Cylinder pressures
14-12-2004, 05:57 PM
#1
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Cylinder pressures
How different are they between 7-1 and 8-1?
Anyway of telling in something like English?
The way i see it is this..the lower the comp the more ignition up top you can run thus more BHP......BUT add race fuel to high comp (8-1) and this then becomes the same as the lo comp (7-1) on optimax yes?......So what factors make a hi comp (8-1) dangerous if det isnt an issue?Cylinder pressures...but by how much?
Anyway of telling in something like English?
The way i see it is this..the lower the comp the more ignition up top you can run thus more BHP......BUT add race fuel to high comp (8-1) and this then becomes the same as the lo comp (7-1) on optimax yes?......So what factors make a hi comp (8-1) dangerous if det isnt an issue?Cylinder pressures...but by how much?
14-12-2004, 06:11 PM
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Explain here then Stu......as all i know is...if i use less advance the car is slower...more and its faster,so i'd say i was losing/gaining BHP
15-12-2004, 02:37 PM
#7
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The pressure of burnin g gas pushes your piston down the bore...
the crank then converts this to rotational movement.
SO
The harder you push the piston the more torque you get.
Alex
the crank then converts this to rotational movement.
SO
The harder you push the piston the more torque you get.
Alex
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15-12-2004, 02:50 PM
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Mike....std Mahles are like chocolate in big bhp engines...proper Cossie race pistons are slightly harder!....So new crank,arrows rods and proper pistons should cope on highish comp and get me some nice drivability low down...which means maybe GT42 for that little kick
Damn this speccing is doin my head in this time round
Damn this speccing is doin my head in this time round
15-12-2004, 03:26 PM
#11
Norris Motorsport
Dear Philip,
If only engine design was as simple as you make it!
The critical aspect on any forced induction engine is cylinder pressure. This is linked to two equally important factors.
The first is the fuel type. This is brought about by the fuel types resistance to self ignition which is of course directly linked to cylinder pressures and temperature. The higher the fuel's resistance to this self ignition factor the higher the cylinder pressure we can run until we run into detonation problems. This is not the same as all pre-ignition situations, which you refer to by advancing your spark advance to the point or pre-igntion.
The second factor is head gasket integrity. Contrary to what you may read on the internet, this is a very significant issue if you want any form of reliability. A mono block design is perhaps considered the ultimate but obviously for us is'nt an option with an old pinto engine. Ultimately if you have a high compression ratio and then increase the relative air density entering the cylinder by say 2 - 3 times depending on your boost and air temperatures, then this has a direct correlated increase in the effective compression ratio. For example an 8:1 CR turbo engine running at an effective 3 times air density is effectively running a compression ratio of 24:1!!!
In the engine design world there are set guidlines with regards to cylinder pressures that you must not overstep. As good as multilayer steel headgaskets are, there is a limit to how much pressure can be contained and this is where the engine must be designed correctly if you wish to have any usefull life. You may hear of isolated cases where people run high CR and huge power but this is for limited life engines or those which do not have to endure many thermal cycles between rebuilds. All engine components move and creep during the huge range of temperatures under which they operate and this does have a cumulative effect of engine life, or more importantly joint life. (i.e. head gasket life)
In essence nothing lasts forever, and its life decreases on an exponential scale depending on how close to its limit you are. A design engineer will take all these factors in to account when speccing an engine and it is all these factors coupled with expected engine life and fuel type that must be considered when speccing such critical parameters as C.R!!
If only engine design was as simple as you make it!
The critical aspect on any forced induction engine is cylinder pressure. This is linked to two equally important factors.
The first is the fuel type. This is brought about by the fuel types resistance to self ignition which is of course directly linked to cylinder pressures and temperature. The higher the fuel's resistance to this self ignition factor the higher the cylinder pressure we can run until we run into detonation problems. This is not the same as all pre-ignition situations, which you refer to by advancing your spark advance to the point or pre-igntion.
The second factor is head gasket integrity. Contrary to what you may read on the internet, this is a very significant issue if you want any form of reliability. A mono block design is perhaps considered the ultimate but obviously for us is'nt an option with an old pinto engine. Ultimately if you have a high compression ratio and then increase the relative air density entering the cylinder by say 2 - 3 times depending on your boost and air temperatures, then this has a direct correlated increase in the effective compression ratio. For example an 8:1 CR turbo engine running at an effective 3 times air density is effectively running a compression ratio of 24:1!!!
In the engine design world there are set guidlines with regards to cylinder pressures that you must not overstep. As good as multilayer steel headgaskets are, there is a limit to how much pressure can be contained and this is where the engine must be designed correctly if you wish to have any usefull life. You may hear of isolated cases where people run high CR and huge power but this is for limited life engines or those which do not have to endure many thermal cycles between rebuilds. All engine components move and creep during the huge range of temperatures under which they operate and this does have a cumulative effect of engine life, or more importantly joint life. (i.e. head gasket life)
In essence nothing lasts forever, and its life decreases on an exponential scale depending on how close to its limit you are. A design engineer will take all these factors in to account when speccing an engine and it is all these factors coupled with expected engine life and fuel type that must be considered when speccing such critical parameters as C.R!!
15-12-2004, 04:00 PM
#12
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Cheers Karl...food for thought 
Say 2 identical engines....but one is 7-1 on optimax and the other 8-1 on say 116ron race fuel both using multi layer gaskets...how much shorter a life would the hi comp have?
Say 2 identical engines....but one is 7-1 on optimax and the other 8-1 on say 116ron race fuel both using multi layer gaskets...how much shorter a life would the hi comp have?
15-12-2004, 04:42 PM
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Originally Posted by Bosch Man
Mike....std Mahles are like chocolate in big bhp engines...proper Cossie race pistons are slightly harder!
Originally Posted by Bosch Man
Damn this speccing is doin my head in this time round
15-12-2004, 04:49 PM
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I stand corrected Mike
You can do 7 - 9k revs ANYWHERE at Bedford where you might miss a gear when changing down.....but of course not in fifth...unless oyu did a "Brom" with RWD
Could the boost creep have happened if air injectors or an electronic boost controller were fitted???
You can do 7 - 9k revs ANYWHERE at Bedford where you might miss a gear when changing down.....but of course not in fifth...unless oyu did a "Brom" with RWD
Could the boost creep have happened if air injectors or an electronic boost controller were fitted???
15-12-2004, 04:50 PM
#19
This was the well documented time when we bolted a T4 onto a T3/4 specced engine running high compression (7.7:1) on four light blues. It all went Pete Tong due to the high cylinder pressures / creeping boost - we obviously found the limit of the Mahles . Mind you, it did 172mph twice before it went bang at Alconbury way back before I had the 500 engine (which was the catalyst for me going to this level).
. Mind you, it did 172mph twice before it went bang at Alconbury way back before I had the 500 engine (which was the catalyst for me going to this level).
15-12-2004, 04:55 PM
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I would suggest you holed through det not piston failure in the sense you say as a T4 creeping up to past say 26psi on 4 light blues would be suicidal 
If mahles were that weak how come they lasted so well in your 500 engine even though it was lower comp...that engine was caned for thousands of miles and no issues 
If mahles were that weak how come they lasted so well in your 500 engine even though it was lower comp...that engine was caned for thousands of miles and no issues
15-12-2004, 05:01 PM
#24
Phil,
You obviously didn't take in Karl's post. The reason the Mahles were okay at 7.2:1 is the fact that the cylinder pressure is reduced by a huge margin 
.
As I said, absolutely no sign of det, and it was No.2 piston that just broke apart. No holes, no signs of det (which if you have seen det, you will know is INSTANTLY recognisable). The fall-out destroyed the block and the head (just on that one bore), the other three were perfect
.
It was the combination of high compression and the boost creeping. It may have then started to det if the piston hadn't decided to give up the ghost before hand.
You obviously didn't take in Karl's post
. The reason the Mahles were okay at 7.2:1 is the fact that the cylinder pressure is reduced by a huge margin .As I said, absolutely no sign of det, and it was No.2 piston that just broke apart. No holes, no signs of det (which if you have seen det, you will know is INSTANTLY recognisable). The fall-out destroyed the block and the head (just on that one bore), the other three were perfect
.It was the combination of high compression and the boost creeping. It may have then started to det if the piston hadn't decided to give up the ghost before hand
.
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