Question about Big Blocks!!!
Thread Starter
Racer
Joined: Aug 2001
Posts: 473
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From: Raleigh NC
Question about Big Blocks!!!
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.
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.
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Re: Question about Big Blocks!!! (hudman)
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?
How much is the seller asking for?
Thread Starter
Racer
Joined: Aug 2001
Posts: 473
Likes: 0
From: Raleigh NC
Re: Question about Big Blocks!!! (RLH)
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).
Melting Slicks
Joined: Jun 2001
Posts: 2,340
Likes: 2
From: Moro IL
Re: Question about Big Blocks!!! (hudman)
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
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
Instructor
Joined: Nov 2001
Posts: 111
Likes: 0
From: St. Paul MN
Re: Question about Big Blocks!!! (hudman)
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
:seeya
Melting Slicks
Joined: Jun 2001
Posts: 2,340
Likes: 2
From: Moro IL
Re: Question about Big Blocks!!! (LS-5 71)
The stock BB's of the 73 - up were severely undercammed and choked through the use of bad intakes and exhaust manifolds. :chevy
:seeya
:seeya
:cheers:
Pat Kunz
Instructor
Joined: Nov 2001
Posts: 111
Likes: 0
From: St. Paul MN
Re: Question about Big Blocks!!! (73 LS-4)
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:
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,
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:
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Melting Slicks
Joined: Jun 2001
Posts: 2,340
Likes: 2
From: Moro IL
Re: Question about Big Blocks!!! (LS-5 71)
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
:cheers:
Pat Kunz
Melting Slicks
Joined: Sep 1999
Posts: 2,738
Likes: 1
From: Deep in the Heart of Dixie Alabama
Re: Question about Big Blocks!!! (hudman)
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.
Racer
Joined: Aug 2001
Posts: 356
Likes: 0
From: Glacier County mt
Re: Question about Big Blocks!!! (hudman)
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.
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.