compression

21st May 2010, 23:06

I'm just curious to know about the saxo's compression ratios from the different models. Why does the 1.1/1.4 engines run higher compression ratios to the VTR engine? I would have thought generally the larger engine = higher compression. Here are the different ratios quoted from Adsayers saving fuel/mpg guide.

1.0 9.4:1
1.1 9.7:1
1.4 10.2:1
1.5D 23:1 - note that the information in this section regarding octane rating does not strictly apply to the 1.5 Diesel engine - Cetane rating take precedence
1.6 9.6:1 (Mk1)
1.6 9.6:1 (Mk2)
1.6 16v 10.8:1

22nd May 2010, 20:13

someone should know...

22nd May 2010, 23:37

Its quite complicated to explain why they would do it as there are so many possible reasons for it. For example the smaller the engine the better it's inherent cooling will be so you can generally run higher comp. Also the numbers are static pressure so the actual cam profile will effect the engines real running pressure. Some engines will be intended for many markets so have to run on different fuels and so on and so forth. But the size of the engine has no direct effect on engine compression ratio its to swept volume vs combustion chamber size.

23rd May 2010, 09:45

I know you would like to think there is some very technical reason why the engine range has so many dif comp ratios .
yes std engines will give best emissions and all round performance running 10-1+
the real answer why the 8v engines differ is lazyness on cit/pug part they use same head acroos the 3 models ,or near anough and its the dif bore +stroke combinations that give the variance -- in comp ratios and they are all lower than ideal ,just to keep away from det when usaing 2 star fuel .
the man that buys a 1.0 is not worried about last 2bhp -,he wants a car he can chug about in and not have to keep changing gears -so why bother complicating the build process is the makers attitude.

what do they use now for a 1.0 --the toyota 3 cy engine ==more modern ,lighter more power --so no way they going to spend millions redesigning an ax engine designed 25 years ago when they can do a deal and get a cracking engine that gives same bhp(68) as the orginal mini cooper S (1275cc) and complys with all emission regs and does 70+ mpg
that is well over 10-1comp ratio +fully squential injection

23rd May 2010, 19:34

Cheers

I just wondered why PSA would make a 1360cc engine with better compression than a 1587cc unit that was used in the "sports" models the VTR's CR seems a lot lower than the others.