robert heywood Posted July 16, 2003 Share Posted July 16, 2003 went to the towcester meet last night b¬??$r me it was hot, driving back it was a lot cooler but the 7 seemed to have gained 20bhp, why???? Link to comment Share on other sites More sharing options...
martinwhitcher Posted July 16, 2003 Share Posted July 16, 2003 because the air was colder it had more oxygen in it? therefore the engine goes better! Martin MW 51 CAT Link to comment Share on other sites More sharing options...
Peter Carmichael Posted July 16, 2003 Share Posted July 16, 2003 Because it was dark and it was a perceptive difference. Link to comment Share on other sites More sharing options...
Simos Posted July 17, 2003 Share Posted July 17, 2003 It's uphill to Towcester and downhill home again... Link to comment Share on other sites More sharing options...
Paul Bowden Posted July 17, 2003 Share Posted July 17, 2003 the car always seems quicker on the way BACK for the Pub. Not that I condone Drink driving 😬 😬 😬 Link to comment Share on other sites More sharing options...
se7enmad Posted July 17, 2003 Share Posted July 17, 2003 Excuses if I appear to hijack this thread but I have been waiting to ask this for a long time. Subjectively I know that high temperatures are a killer to bhp output especially on modified engines. I am looking for objective evidence now. Has anybody done any studies on the variance of bhp output with temperature changes. Conversely has anyone compared rolling road outputs, assuming no change in engine performance in different ambient temperatures. Obviously the rolling road has to be a constant here. I am really interested in your views. Here engines come to life in winter! It is a relief to wake up to a cold dry day here to know you'll here a louder induction roar and feeling the engine respond better. Maybe one day I could do successive quarter mile runs different ambient temperatures to test this but then there are a lot of other variables which would adversely affect the 'experiment' Any pointers guys? Antonella 1998 Caterham Classic (Malta) .. still not spotted ! my site here more photos here Link to comment Share on other sites More sharing options...
Mike C. Posted July 17, 2003 Share Posted July 17, 2003 Ambient temperature can significantly affect the take off performance of piston engined aeroplanes. Mad Manx Link to comment Share on other sites More sharing options...
se7enmad Posted July 17, 2003 Share Posted July 17, 2003 Any literature please or maybe websites of interest? Antonella 1998 Caterham Classic (Malta) .. still not spotted ! my site here more photos here Link to comment Share on other sites More sharing options...
Tony C Posted July 17, 2003 Share Posted July 17, 2003 Engine power will change with temperature because of the effect of temperature on air density. Your cylinders will induct a given volume of air. The hotter it is the less dense the air and therefore the fixed volume inducted will contain less air - and less fuel and give less power. Similarly?, increasing altitude will give a decrease in air density, but this time because of reduced air pressure. One might think that the decrease in temperature with increasing altitude would compensate, but the effect of decreasing atmospheric pressure is dominant. I'll check around to see if I can come across any tables that show the changes in bhp. For aircraft/engine performance considerations it is common the compare the actual conditions with those of the International Standard Atmosphere (ISA) so that apples are being compared with Granny Smith's. Hope this helps in some small way. Tony BRG SV 😬 It is only with the heart that one can see rightly; what is essential is invisible to the eye. (Antoine de Saint-Exupery) Link to comment Share on other sites More sharing options...
Graham Perry Posted July 17, 2003 Share Posted July 17, 2003 Roger King demonstrated this to me on a Rolling road a few years ago. When the engine was hot in a hot dyno cell with the doors shut it produced X horsepower. When we had let the hot air out the dyno cell and ventilated it and admitedly the engine was a bit cooler, the car produced X + a bit more. I cannot remember what the bit more was though, but I know I was impressed at the time by this effect. He explained to me that unscrupulous dyno operators have been known to do this to show a quantifyable improvemnt in power, when in effect they had done nothing at all. Denser /cooler air is always useful for performace, thats why Turbos run intercoolers. Link to comment Share on other sites More sharing options...
TomB Posted July 17, 2003 Share Posted July 17, 2003 I went out in -5 C last winter late one night. It was very very very cold. Apart from the grit on the road, the car was amazing - I couldnt believe how much quicker the car felt. Darkness also helps. There is less aerodynamic resistance at night because there are less photons flying around hindering progress. I could have been driving faster than normal to get home before I froze though. Link to comment Share on other sites More sharing options...
lowerdowne Posted July 17, 2003 Share Posted July 17, 2003 I dimly remember a gas equation which went something like this - P1*V1/T1 = P2*V2/T2. I think this means that given a constant pressure the volume of a gas changes in proportion to its absolute temperature so that air on a really hot day, say 30 degrees C, it is about 10% less dense than at 0 degrees C so that there will be about 10% less oxygen available to be burnt in your cylinders and presumably 10% less power. Someone will tell me this is a load of b******s and there are lots more factors involved but if it is even half right, engine power is drastically affected by the air temperature. Phil Link to comment Share on other sites More sharing options...
Chelspeed Posted July 17, 2003 Share Posted July 17, 2003 It's o level physics (which is why I forget the details..... so long ago....) or chemistry perhaps, either Boyles Law or Charles Law. PV/T is something or other... where these are pressure, volume and temperature If you have a piston dropping in the bore then it'll suck a given volume of air thro the carbs or whatever. If you raise P by 10% then you'll get 10% more oxygen and hence 10% more power, if you raise T by 10% then you get 10% less oxygen (power) as it's on the bottom. Fortunately T is absolute (add 273 to degC) and so 10 degC rise from 10 degC in winter to 20 degC in summer is only 3% less oxygen (and hence power) rather than 50%!! Link to comment Share on other sites More sharing options...
Chelspeed Posted July 17, 2003 Share Posted July 17, 2003 Snap!! Must be right if we both remember the same. Link to comment Share on other sites More sharing options...
Paul Bowden Posted July 17, 2003 Share Posted July 17, 2003 I still think its 1 pint of Ale= 10 bhp *confused* Link to comment Share on other sites More sharing options...
lowerdowne Posted July 17, 2003 Share Posted July 17, 2003 O levels were so long ago I was afraid the gas laws had been changed since then by Brussels Phil Link to comment Share on other sites More sharing options...
se7enmad Posted July 17, 2003 Share Posted July 17, 2003 funny how difficult it is to remember things from school like this, especially when you've forgotten the formulae out of disuse - makes me feel old thanks for the clear explanation guys! Antonella 1998 Caterham Classic (Malta) .. still not spotted ! my site here more photos here Link to comment Share on other sites More sharing options...
Peter Carmichael Posted July 17, 2003 Share Posted July 17, 2003 Snot as clear cut as all that because less air destiny also means less drag... Link to comment Share on other sites More sharing options...
JP Posted July 17, 2003 Share Posted July 17, 2003 Slightly away from 7's but I remember reading David Vizard's book on Mini tuning. He has dyno sheets for same spec A-series engines running different thermostats. They showed bhp gains of 3-5 bhp (on a ~60 bhp engine I think) when going from a ~80 deg thermostat to a blanking sleeve. This is probably completely irrelevant for a K-series! Link to comment Share on other sites More sharing options...
Tony C Posted July 17, 2003 Share Posted July 17, 2003 Agreed about drag, but the effect on engine power will be greater. Not really JP. We are dealing with heat engines after all and most of the heat generated is "wasted" rather than being used to drive the wheels - don't ask me how much, it was a long time ago I studied heat engines, but I seem to remember that a four stroke engine was only 30% efficient. If some of the heat generated can be diverted from the cooling sytem without anything going "Kaaassssprang!", more heat will be available to drive the wheels - VERY roughly speaking. BRG SV 😬 It is only with the heart that one can see rightly; what is essential is invisible to the eye. (The Little Prince by Antoine de Saint-Exupery) Link to comment Share on other sites More sharing options...
Jason Plato Posted July 18, 2003 Share Posted July 18, 2003 The general accepted rule is that for every 10C increase in intake temperature there will be a 3% loss of power . I did a load of temperature tests to identify what happens to temperatures under the bonnet replicating sprint conditions and found that intake temperature can easily be 40C above ambient at standstill . Even when running at 60mph the temperature under the bonnet can be 20C above ambient . This led me to wrap the manifold in insulation and to poke the filter outside of the bonnet . With the filter only drawing air from outside . And to relocate the air temperature sensor to a location which best represents the intake temperature for temperature compensation in the ecu . At 210bhp 6% (+20C)= more than 12bhp . I know the next comment will be well what happens when the cool intake air temp is raised when going through the throttle bodies ?? - well there is no way of cooling this so there is no point in worrying about it ! . The full report was in the archives ..... Dave Link to comment Share on other sites More sharing options...
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