What he is saying is right, but I think he glazes over an important fundamental.

When you drop the compression without changing the swept area you increase the size of 'fillable' area. The cylinder+combustion chamber is simply larger. This increase in size of the fillable area creates an additional volume that you can fill with O2. On a N/A it isn't enough additional O2 to cover the loss in cycle efficiency from the compression, but at high boost pressures, you can multiply the amount of oxygen that is crammed into that space and eventually overcome the cycle loss.

For example, going from a 10.3 to 9:1 motor you actually increase the size of the total 'fillable' area by ~1.5%. On a boosted engine you can double/triple the density of the air filling that unswept area, so you can turn that 1.5% of increase fillable area into 3% of extra oxygen at ~15 psig and 4.5% of extra oxygen at ~30 psig. At some point, this density multiplying of the unswept area overcomes the loss in thermal efficiency from the lower static compression. This usually happens between 20-30 psig because the gain/loss in compression (although not linear) is about 3% per point of compression.

This is the whole reason why I run ~8.6:1 at ~20 psig on pump gas rather than higher compression and meth. Similar power, less detonation issues.

 

Of course I watched from 29 min. BTW, nicely done with the last sentence. Still I don't think he was implying anything about the power output na of either engine. At some point there is a loss of power by raising the CR at some boost level. There is a reason that top fuel engines run 6.5:1 cr. I think it is reasonable to believe that na they would benefit from a higher cr.

 

Exactly. Lowering the compression just makes more room to stuff in more fuel and air to make a bigger bang. I don't think he went thru all of his assumptions and calculations that he went thru to come to his conclusions. He was pretty clear that the numbers that he came up with apply specifically to the 2.0L engine he had in mind.

 

Maybe this is a case of a Holdener test, telling people stuff that is already obvious. Everyone all agrees that double NA power occurs at 1 bar of boost, parasitics and special variables aside. Take any example you want of an NA motor. Does an NA 8:1 motor make more or less power than a similarly cammed and headed 12:1 motor assuming you are not octane limited? No. It's generally agreed that every point in compression is worth about 3% of NA power. So foe his example in the video to be true, either he's assuming the same NA power for the 2 liter motor at both compression ratios, OR you're octane limited at the higher compression. Otherwise you could say that NA power doubles at 1 bar of boost. Running low compression ratios is old school thinking. Many heads up class racing stuff run high compression on their boosted motors. Why don't they run 8:1?

 

If I had to guess, spool time. Above a certain size turbo, transition time to full boost is more important than peak power and static compression has a very noticeable effect on spool time.

Conversely, for a supercharged car, I suspect there are a decent amount of cars out there that may run a smidge faster dropping the static compression a little.

 

Why are we using extremes like a 2.0 engine and top fuel engines? If a turbo motor spools quicker and is more responsive out of boost with higher compression, then it's going to be more fun to drive on the street.

I remember when LME built their motor for the LS vs coyote dyno competition, they went with over 11 to 1 compression and the fuel both were using was E85 and prochargers.. The competition was based upon peak power, average power, and power per cubic inch. Things change at the very extreme ends of anything but who is running 45 psi on a street driven LS? At realistic boost levels for a street driven LS higher compression will make more power all other things equal unless octane limited.

We can speculate all day, but I've driven the same combo with only the compression ratio changed and it's a big difference for a street driven car.

The entire combo including fuel and use of vehicle all factor in so there are cases where lower compression may be the way to go. If we are talking max peak power on a 2.0 or top fuel at 45+ psi then there are definitely different use and factors to consider in those cases.

I'm sure even at 45 psi that little 2.0 with low compression isn't going to be nearly as fun driving around town as a bigger cubic inch LS at 10 to 1 or higher at half the boost level.

Good luck with your head gaskets with a street driven LS at 45 psi anyway...lol...if you want to talk about hypotheticals that aren't based on reality or real world experience then ya that can be fun, but I'd rather talk about realistic scenarios. In my mind 45 psi isn't a realistic scenario for a street driven LS so you would never need to drop your compression so low that you could run 45 psi.

Notice the guy in post #7 is running 12.5 to 1 and running 7's. He's not octane limited with methanol and seems to know what he's doing. Do we really think he would be faster with 8 to 1 compression? I also doubt the OP is considering running anything close to the 45 psi that is being speculated about here.

 

these are cars with trans brakes and 2 steps and all the octane. They're leaving on all the boost regardless.

 

Did you watch the video? I question the video starting at minute 29 which is the example where they have an 8:1 2 liter motor running 45 psi and say they'll lose power going to 10:1 or 12:1.

 

Yup, you also get rid of the serious pressure spike when ignition first occurs and you spread that out over a longer duration with a softer hit. You also need to take into account the amount of fuel that is actually injected into the motor, and if you are running methanol that also takes up a lot of "room" so to speak.

But you lost me here, you certainly aren't making similar power to something else at say 9.5:1 compression @ 20 psi with meth and I would even argue that you would have less detonation issues running meth vs just pump gas.

 

Its a question of how much meth we are talking.

If you isolate it to just pump gas vs. pump gas (ignoring the whole detonation issue) it is only a 3 HP difference between a 9.5:1 at 20 psig and an 8.6:1 at 20 psig.

A 9.5:1 shift down to 8.6:1 creates about 1% more volume, and at 20 psig, 2.4% more oxygen vs a 2.8% drop in power due to lower combustion temperature. Or in other words, the difference in power between someone running 20 psig on a 9.5:1 motor vs 20 psig on an 8.6:1 motor (all pump gas) is about a 0.4%. When you introduce meth into the mix the numbers go something like:

0% meth = 3 HP difference
5% meth = 6 HP difference
10% meth = 13 HP difference
25% meth = 32 HP difference
100% meth = 127 HP difference

You only need about 5% to 10% meth for the octane boost. Any higher than that we are talking HP gains more from a fuel shift than just detonation suppression. So, if you want to keep as true to what I said as possible, a 9:5:1 at 20 psig running 5% to 10% methanol would make 6 to 13 HP more than a motor running 8.6:1 at 20 psig. I didn't think it was worth the extra trouble.

 

It's been a busy week...lol...about 18 inches of snow and 12 hour work days. I did watch the section you are referencing. It seems like he's using a small sample size of one engine and I'm not sure where he is getting his numbers. Apparently, he took one engine and ran it with several different compression ratios and boost levels? Why doesn't he show the dyno charts for all of these combos so we can see the actual curves?

 

I see we continue to only talk about peak numbers with no concern to average power, torque at low rpm, throttle response etc.

 

Can someone reference the point in the video where he shows the dyno curves? Anyone catch how he changed the compression ratio on his test 2.0 engine that he is using as an example? Some of the compression ratio changes he mentions are extreme so I'm guessing he had to change pistons not just cylinder head cc's. Were these chassis dyno numbers or engine dyno?

There is no way you are losing peak power with a compression ratio increase everything else equal unless you are severely octane limited and you sure as heck aren't losing power/torque in the lower rpms.

I don't feel there is anything to lose and everything to gain by running a street driven 1000 rwhp LS combination at 10 to 1 rather than 8.5 to 1. If we get into extremes of 45 psi you aren't holding head gaskets with an LS at that boost level in a streetcar anyway. So why speculate about unrealistic scenarios?

 

There are no dyno curves, he's using a theoretical example and his simple "math" to show that you lose power when you add compression in his example. That's where I disagree. The only way his math makes sense is if the NA motor makes the same 200hp at 8:1 and 12:1 which makes zero sense. It's generally accepted that you increase your power by about 3% when you add every point of compression, so his scenario isn't valid.

 

LOL... so people are citing a theoretical example not based on actual testing as the basis of their argument. LOL..that is what I figured before I watched it because it made no sense and doesn't align with what I've actually seen in my real-world experience.

I agree his scenario isn't valid. Got to love the internet...LOL

 

Well a few points here...
- The "volume" or swept area has no bearing at such low boost levels and standard compression ratios, that's a MAX effort type thing that a street car will never be at
- A full point of compression is ~3% power increase, so typical 400 horsepower engine would gain 12 horsepower, 20 psi would be 28 horsepower just from compression alone with no other changes and assuming octane isn't an issue.
- Now take that 28 horsepower and add a lot more from efficiency gains and timing increases by running meth

Either way, you certainly aren't making similar power to something else at say 9.5:1 compression @ 20 psi with meth and I would even argue that you would have less detonation issues running meth vs just pump gas.

 

You don't know his methodology. It may be purely theoretical but even then you don't know what experience he is bringing to the table. Aren't you doing exactly what you are accusing him of? Either way he is clearly stating that these numbers are referring to a specific small 4cyl at 45 psi. It doesn't take a genius to figure out that the numbers for an LS engine will be different. The concept is the same. I assume also that all would agree that 45 psi on the street would be at least unusual. Do you disagree with NoGo and with specific numbers by how much? You could use as an example the response by NicD.

"For example, going from a 10.3 to 9:1 motor you actually increase the size of the total 'fillable' area by ~1.5%. On a boosted engine you can double/triple the density of the air filling that unswept area, so you can turn that 1.5% of increase fillable area into 3% of extra oxygen at ~15 psig and 4.5% of extra oxygen at ~30 psig. At some point, this density multiplying of the unswept area overcomes the loss in thermal efficiency from the lower static compression. This usually happens between 20-30 psig because the gain/loss in compression (although not linear) is about 3% per point of compression.

This is the whole reason why I run ~8.6:1 at ~20 psig on pump gas rather than higher compression and meth. Similar power, less detonation issues."

 

LOL...maybe reading is hard for you. I posted my real-world experience going from 11.4 to 1 compression to 9.5 to 1 compression. Your entire argument is based upon a video using theoretical examples. If a motor can't handle 45 psi without blowing head gaskets, then it's an unrealistic example and even then, no actual real-world results to back up his claims. His entire argument is flawed with no actual testing to back it up.

You aren't losing power adding compression at realistic levels for an LS engine unless you are octane limited.

Even if you consider the opinions of others and the guy in the video they are all still talking about peak power. I'm not in disagreement with the opinion that peak power numbers don't gain a huge amount with compression. Maybe you haven't heard terms like... average power, power under the curve, torque at low rpm, or throttle response? Those are the areas where higher compression will gain you the most.

I'd rather have an efficient motor with a better top end to make more power at lower boost and lower iat so I can run even more compression for an over all more enjoyable street car driving experience.

What do you think is more fun to drive... the 2.0 at 45 psi at 8.5 to 1 compression and 850 rwhp or an LS3 with 20 psi and 1000 rwhp at 10.6 to 1 compression?

The best thing to do is consider the entire combination... if you can build an efficient motor to hit the power goals you want at lower boost you can run more compression and still make more peak power with less boost while slaughtering the low compression combo out of boost at low rpm as far as torque and throttle response.

There are people with 427's at 11 to 1 compression and the best top end money can buy making 1000 rwhp with under 20 psi on e85. They are a riot with low iat and tons of response and torque. Efficiency of the motor can't be ignored. If you are super inefficient and trying to make up with it by adding more boost you also raise your iat and this is considering you don't get out of where your blower or turbo likes to be to accomplish the extra boost. Higher IAT is going to add to spark advance limitations. This is way more complicated, and his theory doesn't take all of these factors into account.

 

You have no idea if it is theory or if he has dyno runs to back it up. Besides that, all he talks about is peak power. Is that simple enough for your reading skills?

 

My buddies car was a built 5.3 9.0 compression twin s366's would take 5 seconds to make 8psi on the Trans brake and on 30psi went 5.40's at 135 hurt that motor swapped in a sbe 5.3 same heads and cam made 8psi in 3.5 seconds and went 5.05 at 141 doing the math the compression should be around 10.5

The et improvement is not just from the motor but the mph and spool time is