Low Compression Vs High Compression - Discuss
05-02-2004, 01:00 PM
#122
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You all crazy.
People HAVE to go low comp to compensate for the fact that they can't cool the air enuff before it enters the cylinders.
It costs ALOT more to run systems to cool the air enuff to run high boost on high comp.
thats all it is.
on high comp high boost you have to run even bigger coolers to bring the temp down to an operable level (assuming boost at same as low-comp)
thus you outwiegh the "benefits" if you go too far, ie boost is so high, and comp is so high, that cooling system is so big it takes 5000rpm to fill the system to operable pressure.
what you all need to do is get a book on turbo'd dragsters yank style.
methanol injection (if not fully fueled) is the way forwards.
CO2 spray on intercooler (if not methanol) is also the way.
reasonable sized cooler (not truck style like that red escort avec lorry cooler)
on yank dragsters they spray methanol into the system b4 cooler, over the cooler, and run the thing on methanol!
they put like 14:1 CR with 30+psi easy, an they do 1500-2500hp+
all it is is making sure the peak compression temperature does not exceed the natural fuel ignition temperature. and its not an easy thing to do. else we'd all be doing it!
People HAVE to go low comp to compensate for the fact that they can't cool the air enuff before it enters the cylinders.
It costs ALOT more to run systems to cool the air enuff to run high boost on high comp.
thats all it is.
on high comp high boost you have to run even bigger coolers to bring the temp down to an operable level (assuming boost at same as low-comp)
thus you outwiegh the "benefits" if you go too far, ie boost is so high, and comp is so high, that cooling system is so big it takes 5000rpm to fill the system to operable pressure.
what you all need to do is get a book on turbo'd dragsters yank style.
methanol injection (if not fully fueled) is the way forwards.
CO2 spray on intercooler (if not methanol) is also the way.
reasonable sized cooler (not truck style like that red escort avec lorry cooler)
on yank dragsters they spray methanol into the system b4 cooler, over the cooler, and run the thing on methanol!
they put like 14:1 CR with 30+psi easy, an they do 1500-2500hp+
all it is is making sure the peak compression temperature does not exceed the natural fuel ignition temperature. and its not an easy thing to do. else we'd all be doing it!
05-02-2004, 01:06 PM
#124
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Originally Posted by PJay
else we'd all be doing it!
Originally Posted by turbo man
its not an easy thing to do
05-02-2004, 01:16 PM
#125
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I did readed it yes, why is acheaving an acceptable ACT so hard, i think it's not, why is it more expensive than low comp, again an engine build cost would be similar surely? Were not talking methanol, race fuel or another were talking road cars, wheres the infor mation to substanciate your claims?
PJay
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05-02-2004, 01:19 PM
#126
So in conclusion, from what i've read over the past three pages,
High comp offers more benefits and performance but requires more precise mapping and setting up and looking after whereas low engines are easier to set up and are easier to look after and keep running.
Also low comp engines are cheaper to build and run compared to a high comp engine. If thats the case, low comp all the way for someone like myself. Not much money not much knowledge.
High comp offers more benefits and performance but requires more precise mapping and setting up and looking after whereas low engines are easier to set up and are easier to look after and keep running.
Also low comp engines are cheaper to build and run compared to a high comp engine. If thats the case, low comp all the way for someone like myself. Not much money not much knowledge.
05-02-2004, 01:20 PM
#127
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Also low comp engines are cheaper to build and run compared to a high comp engine. If thats the case, low comp all the way for someone like myself. Not much money not much knowledge.
05-02-2004, 01:33 PM
#128
I know little or nothing about the subject, just coming up with assumptions from what other people are saying.
Just trying to figure out the cost side of it all. In the future i'm hoping for 400bhp plus so am wondering what route to take.
Just trying to figure out the cost side of it all. In the future i'm hoping for 400bhp plus so am wondering what route to take.
05-02-2004, 01:33 PM
#129
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what do you consider an acceptable act? 28DegC with 30psi?
*Following are not actual figures, just examples*
on a 7:1CR engine this 28 becomes 196DegC under engine compression
on a 10:1CR engine this 28 becomes a 280DegC under engine compression
spontanious combustion of the fuel occurs at 220DegC
so the air needs to be made cooler BEFORE entering the cylinders, ie 22DegC at least.
now being as people running 7:1CR can have "problems" with detonation it becomes alot more problamatic once the CR is raised.
boost is "free" power, why waste it? high CRs are used to eliminate lag, plus you can only build on what you start with, and a high CR starts higher. its all swings and round abouts. do something, cut back on another. classic examlpe of our lecture on lateral weight load, do we load the car balanced when static, or is the track more suited to an asymetric load when static, ie are there more left corners than right. you give something up for another.
as in, high CR with same boost as low CR needs a bigger cooler to stop det, but at what point does the size of the cooler have to be so big to stop this that the lag is so big that it becomes pointless?
*Following are not actual figures, just examples*
on a 7:1CR engine this 28 becomes 196DegC under engine compression
on a 10:1CR engine this 28 becomes a 280DegC under engine compression
spontanious combustion of the fuel occurs at 220DegC
so the air needs to be made cooler BEFORE entering the cylinders, ie 22DegC at least.
now being as people running 7:1CR can have "problems" with detonation it becomes alot more problamatic once the CR is raised.
boost is "free" power, why waste it? high CRs are used to eliminate lag, plus you can only build on what you start with, and a high CR starts higher. its all swings and round abouts. do something, cut back on another. classic examlpe of our lecture on lateral weight load, do we load the car balanced when static, or is the track more suited to an asymetric load when static, ie are there more left corners than right. you give something up for another.
as in, high CR with same boost as low CR needs a bigger cooler to stop det, but at what point does the size of the cooler have to be so big to stop this that the lag is so big that it becomes pointless?
05-02-2004, 01:58 PM
#130
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But you won't need 30 psi to make the same power on the high compression therefore the outlet temperature of the turbo will be lower so intake temperature will be lower the reason people struggle with det on low 7.?:1 compression ratios is because they are running huge 30+Psi boost to compensate for the lack of N/A power that a low compression engine can produce an 8.0:1 engince can make 550bhp on 25 ish psi i'd be interested to see the similar ouout for a low compression engine running the same boost, not that boost has anything to do with power but is just a by product of the turbo generating increased air flow and mass.
PJay
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05-02-2004, 04:33 PM
#131
Most engines are a compromise and it is only the ones that are live mapped that are getting the full potential of low or high comp / max ignition / high boost anyway.
As long as the car is live mapped, you shouldn't have a problem either way, as you take the engine to the point of det and then back it off, so it is swings and roundabouts.
Each engine has to be specced from scratch and EVERY single component matched to the spec to keep the cylinder pressures at an acceptable level.
The way I went about doing my engine was firstly deciding what power output I wanted to achieve, then I decided what turbo I was going to use (based on my power requirement and the optimum boost that this turbo was most efficient at). From there we were able to work out what was the highest c/r we could use to give accceptable cylinder pressures. It was then live mapped on an engine dyno with the maximum ignition advance the above could cope so that det occurred (and then backed off to a lower "safe" level) and obviously the fuelling matched to this.
The engine exceeded it's remit by almost 30bhp, so I was WELL happy and it feels like a T3/4 car down the bottom end
. EXACTLY what I wanted - a good point and squirt car.
Most people are chasing numbers without ANY consideration to how the car will drive. I have an engine that is considerably less powerful than most T4 equipped cars, but I think it is much more driveable than almost any of them...
As long as the car is live mapped, you shouldn't have a problem either way, as you take the engine to the point of det and then back it off, so it is swings and roundabouts.
Each engine has to be specced from scratch and EVERY single component matched to the spec to keep the cylinder pressures at an acceptable level.
The way I went about doing my engine was firstly deciding what power output I wanted to achieve, then I decided what turbo I was going to use (based on my power requirement and the optimum boost that this turbo was most efficient at). From there we were able to work out what was the highest c/r we could use to give accceptable cylinder pressures. It was then live mapped on an engine dyno with the maximum ignition advance the above could cope so that det occurred (and then backed off to a lower "safe" level) and obviously the fuelling matched to this.
The engine exceeded it's remit by almost 30bhp, so I was WELL happy and it feels like a T3/4 car down the bottom end
. EXACTLY what I wanted - a good point and squirt car.Most people are chasing numbers without ANY consideration to how the car will drive. I have an engine that is considerably less powerful than most T4 equipped cars, but I think it is much more driveable than almost any of them...
on a DIY build 
07-02-2004, 06:51 PM
#135
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we agree i can see, but we are just slightly out with the way we are looking at it with each other. not gonna argue cos there is no arguement pjay, just missing each othas points.
but..
boost is the "simple" way of saying increased air flow and mass.
the mass is increased due to the fact that it is under pressure, "boost", so how is it not related to power?
but..
Originally Posted by pjay
not that boost has anything to do with power but is just a by product of the turbo generating increased air flow and mass.
the mass is increased due to the fact that it is under pressure, "boost", so how is it not related to power?
07-02-2004, 07:03 PM
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boost is the "simple" way of saying increased air flow and mass.
the mass is increased due to the fact that it is under pressure, "boost", so how is it not related to power?
the mass is increased due to the fact that it is under pressure, "boost", so how is it not related to power?
I agree 100% that the mass is incresed due to the boost but i was trying to say that a high compession engine requires less boost to generate x amount of power compared to a low compression engine. and in doing so as i'm sure you are aware less boost will result in less temperature increase of the air under compression in the turbo compresser. Also a larger turbo produces more 'mass' per pound of boost than a smaller turbo
07-02-2004, 08:13 PM
#138
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it really does amaze me that some people are dumb enough to think that engine management systems can change the laws of physics and fundmental engine principals............. hilarious really 
07-02-2004, 08:19 PM
#139
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Originally Posted by Matt
it really does amaze me that some people are dumb enough to think that engine management systems can change the laws of physics and fundmental engine principals............. hilarious really
07-02-2004, 08:26 PM
#140
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where did that come from Matt? engine management doesn't really affect this situation. all it needs to be is setup properly, it won't change anything magically.
PJay-
yeah i know wot ur saying, all i was saying was that boost is "free" energy, so keeping the "free" energy input the same, a high comp engine can produce more power, if ample cooling is induced, but as said, at what point does the system to cool become so vast that it takes forever to reach the same boost level.
PJay-
yeah i know wot ur saying, all i was saying was that boost is "free" energy, so keeping the "free" energy input the same, a high comp engine can produce more power, if ample cooling is induced, but as said, at what point does the system to cool become so vast that it takes forever to reach the same boost level.
07-02-2004, 08:32 PM
#141
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turbo man, it came in response to the usual crap you read on such discussions about low comp turbo's lumps being a thing of the past due to newly developed engine manegement system..............its just total and utter stupidity
my car is low comp and you couldnt get any more pointier and squirtier if you tried
my car is low comp and you couldnt get any more pointier and squirtier if you tried
07-02-2004, 09:17 PM
#142
10-03-2004, 11:14 PM
#144
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Originally Posted by cossiemanden
in sweden there is all ready en 734bhp engine... and a few in norway...
2L engines... high kompression
2L engines... high
kompression
10-03-2004, 11:16 PM
#145
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It is my understanding ultimatly the engines power/boost will define the CR.
A low power engine can run a higher CR, such as 8.3:1 for example to mainatin drivability, and a massive power engine, due to the laws of physics (dynamic compresion and flashpoints etc) will require a lower CR.
Exceptions being WRC cars which dont defy physics, their conditions are altered so they stay within the boundrys, ie, boost tailing off at high rpm (Hence they can run 2bar at 9:1CR)
Thats my brief understanding
A low power engine can run a higher CR, such as 8.3:1 for example to mainatin drivability, and a massive power engine, due to the laws of physics (dynamic compresion and flashpoints etc) will require a lower CR.
Exceptions being WRC cars which dont defy physics, their conditions are altered so they stay within the boundrys, ie, boost tailing off at high rpm (Hence they can run 2bar at 9:1CR)
Thats my brief understanding
11-03-2004, 07:40 AM
#147
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I am stupid when it comes to engines but if rally cars used to run low comp and now run high comp and everybody follows the trend... would time not be better spent in developing the high revving aka Karls engine or develop say a turbo vtec style type engine ???
will the trend stretch to gearboxes They used to run 6 or 7 speed for rally cars and still do but now the likes of Pug team run a 4 speed box so will everyone be going for a 4 speed downgrade conversion I mean havent we come from there allready back in the day 
will the trend stretch to gearboxes
They used to run 6 or 7 speed for rally cars and still do but now the likes of Pug team run a 4 speed box so will everyone be going for a 4 speed downgrade conversion I mean havent we come from there allready back in the day
11-03-2004, 01:34 PM
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and rally cars can be rebuilt after every weekend and dont have to be driven to work on the monday morning.
i dont understand all the issues but it comes accross as having the cash and knowledge etc.
if it was me i would pick my trusted tuner and listen to wot they say, they are the experts and if u cant trust um why are they building ur engine?
it all seems like horses for courses!
i dont understand all the issues but it comes accross as having the cash and knowledge etc.
if it was me i would pick my trusted tuner and listen to wot they say, they are the experts and if u cant trust um why are they building ur engine?
it all seems like horses for courses!
11-03-2004, 01:50 PM
#150
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Originally Posted by scottbrown
it all seems like horses for courses!
11-03-2004, 02:06 PM
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Sounds like an indian restaurant i once visisted. I think both the first course and the second course were horse!!
20-01-2006, 02:47 AM
#154
DEYTUKURJERBS
Turboman makes a LOT of sence, unlike most.
High comp and big bhp per litre only works with race fuel and/or incredible charge cooling.
If any of your believe (certainly used to by looks of this thread) that the high comp american or jap cars run big boost on pump fuel your massivley mistaken.
And most are horrendous to drive as to make half decent power with such high comp and run pump fuel they need massivley OTT exhaust housings and cams compared to a big boost low comp engine.
High comp and big bhp per litre only works with race fuel and/or incredible charge cooling.
If any of your believe (certainly used to by looks of this thread) that the high comp american or jap cars run big boost on pump fuel your massivley mistaken.
And most are horrendous to drive as to make half decent power with such high comp and run pump fuel they need massivley OTT exhaust housings and cams compared to a big boost low comp engine.
20-12-2006, 06:14 PM
#155
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bk up ttt 
we are having a zetec turbo engine buillt but the tuner rekons on a 9 to 1 compression ratio i thought that was a bit high but at the end of the day they are more expierienced at building engines than me.
wot you guys rekon?
we are having a zetec turbo engine buillt but the tuner rekons on a 9 to 1 compression ratio i thought that was a bit high but at the end of the day they are more expierienced at building engines than me.
wot you guys rekon?
29-12-2006, 09:21 PM
#156
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Stavros when you say high comp and big bhp per litre what figures did you have in mind?
Theres people on here running in the region of 450hp yb's on on anything between 7 and 8:1, obvioulsy theres a a few people below and above these figures but generally it seems sub 8 is favoured...yeah?
I think it would make things alot more understandable for and possibly make a better discussion if we had an example to work off rather than throwing different comp ratio's on different engines and different turbos...what do think?
Lets says a yb(obvioulsy), 500hp is a pretty round number, 7.5k rev limit, rs500 cooler and a gt30r. Firstly i'd want to work out as close as i could as to how much air this will need to see than kind of hp, i'd then see where in the comp map the turbo is likly to supply it while factoring in pressure drops and estimated cooling of the air at the given flow rate.........and so on
Looking at turbo mans example of act temp increase during compression surly we can guessitmate
enough of the factors to give a good working out ballpark figure to what comp ratio we should be at?
Then all it would take would be to build and run this engine setup, test it, factor into the first equation the actaul data we would now have and recalculate to a more.....accurate target comp figure for the next build. Surly it would take long to get to a point where it dyno's at just the right comp ratio
Anyone with plenty cash please try out my theory.
Feel free to tell me to shut up if i'm talking pants
Theres people on here running in the region of 450hp yb's on on anything between 7 and 8:1, obvioulsy theres a a few people below and above these figures but generally it seems sub 8 is favoured...yeah?
I think it would make things alot more understandable for and possibly make a better discussion if we had an example to work off rather than throwing different comp ratio's on different engines and different turbos...what do think?
Lets says a yb(obvioulsy), 500hp is a pretty round number, 7.5k rev limit, rs500 cooler and a gt30r. Firstly i'd want to work out as close as i could as to how much air this will need to see than kind of hp, i'd then see where in the comp map the turbo is likly to supply it while factoring in pressure drops and estimated cooling of the air at the given flow rate.........and so on
Looking at turbo mans example of act temp increase during compression surly we can guessitmate
enough of the factors to give a good working out ballpark figure to what comp ratio we should be at?Then all it would take would be to build and run this engine setup, test it, factor into the first equation the actaul data we would now have and recalculate to a more.....accurate target comp figure for the next build. Surly it would take long to get to a point where it dyno's at just the right comp ratio
Anyone with plenty cash please try out my theory.
Feel free to tell me to shut up if i'm talking pants
22-01-2007, 06:14 PM
#157
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Sorry to get this thread again but I have more questions :
I am going to start to plan an engine build, but I don't know yet all the specs. What I have in 1st position is I want torque. Not torque at high rpms, but torque from low rpms to middle, middle-high rpms.
This car won't be used everyday, only once a week or once in 2 weeks.
I will go for antilag of course, the aggressive one, on a T35. Then it is a bit blank. Would it be better to go in low or high-comp engine ? 4 greys I belive, but which camshafts ? Lot of interrogations.
Power would be around 350bhp.
Open to advices, also in PM if needed.
I am going to start to plan an engine build, but I don't know yet all the specs. What I have in 1st position is I want torque. Not torque at high rpms, but torque from low rpms to middle, middle-high rpms.
This car won't be used everyday, only once a week or once in 2 weeks.
I will go for antilag of course, the aggressive one, on a T35. Then it is a bit blank. Would it be better to go in low or high-comp engine ? 4 greys I belive, but which camshafts ? Lot of interrogations.
Power would be around 350bhp.
Open to advices, also in PM if needed.
15-11-2007, 01:52 AM
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Originally Posted by Stavros
Turboman makes a LOT of sence, unlike most.
High comp and big bhp per litre only works with race fuel and/or incredible charge cooling.
If any of your believe (certainly used to by looks of this thread) that the high comp american or jap cars run big boost on pump fuel your massivley mistaken.
And most are horrendous to drive as to make half decent power with such high comp and run pump fuel they need massivley OTT exhaust housings and cams compared to a big boost low comp engine.
High comp and big bhp per litre only works with race fuel and/or incredible charge cooling.
If any of your believe (certainly used to by looks of this thread) that the high comp american or jap cars run big boost on pump fuel your massivley mistaken.
And most are horrendous to drive as to make half decent power with such high comp and run pump fuel they need massivley OTT exhaust housings and cams compared to a big boost low comp engine.
) Like you know, all things in life take a certain amount of time to take place. Detonation occurs when the fuel as enough time to heat up. that said, we can easily have a 2L engine 9:1CR and produce 900hp on pump fuel, simply by increasing rpm!
15-11-2007, 10:07 AM
#159
Originally Posted by paconassa
Originally Posted by Stavros
Turboman makes a LOT of sence, unlike most.
High comp and big bhp per litre only works with race fuel and/or incredible charge cooling.
If any of your believe (certainly used to by looks of this thread) that the high comp american or jap cars run big boost on pump fuel your massivley mistaken.
And most are horrendous to drive as to make half decent power with such high comp and run pump fuel they need massivley OTT exhaust housings and cams compared to a big boost low comp engine.
High comp and big bhp per litre only works with race fuel and/or incredible charge cooling.
If any of your believe (certainly used to by looks of this thread) that the high comp american or jap cars run big boost on pump fuel your massivley mistaken.
And most are horrendous to drive as to make half decent power with such high comp and run pump fuel they need massivley OTT exhaust housings and cams compared to a big boost low comp engine.
) Like you know, all things in life take a certain amount of time to take place. Detonation occurs when the fuel as enough time to heat up. that said, we can easily have a 2L engine 9:1CR and produce 900hp on pump fuel, simply by increasing rpm!
29-11-2007, 03:25 AM
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Originally Posted by Mike Rainbird
Originally Posted by paconassa
Originally Posted by Stavros
Turboman makes a LOT of sence, unlike most.
High comp and big bhp per litre only works with race fuel and/or incredible charge cooling.
If any of your believe (certainly used to by looks of this thread) that the high comp american or jap cars run big boost on pump fuel your massivley mistaken.
And most are horrendous to drive as to make half decent power with such high comp and run pump fuel they need massivley OTT exhaust housings and cams compared to a big boost low comp engine.
High comp and big bhp per litre only works with race fuel and/or incredible charge cooling.
If any of your believe (certainly used to by looks of this thread) that the high comp american or jap cars run big boost on pump fuel your massivley mistaken.
And most are horrendous to drive as to make half decent power with such high comp and run pump fuel they need massivley OTT exhaust housings and cams compared to a big boost low comp engine.
) Like you know, all things in life take a certain amount of time to take place. Detonation occurs when the fuel as enough time to heat up. that said, we can easily have a 2L engine 9:1CR and produce 900hp on pump fuel, simply by increasing rpm!