commander_47

It has to be taken in context. The 70-80's smogmobiles were detuned and made to run very weakly 'BECAUSE OF ' the rudimentary smog device and computer standards of the day.

By far, the most successful smog device of that era was De-tuning and low performance.

That said, the engines were built to conform to the smog devices of the day, and not vice versa.

Todays new Corvettes take essentially the concept of the original small block and make huge horsepower 'IN SPITE OF' smog control!

The gains you get in smog control from an air pump, for example, are miniscule compared to the power loss.

The crummy Q junk carb is a smog carb. Designed to starve the engine, make it run as lean as possible. It has to accommodate the charcoal vacuum system and a slew of other things.

When anyone starts talking about increasing performance in a late 70's, 80's car, naturally the smog stuff has to go.

For example.....headers: These are the stock headers from an 81:




Air pump and spider:



All that useless crap above does nothing to enhance performance and doesn't even help the environment as much as a properly tuned engine and decent headers.

The Charcoal canister from and 81:


Does nothing for an 81 that a vented gas cap won't do.

And it definitely gets rid of the all the plumbing and the horrific Q junk carberettor:

commander_47

Then you have the wrong or defective cap.

Your 67 GM cap should be stamped 'VENTED" on top.
It only vents internal tank pressure above a certain pressure, so it's not open to the atmosphere; it also opens at a certain vacuum level (below atmospheric pressure) to allow air to replace the volume of fuel used.

You can see one here:

http://www.ebay.com/itm/NEW-1963-1969-CORVETTE-GAS-CAP-GM-3952708-W-SM-STAMP-/291204389456?pt=Motors_Car_Truck_Parts_Accessories&hash=item43cd225a50&vxp=mtr
Make sure the gasket on the cap is intact. If it is cracked or split it won't seal.

The dealer locking cap doesn't have a valve. It is just open to atmosphere through a small hole visible in the top of the cap.

You may have a small engine cap which also is simply open to the atmosphere.

David Mc

And when I move the car out into the sun where do you think the expanding fuel vapors go?

I'll just add a quote I found online....."I found a reference in Noland Adams' restoration guide (page 94, I believe) describing the 63-67 vented gas cap as "able to relieve both pressure and vacuum"

commander_47

A vented cap is based on vacuum. In other words, these are technically car specific. Although, quite frankly, most people just buy a one size fits all.


Vacuum refers to the pull on the valve from the gas being pumped out. When it reaches a certain pressure, the valve in the vented cap opens.

Thus equalizing OUTside pressure.

A vented cap does not worry about the tank expanding, the air in the tank expands very little. And it's not the vapor that expands, it's the air. Your tank is plenty strong enough to contain that.

What it cannot contain is a vacuum in the tank. When the gas is pumped out, if you don't have a vented gas cap and no way to equalize the pressure, the tank will crush.

This is what happens if your vented gas cap clogs:

DUB

You can post photos of pre-historic designs fuel tubes...but that is not the same as the system that the person who posted this question. What was done way back when....is just that.

Referring to gas caps that are not in the year model or era of when a vapor line and charcoal canister is like comparing apples to oranges....pointless. And YES...the era of the car makes a difference....which is why on some cars...a loose fuel cap will make it not run correctly....but a late-70's Corvette does NOT fall into that scenario.

Once again...what you do with what you have... and how you do it is all up to you....but I would seriously BET that those that remove parts to gain power do not even run it on a dyno before and after...to actually see the improvement made and how much it cost ....and have the exhaust tested.

commander 47,
I have also seen fuel tanks that look like that....and also have seen what happens when the vapor line is plugged....due to the fuel cap only allows air IN and NOT out.....as you know....and have mentioned....and I acknowledge.

Posting current PCM controlled emission/charcoal canister control information is nice...but has nothing to do with a system that has no PCM. I know a lot about the fuel tank pressure sensors, purge solenoids and all that when it comes to PCM controlled and monitored emission/charcoal canister controls.

I know where the fuel vapors go when you pull a Corvette outside. I will write this and that is coming from a person I know In Simi Valley California who tells me that he can tell the difference in air quality since the State took emission serious. He can tell an improvement.

But as always.....if a person is being held back on what they want to do ( because they saw someone else get to do it)...and they feel that their 'rights' to do what they want are being hindered....boy-oh-boy...do they get infuriated.

DO as you wish with what you have...I do not personally care. Because if you can modify your car and get it burning really clean...I applaud you. Because when the smoke clears...that is what it all about (in MY opinion)...EACH of us doing our part.....unless you feel 'IT' is ALL about YOU.

I wish the best to all... in whatever you do.

DUB

Torqued Off

I am not an expert on any of this but I notice Dub has not commented on power losses of a catalytic converter. I think it is very common knowledge (or maybe just opinion), that they are restrictive and cause power losses.

Also, explain why the mid 70's Corvette has such lower power specification as the earlier Corvettes. Now I already know that they changed how they were measuring HP and torque, but still, my 1977 Vette is advertised at 190 HP!!! This is when the EPA forced GM to strangle the engine...so explain how the emissions were not responsible for the difference between a 1970 Corvette 350 engine and a 1977 350 engine......EMISSIONS!

Mike Ward

The EPA mandated that all cars must run on regular 87 octane unleaded gas. This severely limited the compression ratios, ignition advance curves and cam profiles that could be run.

That's what killed performance. Nothing to do with emissions.

jb78L-82

You are correct!

The reason that the later C3's were such poor performers once strict emissions limits were in place was that GM could no longer offer engines with high lift cams, high compression, big carbs jetted rich, performance ignition timing, etc with a free flowing dual exhaust (NO cat) without exceeding the emissions regulation limits. The 87 octane requirement was done to limit the former and was for emissions….sorry..

As for talk of saving the world making sure that 30-40 year old cars are emissions compliant-WHAT CRAP!!! The number of cars on the road that are 30-40 years old is so small, and even smaller for those that are actually driven often, it would not even show up as a 100th,1000th, 10,000th of a percent of total cars on the road, 99.9% which are emissions compliant. Even our beloved government realizes this fact which is why so many states have no emissions for cars 25 years and older. Let's stop the nonsense!

All of my other cars besides the 78 L-82 are 100% emissions complaint and will remain that way since they all run great with the factory installed emissions hardware-My 94 Mustang GT convertible has performance enhancements but is 100% emissions legal-the other cars are factory emissions legal not because I am trying to save the environment but because they run great with the emissions hardware-Totally not true of the C3 smogged cars….

As mentioned previously a stock 78 L-82 produced 220 Net HP. With no other changes other than Shorty headers, 2.5 inch duals (no cat), performance carb tuning, performance timing, no emissions-no AIR, EGR, Heat riser valve, etc the engine produced 233 RWHP ON A DYNO with a weak #6 cylinder. Another forum member with a stock L-82 with duals no emissions produced 248 RWHP ON A DYNO. In my case that is about 275 Net HP and the other case is about 290 Net HP. A stock emissions legal 78 L-82 could not produce those numbers if your life depended on it with full emissions compliance. The only meaningful differences between the infamous mighty 1971 LT-1 (330 Gross HP) and my emissions tuned and hardware laden 78 L-82 (and the other C3 years with the L-82) are all the emissions tuning and associated emissions hardware on those L-82's-remove that stuff, tune the engine like a 60/70's pre emission engine and you get very similar power numbers. Again, statements that are made that removing 70's emissions hardware and proper pre emissions tuning is dubious about power gains are at best very misleading...

commander_47

Didn't mean to get you mad.

This is a picture of the 1981 charcoal canister and purge valve.



It is identical to the 80 model.

The 80 Corvette has the exact same non computer controlled EVAP system shown in this diagram:


The other material was simply to help put it in perspective, and to show how newer cars operate.

And just to be clear, the charcoal canister cars are a closed system in that they do not have vented caps in the same sense that cars without canisters do.

This is from the 1980 shop manual:
"To protect the tank from mechanical damage in the event of excessive internal or external pressures resulting from the operation of this closed system,a pressure vacum relief valve,located in the gas cap,will control the tank internal pressure".

Like I said earlier, most of these caps are application specific. In canister cars you have a vent that is rated very high and only for safety sake. Under normal operation the vent never opens. It is a sealed system.

In the event of blockage or failure somewhere in the canister purge system, then the pressure goes tremendously high and these caps valves will open to avoid crushing the tank.

It's that simple.

David Mc

I think you're missing the point of the vented gas cap. If the cap didn't vent it would be like opening a shaken soda bottle when you refueled. Besides what would be the point of having a charcoal canister if the tank would hold the "vapors that expand".

Charcoal canisters of the era where only designed to hold vapors generated over an hour or two. Once the canister was full, the vapors would just escape out the filter side of the canister.

Today's evap system only differ in a couple areas. One is that they have a larger capacity and solenoids/sensors to monitor the systems integrity.

commander_47

No, you misunderstand how the system works.

A true vented cap, like on a 67 vette, only allows air IN. It is rated at a certain vacuum, similar to a thermostat, to open and let fresh air in to balance pressure.

It NEVER, never, under any circumstances, (unless it's leaking) let's air OUT. OR, you are using the wrong cap. Very early caps, and current small engine caps, and some work around caps don't vent at all. They simply have a hole open to atmosphere. This is incorrect. Most of these are for lawn mowers, tractors, etc. But they WILL work on a 67 vette.

A steel gas tank, and even plastic, is plenty strong enough to contain any expansion of the small amount of air in the tank. You never see a tank balloon up and explode. But you sometimes might hear the hiss of escaping air on a hot day when you open your Vented cap.

The principle of physics DOES NOT work in reverse! A vacuum in the tank will crush under the weight of earths atmosphere like a roach under your heel.

Some of the problem with this is nomenclature. A charcoal system is a CLOSED system. It DOES NOT use a vented cap. A charcoal system does NOT want any air coming in or out of the gas cap. What it might have is an EMERGENCY valve that will open at extreme pressure just before the tank crushes.

That emergency valve only opens if the PCM (purge valve) on the charcoal canister isn't balancing the pressure in the tank.

The vent tube from the tank to the charcoal canister is a TWO way tube. And the reason is very very simple.

If you had the tube go straight to the carb the motor won't run right. It will be way too lean. There simply isn't enough vapor to combust.

So the charcoal canister stores vapor and the purge valve will open to allow air from the PCV system in to balance pressure in the tank.

When pressure in the canister is of a certain desirable level, the purge valve routes the mixture to the carb.

If you put a true vented cap on the tank, the purge system, hence the charcoal system, doesn't work, and it never opens.

After the effectiveness of the charcoal wears off, the system doesn't work anymore anyway.

Todays system is way different. It is totally controlled by sensors and a billion calculations a second of a computer. 70's systems are mechanical and simple.

David Mc

Sorry you're wrong.

mrvette

'72 350 engine was a muncie.....P/O changed out the tank but the tank here now looks completely stock by any reference I have ever seen, so has a black plastic float valve on the left upper end, that stops raw fuel from going into the charcoal canister.....the cap the car came with was sealed for some reason, and since I am running FI on it, L98 style....I run the PVC for obvious reasons, and had some odd things happen with the tank/cap, so a vented locking cap was installed, for some odd reason my evap canister vacuum controlled valve stuck open, so I just plugged the control line to mani vac....still keep the vapor line hooked up to throttle vacuum along with a line from the pass side valve cover, no pressures building up, no smells.....works fine....



EGR main troubles with the carbs used back then was not the CARBS, but the restricted flow in the intake mani, and so another trick is to pour plaster of Paris into the hear riser crossover to keep the carb from frying in the summer....

commander_47

Ok, enjoy your leaking gas cap.

Maybe you should check out this thread.

https://www.corvetteforum.com/forums...ap-lids-2.html

As for the charcoal system, simply look at the diagram above.....air from the float bowl balances the air through the vent line in the tank. Vacuum controls the purge valve. That's the way it is no matter how anyone tries to spin or obfuscate the issue.

Torqued Off

COmmander 47, are you familar with the 1977 charcoal system. My 77 does NOT have the purge valve, and seems to be very simple. A tube runs from the tank, and I believe there is a orifice in that line, to the canister. Then there is a line from the canister to the carb. I don't see the purge valvue used on later models. Nothing else, other than a drain tube on bottom under the filter. What do you know about this system?

commander_47

I am not an expert by any means on the multitude of systems.

I am familiar with the 79-81 models. And I know the principles by which all of them work.

Your 77 has a purge valve built into the Canister. It does not have the chamber for balancing the pressure.

Also, these earlier systems, especially chevy, purged through the hose in the bottom of the can!!!!

Those models actually have a for real vented gas cap. (just adds to the vented gas cap confusion)

Your opening in the top should probably go to the PCV valve, but may go to the base of the carb. You should check the manual. You more than likely have a one way vacuum valve in the line from the can to the carb/pcv.

BTW I believe that guys who want to keep there canisters can purchase modern aftermarket ones that will work for about 100.00.

Unless you live in CA, then you have to go to the dealer.

commander_47

It occurred to me that you might benefit from a cap similar to the one I use.

It has a push button pressure release on the top. To be honest, I've NEVER pushed the button since it's been on the car. I live in Georgia and it gets mighty hot here. I would think similar to Texas.

I removed the charcoal canister years ago.

I'm not sure you can get a cap for your car like this. Mine is the screw on type. If memory serves me at all, you may have a spade type.

btw, these are simple to test, simply blow through, then draw air at the valve on the bottom. If you can stand the taste

Just a thought.

DUB

commander 47;
I am not mad. By no means. I was just commenting that showing different systems and operation configurations is GREAT...but not applicable when the discussion is specific to a design era. That was all.


I am aware of power loss when the old type of catalytic converter is used with the beads in it. The current technology is not as restrictive...and if some feel that the new converters are...then tell me how GM is getting such high horsepower in the new engines...if some feel that the catalytic converter is suffocating/seriously restricting the exhaust flow. The catalytic is not hurting it so much to get the power than GM can sell off the show room floor.

DUB

commander_47

I believe we are in agreement. I probably didn't make myself clear.

The new systems are far more efficient. My contention is that the early rudimentary systems are marginal at best. 75 to 81 are probably the worst.

Someone above wrote that they have not modified their DD late model. Neither have I! Nor could I as I have no clue how it works.

Someone else wrote that tuning and timing restrictions limited emissions and performance. I totally agree.

If you are going to improve performance on your C3, do you really want to put the limited use smog stuff back on?

My example included a good set of headers will operate as efficiently as the air pump AND raise performance. I do believe that to be the case in a C3.

The charcoal canister in a C3, along with the Q jet, are restrictive and hurt performance. A Holley, Demon, or anything else is better for performance, and not that bad for the environment.


I agree fully with you on the early catalytic converter. The "bead" type. Junk.

I absolutely agree that the new systems are far superior, and that's the point

I mean, seriously, I'd like to have a 700 hp C6

DUB

All I care about is what comes out of the tailpipe. If a person puts on numerous aftermarket upgrades and can make the tailpipe emissions be well within standards set...I APPLAUD THEM.

If they bolt on the stuff and do not do anything at all...and when it is running....it is so rich that you eyes burn. Because they bought the biggest carb...dumping massive amounts of fuel...because they feel it is 'better'......and have no clue on how to set up the timing.....and install a choking air cleaner/filter...because of the intake swap that now makes it hard to get things to fit under the hood...and chose the wrong type of intake....and then add headers and delete the catalytic. WELL....shame on them.

I have had enough Corvettes that have been in my shop that were a total disgrace. And I do not care if a state stops emission test or not. All I would like to see is that people are responsible and at least take the time and effort to achieve the best emission results possible on a car that had emissions...and even those cars that did not have emissions....instead or spending all their time in getting the most power possible.

DUB