Fuel Economy vs. HP, Then & Now

[Clone TIE Pilot]

Wall of text warning.

Modern OBD2 evap systems are tightly sealed like the A/C system with o rings and test ports. Not just hose clamps like the original G body evaps. Modern evap is called enhanced evap and it is all pressurized including the gas tank. A leak anywhere in the system will cause gas tank pressure to drop and set off a SES light. This also makes it easy for an inspector to test the evap for leaks through the OBD2 port with a scanner. The problem is older, non enhanced style gas tanks like g bodies use are not designed to be pressurized and could balloon or burst and can't be made to work with enhanced evap. However, the law is you can't downgrade the emission system on newer engines or doing a mod that prevents inspecting a system like deleting a sensor, causing a paradox with a junkyard LS swaps. Most modern gas tanks will not fit easily into g bodies. The only other way to test evap for leaks is to lock the car in a airtight chamber for two weeks and not even CA is that nuts.

Gas vapors out of the fuel tank is a major source of pollution from cars, regardless if it is even running or shutoff. As long as there is fuel in the tank, it can pollute. Which is why they make such a big deal about evap. In general evap is beneficial by cutting down on sludge forming inside the tank, but designers worry little about backwards compatibility with their design improvements.

 

[CaliWagon83]

So basically you're saying I'm going to have to get a custom-built gas tank with all the evap equipment? I know OBD-II powertrains have been swapped into OBD-I cars, so there must be a way to do it. If I must go with an OBD-I powertrain, I guess my best options are the GN OHV 3.8 turbo, or a Gen-II LT1.

 

[jiho]

Conclusion I came to.

I'm not absolutely sure, so don't quote me, but I think in California you aren't allowed to tamper with your gas tank. I think it has to remain original factory type, even if you swap in a different motor. Kind of a double bind on the tank. But I could be wrong.

There are plenty of ways to swap OBD2 powertrains into OBD1 cars, but I don't know of any that are smog-legal in California. That's why GM came up with the E-rod. Of course, GM played the single biggest role in creating this situation to begin with, from the ground up.

 

[jiho]

Personally, I'm not a big fan of drive-by-wire and computer-controlled transmissions. That's why I like my G-body. I figure if you want that stuff, get a car that has it.

In California, though, it does seem like a drastic trade-off.

 

[CaliWagon83]

jiho, BAR certainly doesn't make it easy, and there is a certain level of subjectivity involved, but my understanding is that if all the hardware is present to make it emissions-compliant with the year of the engine transplant, and there are no other technical issues, they'll approve it. I'm fully aware of the challenge ahead of me. My wish is to do an LF3/LF4, but the reality may eventually hit that I'm biting off more than I can chew. I know I still have plenty of homework to do.

 

[jiho]

BAR certainly doesn't make it easy

GAAAAH! I've been "looking into it" for years and years .... and I'm still not sure about some things.

BAR has an Engine Change Hotline you can call, with staff to answer technical questions. My own experience there was, the guy didn't seem to know much more than I did. You can also connect with your local Referee Supervisor -- the horse's mouth -- but it isn't easy. I got useful info that way, but it was enough of a hassle that I haven't been back.

With your plan, I think the main problem to solve is the gas tank. I don't have a definitive answer there, just a few bits of information mixed with murky assumptions, so I'd be interested in anything you find out.

Go for it! 😕😀

 

[CaliWagon83]

If I do end up going through with the LF3/LF4, I’ll be sure to do a detailed step-by-step on all the necessary details for a CARB-approved OBD-II swap into a G-Body. From some other forums I’ve read, you’re right that the evap system can be one of the biggest stumbling blocks for getting the ref to sign off on it.

 

[jiho]

All three of us diehard G-body fanatics left in California will be waiting with bated breath!

 

[Michael Watts]

a lot is how you drive your car. I have a 1981 El Camino with a 290 HP 350 crate engine ,Turbo 350 trans,true dual exhaust .3:42 gears ,26 inch tall tire.60 mph at 26 rpm And get around 24 mpg.

 

[jiho]

True .... but I think you must have real gasoline, not RFG (California's state-mandated "reformulated gasoline"). RFG does measurably worse.

Aside from that, almost all of it is just how much the car weighs. The engine is a piece of that, but not a huge piece percent-wise. If your car weighs 3500 lb and your motor 500 lb, the motor is 1/7 of the total, and the difference from a bigger or smaller motor is a piece of that 1/7. For example, the difference between an SBC and a Buick V6 is about 1/3 of that 1/7 .... about 1/23 of the total. Which is about what 1 passenger costs you.

I think gearing is also over-rated as a factor. To keep rolling at a given speed, you've either got less fuel per cylinder at a higher rpm, or more fuel per cylinder at a lower rpm. Normally, differences in engine efficiency don't add up to very much.

Edited numerous times. Sheeze, how did I go off on this rant? 😛