SolarFlare
Well-Known Member
LOL....riiiight.
Sponsored
Sponsored
markmurfie
Well-Known Member
Lowering compression on pump gas, allowing for more boost, will not get you more power.
Reliability, maybe, but highly debatable on how will it's calibrated to the octane limit. Margin for error is increased with lower compression, but to me that is different than reliability.
Calculating some limit, based on pre-spark, is the totally wrong approach to this limit. Detonation, the physical phenomenon you are limited by, happens post spark. If pump gas makes high enough cylinder pressure to break a rod in a gen 2/3, before detonating, it was probably octane supplemented in some way.
Any tuner, who has a high degree of confidence in their calibration, would not suggest lowering the compression in any coyote. For gen 1 coyotes it's upgraded rods and piston, using the same compression. Then in the later ones, cylinder sleeves are added to rods and pistons, showing just how good the stock components in them are.
Reliability, maybe, but highly debatable on how will it's calibrated to the octane limit. Margin for error is increased with lower compression, but to me that is different than reliability.
Calculating some limit, based on pre-spark, is the totally wrong approach to this limit. Detonation, the physical phenomenon you are limited by, happens post spark. If pump gas makes high enough cylinder pressure to break a rod in a gen 2/3, before detonating, it was probably octane supplemented in some way.
Any tuner, who has a high degree of confidence in their calibration, would not suggest lowering the compression in any coyote. For gen 1 coyotes it's upgraded rods and piston, using the same compression. Then in the later ones, cylinder sleeves are added to rods and pistons, showing just how good the stock components in them are.
SolarFlare
Well-Known Member
Thanks for posting Mark, Ive seen your post over the last few years, always very detailed and helpful. 
markmurfie
Well-Known Member
Forgot to mention, head studs on gen 2, is also commonly done before rods and pistons.
engineermike
Well-Known Member
In 2000, BMW was testing GDI and turbocharging. They published an SAE article (2000-01-0250) that stated the following: "...compression ratio is a very important parameter for admissible boost pressure and full load torque as well as for part load fuel consumption....The effects show clear advantages for a high compression ratio in part load fuel consumption and combustion stability. ...At high load, however, with reduced compression ratio nearly constant fuel consumption (due to lower knocking sensitivity) and significantly higher full load results...can be achieved. A further reduction of the compression ratio leads to slightly higher full load torque and lower emissions..."
I suppose it's not coincidence that the current BMW 3.0 turbocharged GDI inline-6 is offered in 11/1 compression making 382 hp, but at 9.3/1 compression it makes 503 hp with an increase in boost.
Sponsored
markmurfie
Well-Known Member
"CONCLUSION: The combination of a turbocharged S.I. engine with direct injection and a suitable compression ratio promises potential for further efficiency optimization. For stoichiometric mixture, the compression ratio should not be chosen lower than a reasonable minimum value, even for maximum charging and best torque, in order to limit the exhaust gas temperature and to enhance efficiency. On the other hand, high ε would be preferable for best part load efficiency, but causes higher HC emissions and heavy full load knocking problems. Part load operation with stratified charge may be enhanced to higher specific load by turbo-charging. The effects of the compression ratio on the engine behavior with stratified charge are basically similar like with homogeneous charge. However, parameters like injection and ignition timing and the heat release rate turn out to be more relevant for combustion efficiency and emissions at stratified operation."
ε= Compression ratio FYI.
"In this publication most of the results at different load, compression ratios and charging manners are discussed at an engine speed of n=2000 rpm. All results are presented with ignition timing for highest efficiency. However, especially for high load, the knocking limit had to be considered."
No where in this paper is HP or torque output measured nor does it says they were able to achieve more torque or HP from a lower compression ratio with the higher boost, just that they could raise boost giving the Wi a greater range with the lower compression ratio. Wi is modified by removing an estimated amount of work done by the supercharger/turbo charger in compressing air, there was no actual supercharger/ turbocharger on the actual test engine. Air is conditioned to a specific temperature and pressure then feed to the test engine. Seeing as they where using an optimized ignition value, and not willing to retard that value like any tuner/calibrator would in a real world application, I'm not surprised at these results. Numerous times they do indicate a higher compression ratio is the better option for every except fuel consumption and HC emissions, which usually happen when you make more power and torque. Even in the conclusion minimum value not maximum value is stated indicating the search for benefits from higher compression.
Wi=indicated specific work of 4 strokes 720* of engine rotation.
Maybe buy and read the paper, before citing the abstract like CNN or FOX news to suit your argument. Not likely going to find out how to make more torque and HP, reading studies done on how to improve MPG and emissions. Especially when we are discussing 700+hp 7500+RPM redline coyote applications.
ε= Compression ratio FYI.
"In this publication most of the results at different load, compression ratios and charging manners are discussed at an engine speed of n=2000 rpm. All results are presented with ignition timing for highest efficiency. However, especially for high load, the knocking limit had to be considered."
No where in this paper is HP or torque output measured nor does it says they were able to achieve more torque or HP from a lower compression ratio with the higher boost, just that they could raise boost giving the Wi a greater range with the lower compression ratio. Wi is modified by removing an estimated amount of work done by the supercharger/turbo charger in compressing air, there was no actual supercharger/ turbocharger on the actual test engine. Air is conditioned to a specific temperature and pressure then feed to the test engine. Seeing as they where using an optimized ignition value, and not willing to retard that value like any tuner/calibrator would in a real world application, I'm not surprised at these results. Numerous times they do indicate a higher compression ratio is the better option for every except fuel consumption and HC emissions, which usually happen when you make more power and torque. Even in the conclusion minimum value not maximum value is stated indicating the search for benefits from higher compression.
Wi=indicated specific work of 4 strokes 720* of engine rotation.
Maybe buy and read the paper, before citing the abstract like CNN or FOX news to suit your argument. Not likely going to find out how to make more torque and HP, reading studies done on how to improve MPG and emissions. Especially when we are discussing 700+hp 7500+RPM redline coyote applications.
Last edited:
markmurfie
Well-Known Member
Top fuel nitromethane cars do run 6.5-7 CR, and top alcohol(methanol) cars run the same 60+PSI boost, but with 11-13:1 compression ratio. They probably all say the higher the better or it matches their choice of SC and boost level.
If you think the power increase comes from the drop in compression ratio, and not the fuel they are using, You seriously need to research nitromethane. Hell go buy a hobby RC car and play with it, run different percentage nitro fuels.
The nitromethane to methanol ratio is being limited to 90:10, in an effort to keep top speeds lower in top fuel cars. Nothing mentioned about raising compression ratio to limit the top speed.
If you think the power increase comes from the drop in compression ratio, and not the fuel they are using, You seriously need to research nitromethane. Hell go buy a hobby RC car and play with it, run different percentage nitro fuels.
The nitromethane to methanol ratio is being limited to 90:10, in an effort to keep top speeds lower in top fuel cars. Nothing mentioned about raising compression ratio to limit the top speed.
- Joined
- Sep 26, 2019
- Threads
- 26
- Messages
- 3,057
- Reaction score
- 1,220
- Location
- California
- First Name
- Jaxson
- Vehicle(s)
- 2015 mustang gt
- Banned
- #83
This guy knows what he is talking about.
You can get a Gen3 f150 short block For 1700 brand new from tasca. Put rods and pistons and your good for 1000whp
- Joined
- Sep 26, 2019
- Threads
- 26
- Messages
- 3,057
- Reaction score
- 1,220
- Location
- California
- First Name
- Jaxson
- Vehicle(s)
- 2015 mustang gt
- Banned
- #84
you can build a engine for cheap new block $850
New crank $350
NewRods $200
New Piston kit Mahle 800ish
Bearings 100-250
Opg 20-400
I built mine with boostline rods and I was under 4K including head work
New crank $350
NewRods $200
New Piston kit Mahle 800ish
Bearings 100-250
Opg 20-400
I built mine with boostline rods and I was under 4K including head work
engineermike
Well-Known Member
I didn't think it was kosher to copy and paste to the public out of a paid IHS subscription.
I am bewildered that it seems as though at least two people actually believe that lowering compression ratio and adding boost won't result in more power when octane limited. If that isn’t understood by all participants then I’m not sure we can have a meaningful conversation around the pros and cons of each path.
It is true that most modern technical articles are aimed at increasing efficiency, largely through downsizing. However, they are applicable in such that many are trying to balance compression ratio and boost for obvious reasons. If we can apply even a fraction of the findings from a 2.0 liter 30 bar BMEP 91 octane engine to a 5 liter engine, then all the better for us.
For instance, MIT states "Turbocharging combined with downsizing to keep engine power constant is a relatively easy way to improve engine efficiency". Therefore, if we turbocharge and don't downsize, then we gain power. They go on to say, "However, turbo charging also increases peak in-cylinder pressures making the engine more prone to knock. The compression ratio of turbo charged production engines is therefore usually reduced by 1-2 compared to naturally aspirated engines." http://dspace.mit.edu/bitstream/handle/1721.1/69496/775670245-MIT.pdf?sequence=2 So, they turbocharge and reduce compression, but are still making the same or more power than the larger engines they replaced. Ergo, lower compression + boost = more power. Here's an interesting analysis of the variable compression ratio engine where, at least in their extensive modelling, dropping the compression from 12/1 to 8/1 and adding manifold pressure allowed massive increases in torque and horsepower. https://deepblue.lib.umich.edu/handle/2027.42/140732
If it weren't the case, then why would Nissan have developed a variable compression engine? "...changes its compression ratio between 8:1 (for high performance) and 14:1 (for high efficiency)....A low compression ratio allows for greater power and torque, and avoids knocking." If boost and high compression had no vulnerability, then why would Nissan go to such great lengths to lower compression ratio while under boost? Why not just run boost and 12/1 all the time? https://www.greencarcongress.com/2018/03/20180329-altima.html
SAAB seems to think it's the case as well: "Use high compression ratio during low load for high efficiency, and as the load increases the compression ratio is decreased to match the knocking." https://www.vehicular.isy.liu.se/Publications/MSc/03_EX_3421_AB.pdf
Other MIT folks seem to think that dropping compression from 12/1 to 10/1 allows for about 6 psi boost when both are running at the knock limit. They didn't quantify the power increase. However, I know from experience and my own testing plus prior research that the compression drop would be worth a debit of about 30 hp. 6 psi boost of course is worth over 100 hp on your typical coyote. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.545.7064&rep=rep1&type=pdf
Heck, back in my LT1 days I ran the gamut of boost and compression ratios. After breaking some forged pistons at 10.25/1 and 8 psi boost, I dropped compression down to 8.5/1 and cranked the boost up to 17-18 psi. Power improved tremendously and it didn’t break pistons.
One might argue that none of the above apply to the boosted coyote for this reason or that. However, we see the same trend playing out in automotive (including performance), marine, and powersports, regardless of manufacturer time and time again. Why would Ford only run 9/1 compression in its ultimate street performance machine and only 9.7/1 in its ultimate race machine? Yet, the highest compression F-150 is the base V6 at 12/1 and happens to be naturally aspirated.
Last edited:
Sponsored
SolarFlare
Well-Known Member
Guys from Lund, have said it multiple times. About customers dropping compression to 10:1 or the aluminator (9.5:1) and complaining to them that car is down 50-80whp and their response was and I quote, “you’re not going to make the same whp when you drop compression like that. What you made before on 15psi Will now be attained on 19-20psi”.
engineermike
Well-Known Member
I never said you wouldn’t have to raise boost to reap the benefits of lowering the compression ratio. Luckily, this is all predictable through engine modeling so you can account for it in your build.
SolarFlare
Well-Known Member
IMHO say you make 700whp at 12psi on 93. You drop to 10:1 (to not make it as bad as the aluminator’s 9.5:1). I don’t believe the 10:1 compression will suddenly allow you to push 15psi on same 93oct. I think you’ll be stuck at 13, maybe 14psi if you’re lucky. And break maybe even. The underlying limitation I believe is still octane.
SolarFlare
Well-Known Member
Don’t recall who is OPs tuner is. But I’m curious to what his suggestion is.
markmurfie
Well-Known Member
I can't have a discussing on the topic with you, not because I "think that lowering compression ratio and adding boost won't result in more power when octane limited", but because you are talking about power being limited, and not torque. Showing me clearly you are lacking an understanding of how all this works.
There are things you can do to the engine physically to help it make power when octane limited, lowering compression isn't one of them. In my previous post, I stated 7500+ rpm for a reason. You are not making 700+hp with a coyote at <6500rpm like a GM LS or LT engine, not on 93 with out octane booster/ meth supplement.
There are things you can do to the engine physically to help it make power when octane limited, lowering compression isn't one of them. In my previous post, I stated 7500+ rpm for a reason. You are not making 700+hp with a coyote at <6500rpm like a GM LS or LT engine, not on 93 with out octane booster/ meth supplement.
Sponsored