‘Only if you look closely will you see the Her­itage Throt­tle Bod­ies for what they are’

VANTAGE - - Tech Carbs V Fuel Injection -

po­si­tion of the in­jec­tors. Then we wrapped the rest around those hard points.’

Sounds sim­ple when told like that (and when you have a Solid­works CAD pro­gram in which to cre­ate your vir­tual throt­tle body) but ev­ery tiny nu­ance has to be cal­cu­lated and drawn. Next came a pair of 3D-printed pro­to­types, from which moulds were made for Jen­vey’s in-house sand­cast­ing. With the cast­ings ma­chined and assem­bled into throt­tle bod­ies, only a few mi­nor changes to the de­sign were needed.

De­spite their car­bu­ret­ted looks, the HTBS func­tion just like Jen­vey’s mod­ern TBS. So they have a fuel rail, formed into the lid of the ‘float cham­ber’, and stan­dard Bosch in­jec­tors. The throt­tle po­ten­tiome­ter fits in­vis­i­bly be­tween an HTB’S choke tubes. The HTBS will be avail­able with 38mm, 40mm, 42mm and 45mm di­am­e­ters, and all stan­dard Dcoe-com­pat­i­ble throt­tle link­ages, air­boxes and fuel-pipe fit­tings will fit the HTBS. Fit­ted to an en­gine, they look con­vinc­ingly ‘right’; only if you look closely will you see the HTBS for what they are.

JEN­VEY DY­NAM­ICS was fi­nally con­vinced to de­velop the HTB by Aston Martin spe­cial­ist (and Fer­rari-fet­tler and Lam­borgh­ini-lover) GTC Engi­neer­ing, based close to Silverstone Cir­cuit. Our DB5 be­longs to a long-stand­ing GTC cus­tomer; in­deed GTC’S co-founder, John Win­sor, re­stored it back in the 1980s. ‘It’s the last car I painted in cel­lu­lose,’ he ob­serves. ‘I see there’s a bit of craz­ing in the lac­quer on that front wing now, but it’s lasted well.’

The DB5 is the test-bed for the first of Jen­vey’s fin­ished HTBS, three of them in 45mm form. Their sand­cast fin­ish isn’t quite as smooth as a diecast Weber’s, but iron­i­cally this makes them look more retro. Their in­jec­tors’ se­quen­tial squirts, and the sparks at the DB5’S plugs, are con­trolled by a GEMS en­gine man­age­ment sys­tem. This con­tains its own at­mo­spheric pres­sure sen­sor and takes sig­nals from sen­sors re­port­ing en­gine speed, crank and camshaft po­si­tion, throt­tle po­si­tion, coolant tem­per­a­ture and – for fine-tun­ing of the air-fuel mix­ture – the ex­haust’s oxy­gen con­tent or ‘lambda’ value.

There’s also pro­vi­sion to con­nect to an idle con­trol valve, which ad­mits ex­tra air via small tubes un­der the HTBS to help with idling af­ter a cold start, but that’s not cur­rently con­nected. ‘At the mo­ment we're us­ing spark-scat­ter to reg­u­late the idling speed,’ says James Win­sor, John’s son and a self-taught guru of en­gine map­ping. ‘If the revs drop, it ad­vances the ig­ni­tion tim­ing slightly, and vice-versa.’

As well as GEMS, other sys­tems such as Omex, DTA and Me­gasquirt could also be used – just as they can be with Jen­vey’s reg­u­lar throt­tle bod­ies. The dis­trib­u­tor is re­tained to house a camshaft-po­si­tion sen­sor and, yes, to dis­trib­ute the sparks.

It’s early days for the DB5 in­stal­la­tion, which means it’s still a work in progress. So it will be ex­cit­ing to try it out in this de­vel­op­men­tal stage. To help es­tab­lish a bench­mark, I have driven here in an ex­am­ple of the New­port Pag­nell DB line that rep­re­sents the op­po­site end of in­duc­tion tech­nol­ogy. Bryan Smart’s 1959 DB4 Se­ries 1 has the same uprated, 4.2-litre ca­pac­ity that the DB5 has gained, thanks in the DB4’S case to a re­cent en­gine re­build by Oselli Engi­neer­ing, but it’s run­ning on a sim­ple pair of SU HD8S. Oth­er­wise the en­gines are quite sim­i­lar.

I know Bryan’s car well, hav­ing sam­pled it sev­eral times over the years. It runs beau­ti­fully, pulling with a smooth rush of torque and never show­ing any signs of flat spots or hes­i­ta­tion. But it’s quite a thirsty ma­chine, and apt to emit a puff of black­ness from its ex­hausts un­der sud­den ac­cel­er­a­tion. Its throt­tle re­sponse is de­li­cious, its rich, sonorous bark the stuff of boy­hood fan­tasy (there was no car I ad­mired more as a small child than a DB4). Frankly, it’s hard to see how the driv­ing feel of this en­gine could be bet­tered. The same can­not be said for its ef­fi­ciency in con­vert­ing fuel into mo­tive force, how­ever.

THE TIME HAS come. The old world of ana­logue ap­peal is about to meet the brave new one of dig­i­tal pre­ci­sion, and the gen­er­a­tions need to co-ex­ist. I set­tle my­self in the star­tlingly

ma­genta-coloured driv­ing seat of the DB5 – it’s the orig­i­nal leather – and fire up the en­gine. It catches with an en­thu­si­as­tic growl, qui­eter than the DB4’S be­cause a DB5 has an ex­tra si­lencer, and then stops. Try again.

I need to keep the ac­cel­er­a­tor tick­led, be­cause there’s a dead spot fol­lowed by a snatch as the en­gine comes off idle. We ease out of GTC’S yard and out onto the Dad­ford-to-silverstone road, and once out in the open spa­ces the Aston can bet­ter show what it can do.

Mov­ing up through the five gears – it has a ZF gear­box, whereas the DB4 has a David Brown four-speed – it pulls as smoothly as the DB4 and nearly as strongly. If we re­mem­ber that this later car weighs about 90kg more than its an­ces­tor, the torque in­crease with the HTBS be­comes ev­i­dent; James tells me that dur­ing the map­ping ses­sion on Tim Samways’ dy­namome­ter the en­gine was pro­duc­ing up to 235lb ft at just 1600rpm, ris­ing to a peak of 289lb ft. The same en­gine fed by We­bers would make about 214lb ft at those low revs.

The throt­tle-body en­gine is also very vig­or­ous at high revs, with a sense of free, un­fet­tered breath­ing, and it is crisply blip­pable for down­shifts. Not so good is the surge when the over­run fuel cut-off re­in­states the fuel sup­ply at 1800rpm as I slow down. So it’s back to base for some ad­just­ments to the map­ping.

James tap­ping a lap­top in a DB5’S cock­pit makes for a sur­real sight, and he con­tin­ues with some dig­i­tal tweaks as we set off for another drive. ‘It seems hap­pi­est run­ning at lambda 0.91,’ he’s say­ing, ‘which is slightly richer than sto­i­chio­met­ric.’ That’s to be ex­pected of an old hemi-head en­gine de­sign, op­ti­mised for pace rather than air pu­rity. James has al­ready shown me com­pli­cated en­gine-man­age­ment maps with much ev­i­dence of that 0.91 fig­ure, but it’s the tran­sient re­sponse that needs some fine tun­ing. And on this sec­ond drive it’s def­i­nitely bet­ter, helped by the man­age­ment sys­tem’s abil­ity to ‘learn’ the move­ment pat­terns of the DB5 driver’s right foot as it goes.

Back at base again, with the en­gines of both DBS prop­erly hot, we com­pare ex­haust emis­sions at idle. It’s hardly a de­fin­i­tive test, but it does quan­tify the dif­fer­ences. The DB4 on SUS pro­duces 4.1 per cent CO, with un­burnt hy­dro­car­bons fluc­tu­at­ing be­tween 550 and 700 parts per mil­lion. The DB5 idles be­tween 2.1 and 2.5 per cent CO, with hy­dro­car­bons in the 200-250ppm range. Its fuel is com­bust­ing more com­pletely; it’s a sig­nif­i­cantly more air-friendly Aston. With that tran­sient trou­ble elim­i­nated, as it will be (prob­a­bly via the idle con­trol valve), it will be gains all the way.

The Jen­vey HTB can, of course, be used in any car orig­i­nally fit­ted with Weber DCOES or the Dell’orto or Solex equiv­a­lents, and Jen­vey is also plan­ning an Su-shaped HTB range for the fu­ture. Mean­while, the HTB will cost £288 per twin-choke unit when it goes on sale in Jan­uary, plus £120 for the po­ten­tiome­ter and £89 per in­jec­tor. To that you must add the en­gine­m­an­age­ment sys­tem and its sen­sors, and a ses­sion on a rolling road.

So, is this the fu­ture for clas­sic cars? If airqual­ity reg­u­la­tions start to place lower emis­sions lim­its on his­toric ma­chin­ery, it could be. The HTB con­ver­sion is also good news for those who use their cars in­fre­quently but ex­pect in­stant life when they turn the ig­ni­tion key. Ei­ther way, an ob­vi­ous niche has just been rather ef­fec­tively filled.

V

Above and right James analy­ses the air-fuel ra­tio: the DB5’S throt­tle body set-up is still a work in progress, but it’s al­ready show­ing clear gains over a car­bu­ret­ted car, par­tic­u­larly on emis­sions. Right: DB4 en­gine is a 4.2, just like the one in this DB

Above Back at base at GTC Engi­neer­ing, with the en­gines prop­erly hot, the team com­pares the ex­haust emis­sions of the two As­tons at idle

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