Fuel consumption

[Thomas591]

Gentlemen!
I am interested to find out what are the realistic figures when it comes to fuel consumption on the 8 Cyl cars. Please let me have your figures.

My 411 S5 400ci engine takes uses around 8 mpg / 27l/100 km.

Regards

Thomas Willig

[Nick Challacombe]

I travelled 290 miles today back from Wales. I brimmed my tank at the start of the journey with Shell Vpower and again after 210 miles . I used 51.8 litres which Is near enough 11.38 galls which equates to 18.45 miles per gall.
My engine though is the small 313 cu in although now 318 cu in after a rebuild with 40 th over size pistons. Ports have been cleaned up, head slightly planed , crank rebalanced but not much else. Normally we achieve between 16mpg and 17mpg. Today was mainly motorway work including normal British hold ups.
Shell V power is 99 octane. Your engine will of course use more but at twice as much seems excessive. I did reject the Carter carburettor, I increased the size of the radiator, I use a Facet Red top fuel pump and radial Michelin X tyres.
I expect this does not help much but as it was today I thought I would publish result. KR’s. Nick.

[peter dowdle]

I have a 411 series 2 383 and a 412 series 1 400. The 411 had a 750 Holley and was giving 8 mpg on an interstate trip, I swapped it for a 650 edelbrock (carter) and immediately got 13.5mpg. Nothing I have done has improved that milage.

The 412 with the 400 motor has a carter I think 650 gets about 13 to 14 mpg for seriously less performance.

[AndrewA]

For what it's worth I've got the 318 with a 4 barrel on it and when car is working (!) I get 18 mpg running around locally and around 25 mpg on a run.
Previous owner fitted the later torqueflight gearbox so I've got overdrive and a lock up though. 8 mpg does seem a bit on the low side even for a big block.

[Kevin H]

Thomas,
8 mpg is low, unless you are driving the car very hard. I would expect more like 12-14 mpg from a standard 400cid engine - but that's an engine in good condition with a properly sized carb with the right jets.


Peter, you get poorer performance from your 400 cid engine compared to the 383 because the 400 has "smog" heads, which don't breathe well (by design to reduce emissions). If you put original 383 heads on the 400 you would see an instant improvement in performance. Alternatively there are aftermarket heads you could use, but they will usually be aluminium alloy so they won't look original, whereas it's almost impossible to tell between the original 400 and the 383 heads.


kevin

[dwomby]

Well, I am in the 8mpg club too it seems. That is based on only 87 miles of local driving though.

I am still getting to know this car. It does have a replacement engine - a 360 - and I know it has a 'hotter' cam fitted - but still, 8mpg is ridiculous. It seems even worse here because that 8mpg is per Imperial gallon and equates to only 6.4mpg per US gallon .

The carb is one of these https://www.summitracing.com/parts/sum-m08600vs apparently 600cfm. It may be too much or it may need tuning. There certainly seems to be a smell of fuel in the exhaust.

Anyway, the car is now on jacks, laid up for the summer while we escape to our summer home in Nova Scotia.

David

[peter dowdle]

Kevin the 400 motors do have different heads and you are right this motor with aluminium head different pistons and cam are very popular with the Hot Rod Boys. The biggest difference between the series one and two 383 motor is they are much higher in compression ( approx 10.2 to 1 ) as against the late 383 motors and the 400 which were about 8.3 to 1 . This leads to about 80 more rated horsepower.

[Claude]

Originally Posted by Thomas591

Gentlemen!
I am interested to find out what are the realistic figures when it comes to fuel consumption on the 8 Cyl cars. Please let me have your figures.

My 411 S5 400ci engine takes uses around 8 mpg / 27l/100 km.

Regards

Thomas Willig

In Bristol an Illustrated History (1990 First Edition) Oxley quotes on page 232 a range of 13-17 UK MPG (21.7 to 16.6 l/100)

and on page 282:
12 UK MPG (23 l/100) for full performance driving,
16 UK MPG (17 l/100) under 70 mph
and as much as
20 UK MPG (14 l/100) by some drivers

It's unclear if you are referring to US or UK MPG

27 l/100 = 10.47 UK MPG or 8.71 US MPG

I once visited some club members in the UK, and was in the passenger seat as my host's 411 was galloping along at insane speeds on very narrow roads. With such treatment, one would expect 27 l/100 due to lots of tromping on the accelerator after exiting each curve. Likewise if you are on the autobahn pressing the maximum speed of the car, such consumption may be expected.

But if you are driving gently so as to not spill the lady's G&T, as a Bristol gentlemen is expected to drive, then an engine tuner may need to have a look. I shifted my 411-S1 from a Carter carburettor to a Holley 670 Avenger custom tuned to my engine and it was transformative including better fuel mileage. My car came with the 383 engine, but somewhere along the line, the block was replaced with the 400 engine, but the 383 heads were retained.

It sounds like your fuel consumption is symptomatic of an improperly tuned engine or suffering from drag such as stuck brakes or a clogged exhaust pipe.

[Kevin H]

Originally Posted by Claude

In Bristol an Illustrated History (1990 First Edition) Oxley quotes on page 232 a range of 13-17 UK MPG (21.7 to 16.6 l/100)

and on page 282:
12 UK MPG (23 l/100) for full performance driving,
16 UK MPG (17 l/100) under 70 mph
and as much as
20 UK MPG (14 l/100) by some drivers

It's unclear if you are referring to US or UK MPG

I think it's safe to assume it is UK MPG.

However, Oxley is talking about the 360 engine, not the 400, which of course has a greater swept volume, and is an earlier generation of engine.
Can't imagine a 400 ever getting anywhere near 20mpg ...

[Kevin H]

Originally Posted by dwomby

Anyway, the car is now on jacks, laid up for the summer while we escape to our summer home in Nova Scotia.

David, this may be stating the obvious, but if the wheels are off the ground for a long period of time, make sure you support the front suspension, otherwise you will risk destroying the rubber rebound stops.

[Kevin H]

Originally Posted by peter dowdle

Kevin the 400 motors do have different heads and you are right this motor with aluminium head different pistons and cam are very popular with the Hot Rod Boys. The biggest difference between the series one and two 383 motor is they are much higher in compression ( approx 10.2 to 1 ) as against the late 383 motors and the 400 which were about 8.3 to 1 . This leads to about 80 more rated horsepower.

Peter,
I should have said, it's the earlier "906 core" 383 heads that are much more free flowing. However, I have read that if the 400 heads are ported to same as the 906 core 383 heads, the 400 heads will flow just the same, so you don't have to change them!

Wish I had known that about 18 years ago when I bought a pair of 906 core 383 heads.

[dwomby]

Originally Posted by Kevin H

David, this may be stating the obvious, but if the wheels are off the ground for a long period of time, make sure you support the front suspension, otherwise you will risk destroying the rubber rebound stops.

Thanks, Kevin. I didn't think of that and will ram the roll on ramps as hard as I can under the suspended front wheels. Hopefully that will stop them drooping more while the car is suspended.

David

[dwomby]

Originally Posted by Kevin H

David, this may be stating the obvious, but if the wheels are off the ground for a long period of time, make sure you support the front suspension, otherwise you will risk destroying the rubber rebound stops.

I rammed the roll-on ramps as far as I could under the front wheels. That will have to suffice but I have a question: how do others store the cars for long term? Would I have been better leaving it on its tyres but inflating them to, say, 40psi, to resist flat spots? Bear in mind, this particular car is not in winter storage. It's stored indoors at 25C to 30C for about 3 months.

Thanks

David

[David C]

My two 410s used to average about 18 to the gallon over a single tank, but most of my mileage was fairly long distance with a fair bit of motorway. If commuting across London it could drop to about 15mpg but I don't remember much worse than that. On trips to France I could do as well as 20. One had a stock cam and Carter carb. The other a slightly hotter cam and Edelbrock carb. I was enjoying my driving and not making a great effort to save fuel, but not going berserk.

[Sam410]

According to my app, my 410 has averaged 16.7 mpg (calculated tank by tank) over the last 35,000 miles. High is around 22mpg, achievable if I sit at a steady 70-80mph on the motorway for the whole tank. (I have sometimes filled up from empty three times in a day if travelling long distances.) Low was 7.7 mpg, achieved by filling up in east London one Friday afternoon and after taking well over two hours to get across town I filled up again in west London, just out of interest to see how low the mpg could go.

The overall average reflects quite a lot of short journeys, as I frequently use the car locally, but also my lead footed driving style. The engine is the 318, but was slightly overbored in a 1990s rebuild, and I think it has a slightly hotter camshaft than standard. Gearbox is standard 3-speed.

[PEU186F]

17mpg sounds about right for a 410 on the standard transmission. My experience being that it doesn’t vary much whatever the circumstances.. Be aware that the needle valves on the standard carb do wear over time resulting in overfilled float chambers and an over rich mixture - hesitation on pick up when hot in traffic is one symptom of this issue

[dwomby]

Clearly I need to investigate why my consumption is so high when I get back to the car in October.

David

[John Keighley]

My 410 currently uses around 20-22 mpg but... all that may shortly change - the engine is about to undergo a rebuild and the Carter carb will be ditched, replaced by a Holley - ultimately with a few other changes to internals, it is expected to produce considerably more power.

[David C]

Originally Posted by John Keighley

My 410 currently uses around 20-22 mpg but... all that may shortly change - the engine is about to undergo a rebuild and the Carter carb will be ditched, replaced by a Holley - ultimately with a few other changes to internals, it is expected to produce considerably more power.

Interested in the choice of Holley. Is this a straight swap? Do the linkages line up for the kickdown? Are you still using the original inlet manifold? How many cfm?

[John Keighley]

Originally Posted by David C

Interested in the choice of Holley. Is this a straight swap? Do the linkages line up for the kickdown? Are you still using the original inlet manifold? How many cfm?

David,
I just checked with Sebastian Gross who is going to do the rebuild. It's either going to be the Holley or Edelbrock. Not sure about the linkages or if the inlet manifold will be retained yet. That will depend on whether the mount point on the inlet is the same dimensions. With the original Carter, I have never achieved a successful kickdown - even with a refurbishment kit installed but other than that, it has performed well - considering its age.