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Posted by: Stark 01 Aug 28 2015, 10:54 AM

Hi everyone,

Those of you who have built up a Type IV 2.0 with higher compression, what is the safest compression ratio to use 93 octane pump gas?

Posted by: RoadGlue Aug 28 2015, 11:57 AM

This question invokes a bunch of other questions.

- Stock cooling or are you running a front-mounted oil cooler too with thermostat, etc?
- Stock P&Cs? Deck height can be an issue...
- Stock 2.0 heads?
- Stock cam?

Higher compression = more heat.

Posted by: Stark 01 Aug 28 2015, 12:11 PM

-- Stock cooling or are you running a front-mounted oil cooler too with thermostat, etc?
Ans: stock cooling for now, but am considering front-mount. It is my understanding that I will need to know which before building the engine so I can stub in the external cooler lines, yes? I have a used one in my shed, but I don't know it's history.

- Stock P&Cs? Deck height can be an issue...
- Stock 2.0 heads?
Ans: bored over and forged pistons. Will have to find out what has been done to the heads.

- Stock cam?
Ans: No.

Posted by: Dtjaden Aug 28 2015, 12:25 PM

I'm running a 2056cc engine with a Web Cam #86 camshaft. I also have a rear trunk mounted oil cooler. My CR is about 9.2:1. I use a Megasquirt engine management system. On CA 92 octane gas I have had no problem with detonation.

Posted by: toolguy Aug 28 2015, 02:32 PM

Forget about piston compression ratio. . which does not take into account ignition timing, cam design, cam / valve timing, lobe overlap and separation, all of which reduce maximum designed piston compression ratio. .

Take a compression reading and see where the motor actually is. . anything around 160-175 is still pump gas. 150 or lower is definitely regular gas.

Modern engines can run lower octane, but remember they have ECU's with sensors and do all sorts of magic to get reliability and mileage on cheap gas. My 69 Z-28 had 220 psi , factory rated at 11:1 compression, and wanted 98 or better back in the good old white pump Chevron days.
9.5 : 1 should be good on premium gas these days.

Posted by: yeahmag Aug 28 2015, 06:06 PM

I strongly disagree with the above. Go with the CR that your cam dictates unless you know what you are doing. I've built more than my fair share of motors at this point and I still tend to fall within those CR numbers. While all those variables do change what static CR you can run, all the cams I run (WebCam) come with a recommended CR stated.

Deck height, displacement, and head CC will dictate CR.

Posted by: Dave_Darling Aug 28 2015, 06:23 PM

Yup, unless you are really significantly changing the design of the combustion chamber and adding lots of sophisticated sensors, the cam has the single largest effect on what compression ratio is appropriate.

--DD