CARBURATION

NZ Performance Car - - Promotional Feature Efi Vs Carbs -

The term ‘car­bu­ret­tor’ stirs up a con­ver­sa­tion about en­gines more than any­thing else in the mod­ern world. Carburation re­mains a mys­tery to many, often for­got­ten about as if it were old tech­nol­ogy within the pages of a his­tory book — but that’s sim­ply not true. With older Ja­panese and Euro­pean cars that are still run­ning car­bu­ret­tors from their era seeing a resur­gence in pop­u­lar­ity, peo­ple are chas­ing power on these car­bu­ret­ted en­gines, and, if a glance at what the boys in any his­toric race or rally se­ries are do­ing is any­thing to go by, that power can def­i­nitely be found. And is there re­ally any bet­ter sound than that of a We­bers howl­ing through the for­est at 10,000rpm? To get the ins and outs of how car­bu­ret­tors work and where to start, we spun a yarn with Murray John­son from We­ber Spe­cial­ties.

HOW DO I CHOOSE THE RIGHT SIZE CARBY?

“It’s easy to buy and sell a prod­uct with­out hav­ing any knowl­edge of how it works, and this is where the big­gest mis­take hap­pens. Sizing is crit­i­cal to your ap­pli­ca­tion. There’s a no­tion that elec­tronic fuel in­jec­tion (EFI) is by de­fault bet­ter than car­bu­ret­tors, and that sim­ply isn’t true. Both sys­tems are just ba­sic physics — I often say that this is where pay­ing at­ten­tion in physics class, in­stead of sit­ting down the back play­ing with your­self, ac­tu­ally pays off. Get­ting air into the cylin­der is how you’re go­ing to make more power — the more air you can in­tro­duce along­side the ap­pro­pri­ate amount of fuel, the big­ger the amount of power the cylin­der can pro­duce.

“For in­stance, with older style cars it can ac­tu­ally be cheaper to achieve the same re­sults as EFI with carburation. You’ve got to bear in mind that orig­i­nal carbs, es­pe­cially of the Ja­panese va­ri­ety, were de­vel­oped around econ­omy and emis­sions stan­dards, work­ing on the prin­ci­ple of not a lot in and as a re­sult not a lot out. The carbs were very small. As we don’t have emis­sions stan­dards like that to re­strict us, we can in­tro­duce more fuel and air by fit­ting a cor­rectly sized carb and make nearly dou­ble the horse­power.

“On the other side of that, blow-through boosted ap­pli­ca­tions ac­tu­ally re­verse your think­ing. A much smaller carby is re­quired as the forced in­duc­tion makes ve­loc­ity through the ven­turi higher but the neg­a­tive pres­sure is now greater so it can draw greater vol­umes of fuel — ef­fec­tively turn­ing a two-litre into a three-litre by shov­ing three litres of air in. The trick is al­low­ing it still to func­tion cor­rectly off boost.”

WHAT COM­MON MIS­TAKES DO PEO­PLE MAKE?

“Well, the first is think­ing that carburation is old tech­nol­ogy and can’t stand up to EFI — which is in­cor­rect. A well-sized and cor­rect­ly­jet­ted carb can equal a well set-up in­jec­tion sys­tem at a frac­tion of the cost in terms of power and driv­abil­ity. It won’t have ex­actly the same fuel econ­omy, but re­plac­ing a worn-out carb with an up­graded new ex­am­ple is al­ways go­ing to im­prove the econ­omy re­gard­less.

“The main is­sue I see on a day to day ba­sis is in­cor­rectly sized carbs. They are al­most al­ways too large for the ap­pli­ca­tion, par­tic­u­larly in V8s. Many of those who have pre­vi­ously dealt with them carry over the ‘if big is good, then big­ger must be bet­ter’ way of think­ing.”

HOW DO YOU COR­RECTLY JET A CARB?

“Again, that’s gen­er­ally down to pre­vi­ous ex­pe­ri­ence. We’ve done it all be­fore, but as there’s no magic for­mula for spe­cific ap­pli­ca­tions, be­cause it will vary from en­gine to en­gine, we use an on­board gas anal­yser known as a LAMBDA. It is fit­ted to the tailpipe and mea­sures the parts fuel to parts air ra­tio — take it for a drive and read the mix­tures to de­ter­mine what needs to be al­tered and how. We find that one en­gine may be happy on 12:1 whereas an­other of the same type will re­quire 13:1, and an­other will mis­fire, re­quir­ing more fuel. Ex­pe­ri­ence, driv­ing, and test­ing al­low you to find out what’s go­ing on — the best part is that it doesn’t re­quire a dyno ses­sion to do it cor­rectly.”

HOW IM­POR­TANT IS THE MAN­I­FOLD CHOICE IN RE­LA­TION TO THE CARB?

“The shape of the man­i­fold is para­mount for good cylin­der fill­ing. It is the man­i­folds that ac­tu­ally make the power, not the carb or in­jec­tion per se. Unlike EFI, the air trav­el­ling through a carb man­i­fold is wet, as the fuel has been at­om­ized through the carb. You want the at­om­iza­tion to en­sure the fuel par­ti­cles are very, very fine, as liq­uid fuel doesn’t burn in a cylin­der; it’s the va­por­ized fuel that burns. Get­ting the air and fuel into the cylin­ders with as lit­tle ob­struc­tion as pos­si­ble and best flow is go­ing to help you gain a lot out of your sys­tem.”

IS THERE ANY­THING ELSE TO CON­SIDER?

“Peo­ple often for­get about the air go­ing in — cold air com­ing into the sys­tem is pow­er­ful air. Take a 260z, for ex­am­ple; chang­ing noth­ing else but the in­take to a cold air in­duc­tion sys­tem out to the front with an air fil­ter can net you an easy 12bhp at the wheels. It’s also per­fect for Porsches, where peo­ple often fit a K&N fil­ter and lose 20hp be­cause there is so much heat in the en­gine bay.”

Newspapers in English

Newspapers from New Zealand

© PressReader. All rights reserved.