VERY good writeup on lowering compression
07-06-2005, 08:18 PM
#1
DEYTUKURJERBS
Thread Starter
VERY good writeup on lowering compression
Somthing I noticed earlier, thanks to Jeff the clever American guy I bought by turbo and engine managment off (who also happens to own a 100% street legal car that runs low 8s and aiming for 7s this year) 
Does my feckin head in when people are determined to use high compression when they dont even seem to know why, and thankfully he agrees...
What's better, low compression and more boost or high compression and less boost?
There are certainly reasons to have a high compression ratio, namely when off-boost performance matters, but when talking purely about on-boost power potential, compression just doesn't make any sense.
People have tested the power effects of raising compression for decades, and the most optimistic results are about 3% more power with an additional point of compression (going from 9:1 to 10:1, for example). All combinations will be limited by detonation at some boost and timing threshold, regardless of the fuel used. The decrease in compression allows you to run more boost, which introduces more oxygen into the cylinder. Raising the boost from 14psi to 15psi (just a 1psi increase) adds an additional 3.4% of oxygen. So right there, you are already past the break even mark of losing a point of compression. And obviously, lowering the compression a full point allows you to run much more than 1 additional psi of boost. In other words, you always pick up more power by adding boost and lowering compression, because power potential is based primarily on your ability to burn fuel, and that is directly proportional to the amount of oxygen that you have in the cylinder. Raising compression doesn't change the amount of oxygen/fuel in the cylinder, it just squeezes it a bit more.
So the big question becomes, how much boost do we gain for X amount of compression? The best method we have found is to calculate the effective compression ratio (ECR) with boost. The problem is that most people use an incorrect formula that says that 14.7psi of boost on a 8.5:1 motor is a 17:1 ECR. So how in the world do people get away with this combination on pump gas? You can't even idle down the street on pump gas on a true 17:1 compression motor. Here's the real formula to use:
sqrt((boost+14.7)/14.7) * CR = ECR
sqrt = square root
boost = psi of boost
CR = static compression ratio of the motor
ECR = effective compression ratio
So our above example gives an ECR of 12.0:1. This makes perfect sense, because 12:1 is considered to be the max safe limit with aluminum heads on pump gas, and 15psi is about as much boost as you can safely run before you at least start losing a significant amount of timing to knock. Of course every motor is different, and no formula is going to be perfect for all combinations, but this one is vastly better than the standard formula (which leaves out the square root).
So now we can target a certain ECR, say 12.0:1. We see that at 8.5:1 CR we can run 14.7psi of boost. But at 7.5:1 we can run 23psi of boost (and still maintain the 12.0:1 ECR). We only gave up 1 point of compression (3% max power) and yet we gained 28% more oxygen (28% more power potential). Suddenly it's quite obvious why top fuel is running 5:1 compression, that's where all the power is!!
And before ANYONE says it, lower compression has no effect on what RPM your car begins to boost or hits full boost either....
Does my feckin head in when people are determined to use high compression when they dont even seem to know why, and thankfully he agrees...
What's better, low compression and more boost or high compression and less boost?
There are certainly reasons to have a high compression ratio, namely when off-boost performance matters, but when talking purely about on-boost power potential, compression just doesn't make any sense.
People have tested the power effects of raising compression for decades, and the most optimistic results are about 3% more power with an additional point of compression (going from 9:1 to 10:1, for example). All combinations will be limited by detonation at some boost and timing threshold, regardless of the fuel used. The decrease in compression allows you to run more boost, which introduces more oxygen into the cylinder. Raising the boost from 14psi to 15psi (just a 1psi increase) adds an additional 3.4% of oxygen. So right there, you are already past the break even mark of losing a point of compression. And obviously, lowering the compression a full point allows you to run much more than 1 additional psi of boost. In other words, you always pick up more power by adding boost and lowering compression, because power potential is based primarily on your ability to burn fuel, and that is directly proportional to the amount of oxygen that you have in the cylinder. Raising compression doesn't change the amount of oxygen/fuel in the cylinder, it just squeezes it a bit more.
So the big question becomes, how much boost do we gain for X amount of compression? The best method we have found is to calculate the effective compression ratio (ECR) with boost. The problem is that most people use an incorrect formula that says that 14.7psi of boost on a 8.5:1 motor is a 17:1 ECR. So how in the world do people get away with this combination on pump gas? You can't even idle down the street on pump gas on a true 17:1 compression motor. Here's the real formula to use:
sqrt((boost+14.7)/14.7) * CR = ECR
sqrt = square root
boost = psi of boost
CR = static compression ratio of the motor
ECR = effective compression ratio
So our above example gives an ECR of 12.0:1. This makes perfect sense, because 12:1 is considered to be the max safe limit with aluminum heads on pump gas, and 15psi is about as much boost as you can safely run before you at least start losing a significant amount of timing to knock. Of course every motor is different, and no formula is going to be perfect for all combinations, but this one is vastly better than the standard formula (which leaves out the square root).
So now we can target a certain ECR, say 12.0:1. We see that at 8.5:1 CR we can run 14.7psi of boost. But at 7.5:1 we can run 23psi of boost (and still maintain the 12.0:1 ECR). We only gave up 1 point of compression (3% max power) and yet we gained 28% more oxygen (28% more power potential). Suddenly it's quite obvious why top fuel is running 5:1 compression, that's where all the power is!!
And before ANYONE says it, lower compression has no effect on what RPM your car begins to boost or hits full boost either....
07-06-2005, 08:23 PM
#2
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Re: VERY good writeup on lowering compression
Originally Posted by Itsmeagain
And before ANYONE says it, lower compression has no effect on what RPM your car begins to boost or hits full boost either....
CR, Octane, Head port sizes, cams etc
07-06-2005, 08:42 PM
#3
DEYTUKURJERBS
Thread Starter
Headwork and Cams = Big difference that people dont seem to realise.
Compression Ratio = Small/Unnoticable difference that everyone makes a big deal about cause of the myths
Just like BB turbos 
Compression Ratio = Small/Unnoticable difference that everyone makes a big deal about cause of the myths
Just like BB turbos
Trending Topics
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08-06-2005, 11:10 AM
#13
Originally Posted by MattRS1600i
Why do WRC cars have 'high' comp engines? Is this purely for off boost performance?
08-06-2005, 11:28 AM
#14
DEYTUKURJERBS
Thread Starter
Heres my GTR forum sig now, just to wind em up a bit (this thread isnt posted there, but they all swear on 8.5:1 comp even at mega power (and they dont know why they all need to run race fuel and all got shite powerbands ).......
).......
Raising the compression a full point (eg from 7.5:1 to 8.5:1) gives no more than 3% more power.
Raising the boost just 1psi can give over 3% more power.
Dropping the compression by a full point enables you to run nearly 10psi more while maintining the same effective compression ratio, ie effectivley nearly 30% more power.
Why do you run high compression again? lol...
Oh, and street cars should run PUMP fuel
Raising the boost just 1psi can give over 3% more power.
Dropping the compression by a full point enables you to run nearly 10psi more while maintining the same effective compression ratio, ie effectivley nearly 30% more power.
Why do you run high compression again? lol...
Oh, and street cars should run PUMP fuel
08-06-2005, 12:51 PM
#16
Steve, the reason that the skyline lads run such high CR is a combination of the fact that the skyline revs higher, so an ECR of 14 or more isnt a problem and of course the fact that the VE at any given boost level is higher than a cossie anyway, as the head is so much better designed to begin with and so flows better.
Hope that helps.
Surprised at you getting drawn in by some very generic article and thinking you can apply it cart blanche to everything!
Hope that helps.
Surprised at you getting drawn in by some very generic article and thinking you can apply it cart blanche to everything!
08-06-2005, 12:57 PM
#17
DEYTUKURJERBS
Thread Starter
Chip- All relevant points, and the post IS generic, but totally relevant.
Skylines can make good power and rev high, fact.
But they still HAVE to run race fuel to get that power, and they wouldnt if the comp was lower.
They also all have appaling powerbands that often dont start till 5k or even 6k, and thats on cars with only 600odd bhp
They can be made to rev mega high, but most people arnt comfortable with revving constantly to 8k+
Skylines can make good power and rev high, fact.
But they still HAVE to run race fuel to get that power, and they wouldnt if the comp was lower.
They also all have appaling powerbands that often dont start till 5k or even 6k, and thats on cars with only 600odd bhp
They can be made to rev mega high, but most people arnt comfortable with revving constantly to 8k+
08-06-2005, 01:03 PM
#18
Originally Posted by Itsmeagain
Chip- All relevant points, and the post IS generic, but totally relevant.
Skylines can make good power and rev high, fact.
But they still HAVE to run race fuel to get that power, and they wouldnt if the comp was lower.
They also all have appaling powerbands that often dont start till 5k or even 6k, and thats on cars with only 600odd bhp
They can be made to rev mega high, but most people arnt comfortable with revving constantly to 8k+
Skylines can make good power and rev high, fact.
But they still HAVE to run race fuel to get that power, and they wouldnt if the comp was lower.
They also all have appaling powerbands that often dont start till 5k or even 6k, and thats on cars with only 600odd bhp
They can be made to rev mega high, but most people arnt comfortable with revving constantly to 8k+
And the race fuel they use for the same reason as the cossie boys do, it allows you to get more out of any spec that you have, even people with 7.2:1 cossies run race fuel when they are trying to set records etc, look at all the brunters lads for example!
I think you are completely missing the point TBH
The key thing is that number 12 in there, it just isnt relevant on a RB26 at 8000rpm, 14 or more is fine, then combine that with how little boost you need to make the same power.
If they dropped the CR more, then they would lose efficency at high RPM and would end up having to run more boost to try and get the same power they had to begin with, and more boost = higher inlet temps etc.
Stop over simplifying everything, you know better than that!
08-06-2005, 09:20 PM
#19
Indeed that was an interesting read. Difficult to put into real-life scenario's though.
I mean, without talking about the extremes of CR choice (as in the difference between 6:1 and 10:1), is Low Compression used when their isnt such a good facility to control Fuel/Ignition mapping?
e.g. If you had an engine at, say, 7:1 running 30psi with fairly primitive management, would that engine be less or more safe than one running 8:1 @ 20psi on advanced management if they both made the same power?? Or indeed, could they make the same power??
I mean, without talking about the extremes of CR choice (as in the difference between 6:1 and 10:1), is Low Compression used when their isnt such a good facility to control Fuel/Ignition mapping?
e.g. If you had an engine at, say, 7:1 running 30psi with fairly primitive management, would that engine be less or more safe than one running 8:1 @ 20psi on advanced management if they both made the same power?? Or indeed, could they make the same power??
08-06-2005, 10:02 PM
#20
DEYTUKURJERBS
Thread Starter
Chip- Due to high comp they have to run oversize turbos and wild as fuck cams and run relativley low boost.
All of the above gives them absolutley pathetic amounts of power until 5000rpm or more, and the only time they can run respectable power (even tho the powerband is still horrid) is on race fuel, otherwise its unsafe.
Lower comp means they could run higher boost, taking smaller turbos to their limits more, making the need for most the things that make their cars horrid to drive non-existant.
I can post some graphs up if you like, an average 600bhp 2.6 8.5:1 medium boost GTR engine, and then a graph for your average 2litre 7.2:1 high boost YB.
The YBs powerband is HUGELY better, with far more low down and fuckloads more torque all round.
All of the above gives them absolutley pathetic amounts of power until 5000rpm or more, and the only time they can run respectable power (even tho the powerband is still horrid) is on race fuel, otherwise its unsafe.
Lower comp means they could run higher boost, taking smaller turbos to their limits more, making the need for most the things that make their cars horrid to drive non-existant.
I can post some graphs up if you like, an average 600bhp 2.6 8.5:1 medium boost GTR engine, and then a graph for your average 2litre 7.2:1 high boost YB.
The YBs powerband is HUGELY better, with far more low down and fuckloads more torque all round.
08-06-2005, 10:04 PM
#21
PassionFord Post Whore!!
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In a road car high comp wins everytime..especially when you got a turbo that dont spool up till 5k
Steve are you STILL beleiving your car engine is gonna be a reality?
Steve are you STILL beleiving your car engine is gonna be a reality?
08-06-2005, 10:11 PM
#22
DEYTUKURJERBS
Thread Starter
You gotta be a prick to run a ROAD engine where the feckin turbo dont spool till 5k, so its patheticly irrelivant. Unless you got decent power by 4k at very most its a useless road engine, and compression wont change that.
OFF BOOST a smallish capacity car will be slow at sub 5k wether it 6:1 or 9:1, you must have driven enough cars to know that...
lol@gayboy with no clue butting in anyhow btw whats the pics of my engine everywhere? photoshop?
OFF BOOST a smallish capacity car will be slow at sub 5k wether it 6:1 or 9:1, you must have driven enough cars to know that...
lol@gayboy with no clue butting in anyhow
btw whats the pics of my engine everywhere? photoshop?
09-06-2005, 01:23 AM
#23
SteveN, it depends on what you rev too, an integra type r doesnt really make any real pwoer to nearly 6000rpm, but its still a nice driveable road engien as it has a band from 6-9 that goes like merry fook which is wider than you need to keep it on the boil with its standard gearing.
I think you are too hung up the low rpm stuff, a skyline engine revs further in the first place (in most cases) so its not a problem if everything moves up, plus it means less torque, and hence less chance of breaking things!
Its all down to the individual application.
For my mini for example, in order to make it road driveable i WANT a boost threshold of 4K or so, in order to give me the possibility of driving it around completely off boost, which at 8.5:1 cr will still be "nippy" for normal driving compared to other minis, then on boost it will go mental, a nice split personality for it to have i reckon.
I think you are too hung up the low rpm stuff, a skyline engine revs further in the first place (in most cases) so its not a problem if everything moves up, plus it means less torque, and hence less chance of breaking things!
Its all down to the individual application.
For my mini for example, in order to make it road driveable i WANT a boost threshold of 4K or so, in order to give me the possibility of driving it around completely off boost, which at 8.5:1 cr will still be "nippy" for normal driving compared to other minis, then on boost it will go mental, a nice split personality for it to have i reckon.
09-06-2005, 01:30 AM
#24
PS, you dont really need a bigger turbo for 600bhp @ 9000rpm than you do for 600bhp
@ 6000rpm
Ok the compressor map needs to be a slightly different shape so that its making the sameflow for less boost, but that just means a different configuration in terms of the relative size of the two housings etc, but it doesnt actaully need to be particuarly bigger overall.
@ 6000rpm
Ok the compressor map needs to be a slightly different shape so that its making the sameflow for less boost, but that just means a different configuration in terms of the relative size of the two housings etc, but it doesnt actaully need to be particuarly bigger overall.
09-06-2005, 03:18 PM
#25
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Steve think about it...i still got a 3k powerband 5k-8k...... T34 .48 also has a 3k powerband 3.5k to 6.5k a T4 a 3k power band...4k to 7k
So my car with the higher comp should feel nice below boost to use round town....but bea little bit special up the top
So my car with the higher comp should feel nice below boost to use round town....but bea little bit special up the top
09-06-2005, 04:31 PM
#27
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Originally Posted by Itsmeagain
Heres my GTR forum sig now, just to wind em up a bit (this thread isnt posted there, but they all swear on 8.5:1 comp even at mega power (and they dont know why they all need to run race fuel and all got shite powerbands ).......
).......
Raising the compression a full point (eg from 7.5:1 to 8.5:1) gives no more than 3% more power.
Raising the boost just 1psi can give over 3% more power.
Dropping the compression by a full point enables you to run nearly 10psi more while maintining the same effective compression ratio, ie effectivley nearly 30% more power.
Why do you run high compression again? lol...
Oh, and street cars should run PUMP fuel
Raising the boost just 1psi can give over 3% more power.
Dropping the compression by a full point enables you to run nearly 10psi more while maintining the same effective compression ratio, ie effectivley nearly 30% more power.
Why do you run high compression again? lol...
Oh, and street cars should run PUMP fuel
09-06-2005, 04:40 PM
#28
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Its so true what Steve stated about Skylines with poor powerbands. I have driven quite a few heavily modified (600+hp) examples and they have in the majority been as flat as a witches tit till 4.5-5krpm then whallop straight into warpdrive.
They feel awesome when they kick but day to day a nice stg3 Cos would be all over them on a twisty road/track.
They feel awesome when they kick but day to day a nice stg3 Cos would be all over them on a twisty road/track.
09-06-2005, 05:03 PM
#30
Norris Motorsport
LOL interesting, but total drivel!! 
Firstly I totally agree with lowering compression for normal road engines running high specific power outputs.
However regarding the information Steve has posted there are two blatent errors.........
Firstly raising the compression by a whole point (e.g. 7:1 to 8:1) does in fact increase torque output by around 7% with no other engine changes. (Not 3%)
Secondly the effective compression ratio is most definately NOT the sgrt of the pressure ratio X the C.R.
Firstly I totally agree with lowering compression for normal road engines running high specific power outputs.
However regarding the information Steve has posted there are two blatent errors.........
Firstly raising the compression by a whole point (e.g. 7:1 to 8:1) does in fact increase torque output by around 7% with no other engine changes. (Not 3%)
Secondly the effective compression ratio is most definately NOT the sgrt of the pressure ratio X the C.R.
09-06-2005, 05:21 PM
#32
So, where is the line between those that chose High Comp and those that chose Low Comp??
Is it purely down to Management capability?
Put it this way, if you had 2 engines, running essentially the same spec, but one was, say, 7:1 and the other was 8:1, if the management was very versatile, could the Higher Comp engine make the same/more power than the Low Comp engine? Or will Det spoil the fun?
Is it purely down to Management capability?
Put it this way, if you had 2 engines, running essentially the same spec, but one was, say, 7:1 and the other was 8:1, if the management was very versatile, could the Higher Comp engine make the same/more power than the Low Comp engine? Or will Det spoil the fun?
09-06-2005, 05:48 PM
#33
Norris Motorsport
Christian, unless the management is prehistoric (e.g. Escort turbos) then there is nothing modern management can really do that is of benefit in terms of an engines power potential that for example weber marelli can't.
The defining factor in terms of compression ratio is based on engine power output in respect to its size and the type of fuel used. Just because technology has improved does'nt mean the laws of physics has changed. If you compress any flamable mixture enough it will self ignite and this in essence is the issue with high boost and high compression when using a fuel that has poor resistance to self detenation. (e.g. pump fuel)
The defining factor in terms of compression ratio is based on engine power output in respect to its size and the type of fuel used. Just because technology has improved does'nt mean the laws of physics has changed. If you compress any flamable mixture enough it will self ignite and this in essence is the issue with high boost and high compression when using a fuel that has poor resistance to self detenation. (e.g. pump fuel)
09-06-2005, 11:28 PM
#34
Im lost with the ECR calculation above.
If you take a 7:1 CR Engine and Fire 28psi at it the ECR comes out a 20:1 according to that calculation? I can't see how thats acceptable according to anything I have read on ECR's, yet many people run a set-up along those lines.
Or have I totally missed the point??
If you take a 7:1 CR Engine and Fire 28psi at it the ECR comes out a 20:1 according to that calculation? I can't see how thats acceptable according to anything I have read on ECR's, yet many people run a set-up along those lines.
Or have I totally missed the point??
11-06-2005, 01:29 PM
#37
DEYTUKURJERBS
Thread Starter
7:1 and perfectly happy (as is the turbo!) to run 2bar+.
Relativley mild cams and head ports (compared to everyone else anyhow ) and a not OTT exhaust housing a/r.
Pump fuel.
Cant afford luxurys like race fuel, its a road car and thats where 95% of its hard driving will be.
Relativley mild cams and head ports (compared to everyone else anyhow
) and a not OTT exhaust housing a/r.Pump fuel.
Cant afford luxurys like race fuel, its a road car and thats where 95% of its hard driving will be.
11-06-2005, 01:37 PM
#38
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Originally Posted by Itsmeagain
7:1 and perfectly happy (as is the turbo!) to run 2bar+.
Relativley mild cams and head ports (compared to everyone else anyhow ) and a not OTT exhaust housing a/r.
Pump fuel.
Cant afford luxurys like race fuel, its a road car and thats where 95% of its hard driving will be.
Relativley mild cams and head ports (compared to everyone else anyhow
) and a not OTT exhaust housing a/r.Pump fuel.
Cant afford luxurys like race fuel, its a road car and thats where 95% of its hard driving will be.
I would have liked a Valcon setup but they are not shipping till August so a set of Rods cams which are very conservative are going in.
I am not interested in what mine makes on anything other than Optimax.
Cam
11-06-2005, 01:41 PM
#39
DEYTUKURJERBS
Thread Starter
7.5:1
Cool.
I gotta say, is that a "common" thing for Rod Bell to do (just keeps it quiet), or is this somthing you asked them to do, as all skyline "experts" seem to swear by 8.5:1 no matter what and just use race fuel.
TBH the only reason ive gone for 3litres is much more exhaust gas to spin the turbo up faster, off boost performance matters nothing to me as long as I can get it on boost soon enough
Cool.
I gotta say, is that a "common" thing for Rod Bell to do (just keeps it quiet), or is this somthing you asked them to do, as all skyline "experts" seem to swear by 8.5:1 no matter what and just use race fuel.
TBH the only reason ive gone for 3litres is much more exhaust gas to spin the turbo up faster, off boost performance matters nothing to me as long as I can get it on boost soon enough
11-06-2005, 02:01 PM
#40
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I cant really say if its common practice TBH. When I first approached Rod he spent a good 40 mins talking to me about the available options/specs/costs and also asked exactly how I wanted my car to perform as regard off boost performance, how turbos spun up, power I wanted and what was its main use before he suggested the spec. I cannot speak highly enough about the quality of the service he provides as its the very best I have ever encountered from any tuning firm with the primary goal being totally satisfying the customer...Fuck this is sounding like an advert.
Cam
Cam