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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).

= = = = = = = = = = = = = = = = = = = =
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.

= = = = = = = = = = = = = = = = = = = =
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.

= = = = = = = = = = = = = = = = = = = =
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.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

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

E85 pump is more like 96-98 octane from realistic measurements that I have seen.
I think 14:1 CR is stretching it..
I'd feel more comfortable at 13:1 max or lower.
On a boosted engine..I'd feel comfortable at 12:1 max and must run high octane or very low boost if on 91/93octane.
You can get away with 11.5:1 CR boosted engines on 91 octane with "low boost" (were talking 8-10psi max and a really good tune), and a decent amount more boost and timing with e85 or race gas.[referencing newer aftermarket turbocharged camaros]
Also depends on engine/combustion chamber design.

Side notes: E85 contains more oxygen molecules than standard gasoline and offers high advantage of latent heat of vaporization value which pulls heat out of the intake charge which increases density. Also, the higher octane value (resistance to preignition) allows for higher engine timing.

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

There can be no "rule of thumb".

I have an engine with 11.5:1 compression that runs just fine on 87 octane gasoline. 56mm bore 40.5mm stroke, redline 14,000 rpm. And that's a mid-eighties design. The newer stuff is beyond 13:1 (fuel injection and better combustion chamber shape). I am talking about modern motorcycle engines.

The auto manufacturers have some newer designs that are 11:1 - 13:1 and are designed for pump gasoline. Some are direct injection, some use high tumble intake ports for faster combustion, etc.

It depends on too many factors. Engines with smaller cylinders that rev higher will put up with more compression.

E85 will allow "more" compression. How much more ... depends.

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

(OP)
Some interesting and useful thoughts around using E85 - thanks for those. But only Brian has related this to my search for a "rule of thumb".

In looking for a CR rule of thumb, I was hoping to get some kind of relationship (theoretical, empirical, vague even) between an original CR for a petrol engine and its "equivalent" for E85. Here's my rule of thumb: "multiply by 1.3 (or 1.4 if you're pushing it.)" Well, I did say that "vague" was ok and I'm hoping someone can improve on this and maybe have some theoretical basis for doing so.

Just stating an opinion on a "good" CR for an E85 engine doesn't help much. Brian's last post encapsulates the problem. If we look back at production petrol engines over the past 100 years, there is a huge range of compression ratios. A large capacity side valve engine and a modern high revving multi-valve engine do not demand the same mechanical CR.

But I was hoping that there would be a theoretical basis and a formula. It's a fair assumption that manufacturers are pretty good at deriving a nice balance between efficiency and safety margin in their choice of CR, so if there were a formula to "translate" this to a fuel containing X% of ethanol, it would be useful.

All things being equal, and taking Brian's examples..... let's say we have those 2 engines with 11.5:1 and 13:1 CRs. Each will take "more" compression with E85. The latter engine has the same benefits (modern injection and combustion chamber shape) whether it's running on regular unleaded or E85...... so is there something predictable about this "more" quantity? As I said, my guess - and it really is a guess with only minimal empirical evidence is to multiply by 1.3..... Surely someone with some great theoretical knowledge of combustion processes can do better than that.

@EdStainless
It's European pump E85.
13.5:1 is roughly what I have in mind for a 2v ohc engine which had a manufacturer's CR of 9.5:1 on 95RON
Re. eliminating aluminium from the fuel system - well,

https://www.horsepowerinnovations.com/concrete/ind...

All just anecdotal but the message is the same - if you don't store it for a long time, then aluminium is fine

The stainless fuel filter is essential though.

@gruntguru
Thanks for putting me on to High Performance Academy - good channel! BUT - When you say "E85 burns faster and requires less advance in an engine that is not knock limited." I think it's the wrong inference to make from that video. OK, it does indeed burn faster but I can't see anything saying "less advance" and the shape of the torque curve as well as the very small difference between the 2 runs points to any difference being experimental error. To all intents and purposes, on the low load run, there was no difference between the petrol and E85 requirements.

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

An engine-builder friend of mine posted a picture on facebook, of a current Kawasaki ZX6R engine with the cylinder head off. It is of course a modern 4-valve pentroof design with central spark plug. The combustion chamber consists almost entirely of valve clearance pockets. The rest of the top of the piston is all squish clearance - flat towards the front/back (between the pairs of intake and pairs of exhaust valves) and with a raised projection along the sides of each piston which matches up with the bottom of the chamber in the head (between the intake and exhaust valves on each side). Even if you wanted higher compression, I don't know how you would do it without sacrificing cam timing.

As I understand it, modern superbikes etc have around 14-and-change:1 compression because it is not constructive to go beyond that. The higher losses due to heat transfer, leakdown, and friction outweigh the theoretical improvement. I wouldn't expect changing the fuel to appreciably change that.

I think your "multiply by 1.3" might be closer to the truth to "add 3 but limit to 15:1" ...

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

Again, there's no magic bullet here. Assuming the existence of an ideal static CR is much like correlating higher octane or CR to higher power, they're pretty lousy 2d simplifications of a 12d problem and often not true. Personally I've never looked at static CR as anything more than a rough starting point and rather unimportant vs dynamic compression at critical speeds among other metrics.

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

"BUT - When you say "E85 burns faster and requires less advance in an engine that is not knock limited." I think it's the wrong inference to make from that video. OK, it does indeed burn faster but I can't see anything saying "less advance" and the shape of the torque curve as well as the very small difference between the 2 runs points to any difference being experimental error. To all intents and purposes, on the low load run, there was no difference between the petrol and E85 requirements."

1. As a "rule of thumb" if fuel A burns faster than fuel B, MBT will be less for fuel A.
2. 3 degrees of advance is significant.
3. 2% torque drop on E85 at the same advance (33 deg) as MBT for gasoline - also significant.
4. The guy in the video says there is a difference (and I doubt the car in the video was his first go at tuning for E85). At 9:20 he says "if the engine is not knock limited on gasoline then we can expect the engine to want less ignition timing to achieve MBT when we move to an ethanol blended fuel.

je suis charlie

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

(OP)
@gruntguru

OK, watched that video again, perhaps more awake this time. You're certainly right on some points but let me clarify why I said that it's the wrong inference to make.

1. As a "rule of thumb" if fuel A burns faster than fuel B, MBT will be less for fuel A.

Agreed. But if fuel A is harder to ignite (and that's the case with E85) then there is a longer delay before that faster burning happens. OK, that's a small factor and the faster burning will win out but it is not completely insignificant either.

2. 3 degrees of advance is significant.

It is. But if we actually drew error bars on those torque points, I think the difference we can be sure of is less than 3 degrees. It is probably still significant but see 4. below.

3. 2% torque drop on E85 at the same advance (33 deg) as MBT for gasoline - also significant.

Well, you'd have thought so.... if that's really the case. But see 4. below.

4. The guy in the video says there is a difference (and I doubt the car in the video was his first go at tuning for E85). At 9:20 he says "if the engine is not knock limited on gasoline then we can expect the engine to want less ignition timing to achieve MBT when we move to an ethanol blended fuel.

He does indeed. But at 7:35 he also says "so we're around about 3 degrees [less] ignition advance than what we needed at the same point on gasoline. We can see however that our engine torque is almost identical 116Nm so the variation that we're seeing here is really just the variation that we'd expect with this sort of a test. This indicates really that the engine torque hasn't been affected by the change from gasoline to E85 at a point in the ignition table where the engine was not previously knock limited."

And that's the thing. He does have experience and he is saying that those differences, even the difference in engine torque, is not significant.

Most important is that we can't extrapolate from the minimal data points we have from this video to say that we want less ignition timing on E85 at all points that aren't knock limited. Note that we're at 25% throttle opening in those two low-load runs. So we really can't say anything about requirements at higher loads other than the severely knock limited high compression turbo application in that particular video. OK, he does have experience and maybe that leads him to say that overall E85 will require less advance.

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

"We can see however that our engine torque is almost identical 116Nm so the variation that we're seeing here is really just the variation that we'd expect with this sort of a test. This indicates really that the engine torque hasn't been affected by the change from gasoline to E85 at a point in the ignition table where the engine was not previously knock limited."
Yes the MBT torque is identical for each fuel but E85 loses 2% torque if operated at the gasoline MBT, ie 3 deg over-advanced.

je suis charlie

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