can you get better fuel economy out of a 350 V8

Submitted: Monday, Oct 01, 2018 at 15:51
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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 :-)

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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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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
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Follow Up By: Ron N - Sunday, Oct 21, 2018 at 17:51

Sunday, Oct 21, 2018 at 17:51
You need to be aware that if you buy a Chinese-built Cummins, nothing on them is interchangeable with American-built, or British-built Cummins.

The Chinese Cummins are a breed apart, and you can only source parts and components for them, from China.

It seems pretty obvious Cummins made sure the Chinese Cummins were different, so there could be no "tainting" of their American and British engines by a supply of Chinese parts.

This move not only protects their Western manufacturing base, as regards an inability to undercut parts prices, by the Chinese supplying Cummins parts - but it also ensures the Western-built engines - and the good Cummins engine name - isn't "cheapened" by a reduction in reliability of those Western-built engines, caused by the fitting of cheap Chinese parts.

As to a diesel repower, you can't go past a small Isuzu truck engine.
Buying a complete engine, clutch and gearbox from a wrecked 3 to 5 tonne Isuzu truck means the engine swap is simpler - and you get an overdrive gearbox to boot.

There are plenty of older Isuzu trucks being wrecked, and there's no need to worry about seemingly high kms, they run to over a million kms without overhaul, in most applications.

Cheers, Ron.
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Follow Up By: IvanTheTerrible - Thursday, Nov 01, 2018 at 21:57

Thursday, Nov 01, 2018 at 21:57
All Cummins are made in China. The American model is assembled in America from parts made in China
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Follow Up By: Ron N - Friday, Nov 02, 2018 at 00:24

Friday, Nov 02, 2018 at 00:24
"All Cummins are made in China"

You couldn't be more wrong. Cummins is a global manufacturing corporation, and has manufacturing plants in dozens of countries, worldwide.

Cummins Manufacturing locations

The ISF Cummins is a Chinese-designed engine, specifically built by the Beijing Foton Cummins Engine Co., and it is shipped worldwide.
It is also assembled in the U.S. - using Chinese-built components.

The ISF is a stand-alone Chinese Cummins that owes nothing to any other Cummins engine, apart from the fact the technology in the ISF is BASED on the B-series Cummins engines.

The Cummins ISF

The BFCEC factory

However, the American Cummins engines plants still have the vast majority of their components built in America - particularly the larger Cummins engines.

Yes, they do use SOME parts and components from China, in American-built engines, but not the majority of parts and components.

Cummins grossed US$20B last year, worldwide, and US$2B of their income comes from their Chinese operations. That's just 10% of Cummins total income.

Cummins has 5 engine factories in China, and all 5 factories are Joint Venture companies, with Chinese partners holding 50.1% of the company stock.
This is the only way the Chinese Govt will allow foreign companies to operate in China.
The 5 Chinese Cummins companies are;

Dongfeng Cummins Engine Company - "DCEC".
Chongqing Cummins Engine Company - "CCEC".
Xi'an Cummins Engine Company - "XCEC".
Beijing Foton Cummins Engine Company - "BFCEC".
Guangxi Cummins Engine Company - "GCEC".

The DCEC built the Cummins ISB, ISC and ISL, mid-range 3.9 to 9 litre engines under licence from 1996 - but primarily for the Chinese and East Asian engine market.
These models of Chinese engines have interchangeable components with Cummins engines built in other countries.

FCEC is building the Chinese-designed 10.5L and 11.8L litre ISG series Cummins in China, specifically for the Chinese and East Asian market.
This ISG engine also owes nothing to any other Cummins engine, it is a stand-alone Chinese Cummins.

Now Cummins have "refined" (read - redesigned) the Chinese-built ISG into the
Cummins X12 and X15 - which are being built in America (obviously using a number of Chinese components), and which Cummins are going to sell into the American market.

Cummins X12

2017 Engines from Cummins

What is going to hurt Cummins a bit now, is the 25% tariff imposed on Chinese-made products imported into the USA.
However, Cummins is saying that it's not a huge problem, just one to be worked around - because the largest percentage of American-built Cummins engines don't use a lot of Chinese Cummins parts.

Cummins will have to pay U.S. tariffs on items produced in their own factories in China

Cheers, Ron.

FollowupID: 894547

Follow Up By: 9900Eagle - Friday, Nov 02, 2018 at 07:31

Friday, Nov 02, 2018 at 07:31
Hope you have something to keep your teeth in your head if you install a bone rattling 4bt engine.

Cummins bottom line dollars will be impacted by the half a million engines they have had to recall in the states over failed emissions resulting from faulty cats. Great engines though now they have sorted out the problems with the early signature engines and egr problems. The old N14 was a beauty.
FollowupID: 894551

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

Reply By: howesy - Saturday, Oct 20, 2018 at 07:23

Saturday, Oct 20, 2018 at 07:23
If you already have heads suitable for unleaded with the hardened seat etc then gas is a good saving if you can get it in there cheap enough, it will take a trip around Oz just to recover enough saving to pay for the install and the mods, if you are going to keep it long term it may be an option. I didn't go straight gas because i wanted to have a back up of petrol for remote areas however what i did do was replace the cam with a gas specific cam and had the dizzy altered to run an idle advance of 18-20 maxing to 29-31 degrees. It went awesome on gas and was toey as,,,, I used to run it dry and switch to petrol until i got to a petrol station just to keep the fuel fresh,, but because I set it up for gas you had to drive with only half throttle on petrol or it would ping it's ass off but these days you can get electronic dual mapping circuits. I swear by gas in some applications and opinions vary. Hope this helps as I think that whilst a diesel option is ok its a huge expense to convert it to diesel and even at the current inflated petrol prices you're only saving 20 cents a litre and the advantage in fuel economy will still take sheep loads to recover the install cost
AnswerID: 621674

Follow Up By: garrycol - Saturday, Oct 20, 2018 at 12:46

Saturday, Oct 20, 2018 at 12:46
I dont have a 350 on gas but I do have a FC Landrover V8 truck on both gas and petrol. On gas I get about 10mpg and petrol about 12mpg in normal use and if I go easy maybe 19 l/100km (15mpg).

If you went LPG there should be plenty of second hand LPG parts available to fit fairly cheaply - you can fit tank etc but you will needed a licenced tech to connect it all up.

In normal use I am on gas so the savings despite increased fuel usage are substantial - in fact on the east coast where LPG is reasonably cheap compared to diesel, the truck is as cheap on fuel as my TDV6 Diesel is to run.

As I have a small V8 I need to rev it out in gears on both fuels so I run ignition at 10 degrees BTDC so it is advanced enough for the LPG but not too advanced for Petrol as I still run normal unleaded so I would support dual maps on the distributor so you can get the best out of both fuels.

The advantages of cheaper LPG start to change when you head out west where in some places I have found the cost of LPG to be the same as petrol but normally only 10-20c cheaper. The other issue is availability - if you head NW from Sydney the last gas is at Bourke so if you head remote you need petrol. On a trip I did through SW NSW the fuel locator apps did not tend to show places with LPG but when you get there it was available so I ran on petrol so wasting a lot of money.

Each has their own strategy as far as LPG vs Petrol usage goes - I get about 350km out of a tank of gas so always keep enough petrol in for an emergency. When heading outback where LPG might be problematic, I fill the LPG tank but run on petrol until I am within 300km of where I know I can get LPG and then switch. I always have a few hundred km reserve in petrol.

I do this because the petrol system is more complex and if there is an issue I can always switch to gas and by switching at the end of a trip or segment I still get the best possible benefits of gas.

However I agree with the others, if you have not bought this vehicle I would look for something else with a diesel - just so much less mucking around.

Good Luck

FollowupID: 894266

Follow Up By: Ron N - Sunday, Oct 21, 2018 at 10:55

Sunday, Oct 21, 2018 at 10:55
The major problem with LPG is that it's economic in places such as Victoria - but as soon as you hit the highways to the rural and more remote areas, the cost of LPG absolutely skyrockets.

Even with all the above-mentioned good tuning tricks, LPG still has a far lower energy content than petrol, and is a poor fuel for hauling heavy weights.

I ran a low-mileage 3/4 tonne E series (TK) Bedford truck with a factory 253 Holden V8 for several years, and it ran on both LPG and petrol.

I did quite a number of East-West trips with this little rig - and loaded, it was unbelievably thirsty - and once you left the cities, the LPG price rocketed.

The Bedford's fuel consumption ran around 2mpg on LPG and 4mpg on petrol, towing a 32 foot triaxle trailer. A diesel transplant would have been a far better option for the Beddy, but I didn't realise that, at the time I bought the truck (mid-1990's).

To top it all, LPG is dying as a fuel. LPG installers here in W.A. are virtually non-existent, and the companies that are building new servo's (by the dozen, I might add!), are often electing not to install LPG bowsers.

I see where even Victoria is suffering from a similar LPG impact, driven by the decline of the automotive-manufacturing industry in that State.

Where has all the LPG gone?

Cheers, Ron.
FollowupID: 894272

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