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compressio ratios and octane
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Author:  oklahoma joe [ Thu Apr 17, 2008 7:39 pm ]
Post subject:  compressio ratios and octane
Hi,

I am busy assembling my first slant six and have made it to the point of degreeing the cam and checking compression ratio. I have had the head milled and now have a static compresion ratio of 8.23:1 up from 7.22:1. Also the 198 slant motor calculates out to a 7.78 Dynamic ratio with the cam I have.
I have been doing some internet surfing on compression ratios and pump gas. I want to use the cheap stuff 87 octane. I can not seem to get a handle on what is the recommended octane rating for the different compression ratios. What I have found says 8.0 and above needs to have at least 91 octane and this seem like a low CR to need the higher octane. What do you guys use in your engines with similar compression ratios? :)
Author:  runvs_826 [ Thu Apr 17, 2008 10:12 pm ]
Post subject: 
I run the lower octane on my engine and I shaved .1 off so I should be around 9.x:1 compression I believe. If I jumped the timing I might need medium grade I don't know. However, the v8's we build only need premium when we jump up to 10:1 or 11:1 on aluminum heads. If you factor in the head gasket you should be more than fine. Hope this helps reassure and hope other people agree!
Author:  70valiant [ Fri Apr 18, 2008 3:33 am ]
Post subject: 
My CR is almost 9.8-1 and only need 89 octane. if I back the timing to under 5* BTDC I can get away with 87 but it has less power.
Author:  Dart270 [ Fri Apr 18, 2008 5:24 am ]
Post subject: 
Unless your timing curve is way off, you can run 87 on anything under about 9.0:1. 8.2 is totally fine on 87 and will probably make more power on 87 than higher octane.

Lou
Author:  slantzilla [ Fri Apr 18, 2008 9:42 am ]
Post subject: 
I street drove mine on 87 with 10.8-1. Total timing was 30*. It would pick up a tenth on 91 at the track.

Put some gas in it and drive it. Too many talking heads on Google will lead you astray. :lol:
Author:  sandy in BC [ Fri Apr 18, 2008 1:38 pm ]
Post subject: 
That Dynamic CR should be perfect for 87.
Author:  70valiant [ Fri Apr 18, 2008 6:58 pm ]
Post subject: 
Hey Zilla, how did you get your compression? Mill the head or block or stroker?
Author:  sandy in BC [ Fri Apr 18, 2008 8:39 pm ]
Post subject: 
....long rod 2.2 piston
Author:  Aggressive Ted [ Fri Apr 18, 2008 8:56 pm ]
Post subject: 
Oklahoma Joe,

I am running 9.5 compression on 87, 89 gives it more spunk. Timing is set at 32 degrees. No ping or spark nock with either.
Author:  oklahoma joe [ Fri Apr 18, 2008 9:02 pm ]
Post subject: 
thanks for the input. Sounds like I will be fine no matter what gas I choose. It amazes me to see how far a stock engine is from what it is advertized as. So far:

the 1976 stock cam was only providing a intake lift of 0.36 at the valve.
Had a cam grinder bump it up to 0.41

cam duration at .050 lift was 206 degrees way under

Rocker arms advertized at 1.5 are really 1.41

compresion ratio 8.5:1 was really 7.2:1



I think I have brought everything up to manufactures advertized specification now, except the racker arms. Maybe if I'm luck i will have a nice little reliable engine with a little get up and go. I was planning on completing the engine this weekend and putting a blanket over it until the I get the body work done and paint on the car, but just my luck the water heater went out this morning and now I am going to spend my week end doing that.
Author:  Doctor Dodge [ Fri Apr 18, 2008 10:55 pm ]
Post subject: 
Quote:
...the water heater went out this morning and now I am going to spend my week end doing that.
I hate household plumbing, it always takes waaayyy longer then the job should take. :x
DD
Author:  Wizard [ Sat Apr 19, 2008 12:51 am ]
Post subject: 
This makes me curious beause of high compression have less issues compared to the past as it was to used to be.

Just a aside, People who is more famillar with chrysler 2.5L says I can do high compression on 87 to 89 gas like 9.5:1 to 10:1 with pent roof combustion chamber type like on 16 valve heads (aluminum head).
I'm not after high rpms, I tend to run around 3500 or so, with occasional blasts to pass or hard accelerations.

But I'm still bit leery about that but still interested in that high compression area and want your thoughts on this.

Cheers, Wizard
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