[vertigto]

I have been contemplating replacing the stock exhaust manifolds on my 70 GTO with RARE oversized manifolds. The original 400 will be rebuilt with some mild upgrades, probably no more than 400hp. Don't want to go the header route for obvious reasons, but wondering if the performance gains for going RARE vs. stock will be worth the $$ for probably a 2.5" X pipe system?

What kind of hp increase could I expect? Other benfits over stock?

Lots of other places to route the money, if it doesn't makes sense. Got the Eaton diff, braking and suspension dealt with, but still need to make a decision on the radiator and other things.

Thoughts??

[hgiv]

I think they are great. They look good, perform well and aren't a pain in the buttocks. They are expensive. RARE don't perform as good as headers, but much better than stock. So I just told you what you already knew. I have always heard the ram air manis are worth 20-25hp over stock logs and that sounds about right to me.

Not for nothing but what kind of "mild upgrades" do you think are going to give you 400hp?

[b-man]

On a stock engine the RA/HO manifolds are worth about 8 or 10 hp over the logs.

On a hotter build of course they’ll free up more hp, 20 to 25 doesn’t sound too far off.

[Lee]

Here is a graph representing my own, independent testing of log manifolds vs. a set of good headers. The RA manifolds would fall somewhere in between, but I feel they would perform much closer to headers, than to logs.

This was on a very mild 455, running 87 octane. Same mufflers, tail pipes, etc., no changes other than metering rods in the carb to adjust air fuel ratio.

Note how the HP curve basically goes flat by 3800 with the log manifolds, in contrast to how HP keeps improving with headers.


The second graph is for a 400 with RA manifolds. This motor had ported 6X heads (by Dave @ SD Performance) and a mild HR cam, but only 8.3:1 compression running 87 octane. The HP curve looks like what you would expect with headers, MUCH better than the curve from the other motor with log manifolds.

[vertigto]

Originally Posted by hgiv

Not for nothing but what kind of "mild upgrades" do you think are going to give you 400hp?

Crank, not rwhp. Maybe a little more aggressive cam, maybe a little head clean-up.

So...@10hp for maybe $800 (including downpipes/flanges)...hmmm.

[vertigto]

Originally Posted by Lee

Here is a graph representing my own, independent testing of log manifolds vs. a set of good headers. The RA manifolds would fall somewhere in between, but I feel they would perform much closer to headers, than to logs.

This was on a very mild 455, running 87 octane. Same mufflers, tail pipes, etc., no changes other than metering rods in the carb to adjust air fuel ratio.

Note how the HP curve basically goes flat by 3800 with the log manifolds, in contrast to how HP keeps improving with headers.


The second graph is for a 400 with RA manifolds. This motor had ported 6X heads (by Dave @ SD Performance) and a mild HR cam, but only 8.3:1 compression running 87 octane. The HP curve looks like what you would expect with headers, MUCH better than the curve from the other motor with log manifolds.

Lee...seems there may be more than 10hp to gain (hard to see your graphs on my pc)?

[napster]

Originally Posted by vertigto

Lee...seems there may be more than 10hp to gain (hard to see your graphs on my pc)?

Brian,

Don't forget if you go with a 2.5 inch exhaust and an x-pipe it will add to what ever the RA III manifolds add for horsepower.

Jim

[PontiacJim1959]

In my opinion, more important are the larger exhaust pipes.

I used stock cast iron manifolds on a built Mopar 360 - 9.0 compression, stock heads/larger valve, Six-Pack, and my last Pontiac 400 build - 8.2 compression, 7K3 heads, XE274 cam. In each case however, I did take a die grinder to them and clean them up deburring and smoothing the inside and matching ports.

Like yourself, I look at budget. Pipe size and good flowing mufflers in my opinion are where you want to spend the money to go with the stock exhaust manifolds. I ran a 3" Dia. pipe right off my manifolds, but I made my own adapter coming off the manifold and welded up my own elbows going back with straight pipe to the mufflers - and they were tight along the engine.

I figure if I can smoke the tires all day long, how bad could they have been?

To squeeze the most HP/TQ out of your build, then RA or headers, but RA are much easier to install and real close to headers in HP/TQ output.

Again, my opinion.

[SD455DJ]

I agree with PontiacJim1959 100%. If you plan on more power mods later on, get the RARE oversized (2.45" outlet) manifolds as they are really nice and can support a bunch of power, however, if more power (> 400 hp) is not in your plans and money is a factor, your biggest bang for the buck is a 2.5" mandrel bent exhaust system from the manifolds back to the tips with a muffler like the 18" Dynomax UltraFlo's, or a similar straight-thru design. I would grind your log manifold outlets to 2.25" diameter, or if that removes too much mating flange width, as wide as you can grind and use the Pypes 2.5" mandrel headpipes for the logs at a minimum into a 2.25" system, again with UltraFlo's (w/18" case). The X-pipe is optional and I've run both with and without, and it does quiet the exhaust note a bit for what its worth, but help engines that make more power (> 450 hp).

My '70 small valve ('70 #16 heads) XV code 400 made 380 hp/450 lbft torque with stock (unmodified) log manifolds, 2801 Summit cam, stock 9.9 to 1 compression, factory cast iron intake and a '70 7040262 Q-jet. I did go with forged RPM 4340 A-beam rods & Icon pistons (off the shelf +.030") to support more power in the future with better heads, etc., but that isn't necessary for 400 hp. I think with bigger valve heads (#13's or #12's) for more flow (210 cfm vs. 190 cfm) would get this engine to 400 hp easily, but not much more without a bigger cam.

Dennis

[Lee]

Originally Posted by vertigto

Lee...seems there may be more than 10hp to gain (hard to see your graphs on my pc)?

Click on the pictures, and they will enlarge.

Summary 1st graph: Log manifolds caused HP to go flat by 3800RPM. By around 4300rpm the log HP was staring to drop. Peak-to-peak, the headers made 45rwhp more than the logs. By 4500, the HP difference was closer to 50rwhp.

Summary 2nd graph: Mild 400 with 260+cfm heads and a mild HR cam, but with 2.5" R/A exhaust manifolds, shows HP steadily gaining as the curve approaches 5000rpm. With 312rwhp, this 400 is approaching 400 flywheel HP, and is doing it with only 8.3:1 compression and 87 octane. Also note this engine was tested nearly 7 years ago, and is still running great.

[vertigto]

I need to get an entire exhaust system, so I'll plan for a larger diameter and X pipe to avoid doing it twice, whatever I decide. I may have to wait on the RA manifolds/downpipes...AC went out last night...not fun. Couldn't get anyone to come out and fix, but that's another thread.

What do y'all recommend for mufflers for that open/raspy sound and good flow...on a budget of sorts. Didn't plan on @ $1500 for a full exhaust.

[SD455DJ]

Originally Posted by vertigto

I need to get an entire exhaust system, so I'll plan for a larger diameter and X pipe to avoid doing it twice, whatever I decide. I may have to wait on the RA manifolds/downpipes...AC went out last night...not fun. Couldn't get anyone to come out and fix, but that's another thread.

What do y'all recommend for mufflers for that open/raspy sound and good flow...on a budget of sorts. Didn't plan on @ $1500 for a full exhaust.

vertigo - Dynomax UltraFlo's are $120 ea. and the comparable Magnaflow's were $110 ea. I have both, Dynomax's on my RAIV judge (UltraFlo's with a Torq Tech mandrel system with an X pipe...old and not made anymore) and the Magnaflow's on the green Tempest (no X-pipe). Both are good systems...the Ultraflo's and X are fairly quiet until you open up the secondaries, while the Magnaflows (w/o the X) is noticeable louder at idle and normal driving...I like it...at full throttle the two systems are similar to my ears. Both mufflers are stainless steel. My UltraFlo's are the ones with the seams, not the welded type (DynoMax offers both). These are our go-to mufflers for performance on A-bodies.

Dennis

[69 Limelight]

Dennis, how often do your different exhaust system combinations (Dynomax Ultraflows, Magnaflows, X, no X) on A bodies wind up with drone/resonance?

[70GS455]

Originally Posted by SD455DJ

vertigo - Dynomax UltraFlo's are $120 ea. and the comparable Magnaflow's were $110 ea. I have both, Dynomax's on my RAIV judge (UltraFlo's with a Torq Tech mandrel system with an X pipe...old and not made anymore) and the Magnaflow's on the green Tempest (no X-pipe). Both are good systems...the Ultraflo's and X are fairly quiet until you open up the secondaries, while the Magnaflows (w/o the X) is noticeable louder at idle and normal driving...I like it...at full throttle the two systems are similar to my ears. Both mufflers are stainless steel. My UltraFlo's are the ones with the seams, not the welded type (DynoMax offers both). These are our go-to mufflers for performance on A-bodies.



Dennis

What length muffler case?

Sent from my SM-T817V using Tapatalk

[Tempest T-37]

For what it's worth, both Thrush and Flowmaster have alumnized mandrel-bent kits with H-pipe that are pretty inexpensive. The Thrush kit comes with cheap Thrush Turbos, and the Flowmaster comes without mufflers. I just bought the 2.5" Thrush kit through Advance Auto online a couple days ago myself for ~240 shipped and plan on using RARE/Pypes head tubes, and swapping out the mufflers for Dynomaxs at some point

[Formulajones]

I agree with Dennis, I go with the Ultraflow Dynomax as a first choice. I prefer the larger case version but also run the shorter case on a different car.

Both have X pipes, and very nice sound, no drone issues.

On the RA manifold debate, Paul C. did some interesting dyno testing on a 500hp 455 using both the RARE ram air manifolds and a set of long tube headers. It was very impressive the very small differences between the dyno curves. The ram air manifolds were within a few HP and TQ on the entire curve and in the lower range actually made more TQ. I believe the custom camshaft he specifies likes the manifolds.

I recommend the RA manifolds to anyone that is even remotely interested in making a little power and having some fun with a Pontiac. The only time I'd ever consider running the logs is if I built a car for the PS drags where the rules require it.

[SD455DJ]

69 Limelight - I'm lucky that neither one drones, not that that is always the case for other folks. I've had drone issues on my F-body with the Pypes system (X and Race-pro muffler), but its tolerable and an isolated set of rear hangers helps with that (Formulajones discussed recently somewhere...it works).

70GS455 - 18" for all. (They make 14" too, but I prefer the longer cases as they are a bit quieter and look more factory).

Tempest T-37 - That's an excellent alternative to the expensive stainless steel Pypes systems and your plan sounds great...so good that I'm going to look into the Thrush system for my GT-37 with the Pypes 2.5" headpipes and UltraFlo or Magnaflow mufflers.

Formulajones - spot on as usual! As for running logs, I'm a gluten for punishment!

Dennis

[389]

Just what the transmission needs, a 3000 degree heater under it in bumper to bumper traffic turning a dual exhaust system into a single exhaust system...

[Formulajones]

Huh?

[scott70]

I think its been shown that the 2.45" vs the 2.25" RA manifolds that theres hardly any HP gain. Just in case the only thing you can find is the 2.25". Same manifolds,,,they just open the outlet some. When I dynoed my 455 i had them use 1.75" hooker headers and made 491 HP. I then had them install the 2.25" RA manifolds and 2.5" mandrel headpipes and it made 18 hp less than the headers.

I would definitely ditch the log manifolds. If anything ,,,just cause theyre ugly.