Highest advised compression to run with turbo?

[jgkurz]

Six_Shooter, Wow, you certainly are passionate about your point of view. I'm not sure I have words to form a similar response. Simply put, I agree that a modern engine using the latest technology can leverage a much higher static compression ratio than was possible in past years. Speaking in general terms, is it your belief that a boosted application should NOT have any reduction is SCR as long as modern detonation control methods are used? (electronics, coatings, meth, cam timing, quench, etc...) Maybe our disagreement is that I believe more boost is beneficial over a higher SCR. For the purpose of this discussion higher boost = more lb/min. The VE of an engine can certainly be improved to a point with a higher SCR but cylinder pressures eventually exceed the ability of all detonation control methods. My point is that a 8:1 engine will reach that ceiling later than the exact same engine with 9:1. It also provides a level of safety to keep the engine alive under varied driving conditions. Assuming a turbo has more lb/min to give at a higher boost pressure, an 8:1 engine will make more peak HP since it will be able to injest more air volume prior to hitting the detonation ceiling. More specifically, I believe that the 8:1 engine should make more power at say 25psi then the 9:1 engine at 20psi. That is if you could get to 20psi before breaking ring lands. My numbers are arbitrary and not really the point. My argument is that a lower SCR will usually allow more PEAK HP with a boosted application. That could mean 10:5:1 vs 11:1. I contend that say 5psi of more boost will allow more peak HP than .5 more SCR would. This is a estimate but it should be in the ballpark. As an example, why is it that turbo kits for the BMW M3 and the S2000s seem to limit boost to single digits? These cars have some of the most modern electronics and engine design on the planet but run ~11:1 or more compression. It's because the high SCR will cause the engine to severely detonate with higher boost levels. As for determining the many parameters of a turbo engine, there are some general assumptions that can be made based on experience and where the car will be driven.

 

To summarize, I agree that a higher SCR is desirable, but given two identical engines with ALL the latest technology to control detonation, I will take the 8:1 vs the 9:1 or for that matter a 10:1 vs a 11:1 engine and run more boost hence achieve a higher HP. Maybe we are saying the same thing...???

 

[Six_Shooter]

I don't think we are agreeing, completly.

 

I prefer higher SCR engines, due to better off boost drivability, better combsution in off boost conditions, which can make for a more efficiant engine, and reduce emmisions, and in theory a higher SCR enging could spool a turbo sooner.

 

I've read theories about a lower SCR engine being able to "injest more" air and fuel mixture than a higher SCR engine, but I don't completly buy into it. The difference in volume between TDC and BDC is what is more important, and that difference is larger with a higher SCR engine, hence the increased SCR.

 

Again, I've seen reports where increasing the SCR reduced detonation probability, due to the better quench and control of the mixture in the combustion chamber.

 

I do believe that even old engines can benefit from newer control technology, hense why I have a semi-late model GM ECM running my L28.

 

Those kits that you speak of, do they tune for each installation, or just a general tune? Is there any tuning at all? Using single digit boost levels (that produce a certain amount of desired power) on stock engines is a safety net. If you look hard enough people that own those same cars/engines wil have pushed the limits on thier own, and tuned the controls appropriatly. There could be other variables like piston esign (though I know some BMW forced induction kits come with new pistons, with a better design), or crank limits, etc, I wouldn't look at the SCR as being the sole factor.

 

I think you're still getting caught up in bigger peak numbers = better, and to some people it is, but teh reality is, if we advertised power increasing products with an "average increase" or showing something a little more meaningful like an increase in area under the curve, or even extending it, like forced induction does well, then less people would be interested in the overal peak number and more interested in the gains that matter. A larger peak number does not always mean that there is a large gain elsewhere as well, doesn't mean it doesn't, just isn't an indication that it does.

I think you're also getting caught up in the "boost numbers" too much. Again, the numbers that show up on the gauge are really just for reference, we should be measuring CFM, or some measurement of airflow to really be able to compare numbers in a manner that will tell you something. The single digit boost numbers on those late model engines could very likely exceed in power what double digit boost numbers on other engines of similar design and displacement could.

 

Again, there are many variables that need to be looked at when building a forced induction engine, or putting forced induction on an existing engine, SCR is rarely of any real concern, I look at more of the design and better yet how the engine reacts to the increased air flow, and tune accordingly.

 

Yes you are correct, I am passionate about my point of view, I've seen way too many people follow advice that was well meaning, but then be disapointed with the results, rebuild and follow possibly "less conventional" methods and be blown away by the results. Sometimes a nudge in a certain direction is all that is needed to find a new means to an end.

I'm not saying disregard what has been done previously, just look at why it was done that way and look at what could be improved on, and why something worked or failed in the past.

[RTz]

Start with post 17... http://forums.hybridz.org/showthread.php?t=145024

 

 

The difference in volume between TDC and BDC is what is more important, and that difference is larger with a higher SCR engine, hence the increased SCR.

 

 

How so? Displacement is displacement.

 

For a given displacement, swept volume remains constant, regardless of CR. However, a lower CR has a greater clearance volume. Depending on valve events, that may translate into greater cylinder pressure at a fixed boost/CFM.

Edited August 5, 2009 by RTz

[jgkurz]

I don't think we are agreeing, completly.

 

I prefer higher SCR engines, due to better off boost drivability, better combsution in off boost conditions, which can make for a more efficiant engine, and reduce emmisions, and in theory a higher SCR enging could spool a turbo sooner.

 

I've read theories about a lower SCR engine being able to "injest more" air and fuel mixture than a higher SCR engine, but I don't completly buy into it. The difference in volume between TDC and BDC is what is more important, and that difference is larger with a higher SCR engine, hence the increased SCR.

 

Again, I've seen reports where increasing the SCR reduced detonation probability, due to the better quench and control of the mixture in the combustion chamber.

 

I do believe that even old engines can benefit from newer control technology, hense why I have a semi-late model GM ECM running my L28.

 

Those kits that you speak of, do they tune for each installation, or just a general tune? Is there any tuning at all? Using single digit boost levels (that produce a certain amount of desired power) on stock engines is a safety net. If you look hard enough people that own those same cars/engines wil have pushed the limits on thier own, and tuned the controls appropriatly. There could be other variables like piston esign (though I know some BMW forced induction kits come with new pistons, with a better design), or crank limits, etc, I wouldn't look at the SCR as being the sole factor.

 

I think you're still getting caught up in bigger peak numbers = better, and to some people it is, but teh reality is, if we advertised power increasing products with an "average increase" or showing something a little more meaningful like an increase in area under the curve, or even extending it, like forced induction does well, then less people would be interested in the overal peak number and more interested in the gains that matter. A larger peak number does not always mean that there is a large gain elsewhere as well, doesn't mean it doesn't, just isn't an indication that it does.

I think you're also getting caught up in the "boost numbers" too much. Again, the numbers that show up on the gauge are really just for reference, we should be measuring CFM, or some measurement of airflow to really be able to compare numbers in a manner that will tell you something. The single digit boost numbers on those late model engines could very likely exceed in power what double digit boost numbers on other engines of similar design and displacement could.

 

Again, there are many variables that need to be looked at when building a forced induction engine, or putting forced induction on an existing engine, SCR is rarely of any real concern, I look at more of the design and better yet how the engine reacts to the increased air flow, and tune accordingly.

 

Yes you are correct, I am passionate about my point of view, I've seen way too many people follow advice that was well meaning, but then be disapointed with the results, rebuild and follow possibly "less conventional" methods and be blown away by the results. Sometimes a nudge in a certain direction is all that is needed to find a new means to an end.

I'm not saying disregard what has been done previously, just look at why it was done that way and look at what could be improved on, and why something worked or failed in the past.

 

I'm not sure why you think I'm caught up on boost. I specifically said "For the purpose of this discussion higher boost = more lb/min". More lb/min = more air which = more HP. (assuming you have the fuel to match) Boost is meaningless unless it equates to a higher volume of air though the engine. Often higher boost just equals more heat and no improvement in flow. Also, when designing a purpose built application I fully agree that there are a multitude of variables to consider when designing an engine. However, I think for this conversation we were discussing a street driven and boosted L28 which allows certain assumptions.

 

I think we are going to have to agree to disagree on this one.

[Gixxer Squid]

So how big will your bore be with a 3.2L? I guess he didn't think thin cylinder walls were an issue???

 

 

I asked this question myself, going with the 89mm rather than the 90mm from what I understood. We are using a forged rod as well rather than the traditional 240z rod to get the 3.2.

 

Apparently this is something he has had on the back table and the last project he was going to do this combination fell through so I am the "guinni pig." so to speak, LOL. Hell, could not be any worse than the 2 seats I have dropped already.

 

James

[rsicard]

Gentlemen: The new Corvette LS9 engine is supercharged and has a static compression ratio of 9.1:1. It also has oil cooling to the bottom of the pistons. What REALLY matters when it comes to knock and SCR is the configuration of the piston and cylinder head combustion chamber. Good swirl and quench lead to FAST burn characteristics. The most complete cylinder burn characteristics is what is needed. This comes through CFD and experimentation with differing combustion chamber designs. There is even a form of specific coolant liquid for horsepower.

[Tony D]

I would not put Corvette LS9 Engines, Honda Engines, or any other Engine is this discussion on boost and L28's. They are radically different combustion chamber designs.

 

Six Shooter must have missed my threads in other posts regarding flow through the engine where I used the example of a car making 300HP at 8psi instead of boosting it to the stratosphere on a stock head to make the same horsepower.

 

I think the off-boost drivability is a red herring. If your car is ported for the flow (even with say a 7.4CR), even before boost threshold the compressor will be putting in air and making power well above an N/A setup running 8.5, 9, or higher SCR. Even with a boosted engine, the 1, 1.5, or higher) boost in the SCR may return slightly better power, but not significant power at these 'off boost' areas. I think this is what JGK is getting at...

 

The combustion chamber design of the L28 ain't the greatest. There are things you need to do to decrease it's tendency to knock. If you don't, then things break.

 

Sure Cam Timing and valve events will affect it.

Sure flowing more will decrease the boost needed.

 

But at the end of the day, if you have a knock problem, and it won't go away then you are left with the old standbys of 'the 60's: retard timing, lower SCR, or change your cam'

 

riscard is getting to it saying 'FASTBURN'. Redesign of the C Chamber will really pay dividends. But this starts at $1500 on most heads. And if that design doesn't work...you are out another $1500 for the next design.

 

Putting it back to retard timing, lower SCR, or change your cam...

 

I'll admit I've run some boost numbers on my setup (Non US Market 1977 L28 with an N42 Head) without an intercooler that people said is impossible. But when BUILDING an engine...choices have to be made. And most people will not spend the $$$ for a head combustion chamber rework, or pay for total custom pistons to promote proper burn...but with a sub-standard combustion chamber to begin with, it will still be sub-optimal.

 

JeffP did some experiments with his setup, and it shows some interesting variances. KTM's build was X HP with a stock head. Jeff made close to 75-80 hp more at slightly lower boost pressures because of his cam and better flowing head. But both were at a detonation point. Comparing JeffP and KTM's dyno pulls will illustrate the difference between flow built into the engine. JeffP's 3.0 stroker with an 8.5:1 CR of course has more power, but that is some stroker advantage there... but down low the difference in power 'off boost' or at 'low boost' can be explained more from Stroker Advantage than 8.5CR versus 7.4 CR. If only I had my old dyno curves for a 8.8:1 curve on a stock engine...that might illustrate it better for this discussion.

 

Raising the compression ratio in either of these engines would NOT make detonation any better. And likely this is the situation in this case as well. Eventually BMEP of the cylinder will be to a point where detonation becomes a problem with the available fuel.