Max CR for 91 Octane, California

[Scott M Jackson]

Anyone, I've heard so many stories and it's almost time to build my engine. I'm going to Craft and I need to know the max compression ratio for our California fuel. The final plan is a stroked 482ci all aluminum with stage 3. For planning purposes target a four barrel but I've paid for a Mass-Flo EFI system already. I'd like to hear your thoughts. Just another idea, if anyone knows, how do you calculate this stuff? Thanks, Scott

[Rick Parker]

I think the general concensous is that near 11:1 will work with aluminum heads and a cam with enough duration that it will bleed off some of the dynamic cylinder pressure. However that doesn't allow much margin for error IE: bad fuel load, timing error, etc. Iron heads by nature do not disipate heat as quickly and therefore are a liitle more sensitive to compression ratios at this level and will require fuel with a higher octane rating to avoid destructive detonation and pinging. With iron heads the generally accepted maximum is between 10:1 and 10.5:1. This is with the technology currently available to the hobbyist and does not take into consideration the electronics, knock sensors, and various types of fuel injection used by OEMs. Refer to your engine builder as he is the one you want to listen to.

[ByronRACE]

The objective is to light the mixture in a particular crank angle range to make it possible for the engine to operate efficiently and avoid problems. This range varies depending on many factors, and fuel octane is a large one. By efficiently, I mean more than just peak power, but that is what most people are primarily concerned about. The other factors are exhaust and engine heat, economy, emissions, and other lessor factors. By avoiding problems, I mean avoiding carbon fouling from having to run excessively rich mixtures to combat detonation, avoiding excessive engine heat, avoiding having to retard the timing a bunch...and still having an engine that is detonation prone.

Generally speaking, the earlier you light the mixture, the more complete the combustion process will be. This creates efficiency. The limiting factor for how early you can light the mixture is fuel type and burn rate (octane), compression ratio, engine geometry and rpm (how fast the piston is moving), valve timing (cam profile), etc. When you go too far, detonation and engine damage results.

So, in your case octane is fixed or decreasing; 91 pump gas probably to decrease in octane over time, as history has shown. The next largest factor is compression ratio. The goal is to pick a compression ratio that will allow you to light the mixture at a reasonable crank angle to make an efficient running engine. Generally speaking, the higher the compression ratio you choose, the later you have to light the spark.

11:1 CR on 91 Octane CA gas is not going to put you in the middle of the efficiency range. You'll be lighting the mixture late to avoid detonation. Translate that to "Deg BTDC" and I'd be willing to bet that you would be timing this engine in the 24-28deg BTDC range through peak torque and running at mixtures around 12:1 to make it durable. It will probably detonate at 30-32 total on mixtures around 12.5:1, and will likely give you fits if you pick up a tank of substandard gas.

Build the same engine 10.3:1 would probably put you in the 32-34deg total range and your optimal mixture will be in the 12.7-13.3 range. Detonation will set in at 36-38 total. Much more efficient, much more durable.

If this was my own build, I'd build it 9.8:1 CR and run 34-36deg total. Why? 10rwhp isn't worth the chance of detonation. Our fuel burns so fast, the performance penalty is almost nil and you end up with a more durable engine over-all.

I'll close by giving you an example of what a low compresson engine can do on our 91 fuel. I had a customer show up with a 8:1 CR 347" small block that was destined for a blower build. He decided to drive the car sans blower for a few months to get break-in miles, so he brought it to the dyno and I tuned it for him. It made 338rwhp in a 5spd mustang. The timing advance was around 34deg at peak torque and around 38deg advance at peak power. Normal for a 10.3:1 347 (I've tuned over 100 of them) would be about 320hp to the tires and about 28deg at peak torque and 30 at peak power. In this particular case, the low compression engine made MORE power than average once tuned. More interesting than this is what it did at higher advance values and leaner mixtures. It NEVER detonated, it just stopped making more power. I took it all the way to 44deg total advance and 13.5:1 and at that point, power started to drop off. Considering how easy it is to detonate a performance engine on CA 91 fuel, I found this remarkable considering the power number. Later, we copied this build for a road-race customer and the results repeated. That engine has been the most reliable road race engine the guy has ever run.

edit: comments on Mass-Flo EFI

Unless things have changed recently, the Mass-Flo branded system is OEM Ford EEC-IV EFI with a GM MAF sensor and a frequency-to-voltage converter box. The only thing I dislike about the system is the placement of the air meter; not the best location for a sample-tube based laminar-flow meter. Accuracy suffers. They've calibrated the processor to do a decent job of masking the shortcomings of having the meter in that location, but you may still experience some off-idle behavior that is less than desireable. Not a show-stopper, just something to be aware of. The Ford EEC-IV is quite capable...I'm running a 5.0 Mustang box, 160lb/hr injectors, and a blow-thru univer meter on the blower motor. The only limitation to be aware of is that the processor can't handle more than 7500rpm reliably without extensive modification. I doubt this is an issue for you. As far as drivability is concerned, the algorithms in the Ford EEC-IV are still superior to most aftermarket EFI systems...just harder to tune.

[Excaliber]

Rick over all those comrpession numbers sound a little high for my taste. I'd max out alloy heads at 10.5 myself, and get under 10 for iron heads. Perhaps that is a bit conservative, but only slightly so I think. I chose 9.8 for my iron heads.

With 92 octane on some occasions (really hot motor, idle just a tad to high) I get some 'run on' after turning off the key. I ran a tank of mix race gas (100 octane) and 92 octane and 'run on' totally went away under all conditions. Back to 92 and I have to be careful when I shut down the engine or it might happen.

This tells me I'm right on the 'edge' of max compression for Hawaii 92 octane.

[wtm442]

I have aluminum heads with 10.2:1 CR and I use 89 octane gas with no problems. Total advance of 34 or 35 degrees.

[Excaliber]

Well how much CR you can get away with is related directly to how much timing you want to run. When my motor was 12.5 to 1 and iron heads I used 92 octane and it was "OK" if I was careful (didn't lug it at low rpm, never ran it hot, careful taking off on hills, etc.). But when I ran 100 octane AND kicked up the timing I could drive it anyway I wanted and the increase in power was significant. Just something to consider...

[Rick Parker]

I also believe that combustion chamber design also plays a part in the equation. But make no mistake about it, the ablility of Aluminum to pull heat out of the fuel charge will allow a higher static and dynamic compression ratio with all other things being equal.

[Excaliber]

No question about that Rick! Alloy will certainly allow for a higher compression. I specified my custom pistons for 'quench', something unheard of in 1965, and impossible to attain with 'stock' FE pistons. This was to improve the combustion chamber 'burn' as much as possible. It was a HUGE leap forward in combustion chamber technology when Honda first introduced it's 'swirl combustion' design years ago. That was about as revolutionary as the original Hemi head design in the 50's. Combustion chamber design has come a LONG way since '65.

[Scott M Jackson]

Thanks all, very much
Byron, a lot of effort there, thank you, that helps emmensely. Question, the word "mixture" confirm your talking about air/fuel ratio. I've been following Neverwouldof's Dyno lessons and tracked the AF targets. Thanks again, Scott

[ByronRACE]

Mixture = Air/Fuel Ratio

For a combo like yours, peak power should occur at about 13.3:1 AFR, but you should check everywhere between 11.0:1 and 13.5:1 to be sure. I've seen NA engines make substantially more power/torque quite far from "the norm".

And, for durability reasons, and the fact the the power change isn't that dramatic...I wouldn't recommend tuning it leaner than 12.7:1; there is little to gain and the richer the mixture, the less likely it is to detonate. Too rich and you'll start fouling plugs, pistons, chambers with carbon...don't want that either. 12.3-12.7:1 at WOT is where you want to be.

Sorry about the length, I type fast and get carried away.
A couple more things... The cam plays a pretty significant role as well. Generally speaking, you want peak torque to occur at as high an engine rpm as practical and within reason. Cams that build a lot of cylinder pressure when the piston isn't moving very fast tend to detonate more easily...RV cams, short duration stuff. Think of it this way...you light the mixture, pressure is rising and chasing the piston down the bore. The faster piston is moving to start with, the less likely it is that you'll hit critical pressure/temp and detonate the mixture. Picking the right cam will help as well...and when you're ready, I'm happy to run simulations and make suggestions.

And yes, combustion chamber (both head side, as well as piston side) play a role. Quench, squish, all that stuff...yes. But, it's a lesser role. You will avoid bad combustion chamber design by buying popular/proven aftermarket heads/pistons...so I wouldn't worry about that too much. Make sure to take off all the sharp edges on valve reliefs, around the spark plug hole, flatten/smooth any casting flaws or ridges anywhere in the chamber. Edges and protrusions glow in a hot combustion chamber; and can cause preignition (cousin to detonation), and run-on to occur. If you want to get nutty about it, polish/smooth the J electrode on your spark plugs as well. I had a '93 Cobra (Mustang) that wouldn't quit pinging under load and couldn't take hardly any advance. Eventually I upgraded to better heads, and upon removal...the reason for the detonation was quite obvious. The bottom thread of the spark plug hole in #3 was hanging down about 1/4" into the chamber. That must have created a very effective glow plug.



Byron

[Zoom This]

This is all really good info. For what it's worth, I'm running a 418W with 10.83:1 CR. 34 total timing, 20 initial, aluminum heads, 232 degrees duration at .050 on the intake with .565 lift. , 110 lobe separation, 12.8 A/F at WOT. No detonation/ping, even on the hottest days in traffic.