Compression Ratio vs. Octane (Turbo/non-turbo)

[alex484]

I'm designing a turbo engine to use in an autocross car (i.e. a torque-monster). The engine is based on a 1.6L 16-valve DOHC Fiat engine. I currently run VP C-12 with my 14:1 NA engine and it works fine, revving to 9k.

My question is what kind of compression ratio / boost can I run for VP C-12 (110 Octane Leaded Race Gas) in a turbo motor? My goal is to run up to 2-bar (28 lbs.) of boost and I read here that people here are doing that with 100 Octane unleaded fuels. VP and others have 103 Octane unleaded fuels that sound interesting because I wouldn't have to change out my O2 sensor ever time I want to play with my computer as the O2 sensors only stay good for 20 minutes on leaded fuel.

I found this chart in a book:

Normally Aspirated w/ Carbs (FI can go at least 0.5 higher):
96 octane - 9:1 CR max, standard cams only
97 octane - 9.6:1 CR with fast street cams; 10:1 CR w/ standard cams
98 octane - 11.5:1 CR with any cams

Turbocharged:
96 octane - 8:1 CR up to 12 lbf/in^2
97 octane - 8:1 CR up to 15 lbf/in^2
98 octane - 8:1 CR up to 18 lbf/in^2
_________ - 7.8:1 CR up to 22 lbf/in^2
_________ - 7.6:1 CR up to 25 lbf/in^2

Comments on the chart?

Can you help point me to the math that may have been used to create this chart or help me continue the chart up to 103 Octane and what CR I can run for 25 lb-ft?

Here is the information from VP Racing Fuels

MOTORSPORT 109
Produces more power than any other unleaded fuel. Recommended for higher boost applications with CRs up to 11:1 or naturally aspirated engines up to 13:1.

• Color: Clear
• Motor Octane: 101
• R+M/2: 105
• Research Octane: 109
• Oxygenated: Yes
• Specific Gravity: .722 at 60° F

 

[globi5]

According to the adiabatic compression law the temperature at TDC during the compression cycle only depends on the initial temperature (when the air fuel mixture enters the combustion chamber).
If you could keep the temperature after the intercooler steady (with a more effective intercooler or water injection), you wouldn't need adapt the CR (assuming there's enough remaining exhaust pressure to drive the turbine).
Well that's how I would understand it.

 

[NickE]

Note: I think your data on octane vs compression ratio is wrong or at the least missing some important information.

Are these Ideal values?

I do know that my genIII Jetta 2.0l N/A motor was stock with 10:1 CR and could run 87 or even 86 octane... Quite far from your given vlaue around 96.

Also my current car maxes boost at ~14.0psi MRP and the CR is 8:1. I currently have Maximum spark advance on 94 octane.

 

[Warpspeed]

A am with globi5 on the importance of very effective intercooling. Another factor to think seriously about is the inlet valve closing point.

The compression stroke does not actually begin until the intake valve is fully closed.

The Mazda Miller engine has 10.2:1 compression and runs 15psi boost, and it has rather pathetic intercooling. They get away with it because the inlet valve closes 72 degrees ABDC.

 

[jons999]

I don't think there is any easy answer to that question. Coolant temperature, valve and ignition timing, piston material, combustion chamber shape, even the type of spark plug used has a huge influence in the octane needed to keep an engine from detonating. I have seen guys putting 104 octane racing fuel in relatively mild 400hp street engines, and i know of engines that make over 800hp on 94 octane pump gas. I think its more of trial and error and how your combo works. I wouldn't trust that chart.

-Jon

 

[drwebb]

Seems to me to have a coherent discussion of octane numbers you have to be specific whether you're talking about RON, MON, or AKI (Anti-Knock Index, aka "the pump rating" or R+M/2). For the benefit of us spectators- when engine designers just say "octane," do they always mean RON? And what's the formula for estimating "octane" (whether RON or MON) from AKI?

 

[carnage1]

Does anyone have any information on octane vs. cylnder pressure? It seems like this would be the most usable to start calculations from.

 

[Warpspeed]

I doubt if even that would give a usefully close starting point to guarantee combustion stability.

Rate of pressure rise (once the fire is lit) is going to depend on squish, conrod ratio, and ignition timing.

 

[globi5]

Besides it's high temperature more so than high pressure that leads to detonation.
(Although pressure and temperature are dependent, an engine operated in Northern Alaska during the winter will be less likely to deal with detonation than an engine operated in Mexico city in the summer, even though the combustion pressure in the first example is higher.)

 

[HunterA]

If the rules allow it, Water Injection...the potential is very high.

http://www.aquamist.co.uk/

There is also a forum @ that site. Lots of good info.

There are also other suppliers, and DIY websites.

I would go with the unleaded fuel.

Art