Bristol's Chrysler Engines

[legavroche]

Kevin

Would you please expand on the last point you made about the 383 engines, I take it that if you owned one of these you would have considered rebuilding it rather that replacing it with a more modern unit. I have a 411 /S1 with the 383.

Gavin

[Kevin H]

Originally Posted by legavroche

Kevin

Would you please expand on the last point you made about the 383 engines, I take it that if you owned one of these you would have considered rebuilding it rather that replacing it with a more modern unit. I have a 411 /S1 with the 383.

Gavin

Gavin,

I suppose it also depends on what you want to do with it. If you intend to do a basic rebuild/recondition without any improvement (increase in performance) then it wouldn't make much difference. But if you did want to do some tuning then it is more worthwhile on a pre 1971 383.

The main issue is the cylinder heads. The 400 CID engines used by Bristol have a head designed to lower the compression ratio and comply with the pollution controls introduced in the US in the early 1970s. They have a different intake port design so they don't flow anywhere near as well as the early 383 heads, so there's not much point in increasing the valve sizes and doing any porting.

You can of course buy aftermarket heads for the 400 CID engine, but they're not cheap!

I now have a pair of early 383 heads on my 400 CID engine (with larger valves and mild porting). The casting number of the pre smog 383 heads is 2843906, (sometimes referred to as '906 core' heads).

The other benefit of a pre 1971 383 is it will have a forged steel crank shaft, whereas the 400 CID engine has an inferior (cheaper) cast iron crank. This is less of an issue than the heads, although I don't how much you can do with the cast iron crank in terms of welding, machining and polishing.

I was persuaded to "stroke" my engine, so we put a 440 forged steel crank in it, which was "worked" a fair bit to streamline the counterweights.

That said, if you want multi point fuel injection and sophisticated engine management, then it's probably cheaper and certainly a lot less hassle to simply buy a modern replacement engine, regardless of whether you have a 383 or a 400 engine.

[Kevin H]

I came across an article I wrote back in 2001 for the old BEEF mail list, thought it might be of interest to anyone considering a carby change or rebuilding an engine.

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There must be many a V8 Bristol out there with a tired or non original carburetor, so for those who are thinking about fitting a new carburetor I thought I might share a few facts I which learned recently about carburetor selection, and possibly explode a few myths in the process. This is a fairly simplistic view but it should at least point people in the right direction!

Carburetors are rated according to the airflow capacity, that is the volume of air which they can supply or 'flow' in one minute, this is measured in Cubic Feet per Minute or 'CFM'. The air of course combines with a measured quantity of fuel on the way through, which ideally will be vaporized by the time it gets into your engine's cylinders. There is a 'rule of thumb' formula which can be used to match carburetor size or capacity to engine size;

engine capacity (CID), multiplied by maximum engine speed (RPM), divided by 3456 = Carburetor CFM

CID x RPM
__________ = max CFM
3456

For example for a Bristol 411 series 4 or 5 the formula would be (400 x 5500) / 3456 = 636 CFM

However this formula assumes 100% volumetric efficiency (VE), which means the 400 CID engine actually consumes 400 cubic inches of air/fuel mixture for every two revolutions. Which in the case of a normal 400 CID Chrysler engine produced in the 1970's, it doesn't. In fact of the Chrysler V8's used by Bristol, the 400 CID engine, fitted to the 411 S4 & S5 and 412 S1, probably has the lowest VE because of the anti pollution, or 'smog' measures in force in the USA at that time. It is effectively 'de tuned' and probably has a maximum VE of about 70%. All other V8's used by Bristol probably have a maximum VE in the region of 80% to 85%. An engine's VE varies with engine speed and is highest when peak torque is produced.

So taking into account the VE of the 400 CID engine fitted to Bristols, it really only NEEDS a carburetor which flows about 445 CFM.

CFM ratings are only a guide and it is possible for a four barrel carburetor to flow at least 20% more air than it is rated. At the other end of the scale they can of course flow a lot less, in fact the secondary throttles on some four barrel carburetors are activated by manifold vacuum controlled air valves (commonly known as 'Air Valve Secondaries'). If these are working properly the carburetor will only flow the amount of air required by
the engine. (Manifold vacuum varies with engine speed and load)

However, most carburetors are fitted with jets, metering rods, etc, which provide a fuel supply in keeping with their air flow rating. So the greater the CFM rating the bigger the standard jets will be and vice versa, (to allow the correct maximum air/fuel mixture). Jets are of course changeable (as are often metering rods ) so there's nothing stopping you from re-jetting a carb that is slightly too small or too large for your engine to make it more suitable. There are however, other specifications which may need to be modified to achieve optimum performance from a wrong sized carb. Including but not limited to; metering rods, air valve adjustment, inlet 'needle and seat' size, air bleed, float level, float drop, accelerator pump adjustment, enrichment circuit timing. Most of these modifications should not be attempted by anyone other than an experienced specialist.

When a carburetor is significantly larger or smaller than the engine requires, these issues are exacerbated because the manifold vacuum becomes too high or too low. Larger carbs, for example, have larger throttle bores and venturi area, which means the manifold vacuum will not reach anywhere near that which the carb is rated at WOT wide open throttle (1.5-inch/Hg), unless the engine is highly tuned to a greater volumetric efficiency and probably higher maximum engine speed. Generally speaking any significant variance in manifold vacuum from the standard at which a carb is rated will make carburetor tuning very difficult.

Many people have been, and probably still are, led astray by the carburetors fitted as standard by American muscle car manufacturers in the 1960's. According to the formula given above, it would seem they fitted oversize carburetors to some particular production model Corvettes, Camaros and Mustangs. Well they did! But that's because these cars were frequently entered in sanctioned competition events where the standard equipment carburetor had to be retained. So when competing they tuned the engine 'up to' the capacity of the carb.

The moral of this story is, if you need a new carburetor for your Bristol, unless the engine has been seriously modified for performance, use the formula above to correctly size your carburetor and it will likely work very well 'straight out of the box' with very little alteration.

NB: Carter, and most other manufacturers of 4 barrel carbs, made different models for different engines (Chevrolet, Pontiac, Ford, Chrysler etc). Make sure you get the correct one for your engine or the throttle linkages won't be correct, the choke actuation will likely be incorrect, and it probably won't fit your intake manifold without an adapter plate!

Kevin Howard
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[jimfoz]

I remember seeing a 410 being auctioned at Sothebys in the late 80's or early 90's with a 440. It was well maintained and well used and had a estimate of £10,000. It was also on a set of minilites. It was described as the 'perhaps the fastest road going Bristol' (faster than the turbocharged models?). Haven't seen it in the BOC listings and wonder if it is still on the road???

[Kevin H]

Originally Posted by jimfoz

I remember seeing a 410 being auctioned at Sothebys in the late 80's or early 90's with a 440. It was well maintained and well used and had a estimate of £10,000. It was also on a set of minilites. It was described as the 'perhaps the fastest road going Bristol' (faster than the turbocharged models?). Haven't seen it in the BOC listings and wonder if it is still on the road???

Interesting. Do you remember if it had a modified bonnet?
The 440 was/is an "RB" engine, which some say stands for "Raised Block". Which means it's a bit taller than the 363 and 400 engines used in the 411 and 412. The air cleaner on my 411 with a 400 engine is a pretty snug fit under the bonnet, so much so that it leaves an impression in the under bonnet sound insulation. So unless the 410 had a bit more room it wouldn't be possible to fit a 440 in there with a standard air cleaner.

I was going to say I doubt very much if it was faster than the turbocharged Bristols. For some reason Bristol didn't release power figures in that era, but a Jensen SIII with a 440 engine and would do 0-60 mph in 7.7 seconds. The turbocharged Beaufighter weighed a 1 cwt more than the Jensen, both had 15" wheels and a 3.07 rear axle ratio, but the Beaufighter did 0-60 in 6.7 seconds!

BUT, the 410 was 3.5 cwt (or 10%) lighter than the Beaufighter. Would that 392 lbs (Imp.) make the difference of more than 1 second 0-60 time?

It would be a very close thing!

[jimfoz]

I don't think it did. I say it was a 440, but it was described as a 7.2 which is probably the same thing? Not sure if it was merely an overbored 5.2. I notice Bristol offer an enlarged 7.2 upgrade for the older pre 603 V8's

I will try and retrieve the original catalogue and scan in the picture of the car. It was a well used example so I imagine if the owner has not done much to it since it will be looking very scruffy!

[Sam410]

I seem to recall reading in an old Bulletin about a 412 with a 440 fitted. Does anyone know anything about that? IIRC it had a louvred bonnet to keep it cool.

I have definitely read that Bristol did experiment with using the 440, but decided turbocharging was a better performance / economy compromise.

[Kevin H]

Originally Posted by jimfoz

I don't think it did. I say it was a 440, but it was described as a 7.2 which is probably the same thing? Not sure if it was merely an overbored 5.2. I notice Bristol offer an enlarged 7.2 upgrade for the older pre 603 V8's

440 cubic inch is 7.2 litres, but those small block engines used in the 603 are a thinwall casting and I believe the maximum they can be bored out to is about 365 cid (if my sums are correct). You might be able to get a bit more by 'stroking' it but it wouldn't get anywhere near 440 cid.

However, uprated 440 crate engines are readily available from the US for about USD10k and given that Bristol altered the chassis to accommodate the taller turbocharged engine in the Brigand I'm sure it's not a big deal for them to do the same for any of the earlier V8 cars.