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# Rule of thumb for E85 compression ratio

## Rule of thumb for E85 compression ratio

(OP)
Is there a good rule of thumb for compression ratio where we know the original manufacturer's CR for a given petrol engine and wish to know the desired CR for an otherwise mechanically identical engine running on E85?

Any thoughts and theoretical basis on this would be appreciated.

### RE: Rule of thumb for E85 compression ratio

If the other engine is mechanically identical, then by definition it has the same compression ratio.

### RE: Rule of thumb for E85 compression ratio

(OP)
To clarify: Otherwise mechanically identical.... So it is mechanically identical except for the CR.

i.e. If we have a petrol engine with a particular CR and we wish to convert it to E85, making no other changes than CR (and non-mechanical to ignition and carburation), then, on what basis should we guess or calculate the desired CR for E85?

### RE: Rule of thumb for E85 compression ratio

Pump E85 or controlled E85? What octane rating?
I have seen the stuff around here (WI) rated 103-105, though I have seen it up to 112.
I have seen testing done (try YouTube Engine Masters) have done good work.
Typically people will run 12.5 - 13.5:1 comp in a NA engine. The turbo guys live on this stuff at high boost.
In theory you could go to 15:1, but that is high risk territory.
The apparent FAR is different, say 7:1 instead or 13:1 with gas because the alcohol adds a lot of oxygen.
And you need much larger jets or injectors, Holly sells carbs and injectors sized for E85 use.
And remember, teflon lined hoses and no aluminum in the fuel system (not even fittings).

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P.E. Metallurgy, Plymouth Tube

### RE: Rule of thumb for E85 compression ratio

Agree with EdStainless on all points. Don't fall into the Engine Masters trap and assume more spark advance will be needed - E85 burns faster and requires less advance in an engine that is not knock limited. Link

Here is a guy putting 15 psi boost into a stock (11.5:1 CR) Toyota 2ZZ-GE with E-85. Link

Unfortunately you can't raise CR without changing the combustion chamber shape. The OEM probably spent a lot of time optimising the shape so the usable CR increase with E-85 will vary. E-85 is very resistant to detonation - the power benefits from raising CR will probably start tapering before you see the detonation limit especially if poor chamber shape (lumpy pistons) or valve shrouding come into play. Most engines will only see minor improvement past 13.5:1.

je suis charlie

### RE: Rule of thumb for E85 compression ratio

I should point out that pump fuel rated as (R+M)/2 is bit optimistic for E85.
In E85 there is a larger spread between RON and MON. You will want colder plugs for example.
In many older engines you and mill the deck and heads both to raise CR. In newer engines there is not enough valve clearance to so this without replacing pistons.
Remember that all of your fuel system needs to be larger, pump, lines, filter (only SS elements).
And that E85 does not store well, the car needs to be driven regularly.
But it does let you run 12-13 CR and drive on the street with reasonable price and great power.
Guys that race blown (turbo or super) engines on E85 often use pump E85 around town and dial the tune back a little, then when they race they use racing E85 and crank the tune back up.
E85 is more forgiving of engine tune than gas. Small changes in timing and FAR don't have a huge impact.

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P.E. Metallurgy, Plymouth Tube

### RE: Rule of thumb for E85 compression ratio

And then there is the lack of heat value per unit of the fuel. Meaning efficiency takes a dump.

### RE: Rule of thumb for E85 compression ratio

#### Quote:

i.e. If we have a petrol engine with a particular CR and we wish to convert it to E85, making no other changes than CR (and non-mechanical to ignition and carburation), then, on what basis should we guess or calculate the desired CR for E85?

Assuming this isnt a hot rod project and the desire is simply to create either a flex fuel or E85 capable engine, typically the difference is simply materials used in the fuel system and tuning of the base engine.

### RE: Rule of thumb for E85 compression ratio

enginesrus, be careful how you use the word 'efficiency'.
While the fuel consumption measured in gal or pounds will increase (mileage will go down), the volumetric efficiency of the engine will increase.
There is a benefit to using more fuel of a lower heating value, that is the inlet air cooling effect. It helps keep a denser charge and lets you make full power on hotter days.

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P.E. Metallurgy, Plymouth Tube

### RE: Rule of thumb for E85 compression ratio

Same goes for brake thermal efficiency (kWout/kWin).
BTE will probably improve with E-85 and higher CR.

je suis charlie

### RE: Rule of thumb for E85 compression ratio

#### Quote:

While the fuel consumption measured in gal or pounds will increase (mileage will go down), the volumetric efficiency of the engine will increase.
There is a benefit to using more fuel of a lower heating value, that is the inlet air cooling effect. It helps keep a denser charge and lets you make full power on hotter days.

E85 isnt the proverbial magic bullet, its actually rather counterproductive to a well built, well tuned engine despite the many misconceptions and backyard science online. Yes, VE increases slightly however those gains are more than offset by the fuel's lower energy density. The effect on IATs is another good one, in theory the difference in evaporation works well however in practice the cooling effect is negligible in both DI and PI engines.

### RE: Rule of thumb for E85 compression ratio

Def not a silver bullet.
But if you force charge and want to run higher boost, or if you want to build a higher power engine and be able to afford to drive it often on the street then E85 offers some real advantage.
It is a reasonable way to get 105 Oct without breaking the bank.
And the parts to do it are readily available.
If you are not going to use the octane rating to make more power there isn't much reason to go to it.

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P.E. Metallurgy, Plymouth Tube

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