Question on compression ratios...

[Rob0381]

For you really knowledgeable mechanics, what if any, is the difference in compression ratios? The two I am comparing are 10.3:1 and 10.6:1. Is the .3 difference really significant, or would it seemingly be the same motor?

Thanks for your help!

[tharam04]

Have a read...
HowStuffWorks "What's the connection between compression ratio and fuel economy?"

[Rob0381]

Originally Posted by tharam04

Have a read...
HowStuffWorks "What's the connection between compression ratio and fuel economy?"

Interesting but not quite what I was looking for. Basically Im curious as to what the difference may be if you take an engine with a 10.3:1 ratio and up it to 10.6:1 what differences could be noticed in the operation and performance of the engine? Would the .3 difference in the compression ratio be noticeable in any way?

The reason I am asking is Im trying to understand why the Altima 3.5 only needs regular gasoline, has a 10.3:1 compression ratio, yet the Maxima requires premium and only has slightly higher compression at 10.6:1. Same 3.5 24V V6, but the Max has 20 more hp and requires premium. Looking at an Edmunds road test of the '13 Altima it says regular unleaded for the fuel type, but if you scroll to the bottom where it says "testing conditions", they used premium octane for the road test. 2013 Nissan Altima 3.5 SL Road Test Specs

I have had many cars, none that required premium, but I always experiment with different octane's just to see if the car runs any differently, which in the past they never have. My reason for being so curious is that I honestly feel like using premium in the Altima makes the engine more responsive, and run smoother. It could all be in my head, but this is the first time I have felt a different octane other than regular makes the car run better. Everyone please share your thoughts...

[Rob0381]

Any ideas?

[4DZC]

More responsive at the throttle.

[tharam04]

Maximas are designed to be sports cars. Sports cars normally require higher octane fuel to have increased performance like 4DZC said.

[Rob0381]

Originally Posted by tharam04

Maximas are designed to be sports cars. Sports cars normally require higher octane fuel to have increased performance like 4DZC said.

I understand the Max is a "sport sedan", but the Altima 3.5 could also be put in that same category. The power difference between the Altima 3.5 and the Max is minimal, 270hp on the Altima and 290hp for the Max with torque at 258 and 261. When looking at performance numbers, the Altima actually out performs in 0-60 and 1/4 mi, albeit slightly. Im just surprised that the Altima can use regular but the Max requires premium when the engine differences are negligible.

[PrDm]

I'm not sure the Maxima requires 91. For optimum performance yes but 87 works because the timing will adjust. Then you have a Max behaving like an Altima. Unfortunately the opposite doesn't hold. Putting 91 into the Altima still gives you 270 HP. An Infinity engineer has confirmed this sad state of affairs. The VQ35DE as set up on the Altima cannot take advantage of the extra octane. There may also be some small differences in engine breathing (exhaust and intake) that account for the 8% HP difference.

[2-0 terror]

I just came from a 2010 maxima that was giving me barely 20-22mpg. My new 2013 altima 3.5 SL is putting down 26-27mpg on the exact same drive, not to mention on regular unleaded.

[blueis300]

Premium fuel because maybe they are running the tune different to achieve the horsepower. Might be running the a/f leaner so to combat detonation use premium. As for 1/4 times what is the weight difference,gear ratio etc.. More comes into play than horsepower.