Petrol vs Diesel

www.thegolf.co.uk 1 May 2005
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Can a modified turbo-diesel Golf really challenge the pace of a petrol engine version?
We took a pair of tweaked Mk4s to the test track to find out

Words: Peter Knivett     Photography: Jakob Ebrey /John Colic

Ice blast - low temperatures on test day didn't help traction, but the diesel Golf still recorded some storming sprint times

Petrol versus turbo-diesel Golfs - which is quicker?
It's a question that's been bugging us on The Golf+ magazine, and with good reason, because Wolfburg's oil-burners have been getting increasingly pacey, to the level where they should be nip and tuck in the performance stakes with their petrol drinking cousins.
In the early days diesel Golfs were boring, smoky and slow, but the real turning point was the introduction of the Pumpe Duse engine with the Mk4 Golf. This new high pressure diesel pump technology was a breakthrough as it allowed the use of a new style of injector, one that could be machined to tighter tolerances, which allowed for finer fuel atomisation. That in turn meant a better, more complete `burn' within the combustion chamber, which allowed VW for the first time to throw more fuel into the engine while avoiding the dreaded soot. More fuel meant that the engines could run more turbo boost to unleash more power. so as the technology was perfected, Golf Mk4 TD power outputs climbed from 115, to 130, then 150bhp. In comparison the Mk4 petrol engined Golfs also became more powerful than their predecessors, boasting 150bhp in base 1.8T guise, then reaching 180bhp in the later GTI versions.

So while the petrol engined cars grabbed the headlines with their horsepower figures, not only were the diesels catching up in the horsepower league, but they had a real ace up their sleeves - torque. Always the trump card of turbo-diesel engines, torque has unfortunately always been the poor cousin in the glamour stakes to horsepower.

People tend to forget that it's torque and not brake horsepower that actually accelerates a car.

Understanding the difference between the two can be a tad tricky. Concepts like twisting forces, motions and radii enter the picture, which can seem like a GCSE Physics lesson.

Our man Peter folds his legs behind the wheel of the Allard diesel Mk4.

But don't switch off - I've always found it best to explain the difference between the two forces like this - if BHP is a measure of how much work an engine will ultimately do, then torque is a measure of how quickly it gets that work done.

So an engine with loads of torque and no horsepower will tend to accelerate really quickly but quickly run out of urge as the revs climb, which will hamper top speed, but give great flexibility. Whereas an engine with stacks of horses but no torque will be inflexible - great when it's wound up, on song, but limp and lifeless at lower revs. In practice that could mean a good 0-60mph and top speed time, but poor in-gear acceleration. For fast road use, the best engines produce almost even amounts of horsepower and torque, to give you flexibility and outright performance. The question is, which type of Golf engine is going to offer the best combination - internal combustion or compression ignition?

To settle the question once and for all, we set up a petrol versus diesel showdown, between a pair of tweaked MK4s that offered similar performance on paper. In the petrol corner we have Nick Lormor's tidy Jabbasport-tweaked Mk4 GTI, meanwhile in the turbo diesel corner we have Lloyd Allard, wheeling an Allard Turbosport tweaked 1.9-litre PD130 into the battle. Both are boasting over 200bhp, achieved in very different ways, with very different engines, but would the petrol engine's superior horsepower kick sand in the face of the oil-burner? Or would the turbo-diesel's extra torque sucker-punch the mighty 1.8T motor? Our super-accurate
GPS timing gear would give our contenders no place to hide, so without further ado we left the comfort of Bruntingthorpe's canteen and headed out onto the freezing trackside.

Petrol dreams: Jabbasport Mk4 1.8T GTI

In grip terms these would be challenging conditions to lay down some decent 0-60mph and standing quarter times, but that would only be part of the challenge.

Living in congested Britain, the opportunity for flat out blasts is becoming increasingly limited, which means that increasingly, manufacturers and tuners need to concentrate on real-world performance. 'Real-world' sounds dull, but it isn't, because this means focusing on in-gear acceleration times - the kind of performance you need to squirt past that lorry or tractor without having to change down two or three gears to get things moving. Bearing that in mind, a crucial part of our test would assess the third and fourth gear acceleration times.

Nick Lormor's drive-by-wire 2001 car rolled out of the factory with 150bhp, but seeing as Nick's a full-time employee at ace VAG tuners Jabbasport, it's hardly a surprise that now it's packing quite a few more ponies. These come courtesy of a Jabbasport 'dry' cold-air induction kit and a comprehensive remap. Nick's also wisely added a larger front-mounted, full-width alloy intercooler which helps keep inlet temperatures low - which should hardly be a problem for our test. Spent gasses from Nick's 1.8T exit through a Jetex system which isn't exactly in its first flush of youth, so he's keen to point out that it's in line for replacement in the not too distant future.

Even so, needn't be ashamed of this petrol Golf' output. Missing engine cover aside, its a storming example of the ready tuning potential of the turbocharged 20¬valve petrol engine, which should easily see 220bhp. It also should be packing around 2301b.ft of torque.

Behind the wheel, this petrol contender was a very polished performer, being smooth, torquey and Tree-revving. Okay, it's not the most powerful Jabbasport 20-valve we've ever tested, but there's no doubt it's one of the sweetest, boosting nicely from around 2000rpm, all the way up to nearly 7000rpm. Peddling it around the test track's sweeps was great fun, exploiting the petrol engine's willing mid-range pull, enjoying the strong tug as the rev-counter swept higher, backed by an exciting, stimulating soundtrack - part induction, part exhaust. Pleasingly cog-swapping in the five speed box was a real joy, thanks to the tactile B&M gear linkage. Nick's choice of Eibach coilovers with alloy top mounts, matched to front and rear Eibach anti-roll bars and Powerflex bushes certainly kept body roll in check and afforded a neutral balance, helping to work the Toyo T1 -S tyres to the maximum.

In the bigger dynamic picture, those factors were a mere bonus, because this was a pure and simple contest against the clock, muscle against milliseconds. And when Nick's car stepped up to the pedestal, it delivered nicely, easily recording a 0-60mph dash in 6.91 secs, a massive step forward from VW's quoted 8.5sec time. Thanks to the progressive power delivery, launching the car at 3500rpm seemed best to balance acceleration against wheelspin as you fed more power in and snatched a gear just as 60mph was reached. Nick's car clocked a
respectable standing quarter mile time of 15.5secs, an excellent figure and a testimony to the quality of Jabbasport's engine work.

In-gear acceleration times were going to be very interesting, as we felt that these would be the benchmark for the diesel to beat. On a 3rd gear 30-70mph acceleration test the Golf took 15.4 secs, then clocking a 11.9 secs for a 40-90mph 4th gear trial.

On the flat-out, full bore run, it clocked 134mph, which considering that Nick's car cost around £1800 to get to this state of tune, tops a very decent set of figures which back up a very solid, impressive package. Thing is, would it beat the diesel?

Diesel power:
Allard Mk4 PD130 1.9 TDI

Aurally it's no contest, because the diesel's soundtrack still conjures up images of taxis, farmyard tractors and smoky Inter¬City 125 trains. Round one to the petrol car. Still, once you've got over the shock of the muted clatter that's present once you've let the glowplug light flick out and twisted the key, the interior of the Allard PD130 is a mighty nice place to be, with leather, climate control and heated seats.

Externally the Allard Golf is a real sleeper, running on stock suspension, brakes and wheels, aside from a set of the latest Toyo Proxes T1 -R rubber. Eagle eyed readers may spot the biggest clue to this MK4's performance - that large alloy intercooler nestling behind the bumper.

Pop the bonnet and the game becomes very clear. The bespoke carbon fibre engine cover hints at the potential lurking in the engine bay, as does the silicon hose plumbing. That's only half the story, because the juiciest parts of the Allard Golf don't grab your eyeballs at first. So it's only after enquiry that we learn it's actually in 'Phase 3' tune, running a bigger development AMC 275VNT turbo, originally destined for a 2.5-litre Audi turbo-diesel motor and capable of handling up to 275bhp. Remapped to provide up to 1.75 Bar of boost with fuelling to match, this mounts onto a free flow exhaust manifold, which hooks up to a big bore turbo downpipe, linked to a 63mm big bore stainless steel exhaust system.

The engine inhales through one of Allard's massive intercoolers via a K&N filter equipped airbox and the results are approximately 200bhp at 4350rpm, coupled to over 3101b.ft of torque at 2850rpm. That's enough torque to wilt the standard clutch, so wisely the car uses an uprated Helix pressure plate. With a slightly more aggressive map Allard claims this is quick enough to see off the new Mk5 2.0 litre petrol GTI, which sounds ambitious, but we were about to find out. Immediate impressions are very, very promising, yet the experience is surreally different to the petrol 1.8T. There's less fuss, less revs, less drama, just a mighty dose of serious performance, although unlike some diesels, the power delivery produced by a development ECU map does take a little getting used to. As such there's a period of surge as the turbo's not sure how much boost to produce, clearly audible as a flutter under the bonnet, equating to around 1700rpm on the tacho. By 2500rpm this is all done and the boost really comes on strong, the bonnet lifts skyward, you're pushed back into your seat and the rev-counter races around the clock to 4500rpm, with a massive surge that barely tails off into sixth gear. Best of all, there's no trace of any smoke.

Once you're used to the way it behaves, it's easy to drive around the low-rev hesitation and you begin to revel in a very, very quick Mk4 Golf. In the wet conditions the Golf's ASR traction control system was working overtime, but flick it off and the tyres would light up in third gear if you mashed the throttle exiting lower speed curves such was the grunt on tap. That said, the sharp throttle response and nicely cosseted steering made wheelspin pretty controllable, but careful use of the loud pedal is necessary such is huge pull on tap between 2500rpm and 4500rpm.

Downsides? It's a touch boomy under hard load, but in fairness at an 80-85mph cruise in sixth the noise levels are perfectly acceptable and it's a small price to pay given the intense levels of low-rev urge and top-end fizz that are on tap. Also, the dull engine note is a little underwhelming.

Do I like it though? You bet. Within a few minutes it's being peddled around hard with real pace and the engine's huge mid¬range make it easy and relaxing to drive very fast. What's really impressive is that you feel you could take the Allard car to a track day and really have some fun upsetting the more conventional cars.
Granted, at £3454.50 fitted including VAT it's not cheap, but it more than delivers the thrills in a most unexpected way, as well as giving 38mpg!

But how does it fair against the clock? Weather conditions are getting worse as we strap the GPS gear to the Allard car, so getting decent 0-60mph time is going to be a struggle. Lloyd tries a launch first with the ASR on, then trying with it off. The results raise eyebrows, as he clocks a best time of 6.78 seconds, marginally quicker than the 1.8T petrol Golf, then covers the standing quarter in 16.1, some six tenths down on Nick's GTi.

The real difference comes when I record the in-gear times. Given that the PD130 packs six gears compared to the GTI's five cogs, this isn't an exact comparison because the ratios are vastly different, but nonetheless, the results are a shock. When the data is analysed, the Allard turbo-diesel has recorded a 30-70mph time in third gear of just 7.5 seconds. That's a storming 8 seconds quicker than the petrol car...

The fourth gear 40-90mph times are more evenly matched though, as the Allard turbo-diesel clocked a 10.75 second time, compared to the GTI's 11.9 second figure. On its top speed run Lloyd saw 121.5 mph. Not bad, but some way short of the petrol car, partly explained by Lloyd backing off given the semi-experimental ECU settings, so he's confident of more to come.

Boasting a bigger charger, larger intercooler and ECU remap, the 1.9 turbo-diesel lump provides over 3101b.ft of torque

Much to mull over then. We didn't expect the turbo-diesel to be quicker 0-60mph than the petrol and then be slower over a standing quarter. Scores? Diesel one, petrol one. Given the diesel's low down pull its in-gear advantage was easy to predict, yet not to the tune of nearly eight seconds in third gear. Diesel two petrol one. Top speeds enabled some clear water to emerge, with the petrol car winning out over the PD130. So that's two all. Then take prices into account. Allard's package is the priciest, weighing in at over £3k fitted, but Jabba's petrol 1.8T costs about half as much, so you could argue it's a better bang for your buck. Petrol three, diesel two. But then the diesel is less juicy, so again were back at a score-draw...

Splitting the difference between these two in a head to head dual isn't as easy as you'd think. But that's great news for Golf fans because either petrol or diesel, you've got more choice than ever before in how you get your tuned VW kicks. On this evidence, everyone's a winner.

www.thegolf.co.uk 1 May 2005

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