Cas' 800+ or Bust Build
AYOUSTIN
Greenie Member
Something to keep in mind, compression ratio to cylinder pressure is not exactly a linear trend. Moving from 7:1 to 8:1 will have a much larger effect than moving from 10:1 to 11:1. Simple reason being that a one point increase to 7 is a ~14% increase and the same increase to 10 is a 10% increase. Obviously the more you increase it though your knock tolerance will plummet after a certain point. I think you'd be fine with 12:1, hell Mazda's skyactiv gasoline engines run 14:1 and they can use 87 octane fuel. I know it's not an apples comparo since cylinder head and chamber design have come a long way from 2006 but it's still relevant.
The largest determining factor IMO would be your intended application for the car. If you're going to be doing mostly street driving/roll racing/test n tunes at strips then go up with compression because the car will be more pleasurable to drive. If you're going for purely performance and you want to make crazy power numbers and won't be driving the car around a ton, then lower the compression a bit. To a point, you'll almost always be able to make more power on lower compression. Reason being (don't quote me on the math because it's hip shots) is that for every point of compression you drop, you can run 3-7 more psi of boost. If your peak boost is still in an efficient zone of the turbo's compressor wheel and you're hitting a knock limit, then you would pick up power with lower compression because it would allow you to push further into the turbo's higher efficiency zones without knocking, thus making more power. Once you're out of that zone though you're in the same boat as having higher compression. So in a sense it's a balancing act of compression, turbo efficiency, and where you land in that efficiency.
You might be able to math some of that out but honestly, your best bet is to look at what others have done and see what worked for them. No offense to Clint, but his build and application isn't even close to your car. Being limited by class rules means he's been running on a stock turbo, intercooler, exhaust manifold, and intake manifold. So I'm not surprised he saw as much knock as he did on 93. Not to mention he spools the ever living shit out of that stock turbo and I'd bet more likely than not that engine saw a fair share of LSPI. You should see what others have done on other platforms for larger turbo, high compression, 4 cylinder applications. Other than Realgib3, I'm not aware of anyone who's gone that route with a speed.
I'll also say, if you're going to go to the extremes of sleeving an engine and getting custom pistons you'd be much better able to get good use out of a high compression short block with a ported head and some cams. Hell even if you don't go with higher compression, there's still great gains to be had with headwork and cams. They go hand in hand on this engine so if you're going to do one, do both.
The largest determining factor IMO would be your intended application for the car. If you're going to be doing mostly street driving/roll racing/test n tunes at strips then go up with compression because the car will be more pleasurable to drive. If you're going for purely performance and you want to make crazy power numbers and won't be driving the car around a ton, then lower the compression a bit. To a point, you'll almost always be able to make more power on lower compression. Reason being (don't quote me on the math because it's hip shots) is that for every point of compression you drop, you can run 3-7 more psi of boost. If your peak boost is still in an efficient zone of the turbo's compressor wheel and you're hitting a knock limit, then you would pick up power with lower compression because it would allow you to push further into the turbo's higher efficiency zones without knocking, thus making more power. Once you're out of that zone though you're in the same boat as having higher compression. So in a sense it's a balancing act of compression, turbo efficiency, and where you land in that efficiency.
You might be able to math some of that out but honestly, your best bet is to look at what others have done and see what worked for them. No offense to Clint, but his build and application isn't even close to your car. Being limited by class rules means he's been running on a stock turbo, intercooler, exhaust manifold, and intake manifold. So I'm not surprised he saw as much knock as he did on 93. Not to mention he spools the ever living shit out of that stock turbo and I'd bet more likely than not that engine saw a fair share of LSPI. You should see what others have done on other platforms for larger turbo, high compression, 4 cylinder applications. Other than Realgib3, I'm not aware of anyone who's gone that route with a speed.
I'll also say, if you're going to go to the extremes of sleeving an engine and getting custom pistons you'd be much better able to get good use out of a high compression short block with a ported head and some cams. Hell even if you don't go with higher compression, there's still great gains to be had with headwork and cams. They go hand in hand on this engine so if you're going to do one, do both.
alexwlwsn
BANNED
Even @Captain KR dynoed his 866whp using 4x 1600cc injectors if I recall correctly.
I am curious as to your goals with this car.. it seems you're basically shooting for a ton of HP/TQ?
AYOUSTIN
Greenie Member
Will was probably on 50/50. Once you move to full E85 you run out of injector very fast. Not sure if that's his plan but just thought I'd mention it.
Realgib3
Greenie Member
A ton of HP and getting there the most efficient way possible.
Just a couple things on compression. It was said perfectly on the previous page. If you want to drive the car around and want more responsive, more enjoyable experience on the street with a large turbo, bump the compression. If you think you'll run into octane issues when trying to reach your power goals, you'd have to find better octane fuel or lower compression some.
With that said, I ran just under 500whp on 93 alone on the 12.8:1 pistons on the street. No KR at all, but knowing you're timing tables must be changed dramatically vs a standard compression car. I then ran >40psi on a 6466 with those same CR pistons on a little over 50% e.
Realgib3
Greenie Member
I saw nearly 5mpg highway increase going from stock to 12.8. Also noticed a big difference in off-boost response, but again I was running a 6466, so I had more time than most off boost.
CMajor
Greenie Member
I started to reply yesterday and I was getting that look from my fiancé that I needed get off the computer since she took me to dinner and Fate of the Furious last night haha.
To begin, your assumption is correct on the HP capabilities. I was planning on going with the second option "less boost and more CR". I wanted my car to be more efficient with this build in making this crazy *usable* power. Mark and I had a brief discussion on FB the other day and he said, "I maintain that I believe rpm is the key for drag racing...That's why I think cams/airflow is such a big deal for drag. You have to be able to have lower tq and carry gears out farther to get into the et ranges the cars with this amount of power should be in." Going in with that mentality, wouldn't a higher static compression ratio give me more response down low? My thoughts are to have the HC pistons and the airflow for up top in the RPM band so the engine can breath. I plan on running stock cams in the beginning to set a baseline, then upgrading cams down the road to get before and after results. Another item that I have been thinking about. If I go this route of 12.5:1 vs 9.5:1, I would reach the same HP and/or TQ at a lower boost level.
For example, before I bent the rod in my car I was targeting 28 psi making 500HP and 400TQ. That tune was also conservative with the way the TQ came on for obvious reasons but it gets my idea across. So would I be able to make 500HP and 400TQ at 10-15% less boost? With less boost that would be a lower temperature/pressure. I'd like to sit down and figure out the rough math to see what theoretical temperatures/pressures I'm hitting through the combustion process with each CR. I'm sure the pressure in the cylinder is still more overall with the 12.5:1 even with lower boost but it wouldn't quite be a 1:1 comparison when talking about power and torque. Also I know power and torque aren't everything, this is just the only baseline that I have from my previous setup. This is just something to think about as I make my decision. Hopefully that makes sense, if not I can try to elaborate more.
Not that it matters at this point but I'm trying to keep the drivability somewhat tolerable. I was thinking the 12.5:1 would give me more response and it would be pleasant to drive out of boost as well. Also I want to clarify a little detail, I don't really have a specific number in mind with HP and TQ, I want a fat usable power band. Lastly, I want something different with my car. I want to push the envelope to see what it will do. I want to prove, more to myself, that I can develop a plan then execute it rather than going with a "cookie cutter" build.
As always your info and insight is much appreciated Enki!
[doublepost=1492886267][/doublepost]
Wow I hadn't refreshed my browser before typing all of that so I didn't see any of those responses haha. @alexwlwsn, @Realgib3 pretty much nailed the reasoning for my build and the HP/TQ capabilities.
That sounds like pretty much the same idea from what I mentioned in my previous response. I'd say if I had to label it, my car is being built for the street, roll racing and test and tunes at the strip. I may venture over to the 1/2 mile event to see what happens but it's not being built to sit in my garage. I plan on driving it. Also headwork and cams are in my plan as well. I'm probably going to be ordering my 84lb valve springs today so there's that.
Enki
Motorhead
I think matching parts is the key to success in any form of racing. Engine output, gearing, tire size, etc all matter to varying degrees.
That said, you don't have to lose torque; you tune for what you want, really. Also I think there's a bit of a misconception as to what higher compression ratio actually does when the term "efficiency" is thrown around. For example: the main reasons I went 12.5:1 compression on my build is to make better use of full E85; basically improving my mileage and slightly increasing spool with my decreased displacement. I'm not *really* expecting to get more power from the bump in squish, but I'll take it if it shows up. As far as realistic increase in power goes, I just tested this in the engine sim and HP /// TQ went from 277 /// 283 to 322 /// 320 with only a bump in compression so the gains from this even when otherwise completely stock look promising (however, I wouldn't hold your breath; there are many variables to consider when making power, and some of them *will* hold you back).
Horsepower (like torque), is all about cylinder pressures, and you increase that with more air and fuel in the cylinder to start with; I think that any increase in power output due to compression will likely come from peak pressures and not average (and I think average would be better due to the forces involved on the components). Engine sim says that BMEP peaks about 314 @ 9.5:1 compression and 355.9 @ 12.5:1 compression (same RPM) so the average pressure does go up, but it makes me wonder how much the peak pressure rises (which isn't shown in this tool, sadly). Side note: something I just noticed is that smaller displacement engines seem to benefit more from a bump in compression than larger engines running the same boost...Interesting.
On the subject of boost temps, one thing you need to keep in mind is compressor efficiency. Even at low boost, if the efficiency is lower, temps will be higher. Fortunately, at lower flow rates, intercoolers can pull more heat out of the charge air so it's not much of an issue during spool; not really sure when it comes to high flow rates and low boost, as I've not really looked into that much myself.
That said, you don't have to lose torque; you tune for what you want, really. Also I think there's a bit of a misconception as to what higher compression ratio actually does when the term "efficiency" is thrown around. For example: the main reasons I went 12.5:1 compression on my build is to make better use of full E85; basically improving my mileage and slightly increasing spool with my decreased displacement. I'm not *really* expecting to get more power from the bump in squish, but I'll take it if it shows up. As far as realistic increase in power goes, I just tested this in the engine sim and HP /// TQ went from 277 /// 283 to 322 /// 320 with only a bump in compression so the gains from this even when otherwise completely stock look promising (however, I wouldn't hold your breath; there are many variables to consider when making power, and some of them *will* hold you back).
Horsepower (like torque), is all about cylinder pressures, and you increase that with more air and fuel in the cylinder to start with; I think that any increase in power output due to compression will likely come from peak pressures and not average (and I think average would be better due to the forces involved on the components). Engine sim says that BMEP peaks about 314 @ 9.5:1 compression and 355.9 @ 12.5:1 compression (same RPM) so the average pressure does go up, but it makes me wonder how much the peak pressure rises (which isn't shown in this tool, sadly). Side note: something I just noticed is that smaller displacement engines seem to benefit more from a bump in compression than larger engines running the same boost...Interesting.
On the subject of boost temps, one thing you need to keep in mind is compressor efficiency. Even at low boost, if the efficiency is lower, temps will be higher. Fortunately, at lower flow rates, intercoolers can pull more heat out of the charge air so it's not much of an issue during spool; not really sure when it comes to high flow rates and low boost, as I've not really looked into that much myself.
Similar threads
