can you get better fuel economy out of a 350 V8

Submitted: Monday, Oct 01, 2018 at 15:51
ThreadID: 137306 Views:1711 Replies:10 FollowUps:6
I am looking to purchase a motorhome with a 350 V8 in it.If I have gas fitted to it or change diff or change 4 barrel carby or alter main jet will I improve the economy by much?I do not need the power it has.The motorhome section is great,its just bad luck it has a motor of this size.
Thanks all
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Reply By: swampy - Monday, Oct 01, 2018 at 16:14

Monday, Oct 01, 2018 at 16:14
Yes u need a motor of that size for the torque not the HP . A similar diesel would have been far far better .

Have u confirmed its a 350 not a 305??

Whats your current fuel consumption mpg or L/100km??
Depending on a lot of factors 10-12mpg ??

A good tune up compression test would be a start .
Is it carby or efi ????
The TBI can look like a carby
A carb will be cheaper to tune .
There are performance //economy options for throttle body injection as well.

A small motor does not last in a large weighted m/home .A small engine generally has tobe flogged to move .
AnswerID: 621438

Reply By: Peter_n_Margaret - Monday, Oct 01, 2018 at 18:08

Monday, Oct 01, 2018 at 18:08
We used to have a slide-on on a 1987 F350 4WD. It had a 351 Cleveland on gas and it DRANK fuel (typically 40L/100km). Plenty of power and great to drive though.
Messing with it is unlikely to save you money.
Changed to the OKA which is the same size, 1.5T heavier (at 6T) and 4L diesel and the fuel cost about halved (17-18L/100km).
OKA196 motorhome
AnswerID: 621439

Reply By: splits - Monday, Oct 01, 2018 at 20:08

Monday, Oct 01, 2018 at 20:08
"If I have gas fitted to it or change diff or change 4 barrel carby or alter main jet will I improve the economy by much?"

Most likely no. In order to get the best out of gas on an old carburettor engine, you need to run it on straight gas and do a lot more than just install a gas kit.

Gas burns slower than petrol so it should be ignited earlier. The distributor needs to be modified so it has around 10 degrees more advance at idle and a steeper but shorter advance curve so it does not over advance the ignition at high speeds.

The valve timing should be altered. Websites for high performance camshafts usually list a variety of petrol cams and a gas cam.

It has a higher octane rating than petrol so it can benefit from a higher compression ratio without resulting in detonation.

Exhaust valves are often burnt by by the additional heat caused by gas and need either new valves or harder seat inserts.

The additional heat often calls for alterations to the cooling system.

If you run it on dual fuel and set it up for gas, it will ping its head off when you switch to petrol. Pinging is detonation which is an instant mass ignition of the fuel/air mixture resulting in the pistons and cylinder walls being hit with an impact like a hammer blow. That will soon break the pistons if it is allowed to continue.

The petrol/air mixture is supposed to explode, not detonate. An explosion is a rapid but controlled burning that builds up pressure and applies a continuously increasing pressure to the piston. As an example a rifle cartridge explodes and pushes the bullet. A bomb detonates and instantly shatters everything around it.

The other issue with gas is its cost difference between cities and the bush. I used to work in the motor industry but completed a two week full time gas installation course at Ultimo TAFE in Sydney after I left it. I was commuting around 100 ks one way to and from Sydney each day to work and decided to convert my cars to gas. I did not do any of the major modifications and decided to see what happened while using the petrol specifications.

One car, a Peugeot 504, needed harder exhaust valve seats after 20,000. The other was a Gemini and it overheated. A new triple core radiator and different thermostat made no difference at all. I removed the radiator and made a fibreglass fan shroud that fitted right around it like a glove. That fixed it immediately.

The price of gas was almost always slightly under half the price of petrol in Sydney and I was saving about 45%. That dropped to as low as 10% in many country areas due to higher gas prices. Had I been living in those places I would not have used gas.

That is a problem you are going to face when touring the country
AnswerID: 621440

Follow Up By: RMD - Monday, Oct 01, 2018 at 22:09

Monday, Oct 01, 2018 at 22:09
I agree with splits.
After purchase is too late to try and alter it all if it has a drinking problem. Best to know beforehand.
With the V8, you can't simply reduce the main jet as that makes it run leaner and less power and torque. So self defeating there.
If altering the settings/tune, the timing is important with both petrol and gas but as mentioned they require differences which can't easily be accomodated to get the best out of both.
The torque may be the thing, but unless you carefully drive it in the "zone" or have a diff or tyre sizes which sees it just into the area of reasonable torque, ie, low in the torque curve while operating, the economy will be low. Not good anyway.

If gas, the radiator airflow through the core is essential and just adding rows of tubes doesn't fix basic problems or flow through. Splits mentioned that too.

Ensuring more flow through is the only thing which removes heat, especially for gas developed heat.
Often a single row of suitable area and water flow capacity cools better than the promoted multi row ones.
FollowupID: 893955

Reply By: Notso - Monday, Oct 01, 2018 at 23:14

Monday, Oct 01, 2018 at 23:14
You can't make a silk purse out of a sows ear mate.

They are thirsty and love burning valves.

As most point out, get a diesel!
AnswerID: 621443

Reply By: Ron N - Monday, Oct 01, 2018 at 23:21

Monday, Oct 01, 2018 at 23:21
Stuart - Rip the 350 V8 out and fit a little Jap diesel.

You'll pay out a bit initially, for the engine swap, but over several years you'll end up in front - and over a decade you'll be laughing.

350 V8's are built for America, where petrol has always been so dirt cheap, 7 or 8 mpg didn't bother them.

I owned a short wheelbase 4WD F100 once, it had a 302 V8 in it. I bought the 302 rather than the 351, because I thought it would be much more economical.
That 302 never went over 13 mpg, and often it was around 10-11 mpg when loaded or hooking along at 110kmh. All V8's are simply fuel guzzlers.

The only thing you can do, if you don't want to spend the $$$'s on a diesel engine swap, is fit an overdrive gearbox or a higher ratio rear axle.

I bought a new HQ Holden sedan with a 308 V8 and 4 speed and it had a low ratio rear axle - 3.36:1, which meant the 308 was doing 3000RPM at 110kmh (on 185R14 tyres/rims).

I swapped the rear axle to a 2.78:1 ratio (out of a V8 automatic) - and the engine revs dropped back to 2500RPM at 110kmh.

That old girl developed long legs, right after that rear axle transplant, and the old 308 would often give me 25mpg at 110kmh - compared to the 16-18mpg I got with the lower-ratio rear axle.

With 240HP on tap from the 308, it didn't need to be humming at 3000RPM at highway speed. Your 350 V8 is in the same boat.

You might be able to source an overdrive gearbox in place of a rear-axle ratio swap, it would achieve the same result - but you want an overdrive that gives at least 15% engine RPM reduction.

Cheers, Ron.
AnswerID: 621444

Reply By: Rangiephil - Tuesday, Oct 02, 2018 at 15:29

Tuesday, Oct 02, 2018 at 15:29
It sounds like an old USA built motorhome.

If you were to change to a diesel a Chev 350 diesel would probably fit the bill.
However this would probably cost $20K by the time it is installed etc.

Has it got and old 3speed auto with no TC lockup? If so a later 4speed with TC lockup would be much better for economy.

IMHO, In your position I would ask myself do I want to buy this one and spend bulk money getting it to where I want it, or spend 20K more and get a later model European motorhome with a diesel and modern transmission.

In any case you can do little to improve the economy of a Chev 350. You may get 10% if you do things like fit fuel injection and electronic advance but that is probably all. BTW if you were to go for gas there are modules available that can give different advance curves for gas and petrol. One is Unichip.

AnswerID: 621446

Follow Up By: Greg J1 - Tuesday, Oct 02, 2018 at 18:17

Tuesday, Oct 02, 2018 at 18:17
$20 000 will buy a lot of petrol.

The old spend a ship load of money to save 20 cents.

How many times have you seen this in your life !!!

Just saying

Cheers Greg
FollowupID: 893964

Reply By: Ken - Tuesday, Oct 02, 2018 at 16:30

Tuesday, Oct 02, 2018 at 16:30
Sounds like the perfect job for a Hiclone !! Couple it with magnets on the fuel line to align the fuel molecules and watch the diffence. LOL
AnswerID: 621448

Follow Up By: Frank P (NSW) - Tuesday, Oct 02, 2018 at 17:28

Tuesday, Oct 02, 2018 at 17:28
Add an Energy Polarizer for more bang for buck :-)
FollowupID: 893963

Follow Up By: Alloy c/t - Wednesday, Oct 03, 2018 at 09:37

Wednesday, Oct 03, 2018 at 09:37
Add 4 or 5 and the bus will get 200ks per Litre easy ...[ how it is still legal for the 'shonks' to continue to advertise on TV ? ]
FollowupID: 893968

Follow Up By: Allan B (Sunshine Coast) - Saturday, Oct 06, 2018 at 21:18

Saturday, Oct 06, 2018 at 21:18

Oh, and pop a couple of aspirin tablets into the fuel tank for good measure.


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FollowupID: 893980

Reply By: Jackolux - Tuesday, Oct 02, 2018 at 18:25

Tuesday, Oct 02, 2018 at 18:25
If it's in great nick and cheap as chips compared to something else that's a lot younger , you might be a long way in front just paying for petrol .
How many K's a year and for how many years do you plan having it .
AnswerID: 621449

Reply By: IvanTheTerrible - Tuesday, Oct 02, 2018 at 20:29

Tuesday, Oct 02, 2018 at 20:29
3.9l Cummins 4bt are great engines and have adaptor kits available to fit to a wide variety of drivetrains.
Or get a Cummins ISF 2.8 or 3.8l. You can by them as a crate engine that is complete with everything including starter and alternator.
Cummins brochure

A mate also pointed out you can get them fitted with an air compressor and you can also fit a jake brake
AnswerID: 621451

Follow Up By: IvanTheTerrible - Tuesday, Oct 02, 2018 at 20:40

Tuesday, Oct 02, 2018 at 20:40
FollowupID: 893965

Reply By: Member - Trouper (NSW) - Saturday, Oct 06, 2018 at 18:37

Saturday, Oct 06, 2018 at 18:37
Stuart, I was in a similar position but in my case we are talking a 31foot mariner boat it had 2 x 5litre petrols .. It was $12000 cheaper than the comparable diesel.
Others talked me into the petrol Mariner saying as others previously "you can by a lot of petrol for $12k. Biggest mistake ever...... refuelling it was expensive constantly sticking your hand in your pocket.
Mate, don't buy it, you'll regret it. then try and sell it
AnswerID: 621465

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