My goal is to primarily gain highway fuel economy. My secondary goal is to gain some horsepower in city driving with less regard to economy but again not jeopardize the highway economy.
I have a 1965 Corvette convertible 327 /300HP Power Glide in excellent shape. The only two modifications so far is a GM-approved but aftermarket A/C unit put on the car in the early 70s and upgraded to accept the new coolant. I added an aluminum radiator to handle the stress of California Central Valley heat-A/C and city driving. The car at best gets 16 MPG which is typical.
I plan to have a 4-speed auto installed with the 4th gear as an overdrive and torque converter lock-up. The other two minor changes are an electric fan and a more efficient water pump. From what I can tell the engine as is produces sufficient torque at 1500 RPM to sustain highway speed at 70 MPH. I would like some feedback on that point. I drive I-5 and I am hoping that I can get 25 MPG+ by doing no more than this. The rear end is a standard 1:3.36.

The second question I have is I would not mind a bit more zip to the car around town and around town fuel economy is not too much of a concern as long as it is still in double digits. What I am concerned about is if the modifications I would like to have will affect steady highway cruising with a light foot.
The zippy modifications goals are keeping the fuel economy on the highway, a modest amount of lower-end torque, and horsepower at an RPM range of 3500-5000 RPM. I don't want a high revving screamer. Engine reliability and longevity are important to me.
So my thoughts are keeping the cam for now. Aluminum heads, AFR 180, or something like that, Sniper EFI with a better distributer, short headers, and an increase in size exhaust system with Magnaflow mufflers.
I am open to any comments and suggestions. I really do not know if my goals are compatible.

The last question would be if this setup would work would adding a turbocharger ruin the highway mileage? Again a light foot steady-state highways drive at 70 MPH.

 

Your thoughts of a quest for more power and better fuel economy is something you will find in late model Corvettes but not with your car without spending a lot of money. Multi speed manual transmissions and Holley Sniper induction systems are about the best you can do with your current engine. searches for these two things, they are all over this forum.

 

I would suggest only swapping in a over drive first and just drive it as is for a while before going down a bigger rabbit hole. or just buy a C5/C6 and call it a day

also
they where standard so you went back to OEM not a upgrade

 

Since you state fuel economy as your primary goal (saving $$), do you drive the miles necessary to break even with the costs of the mods?

 

You don't say the condition/milage on the engine. Maybe it's rebuilt? Accurately document your performance and economy (numbers required, not guessed at) for a baseline. Then make one change at a time, otherwise doing more than one, you will not know what each mod/change did. Doing the trans first, as previously suggested would be smart. Then maybe fuel injection. If you are having a cooling issue, (are temp reading taken with an IR gun?) you need to check the operation of the radiator (primary cooling devise highway driving) and if it's not a DeWitts, that will contribute to the cooling issue. The vacuum advance on the distributor effects cooling at idle and very low speed creeping. Test it and meke sure it operates. Let us know about the above, and maybe we can continue to help. Dennis

 

"I am hoping that I can get 25 MPG+"

Personally, I don't believe you can get to 25 MPG+ with a stock 327/300 SBC. I have a 5 speed in my '66 L79 CV, and with the cruise control (Rostra) on @ 70 MPH, the best I can do is @ 21 MPG.

Tom

 

Here's the calculation:

• Current
  • Miles per year driven / miles per gallon = gallons per year X cost per gallon = current cost per year for gas
• Future
  • Miles per year driven / estimated miles per gallon = gallons per year X estimated cost per gallon = Future cost per year for gas
• Savings per year = Current cost per year - future cost per year
• Cost for upgrades
  • Transmission purchase
  • Transmission install
  • Fan purchase and install
  • Water pump purchase and install
  • EFI purchase, install and tune
  • Total = Sum of above upgrades
• Years to break even on upgrades = Sum of cost for upgrades / Savings per year

Speaking for myself, every project I've undertaken has cost twice as much and taken three times as long as my best first estimate.

 

Yes but recouping the investment is not the primary thing. It is however a pleasant secondary thing.

 

The engine is very healthy with about 4000 miles on the rebuild. I plan on doing one change at a time, that is very good advice. Right now the engine cooling is keeping up since the radiator change. But I would like it to not be working in the near upmost range on these very hot summer days when I would like the A/C on in town.
My plan was to change the fuel injection first and then evaluate. Then the heads and exhaust and then evaluate. Thank you for the advice.

 

What the hell does "new coolant" have to do with the AC system?

I've never heard of a "GM-approved" aftermarket AC system.

You added an aluminum radiator? The OE radiator was aluminum - a very high tech piece in the 1960s and an exact reproduction is made by DeWitts. It's the only radiator that fits in the OE space that will keep the engine cool, especially in a hot climate with AC that keeps the interior comfortable without overheating the engine. So tell us more about the aliuminum radiator you installed.

... more efficient water pump? Right, tell me more.

What four speed auto are you talking about and what is the fourth gear overdrive ratio? In fact, specify all the ratios. A shorter first gear ratio than the Powerglide's 1.82:1, like the 3.06:1 of a 700R4 will certainly improve "zip" around town, as will the 200R4, but I think first gear is a little taller, but it's easier to fit than a 700R4

A well tunied small block like my 340 HP SWC with a 3.08 axle can knock down low 20s on a the highway at an average of maybe 70 MPH and mid-teens around town shifting at low revs, which is fine because the engine has plenty of low end torque.

A stock 300 HP engine has enough power to do 65-70 MPH on a level road at 1500, but the spark advance map needs to be optimized. You said the engine was rebuilt, but what are the details? Does it have an OE equivalent camshaft? What's the compression ratio? TRUE MEASURED, if you have that data. Otherwise it's just a guess. Too bad the engine is already rebuilt. Otherwise you could have rebuilt it to the "Special 300 HP" configuration.

Does it have the original 3721SB carb?

There's plenty of performance to be mined from the spark advance map. The OE map is not that great, but you need to measure what it is today. What's the ID data on the VAC and start and stop points, and what's the centrifugal curve.

Search for threads started by me, and download the tuning seminar pdf and the Special 300 HP design, analysis, and test paper. That should get you sufficient education where you can set realistic goals and start gathering data to determine the current engine state and then where you need to go to get maximum performance and fuel economy.

As far as turbocharging is concerned... you are living in Lala land.

And tell us what technical documentation you have for your car like service manuals, AIM, AMA specs, etc. Don't leave home without them.

Duke

 

You are not going to Achieve your goal with that engine,with the modifications that you’ve mentioned. The only way I can see that you can get the goal of gas mileage and performance is to install an LS swap with an over drive transmission , inwitch you will never Recoup the money you spent from the savings of gasoline.

 

Sorry, Duke, I did not want to use the word Freon and I couldn't think of the right word so I used coolant. Imprecise, I know. I was told that the original radiator was not an aluminum one and so it was replaced and indeed the engine did run about 15 degrees cooler on an IR meter.( Not sure what the meter was other than it measured the temperature externally.) The engine was rebuilt by the previous owner. I do not have the details. I can find out the particulars about the carb. I have no problem being ignorant, and so I have done as much research as I can on my own, in order to ask as many questions as I can so that I can learn more. I want to thank you for replying.

 

300HP Powerglide/AC coupe 25% in town 75% highway. 16MPG +/-
Installed 700R4. 16MPG +/-
Installed Fitech. 16MPG +/-

The drivability is MUCH improved, but gas mileage very disappointing. I'm still tweaking mixtures, but it is getting close to a LS/4L60. I wouldn't want a high HP racer but would like the maximum torque that you could reasonably use with the 205/75/15s.

I have had five C2s, they all gave about 14-16MPG except my 64 which gave an honest 20+MPG. It was a 327/365 with 3.08 gears. It was a great little car, it was three years old, I was 19, and I ran the hell out of it.

 

A lot of engines get modified in the process of being rebuilt and rarely is there any documentation. So the first thing you need to do is figure out what you have.

Block casting number and data.

Stamp pad data.

Inlet manifold and carburetor data

Manifold vacuum at 500 RPM in neutral. It should be 18-19" if it's an OE equivalent 300 HP cam.

Also measure manifold vacuum idling in Drive. A 300 HP cam should idle smooth in Drive at 450-500, but with less vacuum.

VAC ID and start and stop points.

Centrifugal spark advance curve

Whoever told you the original radiator was not aluminum is dead wrong.

Some photos with the air cleaner both on and off would also be helpful. Also photos and details of the radiator.

You need to buy a 1963 Corvette Shop Manual, 1965 Supplement, and 1965 AIM. Also download the free 1965 Corvette Vehicle information package from the GM Heritage Web site. It has lots of data and specs that will be very useful in determining what you have.

Duke

 

My wife's '62 has a 3.08 rear and a 6 speed T-56 transmission, 6th gear ratio is 0.5 to 1. At 75 mph in 6th gear the engine is turning about 1500 RPM. The engine is a Ram Jet 350 (electronic fuel injection) that I have tuned for mileage. I can average 32 mpg on strictly Interstate cruising. On JF1644OP's car with 3.36 rear and assuming 215/70R15 tires and assuming a 4 speed overdrive transmission with a 4th gear ratio of 0.74 to 1 you would be turning about 2300 RPM at 75 mph. I doubt that you are going to get anywhere near 25 mpg without at least changing the rear end and transmission.

 

Most people that chase mileage lose sight or the overall cost of it. As others have pointed out if you have to buy parts all of this has to be considered to determine what it REALLY costs you.

The bottom line here is if you are worried about your gas mileage you have bought the wrong car and lost sight of what these cars are intended for.

 

68 Hemi it 100% correct. I spent time and money on a Fitech and 700R4 for performance and drivability. Frankly, I was expecting better mileage but that didn't happen, disappointing yes, but that was not the purpose. As previously mentioned, if you want modern performance and mileage you are talking LS and electronic transmission, and that is were I will probably end up. But I can testify the fuel injection and 4 speed automatic is way better than carburetor and Powerglide.