Max compression for 87 octane?

[ScottWisdom]

I'm rebuilding a VW (Type I) 1835cc prerunner motor. What's the highest compression that can be safely used to run 87 octane Pemex? My #1 priority is reliability, but would like to get as much power as possible. I pump the 91 octane whenever available, but it's not always available in Baja.

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[racer951]

I know a couple guys run their VW powered prerunners at 10.5 to 1 so they can run it on pump gas (91) . I imagine you could take some octane booster with you and at 10.5 to 1 you should have no problems.

[ChuckH]

Generally engines with compression ratios of 9.3 to 1 or less will work with unleaded 87 octane, if you had a programmable ignition system you could work with a higher compression ratio with some heavy tuning, i would stay below 9.3 though with 87 octane, or use a octane booster if you go higher compression while in Mex

[ScottWisdom]

Is there a mathematical calculation or is this one of those things that motor builders know based on years of experience? I've gotten opinions as low as 7.5:1 and more than 9:1. Would prefer not to hassle with octane boosters in Baja.

[Mikey]

Reliability? Don't build a 1835, build a 1776 and keep the CR around 8-8.5. 92mm pistons don't seal well and cause excessive blow-by. In the long run, 90.5mm will last longer and have more power.

[FullsizeFun]

There is nothing wrong with 92mm bores. they are not "thin". they have the same cylinder wall thickness as stock bores. Only the 90.5mm barrels are extra thick. the only "thin walled" barrels are the 87mm slip-ins.

[FlyHiFlyLo]

What cam are you running?

Static compression and piston speed is what really determines correct octane, not mechanical compression.

But if you DON'T want to get in all that: Run your regular old standard 8.5:1 especially since an 1835 will not make a noticeable difference in power from one or two points in compression either way.

FYI.... I have built many nitrous motors from 3.0 Auto Crafts to 500+in BB Chevy's and Mopars. I found this one great toy. It is the MSD adjustable timing computer. It's about 150 bucks and you can tune your timing on the fly for any fuel with just about any compression or cam combo. This will allow you to run 11.0:1 compression for the higher octane and racing fuels. Then when you put crap fuel in it you can instantly pull up to 15 degrees of timing out of it in one degree increments, so you will not detonate. Just play around with different fuels and take plug readings to know what works best for each type of fuel for your specific cam and compression ratio.

[pciscott]

I run 9 to 1 in my prerunner and have the timing at 28 and have had no problems with Mexico gas. I would suggest a water seperator, I have had problems with water in some of the stations along the gulf coast. 8.5 is what Rodrigo Ampudia runs on his hot rod, but 9 to 1 has worked well for me. Good luck!

[Mikey]

From Berg's website:

Our rigorous testing revealed many failures in less than 5,000 miles of street driving. Dyno tests of 92mm/94mm sets showed as much as 25% leakage (blow-by) after only 5 minutes of running. 20% had leakage around the head from distortion after only 45 to 50 hours of running. Blow-by was so bad that extra vents and breathers had to be added to the valve covers just to complete the tests. Temperatures, head and oil, were as much as 30% hotter than tests with factory quality 85.5 or our quality 88 and 90.5 piston and cylinders.

[ScottWisdom]

Good tip about the water separator.

I'm running an 1835 with 92mm's, 119 cam, stock 041 heads and a single 44 IDF w/ timing at 28 degrees. Compression is around 8.6:1.

Had to stop prerunning after the 1st day due to a MAJOR knock. Only got thru Erindira so had to run blind thru Simpson's on race day. Thank goodness for the PCI download!!! Don't know what caused the motor problem so I'm trying to cover all bases.