Re: [Amc-list] Detonation -static vs. dynamic calculator (Slightly OT)

["masheen" <masheen@xxxxxxx>]

Very well said Bruce!  I'm a bit curious though about the psi figures you 
gave regarding the stock, modified and hot engine categories.

When you mentioned Toyota's 22R engine (which you've obviously had racing 
experience with), I thought about my '85 4x4 truck that came with a 22R and 
is, as you no doubt know, carbed.  I have rebuilt the engine once. 
Everything in it is stock.  Stock static compression ratio is 9:1  The stock 
cam specs for a 22R engine are:

Duration - 272 Int and 248 Ex
Lift - .398" Int and .382" Ex

I would, from time to time, check the compression while changing the plugs. 
I have an average of 200 psi per cylinder.  Now according to the figures you 
gave, I appear to have quite a hot engine given the stock cam specs, 
compression ratio and cylinder pressure.  Also running off of regular fuel 
too!

I know this is slightly off topic, but I'm just trying to understand how 
your figures are applicable to an everyday vehicle.  Seems like Toyota is 
doing something different from the norm.  And that was 23 years ago.  They 
sure have good stuff.  It's a shame we can't do as well with our own car 
companies here in the US.

Bob Rapp
Newport News, VA
70  Machine



----- Original Message ----- 
From: "Bruce Hevner" <scramblr@xxxxxxxxxxx>
To: <amc-list@xxxxxxxxxxxx>
Sent: Wednesday, June 25, 2008 8:28 PM
Subject: [Amc-list] Detonation -static vs. dynamic calculator


> While the Silvolite calculator is helpful it won't help you determine if
> what you have NOW is close to right.
> DO A COMPRESSION TEST. MOST stock (8.5-1) motors with stock type cams 
> should
> be in the 120-140 psi area. Mildly modified engines (10-1) should be in 
> the
> 140-160 area and "hot" street motors (10.5) should be in the 160-180 area.
> Competition engines (11-1) should be 180-210.
> This gives you an idea of where you are with cam timing to static
> compression relationship.
>
> In other words,, if you have a stock 8.5 motor with a stock cam that 
> cranks
> 140, but after a cam change cranks 110,,, you need more STATIC compression
> ratio (say 10-1) to get the running compression back up. Running the 
> engine
> with low "running" compression results in a "lazy" motor that does not 
> make
> full power and will run on less than expected octane.
> Running an engine with excessive "running" compression results in a motor
> with an appetite for higher than expected octane that is on the ragged 
> edge
> of detonation (and heat).
>
> If you're close in you're running compression you can adjust the cam 
> timing
> to get best power for the combination you have.
>
> On our Toyota's (22R OHC) we ran cams with about 240'@.050 to 260'@.050
> (depending on the track). Static compression was 10-1 to 11-1 depending on
> which cam we chose initially. Our running compression was always 170-200
> although we would shoot for 180-190 (less heat). We would start with the 
> cam
> "straight up" and then advance it 2' at a time watching the compression.
> Once the compression stopped rising we ran the cam there (or 2' less). We
> found this ALWAYS resulted in the maximum performance for that particular
> combination.
>
> I know this might not be a real exact way to tune your engine but over the
> years I have found it to be a valuable trouble shooting tool.
>
> But HEY,,, that's just ME!!!
> Bruce Hevner
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