Low Compression/High Boost and Pump Gas

[71GSSDemon]

Can someone shed light on the ability to run pump gas on a lower compression/high boost engine? I am trying to get my head around boost adding air to the cylinder but not adding to the overall compression ratio. Please be kind. Just trying to understand as my thoughts were adding, say 15psi to an 8:1 motor would make the need to use race gas, E85 etc. Where is the cut off? 10:1 motor? I would assume efficiency of the cylinder head and its material plays factor. And lets say Carbureted, no EFI or DFI

 

[abdywgn]

I don't know but my thought was this would be the answer for a high compression engine on pump gas: a very SLOW advance curve OR one that is tied into a knock sensor to retard as needed.

 

[71GSSDemon]

So, this is what I learned on the subject from a friend, for anyone interested.

The simplified version is to use the ideal gas law PV=nRT to calculate your starting molar mass "n" by translating to (PV)/(RT) = n. P = Pressure, V = volume, R = gas constant, T = Temperature. Starting volume is cylinder displacement, starting pressure would be 90 kpa for naturally aspirated and 90 kPa + (boost pressure*0.90) for pressure charged. Estimating based on expected volumetric efficiency (90%). Temperature will be expected intake charge temperatures. 35C for N/A and 60C for pressure charged. You can then calculate the molar mass at bottom dead center / cylinder filling.



You know the compression ratio, so you know V1 (volume 1) and V2 (compressed volume 2). So you can use the equation (P1xV1)/V2 = P2. To find your compressed cylinder pressure. Once you have P2, you can again use PV=nRT to calculate your compressed T2 temperature. (P2 xV2)/(nxR) = T. Now that is purely compression based temperature changes, but for spark ignited engines, the ignition is before TDC and peak cylinder pressure will rise well above the static compression ratio pressure rise.



Which is where "late" ignition comes into play. Where you delay the ignition of a pressure charged engine to limit the maximum cylinder pressure (Pmax), which limits your maximum cylinder temperature, which directly impacts the propensity of knock. This is also done on naturally aspirated engines, but not to the same extent. For example, one engine with 10:1 CR runs a maximum cylinder pressure of around 60 Bar. The another with 8.5:1 CR and boost runs nearly the identical maximum cylinder pressure of 60 Bar, but the angle of Pmax is much later in the cycle, the area under the curve is much larger, so it generates more power.



The lower 8.5CR reduces the initial cylinder pressure / temperature of compression, which limits the propensity of knock and allows late ignition. The trade off is a loss in thermal efficiency due to the loss in expansion ratio. Which requires additional air mass flow (more boost pressure) to make up the difference again. That trade-off shows up in brake specific fuel consumption which will be much higher.

 

[408scamp]

Went from - "hey, help a brother out, explain it like I'm 5." Directly to professor horsepower - "be in awe of how smart I am" All in under 24 hours.

Slow Clap my man. Wish I was as smart as you

 

[71GSSDemon]

Went from - "hey, help a brother out, explain it like I'm 5." Directly to professor horsepower - "be in awe of how smart I am" All in under 24 hours.

Slow Clap my man. Wish I was as smart as you

I can't take credit other than posting the answer from a very smart friend/colleague. Best part it was just off his head. sheesh

 

[Coyote Jack]

I am of the opinion that each engine is different as to what is doable. Two engines that appear to be the same will accept different amounts of boost. I can only relate what has worked well for me. I built my 340 eleven years ago and this spring was the first time I had any problems. I pushed out a head gasket but no other damage. This is what I have. 1968 340 bored .030 over. Stock crank, TRW pistons with a static compression of 8.2 to 1. Eagle H-beam rods. The "X" heads have been very heavily ported and I run a custom cam from Steve Morris Racing. The intake is an Indy Mod man that I had to massage a little to get to work. My head gasket is a Fel-Pro 1006 that has a .039 compressed thickness. I run a Dyers 6-71 roots blower at 9 lbs of boost. My timing is locked at 31 degrees. Now back to my first sentence, I have been told by more than a few so called experts that I can't run that boost and timing without destroying the pistons with pre ignition. After 11 years of running this combo my pistons looked pretty darn good when I replaced the head gaskets this spring. I started with 6-7 lbs of boost and my timing at 28 degrees and started to sneak up on it a little at a time by adding boost and timing in small increments. As I said I ended up at 9 lbs of boost and 31 degrees of timing and thats where the engine seems the happiest. On a side note I did up the boost to 12 lbs a few years back and although it was fun while it lasted I did break my harmonic balancer which took out my timing cover. My guess is that the added pressure from trying to turn the blower faster was to much for the stock balancer.

Just my 2 cents from personal experience. And I run 91 octane.

Jack

 

[RustyRatRod]

I've built a few roots blown engines. I prefer spinning the blower. You spend a CHUNK of change on one, so use it. I don't know about any fancy formulas. I would get compression down to about a flat 7:1, maybe even 6.5 and over drive the blower. Limit total timing to no more than 34 degrees.....and back it up to see if it makes more power and it likely will all the way down to 28. Plus, now days we have all this fancy computerized boost control timing stuff like MSD so that makes it easy.