[napster]

I recently purchased a 1973 T/A with a stock 455. How much of an increase could I expect from opening up the shaker, Hogged out ram air III exhaust manifolds and 2.5 inch mandrel bent down pipes to the transverse mounted muffler? Depending on the estimates to the first question I might also consider about going from the stock 067 to 068 cam.

Thanks,

Jim

[SD73]

I think the shaker and those exhaust upgrades are just going to be negligible at best. I don't think it would register much on the "Seat-of-Your-Pants-Meter" and barely a blip on a dyno. Camming and ditching that restrictive transverse muffler will do much more performance wise. I've always stayed away from aftermarket exhaust myself for originality reasons, but I think that would be a good route for increasing performance and opening the car up to future upgrades if that didn't bother you.

[superdutybob]

Minimal at best! I would NOT hog out a set of manifolds. Not worth the effort. An open scoop, RAIII manifolds, a cam (not 068), and good exhaust can make a huge difference. The 068 cam has no lift and really is geared toward a higher compression ratio. There are much better choices for cylinder filling and the 8.1 CR that you will have.

[HWYSTR455]

I've seen some early car mag articles do ignition and carb tuning on those years and pick up like 30-40hp. H-O Racing also picked up about that much back in the day by doing similar mods. (think they also recommended a cooler t-stat, like 160 or 180)

RARE makes the manifolds that are 2.45" exit, and with mandrel 2.5" exhaust is said to flow within 8% of a header setup.

http://www.ramairrestoration.com/rm-...oversized.html

Pypes has a cross flow setup for 2nd gens and it supposed to be pretty good, but personally, I use the 'traditional' 2x muffler setup.

I've back in the day heard that some got 5-15hp at the rear wheels, peak, by opening the scoop, but think that may have been inflated. When they did that, think it was in combo with the carb mods, but digging through some cobb webs on that one.

I think with some attention to detail, you could probably pickup 50+ hp, maybe more, certainly with a cam swap. Some minor intake mods would go a long way too.

The guys who do FAST would be good folks to brain-pick, since they have to keep it under the wire, probably get some good tips there in the race section.

.

[HWYSTR455]

Originally Posted by superdutybob

...The 068 cam has no lift and really is geared toward a higher compression ratio. There are much better choices for cylinder filling and the 8.1 CR that you will have.

Agreed. There are much better choices for cams out there, and the engine in question is a prefect candidate for one of the 'cyl-filling' profiles that build DCR.

To pick up a little more CR, you could swap to a thin head gasket, or even consider milling the heads. That would open you up to an even better cam selection.

You can look/search in the Street section, there are plenty of threads regarding cams for almost every combo you can imagine.

.

[dadsformula]

I have the same engine, and the biggest drawback in stock form is the 8:1 compression
4X heads with the large 114 CC (I think) chamgers. A set of 6X or 4X 98CC heads
would provide a nice seat of the pants increase. This would also increase the compression to the point the 068 makes sense (depending on your goals) I run this
setup with stock log manifolds and 2-1/2 inch Pipes. The thing is beast to drive,
and a completely different animal than stock. Headers or RA Manifolds would be even
better, but I got sick and tired of the starter and other issues associates with them.
Everyone is correct that there are a lot a good Cam choices out there, but I am a firm
believer in the 068. Low lift, but that's an advantage for valve train stress and allows
stock springs and other components. This is a great street
Cam with anything near 9:1 compression and was used in many of the best street
engines ever offered by Pontiac. If your gonna race it, by all means go with the high
lift modern cam, but don't disregard the 068 for a street machine. Great Cam.

[njsteve]

We recently played with my Gramma's 1975 Firebird. Four years ago my son and I rebuilt the orginal 7.6 to 1 compression 350 (all 150 horses). We put on a 1972 intake and Quadrajet, along with a Melling SPC7 cam (the 068 cam). Sounded great from the exhaust note but as for power: boring to say the least.

Just last month we put together a 1971 455 shortblock with forged stock height pistons and used the original 6x-4 heads off the 350 (94cc chambers). Also picked a Summit 2802 cam which came highly recommended for the now 9.5 to 1 compression engine. Currently the car has the original log manifolds and crappy 1980's bent and kinked 2" local auto parts store dual exhaust. The car has a 3.42 rear.

This engine completely blows away our stock '73 SD455. It is frighteningly fast. Now I need to get either a lower rear (3.08 would be perfect) or an overdrive tansmission. The car is crazy powerful now.

So if anything, find a set of heads with a smaller combustion chamber and aim for a 9.5 to 1 compression ratio. It makes serious power and runs on pump gas.


Here's the Gramma car thread: http://forums.maxperformanceinc.com/...gramma&page=12

[bird72]

If you build the engine and want to keep matching numbers, I would use forged pistons and balance engine. Roller will add HP also as option. If you have 1.66 exhaust you can up to 1.77 size. Install screw in studs. Careful build and cam with good exhaust, yes RA manifolds will give decent power. If numbers don't matter, the 96cc ish heads work well. I used #96 heads in my last 455 build. They are big valve, screw in, and have exhaust holes for all exhausts. Premium gas. Crossover muffler is a bad choke. But you can get a better one than stock if want to keep.

[cdoggy81]

In terms of "bolt on" power adders & without removing the heads will adding 1.6 ratio roller rockers to your current setup worth any hp increase?

Does the 2802 & 068 cam only start to shine when the cr gets above 9? I agree the Achilles heel is those dog 4x low cr heads :[

Also, what do you think about that dynomax 17533 muffler out back? Running 2.5" pypes to it plus it has a 3" internal x over has got to be better than the stock tiny original transverse muffler :]

[Formulabruce]

I run 87Cc round ports, and stock 8.3:1 Static CR ( for '72 HO) . I had to used dished pistons. It probably runs similar to NJSteve's SD as far as power. While a great economical cruiser, its no powerhouse. 340 HP and 400 Tq .
Good carb rebuild and a re curve dist, and a different profile cam can help ya some.. Good luck !

[rexs73gto]

I had a 73 T/A & the oil pump went out so I had to rebuild mine. I had the heads cut by 40ths. Installed the 1:77 exhaust valves with screw in studs to. I already had the RA exhaust manifolds on the car. I installed a Crane Blue Print RA IV cam with the 1:65 rockers. I used a 72 455 HO intake with no cross over & a Q-Jet that I tricked out. I also used a thin head gasket along with a bit of porting on the heads. When I bought the car it would only do a 16:55 in the 1/4 mile at about 91 MPH. When I got all the work done on the engine , it ran a much better 13:68 at 104 MPH. I know a lot are going to say no way on the RA IV cam , but I've used that in every one of my 455 builds & love the way it works.