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| 8 & 10 cyl Bristol cars Type 407 onwards - restoration, repair, maintenance etc |
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17-06-21, 05:06 PM
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Fuel consumption
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 
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17-06-21, 05:35 PM
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Fuel consumption V8’s
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.
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17-06-21, 10:36 PM
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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.
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18-06-21, 09:56 AM
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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.
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18-06-21, 01:29 PM
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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
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18-06-21, 06:21 PM
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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
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19-06-21, 08:12 AM
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Quote:
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.
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20-06-21, 11:55 AM
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Quote:
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 ...
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12-09-21, 09:38 AM
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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.
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12-09-21, 09:47 AM
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13-09-21, 10:16 AM
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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.
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13-09-21, 01:49 PM
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I replaced the Carter with 500cfm Edelbrock. Many moons ago but I don't remember any problems. Kickdown connection was a breeze. The main reason for change was because the Carter floats jammed on prolonged hard cornering such as motorway slip roads. The Edelbrock seems to have overcome this.
The only significant change was that the new carb top was too big for the original air filter so I went for an Edelbrock pancake. It wasn't simply a matter of making the hole bigger on the original as the bottom of the housing isn't flat. I also opted for manual choke even though I never really had a problem with the previous bimetallic setup.
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