hudman

I have always heard that if you were looking for a performance motor that you should stay away from the 73-75 BB's. Nothing against the engines themselves, but these were the years that the smog and emissions gurus were really getting rolling and the power of the BB's really suffered. A lot of weight and not a lot of horsepower. I suspect most of the HP was lost through the emissions and the heads.

That leads to my question. Should I stay away from a '74 BB that has been rebuilt without the emissions stuff with high output iron heads from 68/69 with a casting number indicating a vette, chevelle, or camaro? Intake manifold is from a '71 vette, motor has headers, etc. In other words, everything but the block is from a more performance minded era.

Please advise. Thanks.

Guest

David, is this car in the area? Are you looking at others, besides 68-69's now?

Guest

The 73-74 BB went off Net HP rating, and not gross, like in earlier years. Along with the emissions, I think compression changes also decreased the amount of HP and torque. BTW, no BB in 75, 74 was the last year.

How much is the seller asking for?

hudman

Oops. '75 was not meant to reference a vette per se. I do not know the origin of the block. The casting number includes vehicles from 75 as well (sedans).

73 LS-4

I doubt there is a whole lot of difference between 71-74 (no BB in 75) power wise except for the 71 LS-6 cars (that was still a very mean animal) . Hp wise they went from (LS-5, LS-4)365 gross (I'm guessing that's approximately 285 net) in 71, to 270 net in 72, to 275 net in 73, and then back to 270 in 74. All of these engines still made gobs of torque. De-smogging these engines and adjusting the advance curves can do wonders, I was surprised how much my 73 perked up just by doing this, along with the little bit of other stuff I've done (as you can see in my sig) has made this car a pretty potent performer. Actually some of the heads in that era weren't bad at all the 049 heads for example are a descent set of large oval port heads as far as an OEM head goes. I think the biggest drawbacks to BB's of this vintage was the lack of compression most 8.25-8.5:1, since the emmision stuff is easily overcome with some retuning.

An LS-4 block is a 2 bolt block but should easily be able to handle well in excess of 450 horse, start going around the 500 horse mark then you may be pushing it but I'm not an expert on BB chevy's by any stretch, but I'm learning fast.

:cheers:
Pat Kunz

LS-5 71

Nothing wrong with that. Most oval port heads have good potential for a street machine. Stay away from the small oval port(peanut port) heads as they are good for boat anchors only. There are many ways to get the desired compression ratio you want. Using earlier casting number heads is one of them. You can use pistons with a dome to up the compression also. Change the stock 2.06 valves for the 2.19 variety and a little port work. Use a current grind cam.(depending on your application) You now have a recipe for horsepower. The stock BB's of the 73 - up were severely undercammed and choked through the use of bad intakes and exhaust manifolds. :chevy

:seeya

73 LS-4

Not trying to argue or anything but aren't the exhaust manifolds and intake manifolds pretty much identical between the LS-4 and 5's, I list the same part numbers for the exhaust manifolds. The intake manifolds always looked identical to me. There was a cam revision in 73, what they actually did I don't know compared to 72, my guess would be less overlap becouse of the introduction of EGR to the big blocks in 73, still ended up with 5 more hp then 72, I think this gain is more do to the cowl induction setup however. These cars are deffinitly under cammed, I think you can say that for almost all of the hydraulic big blocks however. The only good part of that is that they have an abundance of low end torque.

:cheers:
Pat Kunz

LS-5 71

Hello, Pat


The original ? didn't pertain to Corvette only big blocks. Rather, I think he was trying to figure out if these motors in ? are a complete waste for a build up. Making comparable numbers or greater than stock BB's of the pre-smog era.
On the In. & Ex. manifold question. You have to remember that not every BB used corvette manifolds. So when you're looking at those HP numbers for a 76 GMC camper special(454) in the Chilton's manual. The manifolds, Cam, Compression, Smog equipment and restrictive exhaust all play their role in lowering power.
In my opinion the BB intakes with EGR provisions are inferior to the earlier designs. (especially the 427-390HP Aluminum ones) None of the Vette Intakes are on par with say a Edelbrock RPM Air-Gap. That gives you different issues as far as hood clearance goes.

Hope this helps clear some confusion. Let's not :boxing

Later, :cheers:

73 LS-4

No problem. I've been debating searching for a 427 390 horse intake for quite a while, for two reasons: one it's lighter being aluminum, and secondly it probably flows a little better, how much better is the question? Obviously an air gap would be great but then there's that annoying hood clearance problem again. If I run across one of those 390 horse intakes cheap I'll give her a try and see what happens, worst case scenario is it loses some weight along with my wallet :yesnod:

:cheers:
Pat Kunz

bamavettes

To answer your question. There is no reason to stay away from the ' 74 Big Block. I drove one for five years. It was a outstanding performing Vette. A real pleasure to drive. If you can find a good deal on one. Buy it.

shawn_cake

Dump the crummy cam and heads, get headers.
You're not buzzing the thing to 7000 rpm - so a two bolt
LS5 big block will do quite well in a street machine.
Look at the Holley SysteMAX package - it really works well.