 360 motor, all else being equal, that's an easy +40 hp all due to the extra cubes, but that would theoretically get you to about 300 wheel hp, which I think implies about 375-400 crank hp??? (someone help me out on this, my understanding is that it's typically 25% hp loss from crank to wheel)

However, if that 360 combo produces 400 crank hp, with that small of a cam, I would be surprised.

simply put, HP is airflow. Whatever the bottle neck in airflow is will determine the HP.

Is it the cam? the manifolds? the cylinder heads?

Hard to say since not all else will be equal but I think if you got 300 at the wheel I'd be happy. assuming the 270 whp number as accurate. I think you are likely to see more improvements in the torque numbers, which is what you really want on a driver.

One of the magazines did a shootout a number of years ago dyno testing various manifolds and header combinations on a 300 HP 360 Magnum crate motor. Differences were negligible between long tubes and manifolds at that level. I think there was 20 ft/lbs between worst and best. Anything much past that level and you'll start seeing restrictions get progressively worse.

Edit:

Found it... https://www.hotrod.com/articles/mopp-0307-mopar-crate-engine-exhaust/

Results are in the picture captions.

Probably a 25-35hp gain with headers.

On a combo like that, full length headers are really hard to beat from a $$/hp standpoint.

Actually, it's real easy to beat both in money and hp. From the aforementioned HotRod article

I guess if we’re going to quote magazine articles.......
Stock 340 with stock 210/220-114lsa cam:



And then last year in Mopar Muscle, another stock 340:

That’s 32hp & 27ft/lbs over manifolds.

And:
https://www.google.com/amp/s/www.hotrod.com/articles/comparing-headers-and-manifolds/amp/

I’m sure you could find endless examples on the internet to “prove” either side of the argument.

I’ll go with what I’ve experienced.......
Motors with bigger, better flowing heads, combined with longer duration cams that utilize tighter lsa’s benefit more from headers than motors with stock heads and stock cams.

Like this:
https://www.google.com/amp/s/www.hotrod.com/articles/mopp-0106-manifolds-vs-headers/amp/

And what is the OP building?

A very mild combo that really isn't much different than the crate motor used in the test. He sure is NOT building anything hipo.

Now, I am sure if someone wanted to donate to the OP a set of TTi's then he'd probably consider it. Doesn't need them though.

This is why I will no longer try to answer..."how much more, or how much less" question regarding manifolds. And that does not even factor into all the BS dyno stuff and magazine articles.

Simply pick a HP goal and match the cam to the exhaust system to be used. 300 HP, no problem. Just use the right cam.

Any well thought out engine will easily make 1 HP/cu.in with manifolds.

In the first link I posted the motor is very close to what the OP is building(low cr 360 with an intake and a mild hyd cam).......still gained 17ft/lbs and 33hp from a set of under $200 headers.

Seems like a pretty nice bang for the buck to me.

That article written by a Moparts member too 👍

The cam specs in my 2001 catalog for the 300hp crate motor are 250/264 advertised duration, .385/.410 lift.
Quite a bit smaller than the OP’s cam.
The noticeable increase in duration and overlap, along with the heads having bigger intake valves than the 300hp crate are going play into the headers showing more gains over manifolds than what was seen in that magnum motor.

Will it still run okay with manifolds? Of course.
But to think you’re only leaving like 8-10hp on the table with that build isn’t realistic.



Even on that 300hp crate with the tiny cam, all of the headers beat all the manifolds.