compression too high?

[Rowroy]

I've got about 1000 trouble-free miles on my new 1776 (dual solex's, W110, single QP) in my '69 Beetle. Recently, the weather here has been warmer than normal (low 70's - up from the 30's) and I've noticed that the new engine is now detonating.

The calculated CR is 9:1 and I'm running 32º of total timing. I'm also running 93 octane gas. The engine has all of the tin installed and isn't running hot (according to the "dipstick test").

Today, I decreased the timing down to 28º total advance. This eliminated the detonation, but it seems as though my apron is now getting too hot . . . . possibly due to higher exhaust gas temps?

I would think a 9:1 engine should be okay w/ 93 octane, right?

Any ideas?
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[tumtum]

is this a daily driver? if so i wouldnt run 9:1 compression. engine wont last as long as if you did 8.5:1
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you know what they say about us welders, we do it in all positions, and we have deep penetration!

[Max Welton]

[beetleboy58]

Yes, too much for that cam. Shoot for 7.5 cr in your case.

[SRP1]

9:1 is not the problem, it's either in the tune up or how that 9:1 was obtained. To much deck will cause detonation also.
The spark plugs will tell the whole story, post a pic if you need help on the plug read.

[Rowroy]

[Max Welton]

Believe it or not, .041" is exactly the deck I set on my 1776. But then, it is a very well ballanced assembly and there is a Berg crank at its heart.

Max
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1967 Type-3 Fastback
Under the Knife https://shoptalkforums.com/viewtopic.php?f=24&t=151582
Home Stretch https://www.thesamba.com/vw/forum/viewtopic.php?t=767836

[Rowroy]

So basically, what I'm getting out of this is that the cranking pressure is too high with 9:1 compression and the W110 cam.

It kind of sounds like my only course of action would be to installed thicker cylinder shims to increase my deck. Damn . . . I hate to even think about that . . . damn!
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Suicide is man's way of telling God, "You can't fire me; I quit!" - Bill Maher

[Max Welton]

That's one way. I like my .041" so I got Steve Hollingsworth to cut a 3cc dish in my pistons.

Max
_________________
1967 Type-3 Fastback
Under the Knife https://shoptalkforums.com/viewtopic.php?f=24&t=151582
Home Stretch https://www.thesamba.com/vw/forum/viewtopic.php?t=767836

[SRP1]

.041" is a little on the tight side but still ok. It's a lot better than .080" thats for sure. I still say your problem is in your tuning unless your compression ratio calculations are off. To be at 9:1 with .041" deck your heads need to be at 49cc's. Is that what you calculated it at?

Other little things to try are, a cooler plug, check for vacuum leaks, exhaust leaks, lean condition due to jetting or low fuel pressure, clogged fuel filter, poor or weak ignition spark, and bad fuel just to name a few.
Hope this helps a little, don't give up.

[Rowroy]

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I am compelled to agree with SRP1. It has to be in the tuning somewhere or you are missing something key to cooling or???? I run 10.5 to 1 compression in my street engine with no problems, on pump gas. I run a 009 with 27 degrees total advance. I do have an MSD 6a box though. Just keep checking and rechecking, you will find it.

[Rowroy]

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you should have a 50-55 idle, you are pissing fuel out the tailpipe with 70s.

John
Aircooled.Net Inc.
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[Rowroy]

[COFBack]

I'm with John.
70's sound way fat.
I'd try 65s (max) and mess with your timing.
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[70VWkid]

i was reading the topic and thought that i would make a reply!
im running an 1835 with 10.2 to 1 c/r and im running it on premium pump gas and in california its only 91 octane. my timing is set at 32 degrees total advance with a 009. i have 40 idf's with 200 airs 115 mains and 55 idles. 044's with a major port and polish and this setup seems to work for me. in the summer i dont get and pinging unless im in the neibhorhood and im in 2nd and i slow down for a dip and dont down shift but other than that its all good.

[Max Welton]

I'll ask you the same question Roy has not yet answered.

What cam?

Max
_________________
1967 Type-3 Fastback
Under the Knife https://shoptalkforums.com/viewtopic.php?f=24&t=151582
Home Stretch https://www.thesamba.com/vw/forum/viewtopic.php?t=767836

[68suprise]

My 68 was doing the same thing. I was pulling my hair out until I was told to check my valves. I was using steel pushrods and for some reason they were expanding too much and my exhaust valves werent closing all they way for long enough to cool. I replaced them with aluminum push rods and the preignition went away.
I also had a distributor with bad springs and had preignition problems.

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