I droped a 455 from my grand prix to my bird. I went from 3 tube headers to LBM w 2.5 exahust with X-cross over from RARE and from a single plane torquer II to a stock 69 intake and cliff Q-jet. I have no numbers to back it up but the loss in power felt significant. I was shocked by the power loss.
I have my thoery why but its only a thoery. The cam I had at the time was a H10 Ultradyn on a 110LSA. the closer Lobes from what I understand like the scavenging effect that headers provide. The pulses help pull the intake charge in during the valve overlap. Smaller LSA # typicly is a longer period of time. I say typicly because the overlap changes depend on the lift and cam lobe design. The pontiac RAIV had a 113.5 LSA when used with it RA exhaust manifold.
Looking at the LS motors they make decent power in stock form with their shorty log style manifold and have very wide lobes from 116 to 118 deg. I went with a new roller cam in my 455 and used a 114 LSA (230/236 dur, 560/570 Lift). The stock cam I went with usally has a 112 and I'm sure it would work good too.
Its a very different car now. I changed to a Contiental coverter as well. It has a much smoother idle and drives more like a stock ride until you put your foot into it. Its still a work in progress.
I base my thoughts on the little experience I have had with my motor and this article from SD performance.
Ram Air Exhaust Manifolds or Headers?
November 29, 2008
We get asked this question a lot so we thought we'd outline some criteria to consider when choosing.
Early on we learned that camshaft selection is very critical when using the Ram Air Exhaust Manifolds compared to headers, tighter lobe seps along with long durations just don't work since they need the exhaust scavenging of a longer primary tube runner which are found only in long tube tubular headers.
When using the RA manifolds we suggest a cam with a minimum of a 112 lobe sep if not more, along with keeping the duration on the smaller side to reduce overlap.
The hyd. roller cams are perfect for use with Ram Air manifolds because the fast lobe ramps reduce overlap while providing more power building area under the curve at higher lifts. We have various custom grinds that we have tested that give up very little power over 1 3/4" 4 tube headers. Years ago we tested a 467ci 9.5 to 1 OEM D-port headed engine in which we were running an Ultradyne Solid Flat Tappet cam with 263/271 dur. @ .050 on a 112 lobe sep, the engine made 502hp with 1 3/4" Hooker SC headers when we switched over to A-body Ram Air manifolds, peak hp dropped to 457hp and at 6000rpm we saw close to a 75hp drop, obviously this combo worked much better with headers, the reason being that this particular camshaft has a lot of overlap and needs the long primary tubes of a tubular header.
In the last year we've built a couple of different combo's designed around a completely stock look, using iron heads, iron intake and Ram Air Exhaust manifolds.One engine, making right around 450hp only lost 7 hp and 8 lb/ft of torque in a back to back test switching from 1 3/4" open headers to 2.45" A-body RA manifolds with short 90 deg. mandrel bent dumps. We were very pleased with the results but thought what about a 500hp combo?
Recently we had the opportunity to build a serious "sleeper" engine for a customer. We took a numbers correct 71 GTO 400 engine and turned it into a 498ci torque monster but popping the hood won't reveal anything trick. The compression was set at an extremely pump gas friendly 9.3 to 1, the heads are 260cfm CNC ported #96 castings along with a "Rugglized" CNC machined and port matched 71 OEM iron intake. The Cam is our "Old Faithful" grind with a 114 lobe sep. The goal of this test was to do a comparison on a safe real world tune up so we didn't chase every pony available but needless to say we didn't leave much on the table. We started with the A-body RA manifolds again with the short 90 deg. mandrel bent dumps, the engine produced 496hp @ 5100rpm and over 580lb/ft of torque @ 4000rpm.Switching over to a set of 1 3/4" Hooker SC headers only increased the power to 504hp @ 5100rpm and over 590 lb/ft of torque @ 4000rpm. Considering that some longer head pipes on the manifolds could tighten up the difference even more, makes using the RA manifolds in this application an excellent choice.
Hopefully this will give you some valuable information to help decide which direction to go or you can always drop us a line to discuss your specific combination. We will continue to compare headers to the RA manifolds on other combo's in the future and will share this info with our customers.
