Holley Sniper

[roybatute]

A huge torque converter and no lock up clutch is where a lot of power gets lost. The wonderful driving experience where the car hardly ever changes gear and proceeds so effortlessly is down to the torque converter but unless the torque converter can lock up you will always have the disappointing fuel economy.

[Green411]

Hi all, just a quick update on my fuel consumption and efi thoughts:
Firstly fuel - my car has averaged 13.9mpg , including a c.150 mile drive home from Warminster to near Cambridge on mostly dual-carriageway (what a wonderfully old-fashioned expression!). My intial horror at a fag-packet calculation of 10mpg came from assuming the fuel gauge was accurate, but it seems a tad on the pessimistic side. Unless it doesn't take account of the reserve?
Otherwise I absolutely luuurve the car. I adore idiosyncratic touches like the plaited leather glove-box handle, the weight of the switches, the flip-up front wings and the lovely chrome seat brackets. It's really comfortable, the ride is good now that I've got the (new) rear dampers on their softest setting, the brakes and steering are great and I've solved a lot of the (terrible) windnoise from the driver's door. I'm hoping that the rest of the windnoise (or most of the rest, in view of the age of the shape) will be solved when I get the driver's door hinges rebuilt over the winter. I haven't started on the oil leaks yet, (in a sense it's free rustproofing) so the only really annoying thing is it's reluctance to start when cold......
So I've thought long and hard about the cost/benefits of a new Carb vs an aftermarket efi, originality vs efficiency, and so on. In the end I summed it up in my mind as shotgun/carb vs rifle/efi: both are effective but the rifle is more controllable, more precise and less wasteful. With efi the fuel/air ratio is essentially commanded via sensor input, especially with a wide-band oxygen sensor covering a wider spectrum of A/F ratios. If I've understood a carb correctly the A/F is reactive - dependent on pressure drop & jet sizes. So I've pretty much decided on a Holley Sniper 550-516; and a Holley in-tank drop-in returnless fuel module with a 255lph pump and Hydramat reservoir 12-131. I hope to use the existing supply line and I won't need a return line. The engine bay will still look largely the same, especially if I leave the existing machanical fuel pump in place (without the pushrod, and with a new gasket to stop the oil leak!) so it acts as the block-off plate. Any rubber lines will be R9. So I'll have a stealthy ethanol-proof system that seems to get good reviews and is 40+ years more advanced than the worn-out Carter which is currently in place. Will order from Summit Racing and do the deed over winter. Will report back then. And yes, I will have a grown-up holding my hand in case you're worried......

[David C]

Some interesting points raised here. After much research I too came to the conclusion that a newer transmission is the best way to keep the original character of the car but to significantly improve fuel economy.
For 409 and 410 the obvious upgrade is to a later Mopar 'box with overdrive and lockup. For a few glorious years there were hydraulically controlled models that can be automatically controlled with a couple of pressure switches and a vacuum switch.

Dick Peacock converted his 410, changing to an A518 / 46RH 'box. This was a straight development of the 727 Torqueflite and as far as I know mounts straight on the back of the Poly engine. In standard form the 410 would do between 18 and 20MPG on a run. Dick told me that he was getting closer to 30MPG on long continental runs. I hope the current owner of the car will chip in with some accurate up to date figures.

I discussed the subject with the gents from Classic Bristol Car Parts and they told me that for 409 / 410 the factory preferred the A500 / 42RH 'box from a 1992 to 1996 Dodge Durango. This is a development of the 904 transmission which is less robust than the above unit but is good for the output of the 410 in standard trim and can be rebuilt with more clutch plates to beef it up. The 42RH has a smaller torque converter but still bolts up directly to the Poly in the 409 and 410. The original starter motor bolts straight up as the bellhousing is different. The flex plate and dust shield from the 42RH will be needed as they different to the 727. 407 and early 408 have different crankshaft and will not directly bolt to the 42RH.

Fitting an overdrive will probably not involve modification to the tunnel but will require modification of the chassis crossmember along with shortening the prop shaft by around 3 inches. As the engines have good torque the factory back axle ratio is likely to still be suitable. Non overdrive units used 5/16" cooler pipes whilst overdrive units use 3/8" It may also be necessary to replace or augment radiator transmission cooler.

46RH transmissions can still be found in the US for reasonable money. 42RH are very hard to source but not impossible. STS imports will get a transmission from Florida to Essex for about £200 which is probably a lot less than the cost of crating and transportation to Florida from elsewhere in the States.

If trying to track down a suitable transmission it may help to know that A518 is the series with subsets 46RH (hydraulic operation) and 46RE (electronic operation) Similarly A500 has subsets among others of 40RH, 42RH, 42RE and 44RE.

Back to the topic - inlet manifolds more suitable for fuel injection adapters were made for the Poly 318 - E.g Weiand 7503 - but are very hard to find and quite pricey now.

[AndrewA]

I've got the car that was Mr P's - I get 18 mpg running around locally and around 25 mpg on a run.
That's with a 318, the gearbox with overdrive and lock up, new Edelbrock 500, rebuilt top end, electronic distributor, 2.8 rear, flat as a pancake cam and a fair amount of time on rolling road getting it all spot on.
Good luck with the efi!
Cheers
Andrew

[Thor]

That is a good result. By today's standards a 410 is not a heavy car and the relatively small frontal area of a Bristol makes it (quite) aerodynamic, but a 5.2 litre petrol engine has certain limitations ! With a good deal of fiddling about a Sniper OUGHT to be able to do a bit better, but it is quite a challenge.

[AndrewA]

Thanks - when the heads were off the chap that did them spent a lot of time getting them to flow as well as possible. The engine's quite poky - I'm apparently getting 245 brake at the wheels but have spent enough time with various cars on different rolling roads to know some are more generous than others!
The flat cam suits the car well btw - although the hot rodder part of me wouldn't mind a bit of lope!

[Green411]

A bit of lope on a cam does sound good in the right car, but as you say a Bristol may not be the right home for such a cam! Didn't Comp cams do a ' Thumpr' series which had the lope at idle but with modern lobe design?
If I had a 318/340/360 then I'd probably have a 518 in it now, on the basis of the overdrive improving cruising more than efi would. But as discussed earlier a 383 will need an adaptor, which then opens up further debate about whether to stick with Mopar and the A518/46RH or go GM TH700R4/4L60E for it's relative availability, affordability and ease of getting replacement parts.
Lots of useful specifics in David C's post - thank you!