1969 Pontiac Firebird 350 4brl Th400, needs to get better mileage.What to do?

[Fb1969jesse]

Rebuild or LS it or other on my 1969 Pontiac Firebird??[/h][COLOR=#333333][INDENT]I need to do something to my 69 Pontiac firebird 350 4 brl TH400. Haven’t driven it bc of problems it’s had. Had fumes coming into cabin or perhaps it’s and exhaust leak too. I want to have it give me more than 15 mpg. I was only getting 8mpg when I drove it last. Initially I was going to have engine and transmission rebuilt but the Covid hit and nothing was done. So I delayed it more. I had heard and told by friends to do an LS swap but I wanted to keep it Pontiac. It would be costly with all the mods that would have to be done with an LS. Had anyone done it for cheap?? I was told it could cost upto 20k. Crazy. I’d get more mpgs but it’s at a cost. I don’t have anywhere to keep the engine nor invest that much for it. It would be more reliable. But if I just rebuild what I have and maintain it it could also work well. Has anyone done either? Did u go Holley kit, rebuild everything, put a 2 brl with intake and make it EFI or Ls and how did it work lout?

[vcho455]

You did not mention what your goal is nor did you mention your budget.

I will say that my 69 GTO with a RAIV cam and 3 tube headers got 17 MPG on the highway for years and turned 12.9's on a good day. T400 with a 2500 stall converter and 3.55 gears. Mid 70's 350 two barrel Firebirds were able to get a 25 MPG rating from the EPA with low compression and tall highway gears. If your current short block is in good shape (Have you done a compression test?) You can build an engine based on one using a modern roller cam and a quadrajet to start. Then if your budget allows it move to fuel injection down the road. A OD transmission will help but not as much as you would expect on a torquey engine like a Pontiac. I would watch for a used Gearvendor or 2004r for cheap.

[Fb1969jesse]

When I got it it came with a 2brl but the I ended up buying a 1969 Firebirds as a donar car and used the intake and quadrajet it had in my car. I was told that the compression was 10:1 ����*♂️ I don’t know if it’s true.

[dhutton]

Change your rear gears to something lower.

[vcho455]

Your compression ratio is basically decided by what cylinder heads are installed. If your not familiar with how to identify yours I suggest spending some at the Wallace Pontiac website. There are pages on how to identify Pontiac engine components. Once you have identified them you can make a good estimate as to what the compression ratio is.

http://www.wallaceracing.com/index.htm

If your long block is healthy tuning the carburetor and distributor going to make a big difference in your mileage.

How do you plan on using your Firebird? Knowing that is the starting point to planning what you need to do to the car where you want it.

The LS vs Dinosaur Pontiac (Or other vintage cast iron) is a contentious subject as the thread below shows. It however does have some good information if one can avoid the attitudes and personalities.

https://forums.maxperformanceinc.com/forums/showthread.php?t=854893&highlight=Mpg+tuning

[Mr Nick]

A shop with a chassis dyno, and somebody knowledgeable on tuning your carb and ignition, is where I would start. But there is no guarantee that even perfectly tuned, you'll get over 15 MPG with it. Overdrive will help.

Once properly tuned, if you still don't like the smell a set of high flow catalytic convertors will help with that. But don't install convertors till it's properly tuned, a poor tune can ruin them.

[79 Camaro]

I would start with the basics before going crazy.
Confirm timing with the correct timing advance. Make sure to confirm the timing marks are correct on the balancer and block.
Fresh points, plug wires, distributor cap and plugs. Carb in good shape?
Compression check on all 8 cylinders or even better a leak down test.
Maybe to put it on the rollers and have a dyno shop tune it with ox sensors in the tail pipes?
If everything is in order a 4 speed auto OD trans swap is the way to go.

[Alponcho]

I would start with the basics before going crazy.
Confirm timing with the correct timing advance. Make sure to confirm the timing marks are correct on the balancer and block.
Fresh points, plug wires, distributor cap and plugs. Carb in good shape?
Compression check on all 8 cylinders or even better a leak down test.
Maybe to put it on the rollers and have a dyno shop tune it with ox sensors in the tail pipes?
If everything is in order a 4 speed auto OD trans swap is the way to go.

x100 A Pontiac 350 w/Th400 is about as basic as it gets...so like others have suggested, try to sort it out and then go from there.

[Vimes]

If the basic maintenance stuff doesn't get it, the next mod I would make is fuel injection. Holley can sell you a standalone fuel injection setup, and I would get the one that can handle a 6L90E. Fuel injection, depending on how much you want to spend, can either be a throttle body (pretty much a carb with injector pods) or port injection. The port injection would use your existing intake, a air-only throttle body in place of the carb and you have fuel injector bungs welded into the intake. After that I'd look into the transmission.

However, rather than a 4Lwhatever I'd jump straight to a 6L90E. You will have to massage the tunnel for either transmission, so you may as well get the higher gear count for the same amount of installation work. Combine that with a higher rear ratio (lower numerically) you should wind up with a much stronger takeoff while cruising at just over idle. I'd not be surprised if you came back with a cruising mileage of 25MPG or higher on a car as light as that.

[Fb1969jesse]

Don’t really have too much money to spend. Heard LS swap can b expensive with all the upgrades that have to get done. 15k - 20k.

I want something where I can get really good mpg when cruising and still have some power when/ if I need it.
So I am thinking of rebuilding engine (thought of putting another cam,
But wouldn’t it end up costing me more in fuel?) and transmission (with an overdrive on the Th400). Do I have to modify the rear with different gears? Smaller gear? A few years ago I had to get new axles. It now has some KA axles.
Then thought of possibly adding the Holley carburetor kit that has injectors (which they told me would cost 5k in parts and labor), do I have to change the distributor too or could I just ad a petronix point eliminator to it? Can the point eliminator take 12v? When do people have to use the ignition module box?

I know they’re might be an issue with the wiring because last time I tried to turn car on the coil was only getting 2 volts. So I prob have to replace the resistor wire. Unless previous mechanic could have mixed up the wires in the harness thinking the hot wire was connected wrong.


What do you guys think I should do? And what should be the ideal price for the parts and labor? I’m in Los Angeles.

Thanks in advance

[Olds.PhD]

OK, reality check here: you're not going to get "really good" mileage out of an old car with a V8. You're going to get about 20 mpg on the highway with an overdrive and well tuned engine. You might get up to 25 going downhill with a tailwind, but keep in mind, modern Camaros and Challengers with all kinds of modern tricks are barely getting that, with much better aero than your car will ever have.

At the end of the day, focusing on mileage is a fools errand. You'll spend far more on parts than you'll ever recoup from lower fuel costs.

So start by just getting what you have running properly. Get it driving well. Then, if you want better mileage, overdrive will make far more impact on fuel economy than fuel injection. 15:1 cruise AFR is 15:1 whether a carburetor does it or an injector does it, but 2000 RPM cruise speed will use less fuel than 3000 RPM every time. Won't make a difference in stop and go traffic though, which you'll again notice the modern muscle cars are barely mid-teens in that kind of traffic. As I said in your other post, at 2500 miles per year, it'll take you 10 years with California fuel prices to recoup the cost of an overdrive transmission.

[SPLATT71MC]

Best way to improve mpg in an older car is adding an overdrive like a Gear Vendors or swapping to a transmission with an overdrive along with moderate rear axle gearing.

[vcho455]

Spending larger sums of money in search of gas miles savings is a fools errand.

For example if your getting 12 MPG at present and you spend $2500 on upgrades and the results achieve 18 MPG. At $3.75 a gallon for gas you will need to drive 24000 miles before you have paid off the $2500 investment.

Doing a complete engine tune up and blueprinting the stock distributor will be much less money and the results will be paid off much quicker. Allowing you to enjoy the car sooner without the downtime and complications that major projects entail.

[jaybee]

I'm going to suggest a different route...learn to optimize your combo as well as possible. There still may be some money to spend, but you've said budgets matter and you might find the learning process interesting.

First, get your ignition timing right, as others said. That may include an ignition advance recurve kit, basically springs and bushings to let the timing come in faster.

Second, check your fuel pressure. You want no more than 6psi with a carburetor or it will upset the internal workings of the carburetor and create a rich mixture.

Third, tune your carburetor.

How? Here are some links from a YouTuber called ThunderHead289. Yes, he has a product to sell, but we'll get to that. He has a huge amount to teach. Here's his series on setting ignition timing: https://www.youtube.com/playlist?list=PLZTXQG9e2tNzjrB0YfvCvU2GdixqhNZ90

Here's his series on carburetors. You'll know everything you need to about the different circuits in the carburetor, what they do, and how to tune them. https://www.youtube.com/playlist?list=PLZTXQG9e2tNyzTb6OYTQOTndaw1v5icra

Here's the thing about out of the box carburetors; They're tuned from the factory to be "safe." That means rich. That means less power and less fuel economy than they would have if they provided an optimal air/fuel ratio...but at least they don't go lean and burn up your engine. You can do better with a good tune.

Now, as for potential expenditure. ThunderHead289 and a partner have built a device called the Carb Cheater. It does 2 things. First, it gives you a full suite of diagnostic gauges for tuning, with data logging, easy installation, and no need to drill your firewall. It gives you engine RPM, air/fuel ratio, and manifold vacuum. You should add fuel pressure gauge. https://thecarbcheater.com/

You will tune your carburetor to be as close to ideal AFR as possible, moving up through the idle, transition, main, and power enrichment circuits, but never go lean. You can do all of this without buying a Carb Cheater, but if you add up the price of the various components including wideband O2 sensor, you may find the price pretty reasonable.

Once you have that all sorted, the Carb Cheater you can switch on auto-tune assist. At that point, the carb spacer component of the device becomes a controlled vacuum leak. It will continually provide just enough air to lean out the mixture to ideal. This isn't a substitute for tuning. It's a tuning aid to help get that 350 running at its absolute peak on the carb, coupled with a device to help it stay dialed in despite variance in temperature and barometric pressure...including the effects of altitude.

[53merc]

There's a lot to unpack here.

First, is there a reason to rebuild the engine besides lousy mileage? As someone else said, if the long block is good, it would be cheaper to put heads/intake/headers on and wake it up. If the compression check comes out good, then I wouldn't worry about rebuilding until there is an issue.

Second, when/if the engine is confirmed good, tune up the engine. As others said, verify timing, good plugs/wires/clear fuel filter/all the other dumb maintenance stuff. This might be a time to upgrade to HEI for reliability, if not already done. The second part of this is to rebuild the carb, and I would recommend the quadrajet. They are capable of getting very good power with the small primaries. When you rebuild it I would recommend getting a rebuild kit that is designed to work with the new ethanol fuel, this will give you better seals and help with reliability. This will also be the time to dial the carb in.

Third, what rear gear do you have? An easier way to ask this is, how many RPM's are you turning at 60MPH? With the mileage that you are getting I am guessing that you have a fairly deep gear, 3.73 or 4.10. With the Pontiac engine you have a lot of torque, and you are going to want to get the RPM's lower. On the flip side, you don't want to be turning 1000 RPM at 60 because your engine will be struggling. I would think that 1900-2300 would be a good cruising RPM (please any Pontiac aficionados correct me). Spicer has a good gear ratio/transmission ratio/speed calculator that I use. The other part to this is that the TH400 is a very strong and heavy duty transmission, but might be a little much for a Pontiac 350. The TH400 has a lot of parasitic loss, because it is heavy duty it takes a lot of power to turn. I would think that a 700r4 would be the ideal overdrive transmission. As stated earlier, the 200r4 is an option, but in my opinion that might be a little too light duty. The 200r4 came behind mostly v6's and 305/307s, and I would think if started modifying the engine you would quickly find its limit. The 700r4 is a beefed up version of the 200r4, and I would think it fits this build more.

Now you did talk about adding an overdrive and if you would have to do a rear gear. This depends on the rear gear ratio. If you have a 3.42, you may be able to get away with keeping it for awhile with an overdrive trans, because the Pontiac is a torquier engine. If you have a 3.23 or higher you are going to want to swap it to a lower gear, like a 3.73 or 4.10. But this will all need to line up with the transmission and tire size you have, and possibly the engine if you rebuild/swap it. Swapping rear ends might be a better/cheaper way to go if you do need a different ratio. Something like an S10 blazer rear end would give you 3.42/3.73/4.10, disk brakes, and a posi.

[JLMounce]

Others have already touched on some good points.

That said, your engine should be capable of producing an average 15mpg as it sits. It likely needs a good tune-up.

You stated that your engine was originally a 2bbl. This would make your engine a 265hp model with a 254 suffix cam which is a 269/277 advertised duration on a 113* LSA. The engine should have #47 casting heads with an advertised compression ratio of 9.2:1. Actual compression ratio is probably closer to 8.5:1. You can visually inspect to see if the heads are correct by looking for a 4 and 7 embossed on the center exhaust ports.

If all you did was put the 4 bbl intake and QJet from a 4bbl 350 on your engine, it's not going to run incredibly efficiently. The carb is setup for an engine with likely a full point of compression more than yours and either an 067 or 068 cam (067 for auto, 068 for manual) that is 5-10 degrees larger in duration.

Has the carburetor been rebuilt at all? Given it's age and the fact that it came off a donor car, it very well could be in need of a rebuild. A common issue for QJet's is that the well plugs begin to leak. If this is occurring, your mileage goes out the door and it may also be contributing to the amount of smell you're dealing with in the car.

If you haven't pulled the PHS documents on your car, it's worthwhile to do so. It'll tell you how the car was optioned and delivered and a good bet to what equipment may still be in it. My guess is that with the 350 auto trans and 2bbl 350 engine, that you've got a high 2 series open differential in the rear. Maybe a 2.78. This gear isn't great for performance, but it should be nice for cruising on the highway and given a good tune up on the engine, should deliver in excess of the 15mpg you're after.

That's where my money would be on this. Forget about an LS swap, forget about rebuilding the engine unless it has very poor compression and lots of cylinder leakage. Set out to do a good basic tune-up and get the carb tuned for your specific engine. New cap, rotor, points, coil, spark plugs, plug wires and a carb rebuild/tune. Good chance this gets the car running much better than it may be now and achieves your mileage goal.