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whathaveigotinto 08-13-2010 09:44 PM

detonation
 
I originally thought my heads had around 72 cc chambers they are phaseII bowties ive learned they hace 64 cc chambers but mine have been milled so they may be less than 60 cc now an i prolly have around 11 to 1 instead of the 9 to 1 i thought i had, i run 93 octane and have never noticed any spark knock i find that hard to believe with steel heads an 11 to 1, i run 36 degree total timing ,maybe my pointers way off ? ,How noticeable is detonation in a loud car turnin 6000 ? next time at the track would it be safe to bump my timing up a lil at a time till until i notice somthing or slow down an not worry about messin somthin up ?

palmertree 08-13-2010 11:01 PM

Sound like you are running the same thing that I am running in my truck. It was a must for me to run the 93 octane. I messed up my first build in 3 days buy running cheap gas. As far as timing goes I just had to play with it till it run right at rpm.

F-BIRD'88 08-13-2010 11:18 PM

if you want a informed comment on your motor you might want to inform us as to what you got these heads bolted onto.

What is the real compression ratio? deck height gasket volume, dome volume etc etc.
Just cause your heads are milled does not mean they were 64cc to start with or are 60cc now.

You're assuming a lot. Just because you made it down the drag strip on that day does not mean you are ok on another day when condition are different, or are going to be ok in traffic on a hot day.

Check the TDC pointer using a piston stop.
You will not hear detonation with loud exhaust.

While it is possible to run 11:1 cr on the street on pump gas under ideal favorable conditions, conditions are never always ideal or favorable all the time and it only takes once under less than ideal conditions to cause enough detonation and bust a piston ring land.
When this happens, you won't see it coming.
In other words if you tune it and run it right on the edge, sooner or later your luck will run out.

Knowing that your engine has a high compression ratio and you are going to run it hard at the track (or on the street), it is fool hardy to short the motor on fuel octane.
Shorting such a motor on fuel octane is like filling the crank case with the worst crap motor oil just cause its cheaper.
For 11:1 cr you want a real R+M/2 octane of 98 to 102.
You can mix premimum 92 gas with 110 unleaded race gas to achieve this or run 100 octane fuel. Octane (ability to resist knock) is insurance against engine destroying damage caused by detonation and or preignition.
Ya get what you pay for.

In a high compression engine:

Think of fuel octane and extra cost associated with it, like car insurance, you don;t need it until you need it, then you really need it and are glad you got it.
How close to the edge can you afford to run?

cobalt327 08-14-2010 06:12 AM

Quote:

Originally Posted by whathaveigotinto
I originally thought my heads had around 72 cc chambers they are phaseII bowties ive learned they hace 64 cc chambers but mine have been milled so they may be less than 60 cc now an i prolly have around 11 to 1 instead of the 9 to 1 i thought i had, i run 93 octane and have never noticed any spark knock i find that hard to believe with steel heads an 11 to 1, i run 36 degree total timing ,maybe my pointers way off ? ,How noticeable is detonation in a loud car turnin 6000 ? next time at the track would it be safe to bump my timing up a lil at a time till until i notice somthing or slow down an not worry about messin somthin up ?

Use the following to determine if the timing tab/mark is accurate:

DETERMINE TDC. Then MAKE A TIMING TAPE so you can see what the total timing, etc. is w/o needing a dial back timing light- which may or may not be accurate itself.

The problem w/doing track testing to set the timing, is that you really don't know where you are starting at. One run under severe, unheard detonation can ruin an engine- so you need to start at a safe timing setting and work up from there.


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