CR vs Octane rating?

71Vdubber · Post by **71Vdubber** » Sat Oct 19, 2002 6:55 pm

Hi,
has anyone seen on the web, or know off hand, some sort of chart or graph that gives a correlation of compresstion ratio to octane rating for a VW engine?
Over here 95 octane unleaded is standard and 98 is premium, and I am trying to work out the optimum compression ratio.

MASSIVE TYPE IV · Post by **MASSIVE TYPE IV** » Sat Oct 19, 2002 9:41 pm

Its impossible to calculate the octane needed from static CR, you must know the dynamic compression ratio, and even then every engine is different....

Gene Bergs program says that my 12.15:1 CR 2332(222BHP@8500) should have to run 120 octane, thats weird because it wants 30 degrees full advance on 93 octane..

Post by **Marc** » Mon Oct 21, 2002 7:12 am

I suppose you're looking for something like this: http://www.btinternet.com/~mezporting/compression.html
For a STOCK Type I engine it comes very close to my experience, but note all of the disclaimers - like Jake says, every engine combination is different.
Do you use the (R+M)/2 octane rating system? if not you'll have to compensate for that.

74baja1915 · Post by **74baja1915** » Mon Oct 21, 2002 8:43 am

That site said i need 115 octane!!!!???!?!?!?!

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If its not broke don't fix it, but if it can be made better by all means do it!

Post by **Marc** » Mon Oct 21, 2002 8:59 am

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by 74baja1915:
<B>That site said i need 115 octane!!!!???!?!?!?!

</B><HR></BLOCKQUOTE>

At 10:1, yes it will. It also says you can run 87 (R+M)/2 at up to 7.6:1 and need better than 91 by 8:1.
Note that it was created by a motorcycle tuner who specializes in endurance racing engines (sustained WOT with an aircooled engine) so it will be more conservative than a typical waterpumper or "part-time" high performance engine. Again, read the disclaimers.

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James2 · Post by **James2** » Mon Oct 21, 2002 1:28 pm

I only need 101 Octane.

Searoy · Post by **Searoy** » Mon Oct 21, 2002 3:10 pm

I need 127.

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*** Teach a Man to Fish ***
Darth Searoy

"I tend to lean toward a tighter gap and a
looser skirt....a little slap never hurt." --
Joe Locicero a.k.a. Joe of the West

71Vdubber · Post by **71Vdubber** » Mon Oct 21, 2002 3:53 pm

Thanks for that, It's a start!

Mitch · Post by **Mitch** » Tue Oct 22, 2002 5:03 am

Depends on what cam you run!

71Vdubber · Post by **71Vdubber** » Tue Oct 22, 2002 3:23 pm

I've got flycut 56cc ported polished, blended semi-hemi 040 heads, 90.5mm pistons and an engle 110 cam with a 009 dizzy.
If I set the deck height at .040" then I'll get a compression ratio of 8.1, sound ok?

Eaallred · Post by **Eaallred** » Tue Oct 22, 2002 4:30 pm

Wow, it says i'm going to need 156 octane for mine!
LOL

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Eric
64 Bug
64 Bus

jasonb · Post by **jasonb** » Wed Oct 23, 2002 11:31 am

This program is no more accurate for us than using a magic eightball.

At a very minnimum you would need the valve timing events of your cam to get a worthwhile answer to the question of octane vs cr.

Even better (but nigh impossible) would be accurate VE numbers for your engine.

Mitch · Post by **Mitch** » Thu Oct 24, 2002 5:22 am

Who has the formulas for Static and Dynamic compression with different cams vs octane?

jasonb · Post by **jasonb** » Thu Oct 24, 2002 3:42 pm

There's a calculator at http://www.rbracing-rsr.com/comprAdvHD.htm which will calculate your dynamic compression including altitude and boost. It requires you to know the timing of your intake valve closing at @.050.

I made an Excel spreadsheet that will calculate your dynamic compression and your valve timing events based upon the duration of your cam @.050.

The real question is, what's an acceptable dynamic compression for different octane ratings? No one seems to know. I believe that stock VW engines had a dynamic compression of ~7:1.
My 1776, 9.1:1 static compression with a Web 218 camshaft at 5300ft altitude comes out around 7:1 dynamic compression. I run it on 91 octane gas.

John W. Kelly · Post by **John W. Kelly** » Thu Oct 24, 2002 8:02 pm

I need some 80 proof.