Compression ratio Quandaries????

[Kstang]

Ok I have been doing some number crunching and I am confused. According to the manual, and my own calculations the stock compression ratio for a 1965 ford 200 cid engine is 9.2 to 1. Now I have also read in the ford I/6 performance handbook on page 16, “The generality is to run a compression ratio that matches the octane rating commonly available to you. For example: if you choose to buy 87 octane pump gasoline, then plan for 8.7 to 1 compression ratio.â€￾ So my question is that if the car is running a 9.2 compression ratio and it was made to run on regular gas then how does this work. I basically want to know if I run regular 87-octane gas can I have the “stockâ€￾ compression ratio of 9.2 to 1??? Thank you for all your help!!

thanks
kevin

 

[Anonymous]

I may be wrong but I think that there was more octane in the standard gas back then :roll: that and 87 should run just fine with you`r comp

 

[MustangSix]

Octane was computed differently then as well. In the past, the gas companies only used the Research number which was much higher.

Today in the USA we use an average of Research and Motor Octane numbers (R+M/2) so the pump numbers are much lower. Research Octane is typically 8-10 points higher than the Motor Octane numbers.

As an example, 87 pump octane fuel might have a Motor Octane of 82 and a Research Octane of 92. Yesterdays regular might have had Research Octane up to about 95.

 

[Kstang]

so would you be able to run a 9.2 to 1 cpmpression ratio and still run regular gas today? Part of the things we discoverd was that our car was running a 8.8 to 1 compression ratio b/c the head gasket was replaced at one point so it had lower then stock compression ratio. So we dont know if u could put in todays regular with a 9.2 and it not ping.

 

[Hot 6t Falcon]

Hi Guys,

Kstang, you've got it. Advertised compression ratios are not REAL compression ratios. Madison Avenue types tend to fudge everything.

In building a engine in the 1960's, if ALL of the dimensions were spot on the top of the tolerances, you could have the advertised compression. That's what "blue printing" is - to make sure all of the specs are at the most advantageous spot instead of the worst spot.

You can imagine what compression you would have a 7cc dish instead of a 6cc dish; along with a 56cc combustion chambe instead of a 52; added to .040 deck height instead of .015; added to - well, you get the picture.

I run a 9.8 compression with a Clifford 264 cam with no problem. Notice I added the cam as a qualifier. The hotter a cam, the later it closes the intake valve. When the valve closes after the piston passes Bottom Dead Center (BDC) and starts up in the bore, some actual static compression is lost in favor of packing more gas into the cylinder. Trying to figure theoretical compression is beyond my math skills.

PS I hope you're using the Compression Calulator on our web site. You can get there by clicking on the link in my signature. When you get to it, the calculator is preset to a 8.7:1 200. Click on "calulate" and then you can change any individual factor and "re-calulate" to instantly see the change. It's fun to play with.

Good Luck

 

[BB6Mustang]

Kstang, if I'm not mistaken that "CR=octane/10" rule of thumb applies to moderately modified engines with much better volumetric effeciency than stock. On stock engines that don't breathe as well (especially that log head) less air/fuel gets forced into the engine, hence less cylinder pressure at the same compression ratio. And even in the unlikely event that you do get a slight ping you could always just retard the timing slightly at WOT.

Basically what I'm trying to say is don't worry about it. 87 octane should work fine in a stock or even slightly modified 200.