Corvette: detonation

YT wrote: 

I have been looking at crate motors, and IIRC the 500 wedge is 10.50 to 1
and runs on 93 octane.

--
________________________________
Klecko's Komrades. All the way in 2003

YT...

More info is needed.

Is it pinging under power running up through the RPM range?

What temperature thermostat are you using? I'd suggest a 180 deg.

What is the stock timing and has the timing curve been changed?

If the stock timing is less than 4 deg. set it back 2 deg. at a time.

If the curve has been changed, how fast the timing comes in, slow it up with
heavier springs.

Your time depends a lot on your cam, timing, mixture, compression,
gear-ratio, engine temperature, weight, and resistance going through the
air.

Good luck.

Gerd

"YT" <com> wrote in message
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"YT" <com> wrote in message
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YT, the ZZ4 crate engine which has aluminum heads has 10:1 Compression Ratio
and the Ignition Timing recommended is:

10 degrees BTDC @ 800 RPM

32 degrees total @ 4000 RPM with vacuum advance disconnected

Minimum 92 octane fuel is recommended but it seems to run best with 94
octane

"YT" <com> wrote in message
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YT,

My 400 has 9.6:1 (Edelbrock heads, 0.040" deck height, D-dish pistons,
224/234 cam, 114 LDA) and doesn't detonate on even 89 octane. It has
~155-165 psi cranking pressure. You have some other problem than just
compression, unless you have a very short (or advanced) cam (which I don't
think is the case, but check the cranking pressure - anything over 180-185
could be a problem).

Either:

- your mixture is too lean (try bigger jets). When I ran a Holley 750, it
needed to be jetted a few sizes larger than delivered to run right. If I
recall correctly, it came with 72/75, and now has 74/78. A number of other
changes were required to the power valve, accelerator pump circuit and the
idle circuit, as well as the vents and baffles. (I have since switched to
EFI and a MegaSquirt controller.) Also make sure your fuel pump is operating
well. It may be supplying enough fuel at idle, but not supplying enough when
the demand rises....

- you have oil leakage into the combustion chamber (check the plugs for
signs of oil). This could come past the rings or seals (possibly from
something that was forgotten during assembly, or there are blocked
drainbacks in the head, or the rings have subsequently been damaged by
detonation or over-revving), a 'leaky' PVC system, or from a poor intake
manifold gasket seal allowing oil into the runners.

- your spark advance may come in too far and/or too fast (try heavier
springs, lighter weights). I run 32 degrees max. mechanical advance - it
seems to work with the small chamber, dished piston, .040" deck combo I
have, (I rarely see over 36 degrees recommended for a small block chev, and
newer head designs generally don't require a lot of advance...)

- you have spark plugs that are too hot (get a range or two colder), or
incorrectly torqued (if they are loose, they over-heat because of poor
contact with the head)

- your thermostat is too hot (I have 160 degrees F, and an electric fan that
comes on at 175 degrees), or you have a lot of air trapped in the cooling
system (drill three or four 1/8" holes in the thermostat's flange (not the
manifold or housing, on the actual thermostat itself) to help bleed the
cooling system while you fill it). Make sure your fan comes on at the
appropriate temperature (if electric) and that the front of the rad is clear
of debris.

You ought to be able to run very comfortably at 10:1 and 95 octane, so try
some of the above to see what helps.

Lance.

Question.....If you worked the cam did you have the distributor re-curved ?

I'm currently building a similar engine with a 10:31-1 cr.myself. Try
going to cooler plugs and richer jets. Alum heads like cooler plugs (1to 4
heat ranges)...the tighter the combustion chamber the bigger this problem
will be ie the heat is concentrated in a smaller area. 67 cc or larger heads
"usually " don't have this problem. Also make sure you have a CD type
ignition system not the stock GM HEI setup. Now you see why the custom
tuners get such big bucks to build their engine packages. Hope this
helps...if not e-mail me for more free advice.
truckergeorge
"YT" <com> wrote in message
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Ok some more details :

This is a short stroke, long rod combo with a roller cam of 228/236 at 0.05"
and some 0.52 " lift.
It runs on alu heads (2.05 and 1.60 valves, 215 cc runners) with Holley 650
dp.
Thermostat is 160 ° F and fan is standard + heavy duty L82 radiator.
Roller Rockers on top
Pistons are forged types with flat dome of 3cc positive dome disp.

Detonation occurs even without pulling anything just revving it on the
chassis.

I found this detonation problem when I had the engine turn to operating
temp. on the chassis (car is in rebuild).
I use the stock HEI of my 79 vette right now to do this. Standard springs,
just an adjustible vacuum advance canister. I know this thing is not new but
in the previous engine I had (also a 10/1 CR engine it ran fine.

Thanks for all of you !

Yves

<com> schreef in berichtnieuws
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Barney,

What is the normal weight on these mechanical advance weights. Mines are
stamped with a 103 on it.

Thanks

Yves

Barney <ca> schreef in berichtnieuws
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