Timing vs Compression Ratio Debate

[Fox Slaughter]

Alright I am having a debate with a guy on Tech that is running E85 at 12.4:1 CR.
So the debate he is saying is lower compression more timing makes more power.
I say if this is true engines would run 60* BTDC timing and 5.1:1 CR.
I say higher compression with less timing makes more power.

So I did some digging around on the interwebs and found an article by Stanford.

http://rescomp.stanford.edu/~efroeh/...erformance.pdf

In case you don't want to read it or understand it, here is the end result snippet.

"
The most important trends garnered from the above research are that better

performance is generally achieved at MBT, at slightly rich air fuel ratios and at high

compression ratios.

"

For those that don't know MBT stands for Maximum Brake Torque.

[c0ncEpT]

Higher compression and more timing make more power. The goal is to build a motor that actually has a tuning window so you can put some timing in it...

[Fox Slaughter]

Correct but lowering compression to add more timing does not make as much power.

There is a trade off between your compression and timing.
Running E85 for a 12.4:1 compression ratio is a waste.
E85 has octane between ~105 - ~110.
Basically you are running race fuel and using 93 pump gas compression ratio.

[1BADAIR]

static compression ratio means nothing.

the cam controls it all....



and the chamber/piston design controls timing

[Fox Slaughter]

Get out of here with your dynamic compression ratios.

Dynamic compression ratios are a joke.
I don't know how many times I have to post this but apparently it has to be posted again.

Straight from AIs website as quoted

"Common marketing misdirection and fruitless (for the end user) talking points:

• New vs. old "Lobe Technology"
• Dynamic Compression Ratio ("DCR")
• Subjective qualities (e.g. loping, valve train noise)
• Assertion that X valve event must occur at precisely X degree ideally
• Assertion that one aspect of a lobe form is "the secret" or most important characteristic."

A company that grinds cams says DCR is a misdirection and fruitless talking point.

[c0ncEpT]

static compression ratio means nothing.

the cam controls it all....



and the chamber/piston design controls timing

This ^

[JoeliusZ28]

Haha! I agree TJ. DCR is a useful discussion if you are designing your own cam grinds other than that... Whatever.

Cam manufacturers Make recommendations in terms of SCR because thats what you can control with non-valvetrain related components. Its their job to handle the valvetrain portion of the equation.

[QuickTop]

Timing vs Compression Ratio Debate

It's always been my understanding that you build a car with a lot of compression, which then REQUIRES the use of a high octane fuel. (E85) then you can squeeze more timing in with the higher octane.
I dunno though. I'm just a kid.

[Fox Slaughter]

It's always been my understanding that you build a car with a lot of compression, which then REQUIRES the use of a high octane fuel. (E85) then you can squeeze more timing in with the higher octane.
I dunno though. I'm just a kid.

Octane is basically the resistance a fuel to detonation under compression. Obviously when things are compressed they heat up. The lower the octane, and the more you compress it you get detonation which is the self igniting of the fuel.

So E85 has a higher tolerance for detonation, with an octane rating of ~105 - ~110.
What this allows you to do is compress the fuel more, and add more timing to the mix in order to give you a more complete burn. This higher compression and higher spark timing gives you better thermal efficiency.

If you read through the article I posted it basically says that a higher compression ratio with less timing, as opposed to lower compression and more timing, creates more cylinder pressure which means it is creating a better thermal efficiency. Which then in turn means the engine is capable of doing more work.

[c0ncEpT]

Dynamic compression ratio along with timing create cylinder pressure. Static compression doesn't mean much. A motor with less static compression, more dynamic compression, more efficient cylinder head, and more timing might make more power than a motor with high static compression, no dynamic compression, a garbage cylinder head, and no timing. There is no correct answer to your question.

[Fox Slaughter]

Otto Cycle.

Thermal Efficiency is calculated by E = 1(1/R ^ K-1) R is Compression Ratio and K is adiabatic expansion for air, 1.4.
A higher compression ratio will always produce more thermal efficiency.
The timing also obviously has a dictation on the cylinder pressure to an extent.
When you have a 2:1 compression ratio its potential thermal efficiency will never equal that of 15:1, doesn't matter how much timing you have.
So a 10.4:1 will never be able to produce as much thermal efficiency as a 13.2:1 compression ratio.

[TB2]

static compression ratio means nothing.

the cam controls it all....



and the chamber/piston design controls timing

This is exactly how it works. And camshaft design has everything to do with what the true compression is.

FYI. I'm running 13.8 static with the timing locked at 40deg advanced, and a cam designed for max cylinder pressure. I run it on E85 with no problems at all.

[Nocooler]

Keep in mind higher octane burns slower so it needs more lead to get the fire going.

I can't count how many times I've seen people pump race gas in their cars and go slower because they didn't bump the timing or didn't need the octane in the 1st place.

[Fox Slaughter]

Exactly.