454
Safety Car
Joined: Feb 2006
Posts: 4,855
Likes: 2
From: Columbia South Carolina
I read a thing that said that just adding hedders and a 2.5 inch exhaust was good for 50hp. I built mine with the intention of having stock appearance but Kick a$$ performance. replace the cam but keep your rear end ratio in mind, dont go crazy with lift and duration. also you can upgrade the ignition system with an MSD 6A and a Pertronix electronic ignition unit. everything I mentioned you should be able to get for <$1000 also depending on how deep you want to go bump the compression alittle by changing pistons probably no more than 9.5:1 . 
Burning Brakes
Joined: Oct 2002
Posts: 914
Likes: 14
From: Aberdeen, Scotland
71 LS 5 here
I have a stock, well almost stock 71, LS-5. FYI they are rated at 365 hp, but are the SAME engine I believe as you have. The difference in rating is that in 72 they started rating them as SAE vice gross ... I think. Surely someone will chime in here? 
The point being is that with the stock engine you are limited by: exhaust, exhaust manifolds, intake and compression, i.e. heads. Oh and did I mention that stock low lift and low duration cam?
IMHO if you need 400 hp you might as well swap out a cam and add some headers etc. Otherwise, just hit the basics as said before, good carb work, add an MSD maybe, and tune it as well as you can. And ... better swa out to some real
gears. I put in a set of 3:55's. The BBC is a great place to start, but they need help in 71/72's. Another issue is the hood and the clearance if you attempt to swap intakes. No easy answer there either....ah the joys of BB ownership. Still would not have it any other way!
carbster
The point being is that with the stock engine you are limited by: exhaust, exhaust manifolds, intake and compression, i.e. heads. Oh and did I mention that stock low lift and low duration cam?
IMHO if you need 400 hp you might as well swap out a cam and add some headers etc. Otherwise, just hit the basics as said before, good carb work, add an MSD maybe, and tune it as well as you can. And ... better swa out to some real
gears. I put in a set of 3:55's. The BBC is a great place to start, but they need help in 71/72's. Another issue is the hood and the clearance if you attempt to swap intakes. No easy answer there either....ah the joys of BB ownership. Still would not have it any other way!carbster
Melting Slicks
Joined: Jun 2001
Posts: 2,340
Likes: 2
From: Moro IL
To get 400hp+ in net numbers your probably going to have to do a little internal work but not very much. the 425 number your thinking of in 71 is an LS-6, completely different animal then a 72 LS-5. LS-6 had a solid lifter cam and square port large valve heads and more compression then your 72 LS-5. The 425 number is a gross horsepower number, 425 gross would be approximately 350-375 net horse. You can get in the 350 net horse range without going internal if you set everything up right. The right timing curve, carb setup, ignition setup, headers and a good exhaust, possibly a better intake manifold and you can get close to that 350 number. I think the 400 net number would require a cam change, but change teh cam and put bigger valves in the heads along with the rest that was mentioned 400 is not a problem.
Pat Kunz
Pat Kunz
Melting Slicks
Joined: Dec 2004
Posts: 2,101
Likes: 3
From: Cruising somewhere in St. Louis, Missouri
So what would be a good cam to put into it without changing lifters and heads? I am thinking of doing the same. Can anyone point me in the right direction?
Thanks
Jim
Thanks
Jim
Burning Brakes
Joined: Oct 2002
Posts: 914
Likes: 14
From: Aberdeen, Scotland
Ls-6 ...
Pat,
I sort of "forgot" about the LS-6, 454/425 hp! Man I wish I had one of those. I think only something like 188 were made in 71. They will be or are the next L-88's!
If you "need" 400 hp you'll have to open the engine up, i.e. cam and heads and intake (maybe) with an exhaust. Then it will rip.
carbster
Me, one of these days I am going to go the 4 1/4" stroke route to give me a cool 496 CID with the 0.060 +. Then a nice roller and some good port work, with bigger vlvs should give me almost 1 hp = 1 CID.... maybe.
I sort of "forgot" about the LS-6, 454/425 hp! Man I wish I had one of those. I think only something like 188 were made in 71. They will be or are the next L-88's!
If you "need" 400 hp you'll have to open the engine up, i.e. cam and heads and intake (maybe) with an exhaust. Then it will rip. carbster
Me, one of these days I am going to go the 4 1/4" stroke route to give me a cool 496 CID with the 0.060 +. Then a nice roller and some good port work, with bigger vlvs should give me almost 1 hp = 1 CID.... maybe.
Burning Brakes
Joined: Feb 2005
Posts: 894
Likes: 91
Below is photo of factory "original-appearing" LS-5 from '72 Chevelle. Unit made 410 HP/485 Ft.Lbs. Torque. Made it all happen with stock cast intake, Q-jet carb, stock exhaust manifolds. It appears here as dynoed! Has a maintenance free Comp Cams retro hydraulic roller setup along with their "Pro-Magnum" roller rockers. Runs with 15+ inches vacuum and has only 9.0:1 C.R. Customer extremely happy with unit. I will add however it's now a 496" stroker unit, but as I said above it's in a "high-point" restored show-car. Thanks, Gary in N.Y.
P.S. From this particular unit we learned the "low-profile" intake is the "choke-point" for making any serious HP. It was tested on the dyno with headers and made NO difference at all. If you're not locked in to looking original we can deliver 550 HP easy. We just finished a pump-gas 396 BB with 504 HP. Mostly factory parts, a solid lifter, and NOT a stroker.
P.S. From this particular unit we learned the "low-profile" intake is the "choke-point" for making any serious HP. It was tested on the dyno with headers and made NO difference at all. If you're not locked in to looking original we can deliver 550 HP easy. We just finished a pump-gas 396 BB with 504 HP. Mostly factory parts, a solid lifter, and NOT a stroker.
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Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
Originally Posted by pauvil
headers, exhaust, timing curve, carb tuning, you'll get close and maybee a little more cam, not much though.
BigBlockk
Later.....
Team Owner
Joined: Jul 1999
Posts: 26,545
Likes: 46
From: Huntersville NC
Actually Chevrolet provided both gross and net hp ratings in 1971. The mighty 425 hp (gross) LS-6 was rated 325 hp net. Seems like a huge loss to me too, but that's the number provided by GM.
Safety Car
Joined: Feb 2006
Posts: 4,855
Likes: 2
From: Columbia South Carolina
Originally Posted by luerja
So what would be a good cam to put into it without changing lifters and heads? I am thinking of doing the same. Can anyone point me in the right direction?
Thanks
Jim
Thanks
Jim
Pro
Joined: Jan 2006
Posts: 599
Likes: 0
From: New York
rookie looking 4 my horse...
Hey guys i did a rebuild on my 77' 454 mark-IV block with 427 heads .040 over.....i have a edelbrock performer rpm intake manifold,eps 800cfm carb,aluminum water pump,aluminum radiator with electric fans....speed pro forged dome pistons, hooker super comp headers,comp cam XE268H....my gears R 3:08..with a 700r4 gearstar tranny..i went with the jegs hei ignition with the msd 6al control box......c.r. is about 10.1........with this info can you give me an estimate on my horse..this is my 1st shark and i would appreciate any help or advice.......thank you very much.....
Safety Car
Joined: Feb 2006
Posts: 4,855
Likes: 2
From: Columbia South Carolina
ITs so hard to just throw out a number, almost every guy I have talked to has over estimated his horsepower pryor to a dyno run. 300 Horses is more power than people give credit for and 500 horses is just insane power. Realisticly I would estimate that until you get it dynoed and professionally tuned figure that your somewhere between 3 and 400 just because parts you spec. A proper tune up makes all the difference in the world. Also the Exhaust system diameter that you use makes a big difference esp. for a big block. IMHO