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Date: Mon, 20 Dec 1999 19:04:06 -0800
From: Justin VanAbrahams <jvanabranopsamnet>
Subject: Re: Classic900s - 8v vs 16v performance results


Alan Buckingham wrote: > > To kill some time last night I got together as many old road tests as > possible for the classic 900 turbo ranging from 1978-1989 (including one 99 > turbo). I then put as much of the performance data as possible into Excel > and did some comparisons. Clearly one needs to be cautious in comparing > results because of variations in drivers and conditions. However, findings > are as follows: [snip] As I understand it, there are a mess of things to keep in mind when comparing these various tests: 1. More valves does not always equal more off-the-line power. In fact, oftentimes the reverse is quite true. A good example is the VW 8v engine on a given platform vs. the 16v engine on the same platform. Generally speaking, the 8v is quicker off the line, whereas the 16v has more pulling power as speeds increase. This has to do with the breathing and combustion characteristics of 2v/cyl engines v. 4v/cyl engines. At low speeds, there isn't enough air being forced around to cause a problem with the fewer valves and smaller valve area, but as RPMs increase and airflow is increased, the extra valve quantity and size (in overall area) in a 16v plays a major part. There are other variables here, but that is a start. 2. The 16v engine was introduced to provide added fuel econony more than anything else. The 16v engine features a higher compression ratio for increased fuel economy when compared to the 8v. The early 99s/900s used a 7.2:1 on the turbos and a 9.25:1 on the non-turbos; the APC turbos were bumped to 8.5:1 (IIRC); and the 16v engines had a 9:1 on the turbos and a 10:1 on non-turbos. These compression ratios drastically affected (not in and of themselves, mind you) where Saab engineering the powerband - for example, the non-APC 8v turbos had very good top gear acceleration, but the APC 16v turbos were more potent in the midrange. This is partly due to the sizing of the turbo, the cam profiles, and the electronics involved in the APC system. 3. Boost pressure on the turbo models is all over the place. A non-APC turbo should produce a maximum of 8psi, but since boost is regulated entirely by a wastegate, there is a possibility of 'boost creep' under hard acceleration - I've seen as much as 10 to 12 psi on my 1980 900T. The 8v APC turbos were guaranteed 8psi, and assuming the APC system is working correctly should never see more. The 16v APC turbos normally run up to 11 or 12 psi, but the electronics were revised to start backing off boost much more quickly than the 8v models, meaning that you will normally only see the full boost under very hard acceleration and it won't sustain for very long - the knock detection circuit seems to be much more aggressive. 4. Gearing should not play a major role in this comparison. I do not believe 16v engines had significantly higher gearing than 8v engines, except during the latter part of the '80s in the USA (as Saab was trying to meet our EPA's standards). All I can say is that 3000rpm in 5th is roughly the same speed (about 75mph) in my 1980 900T, my 1986 900S (16v non turbo), and my 1990 SPG. I would suspect that no matter what Saab optimized the gear boxes to keep the engines in their respective power band as much of the time as possible. > I prefer the latter explanation. Perhaps some of the Saab factory workers > were a bit liberal with their adjustment of the APC potentiometers when they > installed them! I would not be so quick to accept Saab fudged their numbers... There is a huge amount of variation in the way people launch FWD cars and turbocharged cars in general. In the U.S., I have read Car & Driver reports on Saabs for many years, and the most noteworthy thing I've noticed is that their review of my 1990 SPG, with 175hp and 195 lb ft of torque, has the exact same performance as a 1978 99 Turbo, with 135hp and 172 lb ft of torque. But, a 1988 900 Turbo cabriolet managed to hit 60mph a full second faster than the aforementioned two, despite it's severe weight penalty (300 lbs) and it's reduce output v. the SPG. I am content to believe that the test drivers or the test conditions changed. > The second finding is that the extra 20-30bhp of the 16vs seems to count for > very little. 16vs may be more refined but they seem to lack low down punch > (does anyone have a graph showing the torque curve of 16vs versus 8vs?). > This is a common characteristic of cars with 18 valves. Therefore for > everyday driving an 8v with the boost turned up a bit is going to be a > better mover than a 16v. That is probably true, since the advantage of a 16v will mostly be apparent at higher speeds, where the better breathing of a 16v engine comes into play. That said, I will bet that the 16v car will get better fuel economy when indexed against specific output... That is, a 150hp 8v will get worse horsepower than a 150hp 16v. -Justin 7 Current Saabs 7 Killed/Sold Saabs

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