compression ratio

pickstock Seriously Crusin Dubber

107 posts · joined Dec 2010 · 24 Jan 2011, 5:43 pm

#822911

just a quick question
i was reading baja bugs and buggies by jeff hibard

with the lower octane ratings of todays gasoline even the stock VW engine will somtimes have problems with detonation

serious??
whats a safe CR i can go to with standard maybe mild cam if i can get one second hand

66brm Seriously Crusin Dubber

140 posts · joined Nov 2009 · 25 Jan 2011, 2:25 pm

#823111

What fuel are you intending on running? What cam? Heads? Exhaust?. The reason its difficult to say is a lot of things will affect what CR an engine can run safely, a cam with more valve overlap can run more static compression and will perform better, lower octane rated fuel will be more prone to detonation (pinging) but can be dialed out within reason with timing alterations.

A stock engine(everything std) should be able to run around 8.5:1 static CR IF the tune is maintained on regular ULP.

pickstock Seriously Crusin Dubber

107 posts · joined Dec 2010 · 25 Jan 2011, 7:06 pm

#823159

yeah it was just a general question if anyone knew what octane he was talking about, ill prob be running normal fuel

cheers for the answer

vlad01 Compulsive Aussie Vee Dubber

4,270 posts · joined Jun 2010 · 25 Jan 2011, 8:15 pm

#823170

pickstock wrote:just a quick question
i was reading baja bugs and buggies by jeff hibard
with the lower octane ratings of todays gasoline even the stock VW engine will somtimes have problems with detonation
serious??
whats a safe CR i can go to with standard maybe mild cam if i can get one second hand

what? lower octane of today!

I think this is a typo. Octane has never been higher.

Even E10 has 95🤪

95 RON was like ultra premium back in the early 90's when I was still at primary school, except for leaded which was 97 RON.

71 notchback,
Past owner of, 70 NB, 73 SB and 72 FB TLE

pickstock Seriously Crusin Dubber

107 posts · joined Dec 2010 · 26 Jan 2011, 9:26 am

#823236

hence why i asked the question =D

Smiley Veteran Volks Folk

2,125 posts · joined Oct 2008 · 26 Jan 2011, 10:05 pm

#823367

I think you'll find that the book was written in the 70-80s so you can't read it like it was published yesterday.

Some of the stuff is quite dated but a lot of the information is very useful.

Smiley 😎

If you said I was a Volkswagen man, you'd be right.

70AutoStik Insano Dub Head

730 posts · joined Aug 2002 · 29 Jan 2011, 12:18 pm

#823726

America had very high octane fuel before the crisis - remember they use RON+MON/2 so their 91 octane was equivalent to about 95RON and then they had a number of grades above that.

The magazines back in the late 70s early 80s suggest they also had alcohol available at the pump and avgas was legal for road use (still not a good idea, but that's another story.)

In Australia, we had 91RON both before and after the introduction of ULP and our stock engines had the same CR. Running a higher CR is dependant on combustion chamber design, volumetric efficiency and how hot you are willing to let your heads get. You probably need to raise the CR if you are running a lumpy cam, but don't get too carried away.

E10 does have a RON of 95, but remember it is causing your engine to run about 5% leaner - it is fine with stock engines, but if you have worked your engine and raised the CR you should rejet your engine to suit it and run it all the time. On a stock engine it should give around the same fuel economy, on a modified maybe you can do better - if your CR isn't too outrageous!

BTW, Premium was 98RON in the good old days! It dropped to 95 about the time ULP was introduced (and the original ULP was awful.)