What grade of gas is best?

Yes, although above 4500 feet 85 Octane is OK

 

Higher octane is not going to kill the car, but not necessary. I would pay attention to brand of gas instead. Top-tier vs no-brand ones.

 

There is no reason to use higher octane, but higher octane will do no harm except wasting some money. No need to be concerned if someone does put in higher octane.

 

Not true at all, especially if your Prius is a gen III(2010 or newer).
The Prius Atkinson cycle engine will not be harmed by the minumum 87 octane fuel, but may not make it's best fuel economy as the operators manual says, use 87 octane OR HIGHER..

A high compression engine can benifit from higher octane fuel, to produce more power and better fuel economy. The Prius engine uses variable cam timing to adjust cylinder pressure as high as the fuel octane will allow without detonation, to produce the highest cylinder pressure and the best conversion of fuel to power / economy.
If you notice in the engine specifications, the Prius 1.8 engine has a mechanical compression ratio of 13/1 ,although the Prius engine uses variable cam timing to effectivly reduce cylinder pressure to that of a somewhat lower compression engine.
The Prius engine can lower it's effective compression ratio to as low as 9-10/1 at low rpm wide / open throttle to prevent detonatiopn, yet adjust it to as much as 13/ 1 at light throttle / high rpm, to get the most fuel efficiency at any throttle opening and fuel octane.

I have driven my 2010 Prius now for 3 years / 30,000 miles, and consistantly find a couple more mpg when using my local stations 89 octane E10 vs their 87 octane regular.
This is with their 10% ethanol blend which in theory should have 3% less energy than 87 octane straight gasoline, yet my fuel economy is better. I suspect better fuel conversion of the higher octane fuel has to be the reason, as both the E0 and E10 use the same gasoline base stock through the companys blender pumps..

An added benifit, I use 10% less imported oil and it is priced 5 cents per gallon cheaper

 

Not true. The 2010 Prius info sheet very clearly labels that 13:1 ratio as an expansion ratio, not a compression ratio. Too bad the 2012 info muddies the waters by calling it a compression ratio.

From the details on the valve adjustment range provided elsewhere, one can compute the real compression ratio. I seem to remember a maximum compression ratio in the range of 8.5-9 to 1, and a minimum around 4:1.

 

Would you post a link to that info sheet ? All the 2010 advertising booklets and operators manual state the the engine has a 13/1 compression ratio. Also, unless the minimum intake valve timing closes the intake valve well (30 degrees +) after BDC on the compression stroke, 13/1 is very possible. 8.5-9/1 might be reasonable with low octane fuel at low rpm and wide open throttle, but sounds rediculous for light throttle cruise. I can't imagine any conditions where 4/1 is appropriate ? Don't confuse EFFECTIVE compression ratio, which is controlled by the variable intake valve timing, with the mechanical compression ratio which is the volume of the cylinder at BDC compared to the volume of the combustion chamber at TDC.
A quick search of Toyota and several other sites confirm that the engine has a 13/1 compression ratio, although the Atkinson cycle valve timing puts it at an EFECTIVE 9.5/1 under certain high load conditions.

 

This is from '2010_Prius_Product_Info_FINAL.pdf', which I retrieved March 9 2009 from pressroom.toyota.com in response to a PC posting. It is no longer there, but Google points to an archived copy at priustouringclub.com, though in a form that I cannot directly link.

 

I appreciate this discussion, as I have operated (blindly?) upon the assumption my 2010 would respond similarly to my 2007. In the Gen II, I would get noticeably lower MPGs with higher octane fuel. I do know with the 2010 that ethanol-free regular has led to higher MPGs, but it is quite difficult to find in the Midwest.

 

According to http://techno-fandom.org/~hobbit/cars/valvetmg.png, intake valve closing varies from 72 degrees after BDC at the earliest, to 105 degrees at the latest.

If I'm doing the math right, 72 degrees limits the compression to 68% of the cylinder volume, or 8.85 compression ratio. 105 degrees corresponds to 42% of cylinder volume, or 5.4 ratio. Am I missing something here?

 

Possibly not, what I was missing is that the engine must meet emissions specs. They may be keeping cylinder pressure low to avoid nitrous oxide formation at high combustion temperature. That conservative cylinder pressure makes no sense from a pereformance / economy standpoint, but may be necessary for emissions reasons. Thanks for posting the valve timing link.
If emissions could be ignored, the already extrodinary Prius mpg cold be even better.

My 2010 Prius does better mpg(avg 2) with 89 octane E10 VS 87 octane E0 base stock from the same supply tanks at the local blender pump.
Other E10 fuel is hard to compare because the base gasoline stock can vary so much in quality and BTU content.

 

If you are going to use "effective" compression ratio, then you should use it for all cars. The average car is at about 7 to 7.5 when valve timing is considered.

 

This is a nice back ground article on why valve timing is not just top dead center and bottom dead center as you might expect, but allows for the fluid properties of air.

Valve Timing & Engine Performance,car racing tips on blow down overlap and ram effect via camshaft

In an Atkinson Cycle engine, the intake valves stay open much later, more than 'ram effect' would procribe, allowing intake air to flow back out into the manifold.

 

So If I fillup with 85 Octane in the Colorado Mt's. and use that
same 85 Octane gas at sea level would the engine still run
as if on 87 Octane???

 

No. 85 Octane is for above 4500 feet.

 

E30 works well too

Mike

 

However, Toyota specifically warns you not to go over E10, repeatedly.

 

Yea I seen that. Wonder if the tank will melt or something strange? I do know Prius likes that E stuff. Oh well

Mike

 

But the fuel system seals don't take kindly to E30. as mentioned above, stick with a max. of E10.

DBCassidy

 

Oh for sure run E10 or straight dino-gas. Wonder how long the Prius will go until those seals give it up?

Mike