Another altitude question!

I am currently rebuilding my motor, and I'm putting in some CP pistons. I've elected to go with 9.8:1 c/r for the pistons. With the decked block (4 thousandths off) and 6 thousandths off the head, I'm shooting for somewhere around 10:1 c/r. I am planning on putting some decent boost through this motor (goal is 450WHP).

I believe that since I'm running everyday at 6000ft elevation, bumping up the compression ratio a bit (to 10:1) could make up for some of the altitude losses. My goal of this post is to see if its possible to equate a 10:1 c/r at 6000ft to a xxx c/r at sea level. If you do a compression test on a healthy 9:1 c/r motor at sea level, what kind of #'s do you get? My theory is that it will be similar to a 10:1 c/r motor up here.... any numbers you guys can provide will be great.

 

good question , i would like to see the answer to this

 

Hehe you're great Servion. I was wondering the same thing for when I upgrade mine.

Bump

Bottoz

 

any thoughts?

 

Your theory IMO, is correct. You will be able to account for some of the lost power created by the increase in altitude. However, you will still need to deal with the fact that a High CR leaves a small window for reliable tuning.

The reason that I'm going with 10:1 on my Built ZC:

- To gain more power off boost.
- To correct some of the lost HP at altitude.
- To be able to reasonably spool up a large T3/TO4E with a 78mm bore.

However, I am going to make sure the car is tuned a little richer than it normally would, to aleviate any detonation problems.

 

Cool... I think we're going along the same reasons for a higher c/r. If I was at sea level, I would try 9:1..... but I think that might equate to somethign significantly lower down here....

I found a set of equations to answer the quesiton here... All I need to know is what the atmospheric pressure is here at elevation (~6000ft). Sea level is 14.7 (I forget the units)... does anyone know the number I'm looking for? I'll post up my answers once I get the atmospheric pressure # at altitude

 

I haven't done any calculations or anything, but I think what you'd want to do rather than changing the C/R of your pistons is to run more boost to make up for the altitude......your effective compression drops because you're not actually getting as much oxygen in there at the same amount of boost, so up the boost....I think it's around 2-3 psi to compensate for sea level compression....
This sound right?
Brian

 

From what I understand, you can go either way.... I'm choosing to go the higher compression route because the extra psi of boost will only be effective under boost... whereas the higher c/r will add more power everywhere (providing I can tune it well and aviod detonation)

 

I'm not much help, but my ITR is now sporting 9.0-1 Compression. I am awaiting Hondata, and I'm hoping to meet up with you and/or Justin to get it semi-tuned. So maybe my car will be of some help to your questions.
Mark

 

Well, I found this:
http://www.slcc.edu/tech/mecha....html

Our altitude atmospheric pressure (~6000') should be around 11.478053 psi.

This page says that my old JDM b18c (10.6:1) was really at 13:1.... I don't think those equations are accurate.