360 fuel consumption - considering reverting to stock cam?

[64 Sportfury]

I will share my situation for a bit of reference. I have a 68 Coronet Station Wagon that weighs about 4,000 lbs. It had a 318 with a very mild hyd cam, 4 bbl carb, 2.76 gear and a 904 transmission. I got 17 mpg on when I drove from NY to Florida (approx. 1,000 miles). This was with 27" tires. I swapped in a 390 (318 stroker), added EFI and 28.5" tires with the same 2.76 rear gear. I now get 12 - 13 mpg Highway. The 390 has a mild hyd roller cam. I imagine she weighs about the same as I went from steel wheels to aluminum during the engine swap. I was surprised at the difference but cubic inches burn fuel. My next move is an A500 overdrive transmission and a Gen III Hemi and 3.23 gears. I will be happy if I get high teens but that will be with premium fuel. So Its a push as far as fuel costs. But she'll have about 500 hp with the Hemi. Do you know your vehicle weight?

 

[Cheapsunglasses]

My 78 Lil Red with a factory cam 360 gets 10mpg, is that better fuel economy then what you’re getting?

(Actually it’s really a 340 cam in it, but its a factory 340 cam, that the factory put in)

 

[Sweet5ltr]

As I have posted in previous threads, I get pretty poor fuel consumption and a Muscle Car sounding idle. The idle would be great on the right car but not really suited to mine. My Bristol 408 had its engine replaced with one rebuilt in 1991 that I have the following info. for. I know it used .030" oversize pistons so it must now have slightly more than 360 Cu In. The transmission is a 1962 push-button 727 and the final drive is 3.31:1.

1978 Monaco 360 engine.​

Mopar Performance Cam 4452761.​

Mopar Performance Intake Manifold P4529116.​

Piston rings 4.030"​

Pistons unknown​

Springs 998?​

I am attaching a pic the specs I can find comparing the cam that is fitted and the stock 1978 Monaco 360 cam.

I think swapping my cam for a stock one is just about within my abilities but, before I try it, I expect a stock cam would reduce the idle noise and loping but would it also help fuel consumption to any significant degree?

I also wonder if any other type of carburetor (it has a relatively new Summit Racing Carb SUM-M08600VS. 600cfm) would help? Any advice or opinions would be appreciated.

David

View attachment 1358088

View attachment 1358089

Well, there is something definitely off with the combination (which to me, isn't the camshaft), as my 318 LA engine in the W200 gets on average around 10 MPG with standard 87-octane. It's built like a brick, and is full-time 4WD with 33" tires. It's also tuned near perfectly in regards to air-to-fuel ratio at idle & cruise.

As many have already reported, first step would be to have an 02-bung welded in the exhaust pipe (12 - 18" after the collector) and tune the carburetor to the best of your ability using a Wideband 02 gauge.

 

[Geoff 2]

Compression ratio is a big player with fuel economy.
8:1 CR is not going to give good economy with any cam...or carb.
20* initial timing could be a clue: it could be that a sharp engine tuner built that engine with 10:1 pistons........or it was built with 8:1 pistons by a dope who had to bump up the timing for any semblance of idle quality.

The bottom line: very hard to give advice on an engine built years ago with no real info on the build. While you can change cams, carb etc, these might only yield very minor improvements & might not be worth the trouble if there is a basic internal problem with the engine.

 

[BSB67]

Usually around 8mpg. Never above 10mpg. I'd like 15 but 12 would be OK.

David

if you are getting only 8 mpg, something is wrong, and its not because of the cam

 

[David Womby]

Take a look around the Mopar si.te for an old Edelbrock sp2p intake manifold and a holly 4175 650 CFM spread bore which will be much easier to tune for you then a quadrajet. Thermal quad parts are really difficult to find and are about as hard to tune as a quantity. The sp2p and 41 75 will give you about as much and increase in mileage as you can expect from any bolt on I really don't think swapping cams would be worth all the effort. Put your time and effort into your top end . Much easier and a very noticeable mileage increase.

Yeah, it seems off. I would be scared to change cams with parts available today. Lots and lots of lifter problems we keep hearing about. Myself I wouldn't downgrade to that 2bbl, stock cam anyway. 360's aren't natural gashogs IMO, I think something is wrong, vacuum leak or something. A 3.31 with a 27 inch tire should work quite well with your combination IMO.
I've gotten better than 8MPG with a street Hemi.

Thanks, both. I am beginning to get the message that something other than the cam is the main source of the poor economy

I will share my situation for a bit of reference. I have a 68 Coronet Station Wagon that weighs about 4,000 lbs. It had a 318 with a very mild hyd cam, 4 bbl carb, 2.76 gear and a 904 transmission. I got 17 mpg on when I drove from NY to Florida (approx. 1,000 miles). This was with 27" tires. I swapped in a 390 (318 stroker), added EFI and 28.5" tires with the same 2.76 rear gear. I now get 12 - 13 mpg Highway. The 390 has a mild hyd roller cam. I imagine she weighs about the same as I went from steel wheels to aluminum during the engine swap. I was surprised at the difference but cubic inches burn fuel. My next move is an A500 overdrive transmission and a Gen III Hemi and 3.23 gears. I will be happy if I get high teens but that will be with premium fuel. So Its a push as far as fuel costs. But she'll have about 500 hp with the Hemi. Do you know your vehicle weight?

Unladen 3500lbs

Well, there is something definitely off with the combination (which to me, isn't the camshaft), as my 318 LA engine in the W200 gets on average around 10 MPG with standard 87-octane. It's built like a brick, and is full-time 4WD with 33" tires. It's also tuned near perfectly in regards to air-to-fuel ratio at idle & cruise.

As many have already reported, first step would be to have an 02-bung welded in the exhaust pipe (12 - 18" after the collector) and tune the carburetor to the best of your ability using a Wideband 02 gauge.

Compression ratio is a big player with fuel economy.
8:1 CR is not going to give good economy with any cam...or carb.
20* initial timing could be a clue: it could be that a sharp engine tuner built that engine with 10:1 pistons........or it was built with 8:1 pistons by a dope who had to bump up the timing for any semblance of idle quality.

The bottom line: very hard to give advice on an engine built years ago with no real info on the build. While you can change cams, carb etc, these might only yield very minor improvements & might not be worth the trouble if there is a basic internal problem with the engine.

I am definitely hampered by lack of knowledge of exactly what else the builder did to this engine. It seems the 78 360 was 8.4:1 from the factory. With the change to the cam, is it likely the builder changed the CR too? I can make out the following on the receipts "Basic block prep, Basic head prep, Cut/Polish crank, R/R pistons" so I am thinking the CR was probably left as stock????

if you are getting only 8 mpg, something is wrong, and its not because of the cam

So my overall takeaway on this is - check everything else again before blaming the cam. It'll probably be next week but I'll get back to post any findings.

Thanks, everyone.

David

 

[rumblefish360]

185R16 - 27.75" diameter

So you happen to know at what speed you travel at? The rpm being turned?

Mine has stock cast iron manifolds then dual 2.25" pipes - no crossovers, one muffler per side.

Small exhaust pipe size.

My sloppy wording. By 'idle noise' I was referring to the loping and loudness at the tailpipes.

Change mufflers for a quite softer sound. The cam is producing the rhythm.

Current carb is 600cfm Summit SUM-M08600VS.


Fitted with
Distributor Pertronix PNX-D141701
Coil Pertronix PNX-45001



I'd be happy to get 12mpg (btw, I run only non-ethanol gas)

David

Are you using a vacuum advance with the distributor?

The MP 0.050" cam specs for all of their cams are BS. This cam would probably measure 218/221 @ 0.050" based on other MP cams that I have mesured. It only has 50 degrees overlap. I would characterize this as a nice upgrade from the factory cam but still relatively mild.

This information below is accurate.
You said “Probably”
Below is definitely.

268°/272° Hydraulic: P4452761​
Intake Duration, Nominal​	268°​	Exhaust Duration, Nominal​	272°​
Intake Duration @ .050”​	228°​	Exhaust Duration @ .050”​	231°​
Intake Lobe Centerline​	108° ATDC​	Exhaust Lobe Centerline​	112° BTDC​
Lobe Separation Angle​	110°​	Overlap​	50°​
Intake Opens​	26° BTDC​	Exhaust Opens​	68° BBDC​
Intake Closes​	62° ABDC​	Exhaust Closes​	24° ATDC​
Intake Lift​	.450”​	Exhaust Lift​	.455"​

mild.

 

[rumblefish360]

I would not blame the cam. Though it’s not helping matters.
What is the total timing? Mechanical, vacuum and initial at cruise. I was running 52*. Under full throttle, it was 36* with the iron head. That was the best result I found d on my particular engine.

 

[rumblefish360]

Perhaps better luck following this?

 

[David Womby]

I would not blame the cam. Though it’s not helping matters.
What is the total timing? Mechanical, vacuum and initial at cruise. I was running 52*. Under full throttle, it was 36* with the iron head. That was the best result I found d on my particular engine.

With vac plugged the car idles well in neutral at 900rpm with20BTDC timing and at about 750rpm in drive. I have limiters set to 14degrees. The mechanical advance starts at about 1100rpm and reaches 34 (the limiters hold it there, of course) at just before 3000rpm.

I'll read the article you sent. Thanks

David

 

[rumblefish360]

That is right out of the MP Engines book.
Your curve reads good. No vacuum advance?

 

[David Womby]

That is right out of the MP Engines book.
Your curve reads good. No vacuum advance?

Yes, I am using vacuum advance on distributor.

David

 

[Don Frelier]

I agree Follow the listed tuning procedures.
You should be able to get 15 mpg with the cam that's in it.
Something is wrong.
I would also suggest:
Maybe start by verify that TDC on the balancer is correct.
Do a compression test.
Your base timing should be okay. But I would suggest a lighter spring and quicker mechanical advance.
I didn't see any information on what the vacuum advance was doing? This really helps fuel economy and low speed responsiveness.
I'm sure the engine was properly done.
But it's 30 years ago could be something as simple as the valves need lapping.

 

[padam]

a vacuum gauge is an essential tuning tool.
Two things that come to my mind:
Are vacuum secondaries open too soon? Try heavier springs.
Is the power valve open at idle?
Must be matched to idle vacuum.

 

[BSB67]

So you happen to know at what speed you travel at? The rpm being turned?

Small exhaust pipe size.

Change mufflers for a quite softer sound. The cam is producing the rhythm.

Are you using a vacuum advance with the distributor?

This information below is accurate.
You said “Probably”
Below is definitely.

268°/272° Hydraulic: P4452761​
Intake Duration, Nominal​	268°​	Exhaust Duration, Nominal​	272°​
Intake Duration @ .050”​	228°​	Exhaust Duration @ .050”​	231°​
Intake Lobe Centerline​	108° ATDC​	Exhaust Lobe Centerline​	112° BTDC​
Lobe Separation Angle​	110°​	Overlap​	50°​
Intake Opens​	26° BTDC​	Exhaust Opens​	68° BBDC​
Intake Closes​	62° ABDC​	Exhaust Closes​	24° ATDC​
Intake Lift​	.450”​	Exhaust Lift​	.455"​

If you got this from Mopar, or the Victory Library, it is incorrect.

 

[rumblefish360]

Is the power valve open at idle?
Must be matched to idle vacuum.

No no! Minimum, more than 1/2 the idle vacuum.
Normally the Holley’s come with a 6.5 PV. This is even a bit much at times.

If you got this from Mopar, or the Victory Library, it is incorrect.

OK. Where is your information from?
You did say probably.

 

[Geoff 2]

I might have missed it.......
But if the Summit carb is a brand H clone, it will not be best for economy. One that got good economy would be a one off, should be a museum piece....

 

[David Womby]

I might have missed it.......
But if the Summit carb is a brand H clone, it will not be best for economy. One that got good economy would be a one off, should be a museum piece....

I beleive the Summit is based off a Holley design but people more expert than me need to confirm.

David

 

[padam]

No no! Minimum, more than 1/2 the idle vacuum.
Normally the Holley’s come with a 6.5 PV. This is even a bit much at times.

OK. Where is your information from?
You did say probably.

What I meant to say is the proper valve for the vacuum at idle.

 

[rumblefish360]

I beleive the Summit is based off a Holley design but people more expert than me need to confirm.

David

This is correct. Holley makes this carb for Summit. The only difference besides the Summit logo is the fuel entry was angled downwards.