911 Project - On The Rollers

I mentioned in my last update that I still wasn't entirely happy with the way the newly revitalised motor in the 911 was running.  Although a huge improvement on its previous incarnation, some 700 miles into the running in process it was clear by state of the sparkplugs the rate that the fuel gauge plummeted that it was running rich, the idle wasn't very stable and it was hanging onto revs off the throttle.

All in all, there were issues to drive around, removing much of the joy of driving. It was also something of a financial disappointment as I'd already spent several hundreds pounds supposedly having the carbs set up.

Black plugs = rich mixture

It looked as if I'd have to bite the bullet and find a local rolling road tuning specialist who was familiar with the intricacies of Weber carburettors (the US-made PMO's I am using are essentially an improved Weber IDA 3C), so I was interested to read in the latest copy of Octane magazine of the editor's good experience at Airey Tuning, over Petersfield way.

One Monday morning a couple of weeks later I was up early to take the 911 to Cheriton. The weather was properly foul with wind, heavy rain and standing water, and I almost phoned to re-arrange, but I took a MtFU pill and headed west along the A27, up the A3 and along the A272.

I'm glad I did; it was a very interesting morning's work. Tom Airey is a character from the old school with a stack of stories to tell, but more importantly, with over five decades of tuning engines under his belt he really knew his stuff. 

Magician at Work


After lining the car up on the rollers in Tom's small workshop Tom climbed in (it's a two man job to 'drive' the car and operate the dyno) and we ran the car up through the gears to 4th to get some benchmark readings before making any changes. That first pull got us to just under 100bhp on the screen facing us, before Tom aborted.  At that point - some 5000rpm - the engine was already starting to run dangerously lean. 

As I half expected, the settings and jetting on the newly installed carburettors were all over the place. It was running far too rich at low and mid range, but leaned right off at high revs. 

The remedy meant the main jets, located deep inside the carbs, had to be changed. To get access needed some dismantling so I helped take the air-cleaners and trumpets off and between us we eventually were able to remove the jets they appear to have been tightened in place by an angry American.

A good way to warm fingers
on a November's day


Tom then went through a methodical process to improve things. He made some significant changes to main jets (up two sizes), the mid/low range air-correctors (down 4 sizes), plus a slight timing adjustment, changes to various other ports, bores, jets and bypasses - I'll admit the workings of an IDA/PMO are still mostly a mystery! He also fiddled with the various idle settings, and after putting things back together we went for a second run on the dyno. Now the dials showed 130bhp at around 5,500rpm with a better mixture and improved throttle response.

A 40 year old 911 engine getting
a good seeing to

This time Tom pronounced it too lean, and we needed a slightly smaller main jet, so the air cleaners and trumpets came off again and he changed all six jets for a matching set, and after a bit more fiddling, we put it all back together. He also suggested using an additive in the fuel to raise the octane to 100, so we poured a suitable dose into the tank and ran it up the gears one more time.



Now the number showing on the dial was just over 160bhp at around 6,000. This is what Porsche claim for a 2.4litre 911E, and was the level I had been hoping for. Fuelling was good all the way through, it no longer hung onto the revs off the throttle, and the idle was nice and stable. I examined a plug and it looked to be a bit cleaner than before, although Tom told me they were really a bit too fouled to clear themselves and should be replaced. I was then invited to take the car up the road and back. 

It was immediately obvious that there was a big difference in the way the car drove; pick up was excellent, the idle rock steady and off-throttle behaviour is much improved.  There was also a lovely snap as I blipped the throttle on down-changes and - as you'd expect - the performance felt much stronger too. I took it easy as the weather conditions were still horrible, but even at 3/4 throttle it ran noticeably harder through the gears. The leaner mixture and petrol additives should help the running-in process; bedding the rings and reducing crank-case pressure

I'd been reluctant to spend more money on the car, but this was definitely excellent value. Returning to Cheriton I waited for Tom to complete the paperwork, and while he was doing so we chatted about stuff - I found out that as well as building superb flying model aircraft he's also an astrology nut, so I found myself having a 'free' consultation (I should be looking for an Aires partner apparently). 

Looking at the completed report later on, I was surprised to see he'd written 'Engine 205bhp at 5,500rpm'. The lower number I'd seen on the big dial in the workshop was an 'at the wheels figure', and 205bhp is the more normal measure of engine output - at the flywheel and corrected for losses through the transmission. 
I've learned to take dyno power figures with a largish dose of salt, but if it's anywhere near accurate its an excellent result; by way of comparison, the legendary 2.7RS with its longer stroke is rated at 210bhp. 

Conditions driving back (the scenic route this time) were biblical, so I had to take it easy. Probably the best thing I could say about the motor is that it now feels much more modern; I no longer have to drive around problems and it just gets on with its business, and is wonderfully sweet in the critical 3-5,000rpm range. The fuel gauge now no longer plummets, so all in all it's much more usable, and I feel happier about planning the car's first service and sorting out a new silencer. 

A good result.

SS7