The Jensen GT was,
alongside the Jensen Coupe, the last model that Jensen Motors built before the
West Bromwich factory closed down in 1976.
The GT was a “shooting brake” version of the Jensen-Healey, carrying
over the Lotus 907 engine, the Getrag 235/5 gearbox from the BMW2002tii,
suspension / live axle from the Vauxhall Magnum, and the love-it-or-hate-it
styling of the Healey, but with a more upmarket Interceptor-type interior. 511 GTs were made, of which over half went to
the States, before Jensen went bust, leaving many unfulfilled customer
orders. Only 12 are left on the road in
the UK today (Q1 2015 figure).
I bought my GT in May
2014. It had just come to the end of a
20-year two-owner restoration. Bodily it
was in very good shape, and it seemed mechanically sound. In the six months after buying the car, I
sorted out some residual electrical issues and got everything working, changed
all the weather seals, and replaced some worn suspension parts. The car was fun to drive on winding roads, but very tiring on dual carriageways, because the 1:1 5th gear of
the Getrag gearbox meant high revs at cruising speeds and a very noisy engine
and exhaust. I unsuccessfully investigated
various options to drop the revs, including standalone overdrive units and
finding a similar dogleg pattern gearbox with a proper overdrive 5th
gear (the ZF S5-18 is the only one).
I had read Mike’s “Improving
your Lotus” presentation on the website, and Lotusbits’ work had been
recommended by a fellow GT owner. After
my son told me that my GT “made a lot of noise when I put my foot down but didn’t
actually go any faster”, I called Mike to ask him to price up an engine upgrade
to Lotus spec 5. Mike suggested it would
be better to go for a fuller upgrade to spec 10, and also suggested I get the
Toyota W58 gearbox installed. I agreed
to the spec 10 upgrade and the Lotusbits large bore exhaust system, but
reserved judgement on the gearbox swap.
The mandate was fairly straightforward – more power, less noise, and keep
the car looking as original as possible.
In early February
2015, I drove the GT up the Fosse Way to Lotusbits. The pre-upgrade dyno run confirmed that my GT
was down on power – 119bhp peak (and 116 lb-ft of torque) compared to 140bhp
(130 lb-ft) for a Jensen spec 907 engine in good condition. Mike also found that the clutch had not been
properly installed, and that the gearbox input shaft had been damaged as a
result. Given the lack of Getrag
specialists in the UK and the likely cost of repair, I agreed with Mike that
the Toyota gearbox upgrade was the way to go.
Mike also noted the car’s lean to the driver’s side, which he thought
might be a bent chassis. At this stage,
my heart was really sinking.
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Engine Bay Before and After –
The Lotus Excel Airbox Setup which Unleashes the Horses
|
There then followed
what seemed like an interminable wait for the engine to come back from Mike’s
engine builder. Actually, the hold-up
was partly my fault – I had insisted on keeping the original cam carriers and
cam covers (the ones that leak oil) and it took ages for the cam covers to come
back from the powder coaters. We also
found that the car had been fitted with three 9.5:1 CR pistons, and one 8.0:1
CR piston. Potentially an expensive
disaster waiting to happen. Towards the
end of April, the engine finally came back to Lotusbits, and Mike told me
progress would speed up. Neither of us
had, however, counted upon the time and effort to resolve a previous poor
repair of one of the chassis rail suspension mountings, or to fettle the gear
lever to fit the transmission tunnel.
It’s always the smaller jobs that take the longest.
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| Retaining the Original Jensen Look – The
Lotusbits Large Bore Exhaust System is on the right |
Finally, at the end
of July 2015, with the GT MOT’d and works nearly completed, it went back to
Northampton Motorsport for its post
upgrade dyno run. Mike called me at
work, rather sheepishly, to tell me that peak power was now 155bhp, or 174bhp
without any airbox and filter. This was
pretty disappointing, as both of us had been expecting somewhere north of
180bhp. Mike suggested fitting the
airbox setup from the earlier Lotus Excel.
I agreed, on the basis that something similar had been fitted to the
last GTs (actually the Elite / Eclat airbox went into the last Stromberg carb
T75 Jensen GTs).
A few days later, and
another dyno run gave the figures we were both expecting – 184bhp peak and 167
lb-ft, a 50% increase over the pre-upgrade figures. A week later, Mike dropped off the car and I
drove it for the first time in 6 months.
It was transformed – much quieter at lower revs, but when I put my foot
down it pulled like a train and gave off a pleasingly throaty 4-cylinder
rasp. It now cruises comfortably at
70mph and 2,900 rpm as well.
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Dyno Run Comparison – Before
Upgrade in Black, After Upgrade in Red
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A few days
after getting the car back, I took it to the UK’s resident Jensen GT expert,
and he confirmed what I already knew – I now have the nicest driving GT around. Perhaps, if Jensen had managed to stay
solvent, the GT would have benefited from Lotus’ development of the 907 engine
through to 912 HC spec, and it would have ended up like mine. We’ll never know. I just like to think that my GT is now the
“baby Interceptor” that Jensen always meant it to be, courtesy of Mike and the Lotusbits team.
© Kindly written by: Nigel Kieser - proud owner of NUY 903P
