on my 72 skylark, 455, new 455, alot of head work, howard cam (225-235 @50) edelbrock, forged pistons, balanced, etc... I did hone the stock exh manifolds, but........have a set of Postons headers my concern is do they hang to low? car is lowered some,, and not a daily driver, would like to use them , as the power benefits over exh manifolds is substantial! but will I regret the low hanging tubes?? anyone have experience with postons? pic of headers on the motor,
Its pretty much guaranteed that you will bottom out on the headers. Even on a stock height car they're going to hit at some point. About the only headers I haven't scraped bottom with are the TA Stage 2's on my 70 GS. The Stage 2 headers have higher clearance. Had Poston's on an 84 Regal with a 455 and also on a 71 GS 455 and they both got familiar with pavement and speed bumps....
You just need to be careful, that's all. Short, high speed bumps are the problem, you need to go over them slowly.
I will have to check how my dougs headers sit under vehicle. They were a struggle tho w cylinder 1,3 pipes hitting cross member. Gentle persuasion and a lot of it clearanced them.
I had postons on my car, 455 but small block springs, I slowed down over bumps and took them at angles and only had a very few clearance issues
I worked as a designer for Hooker headers for years, and not in one instance, in testing, did we ever get less than 20hp AT THE WHEELS, (when installing headers) I'm talking v8 cars, all stock, later I did dyno testing ,both chassis and engine dynos, even on stock motors (all brands) headers increased HP, broadened the torque curve, and decreased heat / temps. on modified motors, headers increased power output substantially. so I question where you get the "15hp" value.
Just a ball park response. Shorties make less than full length. If you worked with Hooker, you must know this and that the results vary with individual engine specifications.
Stock 455 Buicks benefit only slightly from the addition of headers. This engine is stock, save for a small camshaft upgrade. It served as the Workhorse in my 3/4 ton Suburban for about a decade. We tested with and without headers, my concern was not HP, it was torque, as this vehicle was used to tow heavy loads at times. Engine spec 1970 Buick 455 Stock bore and stroke Stock pistons (60K mile engine) Stock Rods with ARP bolts Stock heads with Chambers opened up to relieve valves, and reduce compression for 87 octane fuel use. TA 212 cam Iron intake, exhaust manifolds, untouched 750 Q-jet carb Actual Static compression ratio was 9.1-1 Test Performed in Nov of 2008 on a 2002 vintage Superflow 9002 dyno, at one of the most respected testing facilities in the USA. First tested with headers, I had intended to make them fit the truck.. Just to see how much improvement their was, we finished the day by bolting on a set of completely stock manifolds. About 20 ft lbs and 10 ish HP on average. The last test of the day was run 400 rpm higher, as is my custom to insure that we did actually find the tip over point where it quit making more power. I did not invest the time and money in headers for the truck, for that increase in power. I have done this testing many times since, and the crossover point, where you start seeing significant HP increases is with a build in the 425-450 HP area. Those motors will show 20 HP or more typically with headers. To get those HP numbers, the intake ports have to be opened up. A word on the famous 510 Torque number..: I have never seen one make 510 Torque, but then again, I also have never tested a truly stock 455 with the tiny camshaft in it. The last true 455 with a STG 1 grind I did, made something like 480 Torque with the iron exhaust manifolds.. The 510 number was a result of block heat crossovers in the test engines, and correction to the SAE specs at the time, which was 60* dry air not the 72* air that these tests are corrected to. JW
So that's the nuts and bolts of the numbers.. but the question to use headers or not on your build is much more complex than just the numbers. Ask yourself: How am I going to use the car? A cruiser, with just the occasional blast down the country road or the drag strip, with a mild build.... perfectly fine to use manifolds on that motor. They are low hassle and quiet... Spent big money on free breathing heads and complimentary parts?... then ex manifolds are best used to prop the shop door open ... And whatever you do... unless your running in one of those "money is no object stock appearing" classes, don't waste your time or money porting exhaust manifolds... a fully ported set is worth 8-10 HP over the old rusty stock set you have on the shelf right now. On a 550 HP engine... much less on your 450 HP one... If your going to use cast iron exhaust manifolds, my advice is to clean them up and coat them, have them machined so they are flat, make sure the bolt holes have threads in them, and bolt 'em on.. I charge $200-250 to completely re-do exhaust manifolds.. blast, bolt hole repair as required, coat with cast iron color hi heat coating, and sand flat... Do it yourself, and your going to have half that in it... Spend the other $300 to $500 that you will spend on ported manifolds, on something else. JW