'71 big block

[steve]

I have a few questions on my 71 LS5. The car is pretty much original, with only 16,200 miles. It's a coupe with a 4spd and no a/c.

1. Can somebody explain the emission system to me? i asked this before and don't recall all of the details.

2. I need to replace the exhaust. I've seen aluminized systems as well as stainless. Which one is better? I would like to get it top flighted or gold certified at some point so does it matter which type i get? Also, is there any one vendor that's better than the others? I've got catalogs from MidAmerica, Corvette Central and others and I'm wondering how they stack up. This is my first vette and I want to do things right.

3. Is there an easy way to tell the difference between the M20 4spd and the M21 from the outside of the tranny?

4. The previous owner didn't have alot of documentation on the car. I heard of services or people that are able track down things like the original dealer and such. I know the car came from some where in Minnesota.

[Joe L.]

Re: '71 big block

Steve-----

Your 1971 LS-5 did not use an Air Injection Reactor(AIR) system. Instead, it used what Chevrolet called a "Controlled Combustion System"or "CCS", for short. The CCS was a system of carburetor and ignition modifications designed to allow the engine to meet the emissions standards of the period without the use of AIR. In general, this system was only used on lower performance engines since the carburetor/ignition mods were too "power robbing" for special hi-performance engines. That's why 1970-72 LT-1s and 71 LS-6s used the AIR system and NOT CCS.

In addition to CCS, your car has a Transmission-Controlled Spark (TCS) emissions system which delays the application of distributor vacuum advance depending on transmission gear selection/operating conditions.

Also, 1971 models used an Evaporative Emissions Control(EEC) system which routes excess and otherwise vented fuel system vapors to the charcoal cannister located in the left fender well.

Lastly, like earlier Corvettes, you car is equipped with a Positive Crankcase Ventilation(PCV) system. This system causes crankcase vapors to be returned to the intake manifold and burned, rather than exhausting them through a road draft tube to the atmosphere as most pre-62 vehicles did.

[Wayne M.]

Re: '71 big block

Steve --- To answer your question # 3, check for a stamping on the case flange which bolts to the tailshaft. Stamped vertically, along the flange (I think on the passenger side) will be the code P 1 x yy z. P is for Muncie 4-speed, 1 is for 1971 (unless your's is a car produced in late '70, in which case it will be "0"). "x" is for the letter of the month A = Jan, etc. "yy" is for the calendar day of that month; and "z" is either A, B, or C, meaning wide ratio, close ratio M21, or heavy duty M22.

[Jerry Clark]

Re: '71 big block

Hi Steve:

In answer to question #2 your exhaust should be replaced with a carbon steel system if your intent is to obtain maximum points in NCRS judging. The "Best " system of the three is the stainless system, non corrosive and should last the life of the car however you will take a point hit in NCRS judging, this is true of aluminized also.

jerry

[Terry M.]

[Jack H.]

CEC -- to what extent?

A few more words on what you're looking for with the "explain the emission system to me" comment please. The '71 Chassis Service Manual (CSM) contains a dedicated emission section that gives MORE than a passing overview of the one year only Combined Emissions Control system used on your '71 (I have it's little brother). This will give you 'big picture' information....

You will NOT find detailed troubleshooting guidelines in this book as the system came apparently too late to production release to 'make' the CSM for insertion. I have an original copy of the phamphlet Chevy used to orient dealer mechanics with and it gets even more specific than what's in the '71 CSM (nice reading). But, this only provides generic troubleshooting vs. step-by-step detail. Also, you'll find ERRORS in the '71 CSM (incorrect wiring diagrams, Etc.) as well as in the Assembly Instruction Manual (AIM) -- pages/drawings showing 'CANCELLED' and some items (like reversing and delay relay) kind of 'tossed' into sections that don't really make sense....

If it's hard core step-by-step troubleshooting for the CEC you're after, you have to be shrew/clever and read between the lines. You do this by using the theory of operation from the '71 CSM, buy the '72 CSM and look up 6-cylinder Chevy passenger car (Nova, Etc.) because that's apparently where Chevy 'burned up' residual '71 inventory. Now, you lay the '71 circuit diagram next to the '72 6-cylinder circuit diagram, cross out the 1-2 minor changes that don't apply to '71 and BINGO! you've got the documentation you need to understand, diagnose and fix da sucka!!!!

[Mark J.]

Re: '71 big block: M20 or M21

If you don't want to crawl under the car to look at trans numbers, the following road test was explained to me: Drive in 4th gear at 2000 rpm. Downshift to 3rd gear keeping same speed (mph). If rpm jumps to 2800 you have M20 (wide ratio), if rpm jumps to approx 2500 you have M21 (close ratio). I have M20 on my 68. If gears (and these numbers) are different for a 71 somebody pls jump in. MJ