I know there are low compression ratios for boosted engines but would there be any benefit for an engine that has upgraded connecting rods and pistons that is boosted to have a 10:1 ratio?

 

yes. more torque

 

You'd have the ability to make more power. Just keep in mind that you need to keep engine knock in check! There's a reason forced induction engines have lower compression ratios. Knock is more likely to happen the higher compression ratio you go, so timing, A/F ratios, boost pressure, intake air cooling (intercooler), and fuel octane become even more important.

Even with strengthened internals, if you have knock, not only is it really hard on the engine, you're literally working against yourself since the combustion is now trying to turn your crankshaft the wrong way.

Keep the knock at bay, and you could have yourself one sweet machine.

 

ya, detonation is a big concern I might have aside from my tranny. I'll have to be a little meticulous about tuning and the right ignition system setup.

 

basically with a well built motor youl make more power on any given level versus a lower compression motor...you just cant go crazy and run high boost numbers

 

even at variable rpms? I was thinking of gradually decreasing boost after 5500rpms

 

LS1s run 11.5:1 compression and people boost the stock engines just fine.

 

They also run 6 psi on pump gas

 

if you are going to run a higher compression, don't skimp on your cooling mods. Especially look into water/meth injection as it makes your gas act like it has a much higher octane.

 

 

so with a lower compression u can run higher boost levels? wouldnt u want lower compression pistons if this is the case?

 

Well, you could look at it this way... If you're running higher compression, then you may not need higher boost to get a certain power level. The higher compression will already be making more power with less boost. Now if you want to boost it for the heck of it or are planning for high boost anyway, then sure, go lower compression.

This is by no means a scientific explanation though. There is most likely a good combination of the two, and that I'm not familiar enough with it to be able to tell you.

 

kind of..but you're sacrificing low end power when you do this. the LSJs who went turbo with 8.9:1 CR pistons made numbers like 360whp 290tq, whereas the ones who went stock compression were making numbers more like 390whp 370tq

 

im not sure to be honest.....i dont want to spew inaccuracy...some stock exhaust flow charts would probably settle that question..then again...

 

Depending on how much lower you go in compression, you will always make more power from boost than compression due to thermal efficiency vs airflow gains, however, like stated above there is a point where you trade off some of that power. When building turbo you will get slower spool times w/ less compression and less torque, however with a supercharger set up right you will have near instant boost and plenty of low end torque. With the lower compression it usually is easier to dial in your timing as well as the dynamic compression (w/boost) is not as prone to detonation or pre-ignition.
It is alot more complicated than what I want to write here, but you get the jist of it I'm sure.


For your consideration courtesy "Black2003Cobra"

This question seems to come up time and time again. Is it better to increase the static CR or boost pressure. There are a couple reasons why supercharged or turbocharged engines run lower static compression ratios. A static CR in the range of 8-9 is very common. Here are a couple considerations.

Consideration #1
Heat from compression by a supercharger or turbo can be removed (for the most part) through use of an intercooler. Heat from compression within the cylinder cannot. Also, the cylinder pressure at the end of the compression stroke (prior to ignition) goes up exponentially with an increase in static compression ratio, versus a linear increase with boost pressure. Therefore, increasing the static CR is going to unavoidably push you closer to the knock limit for a given fuel. In other words, the octane requirement goes up more by increasing the static CR than it does by increasing boost.

For example, increasing the static CR from 8.5 to 9.5 increases the temperature within the cylinder at the end of the compression stroke (but before ignition) by ~63°F, (assuming IAT2 = 130°F and ideal adiabatic compression with γ = Cp/Cv = 1.4. I won’t bore anyone with equations. The situation doesn’t change much even if IAT2 were only, say, 100°F. In that case, the increase in temp at the end of the compression stroke goes up by ~60°F for the same increase in static CR). Also, the pressure at the end of the compression stroke (before ignition) goes up by ~97 psi from 574 psi to 671 psi, assuming atmospheric and boost pressures of 14.7 and 14 psi, respectively. On the other hand, increasing the boost pressure from 14 to 15 psi increases the outlet temp of the compressor by only ~11°F, assuming AE=60% and IAT1 = 90°F. And by further assuming an intercooler efficiency of 80%, the increase in IAT2 is only ~2°F. Hence, the increase in temp at the end of the compression stroke will hardly change at all. Also, the increase in cylinder pressure at the end of the compression stroke only goes up by ~18 psi (from 516 to 534 psi) with this increase in boost pressure.

So summarizing the effects of increased temp and pressure at the end of the compression stroke for the two cases:
Increased CR from 8.5 to 9.5: ΔT = ~63°F and ΔP = ~97 psi
Increased boost from 14 to 15 psi: ΔT = ~2.4°F and ΔP = ~18 psi

A higher temp and pressure increase the likelihood of deadly preignition for a given octane fuel. And for those astute observers that know the physics I’ve applied, yes, although I’ve idealized things to keep it simple, (by not including effects such as heat loss thru the cylinder walls during the compression stroke or ignition and valve timing in the calculations), I’m sure they’ll also recognize that this doesn’t change the conclusion.

Consideration #2
Power is increased by two completely different mechanisms for the two approaches. Increasing the static compression ratio increases power via an increase in thermal-conversion efficiency. Increasing boost pressure increases power via an increase in mass-air flow rate. There’s less gain in thermal-conversion efficiency (and hence power) via an increased static CR compared to the power gain by increasing the mass-air flow rate via an increase in boost pressure. For example, increasing the static CR from 8.5 to 9.5 results in an increase in thermal-conversion efficiency (for an ideal Otto cycle) of about 3.2%. On the other hand, increasing the boost pressure from just 14 psi to 15 psi, increases the mass-air flow rate by about 3.5%. If boost pressure is increased by 2 psi, (from 14 to 16 psi), the increase in mass-air flow rate will now be more than twice that compared to the increase in thermal-conversion efficiency, (~7% vs ~3.2%), and ΔT and ΔP still won’t be as great as they are when increasing the static CR from 8.5 to 9.5. Therefore, not only can it be “safer” from the knock point of view, but a little more power is gained as well, (relatively speaking that is).

In conclusion, I would contend that for a forced-induction application, that low compression is in general, the better way to go.

Boost vs compression ratio Part II

Since writing part I, there have been some comments made that I felt warranted a part II. Comments such as, “Good stuff, but peak combustion pressure and temperatures are far higher than they are pre-ignition.” Or, “I have more gauges than a pimply faced teenager in his Honda Civic, so I can run closer to the ragged edge.” Or, “The engineers didn’t put 8.5:1 compression pistons in the Terminator, or 8.4:1 in the FGT & GT500 for any thermodynamic reasons.” Or the classic, “It’s all in the tune.” Although most of these statements are not totally without substance, the basic conclusion is unchanged. Even though adjusting timing and AFR can reduce the tendency to knock, and although peak combustion pressure and temperatures post ignition are significantly higher than pre ignition, or in spite of how close one cares to run to the knock limit of a given octane fuel, (for a given fuel and AFR, etc.), the engine can make more power by reducing CR and increasing boost pressure, than the other way around for the same peak cylinder pressure. This is why it is common to see lower compression ratios on SI forced-induction motors. Obviously there are tradeoffs, however, as a direct result of the lower thermal-conversion efficiency. But when it comes to maximizing power and torque output at wide-open throttle for a given octane fuel, etc., lowering CR and raising boost pressure is the safer approach.

This conclusion is based primarily on two basic facts:
Mean-effective pressure (MEP) goes in direct proportion to the mass of air ingested, (which is directly related to boost pressure), but goes up “sublinearly” with compression ratio, CR
To good approximation, peak cylinder pressure goes in direct proportion to both.
And as mentioned in Part I, while heat from compression by a supercharger can be effectively removed through use of an intercooler, heat from compression within the cylinder cannot. As a result, peak combustion temperatures do not tend to rise significantly with increases in boost pressure, whereas they will go up with increased compression ratio. And as we all know, a lower peak combustion temp also reduces the likelihood of knock. One also needs to recall that power and torque are directly proportional to MEP, where the mean-effective pressure is defined as the “effective” pressure over the cycle, which is equal to the work generated over the cycle divided by the displaced volume. In other words, if the indicated MEP goes up X%, indicated power and torque will also go up X% at a given engine speed. Additionally, one needs to recognize that for a given octane fuel, AFR, etc., that as peak cylinder pressure and temperature are raised, eventually the engine going to knock, or detonate. This shouldn’t be any surprise since this is exactly how a fuel’s octane, (i.e. its resistance to knock), is measured. It is put in a special test engine whose compression is raised until the engine knocks. (The reference fuels used for comparison are iso-octane defined as having ON = 100, and normal heptane having ON = 0.) References: http://en.wikipedia.org/wiki/Octane_rating. Or section 6 here => http://blizzard.rwic.und.edu/~nordli.../gasoline.html

For the ideal Otto cycle, it is very easy to derive expressions for the peak combustion pressure (P3) and temperature (T3), and mean-effective pressure. As in Part I, I’m not going to derive or show all the math, but simply get to the bottom line and show the results. For those that want the details, the interested reader is referred to any number of good text books on engine fundamentals, (Taylor’s, Hewood’s, etc.).

Taking the ratio of mean-effective pressure to peak cylinder pressure, or vice versa, one will find that the dependency on boost pressure drops out and the ratio only depends on CR for a given fuel and AFR. (Note - timing does not factor in simply because MBTT at TDC for the ideal cycle, but this does not change the conclusion.)



where the thermal-conversion efficiency for the ideal cycle is given by, ηt = 1 – CR^(1-γ), cv is the constant-volume specific heat for the mixture, ηc is the combustion efficiency, Qhv is the fuel’s heating value, and γ is the polytropic exponent which can be taken to have a value of around 1.25-1.3, over the full cycle, (Ref., H.M. Cheung and J.B. Heywood, SAE paper 932749).

Using these results, one can plot the ratio of indicated mean-effective pressure to peak cylinder pressure, iMEP/P3, vs compression ratio. From this plot, one will see that as CR goes up, the ratio iMEP/P3 will go down. (See plot below).




What does this mean? It means for any given maximum tolerable peak pressure (for a given octane and AFR), that iMEP will be higher at a lower CR than at a higher CR, independent of boost pressure. Said another way, this means peak cylinder pressure climbs faster than indicated mean-effective pressure does as CR is increased, whereas both P3 and iMEP will climb at the same rate with boost pressure. Therefore, for any given fuel and AFR, etc., one can make more power & torque at any given engine speed by reducing CR and increasing boost pressure, than the other way around for the same peak cylinder pressure. The tradeoff is a lower thermal-conversion efficiency, which translates to a higher specific fuel consumption (pounds per hour per horsepower) and a “doggier” response at part throttle.

Although the above conclusion was based on analysis of the ideal cycle, a more complete thermodynamic model including finite burn duration, heat loss, spark timing, etc, will show the same trends and lead one to the same conclusion. As an example, cylinder pressure and temperatures vs crank angle for two engines with the same iMEP, but different CR and boost pressures are shown below. As can be seen, the engine with the higher CR has higher combustion pressures and temperatures, making it more likely to knock for a given octane fuel.


I hope this is good food for thought, sorry I cant get you the plots.