On a (rare) beautiful December morning, six 928 owners headed for Surrey Rolling Road in Chobham. As it turned out, we had a fantastic mix of pretty much every engine configuration that Porsche made for the 928. This was going to be a rare opportunity to compare the power and torque of the different engine versions fitted to 928's.
The earliest example we had was Joel's '79 928 with a hybrid 4.7 engine (courtesy of Dr A!). This engine was only 500 miles old from a fresh build-up and understandably, Joel was keen to see the results. The rest of the pack were all S4's with the following configurations:
- Chas Behl 94 GTS manual
- Joel Hopwood 79 4.7 manual
- Charlie Edgley 91 S4 auto
- Stuart Rankine 88 S4 manual + x-pipes
- Tony Macpherson 87 with 91GT that had a been fitted with a '91 GT engine and manual gearbox + x-pipes
- And Simon Painter 91GT with x-pipes
Joel's was first up as he needed to get on his way (although he ended up staying for the duration after some pleading phone calls!). Although his torque figures were completely text-book, his power was down across the whole rev range. Mr Dyno man suggested that it looked like it could be air flow restriction somewhere in the inlet or exhaust system?? Anyway we all took turns on the rollers with varying degrees of success.
When each of our cars were finished, we each got a set of graphs showing maximum rear wheel horsepower (rwhp) which are the most important figures, the torque figures, air/fuel ratio figures (AFR) and the estimated engine flywheel hp (fwhp). The estimated fwhp is a much debated figure with the most common figure of 15% being the loss incurred through the drive-train to the wheels. So if an engine produces 300 fwhp, then the rwhp will be about 15% less at 255. Given this, our estimated rwhp figures seem a little optimistic?? But hey, this is what the Dyno system estimated.
When we all finished, I got the Dyno guy to print out a single graph with an overlay of all our torque and fwhp curves. This then allows easy comparison of each engine onb a single graph. Some of the curves are so close that it is difficult to identify which is which, but I have had a go at labeling each cars curves - hopefully it will be reasonable clear.
Although all power curves were slightly different, the takeaway here is that all the engines were completely standard (apart from a few cheeky x-pipes) and were all pretty old (OK, except Joels!) and all produced excellent power (Ok, again Joel's was a bit low on the day!!). These engines are all fantastic pieces of german engineering and are proving to last the tests of time
There are a few of us planning to optimise the fueling and ignition (courtesy of Mr Speake's Sharktuning) a then re-run the dyno's to see the differences achieved. So early on in the New Year there will be another session if anyone cares to join us. I will send out an invite closer to the date.
|Angus, Silver S4 SE||346.2 BHP|
|Matthew, Green 91 GT||341.3 BHP|
|Simon, Green 91 GT||321.9 BHP|
|Peter, 91 S4 (Tested in September)||315 BHP (Estimated, Auto)|