Differences between '70 Chevelle LS6 and the '71 Corvette LS6?
 

This is my first post. I was wondering exactly how the '70 Chevelle LS6 was modified to create the '71 Corvette LS6? There seems to be a lot of misinformation on the web and in print about this. Many books just say that the '71 Corvette engine was created by just switching from the Chevelle's cast iron open chamber heads to aluminum closed chamber heads. Many books and web sites state that the '71 had a hydraulic cam and lifters, I am sure this is incorrect, because the redline on the '71 LS6 tach is 6500 rpm. Did both engines share the same cam? I noticed that the Chevelle used an 800cfm Holley and the Corvette used a 780 cfm Holley. I know that the Corvette used much better free flowing exhaust manifolds, and I wondered if these are still available, or reproduced? The '71 Corvette LS6 seems like it would be a great engine for today due to it's 9 to 1 compression ratio (vs the Chevelle's 11 to 1). I wondered what it would take to reproduce it (without the A.I.R. pump). I read in a book that it made 325 net HP @ 5600 rpm and 390 net foot pounds of torque @ 3600 rpm vs the '71 LS5 which made 285 HP @ 4000 rpm. The '72 and later LS5's dropped to 270 HP @4000 rpm.

best regards,

RDave

My book shows 5600 rpm

The BB Vette exhaust manifolds are all basically the same & a big improvement over the Chevelle manifold. The 25hp lower rateing in the vette as I understand it came from having to use the low rise intake manifold which was much more restrictive than the velles aluminum high GM highrise! Not sure on the comp specs!

SAE rated HP @ given rpm vs maximum recommended engine speed
 

Hi dwncchs,

the 1971 LS6 was rated 425 HP @5600 rpm, but I was referring to the latter which indicated on the Ls6's tachometer:

http://www.supermotors.net/getfile/4...s6-redline.jpg

1971 CHEVROLET CORVETTE LS6 SURVIVOR TOP FLIGHT 2006

RDave

Vehicle weigh. The Chevelle has to big a pig. As far as the differance in the engines.......not a clue.

Dave I think that was the LT1 tach. they through in there because there was barely a couple hundred built.L88 had them also.Ive driven both the LS6 and the L88-no comparison!

1971 Ls-6 tach redline photo
 

Hi dwncchs,

the photo is from this Ebay auction:

http://cgi.ebay.com/ebaymotors/Chevr...QQcmdZViewItem

RDave

the velles aluminum high GM highrise!
 

I found this on the web:

LS-6 1970 454/450 HP Chevelle, 4-bolt block, steel "Tuftride" crank, 7/16" "dot" rods, forged pressed pin pistons, closed chamber square port heads, 11.0:1 comp, solid cam, Holley 800 cfm carb on aluminum lo-rise intake.

1971 454/425 HP Corvette, same as above but with open chamber aluminum square port heads, Monte Carlo and Chevelle were to get cast iron open chamber square port heads and 9.0:1 comp.

This indicates that the Chevelle had a low rise manifold (which the Corvette also probably had). The Chevelle had the advantage in compression ratio, while the Corvette had superior exhaust manifolds.

RDave

I built a 70 LS6 last year. They had a 780 CFM carb. and a low rise intake. The 450 HP is underated for insurance purposes. Well over 500 HP. That engine is an animal!! The heads were the closed chamber IRON (last year), different pistons, and heads 70 to 71. The 70 LS6 was the highest HP rated production GM engine for over 30 years. With headers, high rise intake, attention to tolerances with modern machining and some tuning they were capable of 600 HP with stock components. Mine was correct down to the date codes. I was building it for my 63 custom but had to sell it due to medical expenses. $11,750. I wanted the most powerful big block ever produced by Chevrolet in my car but it didn't happen.

I'm running a 1970 LS6 block but nothing else is "original" the best GM platform you could build off of.:thumbs:

Not meaning to hijack, just a quick note......saw your car at A&W, awesome machine.....mine was the '69 red coupe among the others. Seems we live in the same zip code. :seeya

Let me add some info. The LS-6 Corvette was rated 425hp (GROSS) and was rated 325hp (NET) by GM. As some of you here may know, 1971 is a good year to use as a reference point between the GROSS/NET hp figures since GM rated their engines BOTH ways. I believe this was the only year that they did this. The LS-6 Chevelle (1970) was rated the 450 Gross, no net stated by GM. The drop in hp from70 to 71 was mainly due to compression, from 11.0 to 9.0 Of course there were other minor differences ( carb specs). And as others mentioned the 71 LS-6 had the aluminum heads. I have a friend who owns an original LS-6 Vette (and is the original owner) convertible-great car, dark green and beautiful.

First post here too.

Both engines definitely used the same solid cam. The exhaust manifolds on the Vette flow much better but will only fit the Vette, might fits some trucks but won't fit other passenger cars. The tachometer redline is 5600 on the LS-5 but was definitely 6500 on the LS-6. Saw one at Carlisle a few years back; awesome cars.

Dave is this a car your buying or selling ?Hmmmmm where did he go?This isnt the guy from MidWest Muscle is it ? MWclassic hmmm

Dave is this a car your buying or selling ? Neither
 

I just used the photo from the auction. I am interested in building an LS6 replica motor and putting it in a C3, preferably a 1971 LS5 with air. power steering and power brakes. Through my post I am trying to clairfy exactly what went into the '71 Corvette LS6.

RDave

Dave, got some numbers from the NCRS spec guide

block - 3963512 (LS5 & LS6 used the same block)
heads - 3946074 (aluminum, same heads used on ZL1 & 2nd design L88)
intake - 3967474 or 3963569 (low rise aluminum)
exhaust - 3880869 (L) & 3880828 (R) (drilled and tapped for AIR)
carburetor - Holley 4150 R4803A (man) or R4802A (auto)
distributor - Transistor Ignition 1112076 (man) or 1112053 (auto)
coil - 1115263

The camshaft was p/n 3904362 but I think has been discontinued. It was a solid lifter with .496/.492 lifts, 242/242 duration and 114 lobe centerline. Crane still makes a blueprint of this same cam.

The LS-6 also I think used a unique fuel pump but not sure what because it is the only Holley equipped big block to have dual fuel lines.

Not sure if you will be able to run it up to 6500 rpm with the A/C. On the 72 A/C LT-1 Chevy used the tach off the LS-5 with the 5600 rpm to help keep the belts on. Sounds like a fun project.

454 H.O, 454cid /425HP Crate Engine = 1971 Corvette LS6?
 

wfo76,

thanks for posting the numbers! One of these big blocks came new with an interior sticker that instructed the driver to turn the A/C off above a certain RPM.

I wondered if this engine would be equal in performance to the
1971 Corvette LS6? :

http://www.paceperformance.com/index...D&ProdID=26941

Chevy High Performance did a baseline test on this engine using cast iron exhaust mainfolds (I'm not sure if they were the Chevelle LS6 or the Corvette LS6 type) and a generic stock exhaust:

http://www.chevyhiperformance.com/te...ne/index4.html

It looks like the cam is pretty conservative. I guess Chevy's HP and TQ ratings were done with headers, which explains the difference with the Chevy High Performance testsAny thoughts?


RDave

exhaust
 

The big restriction of the power of these motors is the exhaust. Add headers and tune the carb, timing and advance curve to work with the headers and you will see over 550 HP out of the 1970 LS6 stock. Add a little higher lift roller cam on top of the headers to really see a difference. :thumbs:

It's a better cam for a low compression motor. Supposedly the LS6 cars are kind of doggy. The low compression and street mechanical cam end up giving a pretty low dynamic compression ratio. It you built one yourself you could go with all the original exterior specs and bump the compression to about 10:1 or so. That would make a nice motor.

The lifts wfo76 gave include the lash so that's what you actually get.

If you post in the Engine Mods section you'll get a ton of advice - mostly from Jim (427Hotrod).

454 HO specs:
 

454 HO ENGINE ASSEMBLY SPECIFICATIONS
Part Number: Old P/N 24502618, Current P/N 12568774
Displacement: 454 cubic inches (7.4-liter)
Horsepower: 425 @ 5,250 RPM
Torque: 500 @ 3,250 RPM
Max. Rec. RPM: 5,500 RPM
Comp. Ratio: 8.75:1
Block: P/N 12550307 Cast iron four bolt main caps.
Cylinder Bore Dia.: 4.250”
Crankshaft: P/N 14096983 Forged 1053 Steel
Crankshaft Stroke: 4.00”
Connecting Rods: P/N 10198922 Forged 4340 steel, 7/16” bolts, shotpeened
Pistons: P/N 10215228 Forged aluminum
Piston Rings: P/N 12523921
Cylinder Heads: P/N 14096801 Cast iron rectangular port, open chamber, 118cc combustion chambers
Valve Diameters: 2.19” intake, 1.88” exhaust
Camshaft: P/N 24502611 high-performance hydraulic roller tappet
Valve Lift: .511” I, .540” E
Valve Springs: P/N 14097002
Valve Lash: Net Lash
Camshaft Duration: 211° I, 230° E at .050” tappet lift
Rocker Arms: P/N 12523976 1.7:1 ratio
Flex Plate: P/N 10185034 14” for automatic transmission
Timing Chain: P/N 10114177 Single roller
Damper: P/N 10216339 8”
Intake Manifold: Aluminum high-rise, for Holley four-barrel
Oil Pan: 6 quart with windage tray
Oil Pres. (Normal): 40psi @ 2000 rpm
Oil Pump Drive: P/N 3860365 Shaft and P/N 3764554 Sleeve
Fuel: 92 Octane
Fuel Pres. Required: 5 to 8.5psi
Spark Plugs: MR43T Gap .045 HEI or .035 for STD distributor
Spark Timing: 4° BTDC
Technical Notes: For manual transmission applications, use flywheel P/N 14096987 and a suitable 11” clutch assembly. Distributor P/N 1104067 or melonized distributor

The redline is only 5500 with the supplied rocker arms, roller lifters and cam. It would need these changed to compete with the '71 LS6.

RDave