Inliners International Forums General Discussion Engines Smogged '77 250 I6 - Retrofit to pre-smog specs?

I recently purchased a 1977 Nova 250 I6 automatic. I bought it from the original owner, who had kept all the records, including the window sticker, purchase contract, etc.

I plan to keep it as original as possible, but with one exception. I would like to strip off all the smog-abatement parts and retrofit parts from a pre-smog era 250. I am not looking for a performance increase; I just can't stand to see all the vacuum hoses, diaphragms, and solenoids tacked on to such a simple engine. Of course, I will keep them stored in the event that I resell.

I'm thinking that maybe the only changes I would have to make would be to recurve the distributor, change the timing, and replace the Rochester carb with an earlier version.

The cam specs look pretty similar and although the head has an integral intake manifold, I'm wondering if it would still be OK since I am not looking for a performance increase.

Has anyone out there done anything like this? Am I close, or am I missing something? Any ideas would be appreciated.

I've got a '78 Camaro with the 250 L6 and 3spd manual. The pre-smog (depending where you draw that line, as different emissions controls have been introduced over time since '67) motor of '74 and earlier had a completely different top end, as the integrated cylinder head and intake manifold was introduced along with the catalytic converter in '75. So you would need to swap the entire top end out to create an earlier 250; cylinder head, valve cover, intake manifold and exhaust manifold. The exhaust manifold is also a 2" outlet, versus the 2.25" '75 to '78 outlet, and exits at an angle on the driver side instead of downward and around the oil pan to go back the passenger side. So the '75 to '78 exhaust pipes will not fit the earlier exhaust manifold.

However, if all you're looking to do is take off the EGR, EFE, and catalytic converter. You can probably just disconnect them, cap off the vacuum ports, and put a block off plate over the manifold hole for the EGR if you remove it. I'm not sure if you are looking to replace the mono jet with an earlier one without the adjustment caps, or rebuild it with all the fixed factory set screws exposed? I haven't desmogged my Mono-jet. The Thermac air cleaner has been on the 250 quite a way back and has a ducted hose to let cooler air in, so I'm not sure if you'd want to remove that. There is also the PCV which should be left on for crankcase circulation. The EVAP canister is different from the small block as it has an extra valve on it hooked into the EGR system. So I don't know how disconnecting the EGR would effect it? I'll post some more ideas with pics for you when I have time later.

Thanks for your thorough reply. After doing quite a bit of detailing and inspection of the engine compartment this morning, I have decided to leave the engine unmodified.

Like Lifegaurd, I have a 78 nova. The only things that will help are pulling the line from the egr valve and removing the cat. I picked up a little gas mileage when I did both. Jay6155

Thanks for the input!

From what I can gather about the integral head, its not so bad- probably as good as the non-integral head, but you just cant really work on it. So to stay stock, no real reason to change that.

A good spark curve will help, maybe see if it likes a little richer jet. Get rid of the hot air tube to the air cleaner especially for the summer, and run a taller filter element that leaves a gap at the lid like an open air cleaner.

That's really about it, other than the EGR the smog stuff doesn't really hurt performance or mileage much. The air pump takes very little power below 5000, I clocked a rig once with and without, not even a tenth of difference in 0-60mph.

You can do that simple stuff and it will still look stock, and not sink a bunch of $$$ into it either!

Excellent tips; thanks.

Yeah, like Integrated-J, my Camaro did not come with a cat. The previous owner removed it. But I still have a functioning EGR valve. I might try disconnecting it to see what improvement I get in in fuel economy, since DueceCoupe and Jay recommended it. I plan to swap out to a non-integrated head and 292HD exhaust manifold in my stepwise build plan, at which time I would loose the EFE and EGR anyway as the stock Mono-Jet manifold or Offy manifold do not have provisions for the EGR (and THE Co 292HD 2.5" exhaust or Stovebolt cast headers don't have EFE provisions on them).

I hadn't thought about taking off the hot air tube in the summer time. That makes sense. I think the 3rd gen Nova may have more space under the hood than the 2nd gen Camaro. My Thermac air cleaner lid barely clears under the hood of the Camaro, so a taller air filer won't fit. Part of it is probably due to the dip in the contour of the 2nd gen hood which limits it further. On my old '76 Camaro which had a 305 with a 2GC 2bbl, you had plenty of room to swap out the 2.5" filter for the 350's 3 7/16" air filter and lid with room to spare.

I see DueceCoupe is in CA, so he has an air pump on his. Those of us in the other 49 states didn't have the air pump on ours.

My best results so far is in the HEI. Mine had dried out caked on grease on it, so the advance was really sticky. I rebuilt it and used wheel bearing grease to replenish the top end and the advance worked fantastic. I added a quality cap and rotor to it, along with replacement plugs and wires. Then I switched out the stock springs for the stronger set of springs from an Accel advance kit, and there was a definite seat of the pants improvement and fuel economy, with no knocking. I'm putting an adjustable vacuum advance on it next set at 20 degrees advance, and changing the 6 degree initial advance to 10 degrees next. Then trying the next weaker springs after that if there's no knocking.

Also consider the cooling system too. The previous owner had already swapped out the stock brass and steel radiator for a replacement plastic and aluminum one, which shaves some pounds off. And you can swap out the fixed factory 4-blade fan for the later truck 3-blade clutch fan like Integrated-J did, or take the fan off and wire in an electric fan or two like I did. I may do a comparison between the stock fan and those two options this month, to measure the fuel economy improvement between each.

I forgot to mention my air cleaner duct hose. I'm not sure where the Nova is drawing the air from, as it appears that the hose hooks into the left fender. But the Camaro L6 has the hose running to radiator brace and draws air right behind the left turn signal. So I've pulled the lens and reflector over to the side and turned the open into a custom ram air scoop. So I'm avoiding an open element air cleaner to take advantage of that. I have to use the stock size K&N air filter, but my air cleaner size calculations for engine size and rpm under 4500rpm say that the stock size is good enough (although bigger probably wouldn't hurt).

Could you take pictures of what it looks like stock? I'm trying to get emissions back on a "1979" engine.

Dan

I'll try to post some pictures in the next day or two.

The '79 car 250 is different from the '77 and '78. GM dropped the integrated head and returned to the non-integrated head and manifold. Since '79 was the final year for the L6 250 in the Nova and Camaro, this setup was only one year. I assume it was a complete swap over from the 292 truck engines of the time, using the 2.25" 292SD truck exhaust manifold, but I've never seen a picture of one before? The older non-integrated Mon-Jet intake manifold did not have an EGR provision, so I'm not sure if the 292 truck and '79 250 car intakes were different and had a port for the EGR? Otherwise I would assume your components are the same as my '78 Camaro and the '77 Nova.

Here are some pics of my '78 integrated 250 that I had already posted that might help you see the layout of the EGR, EFE, and HEI vacuum advance. Both the EGR and HEI lines have a heat valve on the water neck that they hook to. The EGR lines also go to one of the valves on the EVAP canister. The EFE lines go from the intake manifold, around the back of the valve cover and then down to a boss in the engine block above the starter. The EFE lines are mostly steel, while all the rest of the line are regular rubber vacuum lines. If you need shots of the vacuum connections on the Mono-Jet I can probably take a better pic of it to show where the THERMAC air cleaner plugs in and the EVAP lines.

Here's the hose diagram for the '78:



The EGR and EVAP are tied together. There's a manifold vacuum hose that the PCV shares with it, that runs a line to both of the two vacuum valves on top of the EVAP canister. The one on the passenger side connects on top of the valve, and the one on the driver's side connects to the bottom port on the driver's side valve. There's a thicker line that runs from the carb to the passenger side EVAP valve (I think this is the line to direct unused fuel vapors in the carb when stopped to the EVAP canister). Then another carb vacuum hose which runs to the lower thermal valve on the lower water neck, which then runs off to a red plastic splitter that directs one hose to the EGR valve and the other to top port on the driver's side valve of the EVAP canister. A bit complicated, and I'm not sure how all that works together, and which part you would cap to deactivate the EGR valve.

The PCV just runs one line off the manifold to the valve cover. The EFE runs a manifold port that has a steel line running behind the valve cover and around to a thermal valve on the passenger side of the engine block, and runs back around behind the valve cover to the EFE on the exhaust manifold. The vacuum advance for the HEI runs off a carb port to the top thermal valve on the water neck and around to the vacuum advance.

I'll try to get some pics of the mono-jet to show where the hoses hook up better.