how many hp do you think i will loose if i go back to exhaust manifolds.Thinking of making things more original under my hood

Not just HP but mileage also and increased underhood temps.As for the power loss..it is not unheard of for BBC to pick up 50 HP with a free flowing exhaust system.Obviously a milder combo will show lesser gains and a larger cube bbc with an aggressive cam will skew the numbers higher.Cast iron manifolds will suffice if appearance and noise reduction is your goal and performance and mileage is not.

I picked up 5 MPH and almost 3 tenths in the 1/4 mile swapping from stock manifolds to headers with my 454..it was honest test too..I even reused the headpipes,mufflers and tails.
5 MPH in a 3945 lb. car w/driver is a decent HP gain

Key to stock manifolds is a wide LSA cam. You can not use a narrow LSA with stock manifolds effectively.

FAST class W31 small block Olds uses a 108 LSA cam with factory manifolds..runs mid 13's around 105 MPH last time out at Lebanon (Musclepalooza)

Watch 'Horsepower TV' & whenever they do a header swap with a before & after dyno run, they usually only p/u 5-20 hp @ the wheels.

worth it?

you decide....I'm keeping my manifolds

Watch 'Horsepower TV' & whenever they do a header swap with a before & after dyno run, they usually only p/u 5-20 hp @ the wheels.

worth it?

you decide....I'm keeping my manifolds

I picked up 5 MPH and almost 3 tenths with my car at the track when I swapped to headers from manifolds.
5 MPH is quite a bit of HP in a 3,945 lb. car.

I picked up 5 MPH and almost 3 tenths with my car at the track when I swapped to headers from manifolds.
5 MPH is quite a bit of HP in a 3,945 lb. car.

that IS a large gain, no doubt!

But I've raced on 2 different days at LVD & had a 2-3 mph difference with no changes at all to the car.(literally without opening the hood during the week) The air up there changes radically!

my point is, since I don't race the car, I don't miss the extra few HP!

I can see a 108LSA on an Olds (Olds heads have very large exhaust ports un relation to the intake), also Mopar motors runs best with tight LSA.

i've ran 12.2@113mph with my 70' Pontiac LeMans 455 and exhaust manifolds. Pontiacs are a PITA to put headers on so I never did. Cam was a small Crower hydraulic piece with .221/.229 @50 .455/.470 lift with 1.5's and 112LSA.

On a very mild 396 approx 30hp on avg but on a hotter 396 with more cam,better heads,carb,higher comp,free flowing ex 50+hp.

You could have your stock exhaust manifolds extrude honed for better flow,not as good as headers but better then ex manifolds in stock form .

But extrude honing is pricey for the small 15hp maybe in some cases 18-20hp gain depnding on setup. But you need to run a 2.5" mandrell bent exhaust pipes and free flowing mufflers behind the extrude honed manifolds to get the most out the extrude honing.

Scott