Hi-

I have been reading a ton online, and on the forum about rebuilding the engine for my 1971 Big Block 4spd... 3:08 rear gears, but its decision making time, so hopefully I can get some help from the forum.

Here is what I've got to work with: The matching number block (bare right now), the original (oval port) heads, original intake, as well as square port heads (was told they were rebuilt when i bought them, but surface rust from sitting around... may need work to them), as well as an LS6 intake for the square ports. I have the original carb etc as well.

I have gotten a few opinions on what build would suit my needs, but some being contradictory... I figure posting here will give me some answers and help me make my final decision.

My needs: I need the car to be able to run on pump gas. I was told that out gas in Canada isn't great, and to make sure I am careful when choosing my compression ratio.
I need it to be street drivable comfortably and reliably.
Power wise- I would like it to have low end torque, for the street, preferably keep my 3:08 rear end for now.... however I am a 21 year old, so the Hp numbers would still be nice still if it is doable. These depend on which build I choose. (425? 500? at the crank maybe?) I know its a 40+ year old car with old technology... so I understand that I can't have the same numbers as a new car, and that it can't handle it as well.
Also I prefer originality, and would rather use my original heads/intake if possible.

Option 1: Do a stock rebuild, with my original heads cleaned up, intake, and increase the compression. I am not sure what HP i would get, or if this would be fast enough for me. It might be the best option. I need to figure out the ideal compression and Hp/torque I can get before I decide on this option...
Which brings me to option 2: One person recommended that the best build for me would be to port my oval ports, clean them up and use them at 10.25:1 compression, which he said should get me around 425hp at the crank, and a ton of low end torque... This method seems ideal, but it seems to cost a lot of money to port and spend time on the heads. He said the square ports won't have torque until 4500RPM which isn't good for the street. (Is this true?)

Another opinion said that 10.25:1 is too high, and since I need a crank, buy a full rotating assembly, build it to a 496, with 9.2:1 compression (which he said was the most for our gas)... and use my square port heads and LS6 intake... This would cost a lot as well, and seemed to be out of my budget. This would make 550+hp and around 550lb-ft or so as well I believe. This might be too much for me, as I don't have the money for a tremec 5 speed yet.

Firstly: What is the maximum compression, that I can safely run on 91 Octane gas, and never have to worry at all.
Secondly: What would be the pluses/minus to each sets of heads? Can I get what i want out of the stock heads? Will this require a ton of machining and cost a lot?
Lastly: Any other input would be appreciated!. I plan to keep this car a while, and I only want to do the engine build once, so I want to ensure I'm happy with it, and will research as much as I can before I decide.

I had the assumption I would be able to get the build done for around $3,500 CDN... then spend on the carb, water pump, fuel pump etc.. but I would be willing to spend a bit more if i have to, depending on what it gives me. I am not sure if this is still realistic.

Thanks in advance!
- David

 

Compression you can run on 91 depends on your cam selection and quench mainly. Your oval ports cleaned up with 9.5 actual compression and all stock stuff will make 450 HP and run on pump gas with a hydraulic roller cam. That is where your best off spending some money. Long tube headers and 2 1/2" true dual exhaust with low restriction mufflers is needed if you are willing to vary from stock. Keep your stock manifolds in storage. Your Quadrajet is a good carb for this. Send it off to Lars here on the forum once you figure out exact parts list to rebuild it and set it up correctly for your application.
If you keep the stock deck height to save the numbers which I would do, and use these pistons:

http://www.summitracing.com/parts/sl...make/chevrolet

this head gasket:

http://www.summitracing.com/parts/cg...make/chevrolet

And this cam kit:

http://www.summitracing.com/parts/cc...make/chevrolet

And these rockers:

http://www.summitracing.com/parts/cc...1-16/overview/

You are at 9.54 to 1 compression, 7.77 to 1 DCR and .047 quench. It will have an operating range of 1600-5400 RPM. It will pull to redline and have great bottom end and fat midrange. This should be a great match for your 3.08, 4 speed and run fine on 91 octane.

 

There is a lot you can do with your stock set-up (heads, intake) but regardless you will need to put a fair amount into machining and purchasing of parts for the rotating assembly.
Personally I do not believe you can do this for $3500 unless you are able to do much of the work yourself.
Having said that I was able to achieve 460 HP and roughly 520 lb-ft torque on 91 pump octane no problem on an engine that used the original heads, exhaust manifold, block, intake and carb on my 72 BB. I was worried about keeping the engine looking original.

As 63Mako said, do your homework on a suitable cam, do some modest port work on your heads, pay attention to achieving desirable quench and this can easily be achieved.
But be forewarned, parts and paying someone to assemble an engine properly if you cannot do the work yourself are not cheap.

 

Thank you both for the info.

I would prefer to stick with the factory manifolds, and I purchased a 2 1/4 inch chambered exhaust from Allen's before I decided to do the build.

C2C3PO could you post the details from your build? Those numbers sound great. I agree to get the build I want I won't be able to do it for 3500.

If I can get 450hp on the stock parts and the 2 1/4 inch exhaust that would be ideal.

 

I could PM you the majority of the details if I dig up my build sheet - but if I could do it all over again I would do a crate engine.
While the numbers seem impressive I guess I'm spoiled as I was expecting more "seat-in-the-pants" power.
I've been unable to shake a problem with my engine eating rear main thrust bearings despite multiple attempts to correct and advice from others.
In the end only you know what you want to accomplish but there is something to be said for buying something that is tried, true and proven and may have a warranty too !

 

The parts I listed are about $1800. Should be able to recondition your rods and crank, rebuild your heads and all the valvetrain stuff is included. This figures your cylinders will clean up @ .030. Seat of the pants power will be there. About $4000 assembled if you got a good shop close by.

 

FYI - The 450hp or so that the above have mentioned will be "gross" hp, not what you'll see on a chassis dyno once installed in the car.

Stock OEM exhaust manifolds really hurt a big block. IIRC I saw an article where a stock LS-5 picked up around 40-50hp on a chassis dyno by swapping out the stock exhaust manifolds for a set of headers.

Edit: Here's the thread with the stock exhaust manifolds vs headers.
https://www.corvetteforum.com/forums...ls5-454-a.html

 

Definitely stick with the ovals. Rebuild the engine using the stock crank and rods. If at all possible, zero deck the block but stop at the pad so it preserves the numbers. Have the builder calculate the volumes needed to get you a true 10.0-1 compression ratio. Then put a decent cam in it to make power in the rpm range you need and still have good bottom end torque. Like 63Mako said, an HYD roller conversion would really help out everything power/reliability-wise but... Honestly it's very pricey to do it and the gains may not warrant the expense.. Especially considering the fact it's neutered with stock manifolds. A good HYD flat tappet with all the break in procedures followed should be plenty good.

I would at a minimum recommend changing from the stock intake manifold to a Performer rpm if you can fit it under the hood, that and a 750cfm Holley vac sec carb should provide good low to midrange power and still make an honest 400+ crank HP thru stock exhaust manifolds.

We just built a stock appearing '65 396" that made 441hp thru stock intake, heads and exhaust manifolds and spent a fortune to do it... It had more in it due to the wrong carb being on it and an ignition misfire that showed up past 6300 rpms... It likely would have made 460 ish. I can't say that I would spend that much money again trying to build something similar...It had a custom HYD roller, scat rods, custom RaceTec pistons, steel crank.. Lots of blueprinting.. Bang for the buck it was not...

A 450hp 454 would be MUCH easier to do, but the intake and exhaust manifolds are the killers.