Poor mileage on 63, 327/340HP

[Peter L.]

I had my 327 completely rebuilt 7 years ago, as a 340 HP, with hydraulic lifters instead of solid, for the convenience and to eliminate the solid lifter clicking. Not sure what cam was put in, as the rebuilder (who has since lost all his records to a flood) simply put on the invoice "340 HP Cam". I get about 10 to 11 mpg, no matter how I drive it. I've had the carb checked several times by the rebuilder, and had it tested on a 350 test engine, and was told it was fine. I've triple checked the timing, vacuum advance, and a bunch of other stuff, with no problems being found. I know there are different cams for solid and hydraulic lifter engines with the same HP ratings. I'm wondering if it's possible he put a solid cam in with the hydraulic lifers, and if he did, could this cause the poor mileage. I'd like opinions as to whether or not this could be the problem, before I go tearing apart the front end to get at the cam to see what is stamped on the end of it.

[Edward J.]

Re: Poor mileage on 63, 327/340HP

Peter,
I am not sure what the fuel mileage was in 63 (13/14?)for the solid lifter cam, but can tell you most of the gear Ratios were on the higher side for shp engines, 370's or 411's were the normal with the 4 speed close ratio transmission. which relates higher RPMS when driving at highway speeds, fuel econ. was not the best, I am sure there are many things to look at to consider fuel mileage, what type of terrain is it driven on, hills mountains? radial tires, correct air pressure, just how much stop and go. Another factor is to take fuel econ, over more than a few tanks to get the average.

[Peter L.]

Re: Poor mileage on 63, 327/340HP

The rear is 3.36:1. It's a close ratio trans as far as i could determine while restoring the car. The 10 to 11 mpg is over 4 years, and aprox 5000 miles (I've got the Excel spread sheet tracking it since I put it on the road 4 years ago to prove it). Doesn;t matter if I;m ding highway at 65, or around town at 35 to 40. I rarely (maybe 3 or 4 times) get on it, to really push it, just normal driving with no hard accelrations.

[Harry S.]

Re: Poor mileage on 63, 327/340HP

Peter, I have a 250hp and a 300hp 63. On average I get 15 mpg on both. Each has a 3.36 rear.

I remember getting 6 mpg back in 66 on my 300 hp Impala SS. You 11 mpg on your 340hp car is probably normal.

Just my 2 cents.

[Richard G.]

Re: Poor mileage on 63, 327/340HP

Peter;
I have gotten better than 15 MPH (highway driving) with my 340hp and 3.55 gears.
I have never checked it around town.
My friend with a 302 in a 69 Z28 and 3;70 gets about the same mileage you do.
Car could run fine and still be running somewhat rich.
As little as you drive it likely would not be worth it but a dyno tune might identify any issues if there is one.
I think we get used to modern cars and have higher expectations than we did as kids.
I can remember watching the fuel gauge dropping in real time hot-riding around town in a 70 396 SWB C10.
This truck got 10mph when his dad drove it. My guess we got maybe 1/2 that!
I hear a 2006 Corvette gets about 4mph at 180 mph. Almost the same as the truck hotrodding around town. < LOL>
Rick.

[Frank D.]

Re: Poor mileage on 63, 327/340HP

Like Harry, I have the -- 63 250hp, Muncie, etc -- but, the car had an Eaton posi swapped into it in the mid-70s...

Two owners ago the driver was pretty fanatical about record keeping and I have a small spiral notebook in the glovebox with page after page of copious record keeping. So - if you want some real world measurements; here they are.
I'm only posting two pages (approx 8 years worth of data) but there's a ton more with similar numbers.

So I'm gonna say that 14MPG-16MPG is pretty typical with rare excursions higher to 17MPG-20MPG..

The car was typically driven 100-200 miles per month over the time span. However in December of 2004 the car was driven nearly 1,000 miles in one month so I'm assuming a highway road trip with significantly increased mileage.

As an interesting side note its enlightening to see the cost of gas rising over the 8 years in the data...

[Terry D.]

Re: Poor mileage on 63, 327/340HP

Have you done a compression test? Have the valves been set recently? Just because the carb seems to run fine does not mean it is adjusted right. Could be running rich. When idling in the driveway does it leave a black mark under exhaust? I think if he put hydraulic lifters on a solid lifter cam you would have bigger problems then gas mileage. My 340hp with 3:70 gears got about 12-14 around town and 14-16 highway. The fact that you don't get any better mileage on the highway suggests to me engine not putting out proper hp. As suggested if nothing else a dyno tune would point to problems because something is not right.

[Duke W.]

Re: Poor mileage on 63, 327/340HP

[QUOTE=Peter Loscalzo (48040);857440] I've had the carb checked several times by the rebuilder, and had it tested on a 350 test engine, and was told it was fine. I've triple checked the timing, vacuum advance, and a bunch of other stuff, with no problems being found.


What do you mean "350 test stand"? Was the engine run on a lab dyno? And what's the actual timing number? It's very difficult to set initial timing on a SHP/FI engine because the centrifugal advance starts below the speed that the engine will idle stably long enough to set it. If you set it higher than 700 a few degrees of centrifugal is added, so initial and total WOT advance is less than you think with resulting poor performance and fuel economy, so you must use the total WOT method, but put some light springs in the centrifugal so it's all in at less that the OE 4600.

What's the number on the VAC. What's idle vacuum? Does it meet spec. Does the VAC pass the Two-Inch Rule?

What about the carb? Is it the OE 3461S? Is the jetting OE or modified?

No one can even begin to approach solving your problem without data?

Duke

[Ed D.]

Re: Poor mileage on 63, 327/340HP

Pete,
I can tell you what I experienced back in the day, with different fuel than today. 1967 L79 327, 350hp purchased new, drove 50,000 miles as everyday driver, got between 14-16mpg, same motor converted to 365 solid's with headers, drove 70,000 miles as 365 motor,got 13-17mpg again drove every day and vacations, Fl to Nova Scotia. Descent gas mileage, but only 1000 miles per plug change.
Ed

[Stephen B.]

Re: Poor mileage on 63, 327/340HP

I know this is a somewhat different engine, but I was getting 11 to 12 mpg on my 70 350/350, wide ratio Muncie, 3.55 gear car until I lowered the carb jet size from the factory .076 to .069. The mileage improve 2 to 4 mph, and the rear of the car stop accumulating so much soot. Just a simple idea.

[Timothy B.]

Re: Poor mileage on 63, 327/340HP

I know this is a somewhat different engine, but I was getting 11 to 12 mpg on my 70 350/350, wide ratio Muncie, 3.55 gear car until I lowered the carb jet size from the factory .076 to .069. The mileage improve 2 to 4 mph, and the rear of the car stop accumulating so much soot. Just a simple idea.

I agree with Stephen, and on the AFB carter you can tune the main circuit by changing the metering rods so you don't have to take the carburetor apart. If you keep the diameter of the low step the same the WOT ratio will not change.

[Peter L.]

Re: Poor mileage on 63, 327/340HP

Guys,
I want to thank all who responded with ideas and thoughts on my question. To answer a few of your questons about things to check, I had the engine dyno'ed when rebuilt (approx 7 years ago), and it tested 340 and 345 HP at a max 5500 RPM (245 at 3500 rpm) on two seperate runs. The vacuum advance is a stamped 201, and I don't remember the exact values, but we checked spark advance with a timing light, and as I recall, it was within proper spec's at idle and about 3000 rpm. Fuel pressure is in the 6 to 7 PSI range. Already put leaner metering rods in, with no change.

Some of you thought the 11 to 12 MPG I get was about right, and some of you thought I should get 14 to 16. I m

ight think apout smaller jets (0.069), but there isn't any telltale black soot either on the exhaust tips or on the ground under them after idling in the driveway for several minutes, or any soot on them after a 30 to 60 minute drive. So for now, I guess I'll just live with it. Other then the low mileage, the car runs fine. I only put between 1500 and 2500 miles on it per year, so the cost is not something I can't live with. Again, thanks for the time you put into this, and your thoughts. They were very helpful.

[Duke W.]

Re: Poor mileage on 63, 327/340HP

The 201 15 VAC is likely the original part and whether it actually meets spec, which is in the '63 Corvette Shop Manual and AMA Specs, or not is somewhat irrelevant because it was an improper selection in the first place by GM. It doesn't pass the Two-Inch Rule. You need to measure idle vacuum and select a VAC that does.

Also, the OE centrifugal is lazy and the timing tab is not accurate with the big 8" balancer, so if you set it at the recommended 10, it's really only 8, and if you set it at 700 over it will be less than 8 because the centrifugal starts at 700. Bottom line is that the total WOT advance ends up well short of the optimum of 36-40, all in at 2350.

All this is explained in detail in my Tuning Seminar. Get it set up properly with a functionally correct VAC, an aggressive centrifugal with total WOT advance set as high in the 36-40 degree range as the engine will tolerate without detonation, and both performance and fuel economy will improve.

One other note is that 6-7 psi fuel pressure is probably a bit high for an AFB... 4-6 is probably better. Too high fuel pressure can overcome the float force on the needle and rise the fuel lever to the point where is spills out the venture discharge nozzles.

One thing that drives me nuts is when guys say something like "I checked XYZ and it's okay". Right it down the numbers and specify it. The other thing is that most OE engine spark advance maps are far from optimum and improved performance and fuel economy can be had by optimizing it, and I provide recommended starting points for all OE engine families and guidelines if the engine is modified.

As far as the '63 340 HP engine is concerned I figured out that the OE spark advance map was totally screwed up 54 years ago when I was a 19-year old sophomore ME undergrad. I optimized it, which solved the idle instability problem, added noticeably more low end torque, and improved fuel economy (mid teens around town, low 20s on the highway) and it's been the same ever since... and I wasn't even yet rocket scientist back then.

Get the spark advance map dialed then, if you wish, work on the fuel map. Look at the last couple of slides in the Tuning Seminar. That's the graduate course in carburetor tuning. I ended up slightly leaning the cruise mixture and slightly richening the power mixture. That helped both fuel economy an SOTP power. Around town I shifted into top gear at 30 MPH and didn't need to down shift until below 25 (about 900), and it would respond smoothly without protest. The most efficient operating points of an IC engine is low revs and relatively low load (below the fuel enrichment point), so that's how I drive in around town and I have no problem keeping up or staying ahead of traffic.

Duke

[Timothy B.]

Re: Poor mileage on 63, 327/340HP

The thing that many don't understand is that most street driving involves to a large degree the idle circuit which is where the transfer slots get the fuel. That's the problem with the AFB carburetor, in order to tune the idle circuit you need to change the booster venturi cluster or solder the idle jet hole and Redhill smaller.

If you install larger diameter metering rods you most certainly will lean the MAIN circuit when it's providing fuel.

All that being said, I like the AFB much better than a Holley except the Holley is easier to tune/modify in my experience.

[Peter L.]

Re: Poor mileage on 63, 327/340HP

Duke,
How would I get a copy of your "Tuning Seminar"?
Thanks,
Pete