Author Topic: Supercharging, what's the best compression ratio?  (Read 3414 times)

0 Members and 1 Guest are viewing this topic.

Offline thefrenchowl

  • Sr. Member
  • ****
  • Posts: 414
Supercharging, what's the best compression ratio?
« on: January 07, 2014, 10:25:20 AM »
Hi,

While doing the calculations about supercharging my bike some time back, it dawned on me that there's very little info out there on best static CR.

Here's what I came up to. I wonder if some of you out there have done the same or thought about it and I welcome your various inputs...

Since static CR = (V plus v) divided by v (V being the capacity, v the combustion chamber volume) for any chosen boost, the volume ingested by the cylinder will be bigger if v is bigger, since V is the constant for that engine.

Which implies that for the lower static CR and the bigger v, the fill will be bigger for a same boost.

Has anybody out there done some tests to show which is the best, burn a bigger volume with a lower static CR or burn a smaller volume with a higher static CR?

Bye for now, Patrick
« Last Edit: January 07, 2014, 10:29:14 AM by thefrenchowl »
Flat Head Forever

...What exactly are we trying to do here?...

Offline panic

  • Sr. Member
  • ****
  • Posts: 845
    • My tech papers
Re: Supercharging, what's the best compression ratio?
« Reply #1 on: January 07, 2014, 01:23:39 PM »
I don't have an answer, only more questions:
1. if the VE is really bad, your IVC has to expand to get any off-the-seat time for the intake valve, which means your static CR should be high or the engine will be really soft at low to moderate speeds.
2. there's probably a drop-off point in the parasitic loss-vs-boost curve of any specific blower where turning it even faster produces little in boost per HP consumed.
3. higher static CR may improve chamber "activity" (swirl/squish etc. motion), but is any required with boost - or does the internal homogenization by the rotors pass through to the chamber?
4. side valves are a special case because of the effect of long stroke on the maximum CR (displacement goes up in proportion but chamber volume doesn't).
5. the Otto cycle has big jumps in BTU vs. size with any change in CR at really low numbers, and begins to fade around 8:1
6. your static CR is tough to change without modifying the chamber shape.

I'd be tempted to use the ratio you have when the top end is prepped, whatever it comes out to be.

Offline rouse

  • Sr. Member
  • ****
  • Posts: 563
  • Impound is the place to be
Re: Supercharging, what's the best compression ratio?
« Reply #2 on: January 07, 2014, 03:22:37 PM »
There are a lot of factors to consider, so the answer would depend on the engine combination you are dealing with and where you want to make your most power.

I ran a low compression (10.5) combination with a early model Hemi that work very good for me, NHRA racing. In order to make it work I had to up the boost, too end with the same are higher final CR.

Result was moved more air, had to run more fuel, More AF = More HP. it worked for me. The fact is though, when better engine blocks came out that would stand up to High compression and big boost I was SOL.

Rouse
Johnnie Rouse
Bike 4680 P-PP2000 SCTA record 153.325    A-PF3000 182.920
                              Texas Mile 152.518 PP class  186 A-PF Class
If you love your freedom thank a vet.

Offline Crackerman

  • Full Member
  • ***
  • Posts: 206
Re: Supercharging, what's the best compression ratio?
« Reply #3 on: January 07, 2014, 04:24:54 PM »
I guess a good answer would be...
As much as you can without knock or preignition.

Fuel is a huge consideration ( alcohol vs 87 octane)
Chamber efficiency( how fast the burn propogates due to swirl and quench)
Air temp( intercooled vs non intercooled, water spray or not)


500 hp takes 500hp worth of cylinder pressure, you can make it with more compression or more boost (same goes for airflow or fuel)
An engine with a higher static compression ratio will generally behave better off boost.

Offline desotoman

  • Hero Member
  • *****
  • Posts: 2815
Re: Supercharging, what's the best compression ratio?
« Reply #4 on: January 07, 2014, 07:46:34 PM »
What fuel will you be using?

Tom G.
I love the USA. How much longer will we be a free nation?

Asking questions is one's only way of getting answers.

The rational person lets verified facts form or modify his opinion.  The ideologue ignores verified facts which don't fit his preconceived opinions.

Offline thefrenchowl

  • Sr. Member
  • ****
  • Posts: 414
Re: Supercharging, what's the best compression ratio?
« Reply #5 on: January 08, 2014, 01:11:47 PM »
Hi, thanks for the interest...

My question was more rethorical than for a specific application.

Here're a set of calcs as an example, for a 450cc single with a 300cc per rev supercharger.  Line 3 is with 5 to 1 static CR, line 4 is for 9 to 1 static CR. Same 10 psi boost for both CR.

Column J shows for that 10psi boost, the charger will turn a touch faster and there's more going in the lower CR set up as shown in column J, hence more methanol. The difference is about 15% more in the lower CR set up. The resultant boosted CRs for both is in column P

My querry, all else being the same, what is the best horse power solution, burn more in low CR or burn less in higher stress CR.

If BHP was just related to AFR, low CR is the winner, but BHP is also related to pressure, so where is the balance point when CR overtakes volume ingested?





Bye, Patrick
Flat Head Forever

...What exactly are we trying to do here?...

Offline panic

  • Sr. Member
  • ****
  • Posts: 845
    • My tech papers
Re: Supercharging, what's the best compression ratio?
« Reply #6 on: January 08, 2014, 06:04:55 PM »
Since I don't have an answer, I hate to make this more complicated, but:
1.  I'd be using a number closer to Bonneville as the ATM for the pressure ratio, like 12.59 psi; this is just the elevation, the D/A is better but IDK it. PR ((B+ATM)/ATM) for 10 psi boost based on 4,214 ft. elevation = 1.794:1.
2. You have the pressure ratio, but you won't have the density ratio until you know something about the discharge temperature (Delta T), which will increase with either boost or blower RPM (duh). Cooling effect of methanol evaporation in the system = ??
3. Do you have a factor for parasitic loss in the rotors vs. blower RPM etc? IIRC this varies with the square of blower speed. This is in addition to the drive loss from the engine, which is probably linear WRT RPM.

Again: I can't see any practical (inexpensive + fail-safe) method of significantly changing your static CR.  Both up & down require severe mods to the basic chamber shape, and introduce a new and frightening variable: "What part of the result was the chamber mod?"
Your current CR is well within the bracket of "good starting point" for blown motors. If it was now 5:1 or 10:1 the answer would be different.

Offline thefrenchowl

  • Sr. Member
  • ****
  • Posts: 414
Re: Supercharging, what's the best compression ratio?
« Reply #7 on: January 08, 2014, 07:07:52 PM »
Hi Jeff,

Thanks for interacting!!!

You have probably noticed this theorical single is 1/2 of a Sportster engine... So the 9 to 1 CR would be achieved with std high dome pistons, while the 5 to 1 CR could be achieved by the simple expedient of a flat or slightly reversed dome piston as in some diesels. Not to mention the loss of the dome which is an impediment, both in NA or supercharged set up. It is most likely as well that the lower the static CR, the less advance would be required to burn the contents...

Yes, I can do new calcs to show average Bonneville elevation and heat, but that's not what I'm aiming out!!!

I'm just trying to find, ""all other things being equal"", where is the sweet point, ie the most efficient static CR versus the loss of fill that the higher CR induces in the equation. (at this stage, not too worried about variations in temp, pressure or other external interference that can be assumed to be the same irrespective of where the CR is, on this charged engine)

Patrick
Flat Head Forever

...What exactly are we trying to do here?...