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Old 10-27-2010, 09:54 PM   #1
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N00b here, kammback calculations

I'm new to this site and ecomodding, my 120 mile commute is what got me into this. Anyway Ill explain what I have done, and then ask about the kammback.

Vehicle: 1991 Honda Civic Si, 245k miles, the bottom of the car is rotted in spots, mainly down the entire length of the bottom sides.

Parts I have replaced for fuel economy:

O2 sensor, cone air filter, gm synchromesh for 5-speed trans.

Aero mods:

Removed moonroof deflector, spoiler, passenger side mirror
Partial undertray(coroplast) and a coroplast air dam.

I didn't do any of that a-b-a or tuft testing, but I have always calculated my mileage at each tank. My best tank before any modding was 35mpg, mainly hwy miles to and from work during the summer. Winter I usually see 30-32mpg.

Subsequent to my aero mods, I have consistently seen 39-40mpg where I would normally see 33-35mpg, with warmer temps. Since it is colder out I'm not sure if thats why I didn't see more of an increase.
I filled up the day of the bombogenesis windstorm(Tuesday), I drove 140 miles, and drove 120 today, filled up, only averaged 35.5 mpg. I am attributing that to the horrid winds.

Now to my question, I haven't done any trig or geometry calculations since I was in hs(10yrs ago) and I am reading acronyms on ecomodder.com and on here I do not fully understand.

I am trying to figure out what angle I should use for my kammback, and the dimensions for it.

I have been using this calc to get rough estimates, even though I am not good with math, I can visualize what I wish to do very easily, however.

The height I would be starting my kammback at is 51" from the ground. What is the sharpest angle I can use for the kammback? I am trying to keep it fairly short, but I realize if it's too steep it will render the kammback inutile.

Also, this car is fairly boxy, would it create too much turbulence/vortexes if I didn't use curves on the side parts of the kammback? This isn't my car, but it's to give you a rough idea.

Any help would be greatly appreciated.

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Old 10-27-2010, 09:57 PM   #2
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Re: N00b here, kammback calculations

This forum doesn't allow editing? lol


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Old 10-28-2010, 06:12 AM   #3
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Re: N00b here, kammback calculations

I fixed your image embed. The forum allows editing but not for brand new members. We were getting a lot of spammers who post something that looks legit, then later when we're not looking at their old post anymore they'd come back and edit spam into it.
This sig may return, some day.
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Old 10-28-2010, 01:19 PM   #4
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Re: N00b here, kammback calculations

easy answer and it is proven to work


that is a good site. he used to frequent here a lot. every now and again he will pop up (user name: basjoos)

***edit**** dang it, I didn't even answer your question. several people have said that the best slope is somwhere from 11 degrees to 9 degrees. I would split the difference and go with 10 but that is just me. realize that this will not be an easy task and that you may be subject to inspection issues because of it. the guy that I referred you to lives in south carolina and there is no annual inspection there.
Be the change you wish to see in the world
--Mahatma Gandhi

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Old 10-28-2010, 01:38 PM   #5
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Re: N00b here, kammback calculations

I believe the technical answer is no more than 11 degrees slope, or the airflow won't stay attached.

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