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Old 07-26-2010, 05:31 PM
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Detonation...

Well as the title says i'm pretty sure the noise I have is detonation. I'm new to putting motors together & well here's the deal. I bought a 350 short block & it has a cam(don't know the specs) & high domed pistons in it. I added a Edelbrock aluminum intake & Quadrajet carb Well it came with some heads that I put on & found they are 305 heads & now I have detonation. I put 91 octane fuel in & set the timing best I can but I can't get rid of it completely. Should I yank those heads & put on my set of double hump heads with 2.02 stainless valves or should i use a different set of heads altogether or is there something I can do to help get rid of the detonation?

Thanx, Cory

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Old 07-26-2010, 05:50 PM
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Best thing to do would be to change out the heads,hard to recommend which heads without specs on the cam ect.
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Old 07-26-2010, 06:08 PM
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Quote:
Originally Posted by yttaf
high domed pistons
OK, that identifies the problem. Depending on the dome displacement, you may never find a set of heads with a chamber that is large enough to bring the static compression ratio down into the pump gas range. Even if you did, the flame propogation across the chamber would be horrid, like it is now. Start over with some dished pistons such as Keith Black D-cup hypereutectic pistons.
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Old 07-27-2010, 01:25 PM
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Changing the 305 58cc Chamber Heads to the D-Humps 64cc will drop compression about 1 point, 76cc Heads will drop Cr about 2.5 points.
As Tech said, best way is D-Dish pistons.
As Canuck said about the Cam, you'll need to change the Cam for lower Cr.
After you pull the Heads, measure the Dome height & try to find any numbers on the Piston's top for more help.

Just an example:
(4.030" Bore X 3.48" Stroke), (.100" High-2.4cc Dome Piston), (.041" Thick Head Gasket), (0 Deck Height)
58cc Chamber Head = 12.34Cr
64cc Chamber Head = 11.37Cr
72cc Chamber Head = 10.31Cr
76cc Chamber Head = 9.85Cr
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Old 07-27-2010, 05:58 PM
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Well I pulled the heads today & they are the domes with valve reliefs & flat tops. I cleaned them up but there are no numbers on them. Just a circle in the middle with a dot inside it.
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Old 07-27-2010, 06:43 PM
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How can they be a dome and a flat top at the same time.?
All have valve reliefs. They are either a dished , domed or a flat top piston.
If a dome, how high is the dome? .100" .150" .250" .350" .400" ??

How far is the flat part, (not the dome) sitting below the deck, when at TDC.
(.025",, .045" less, more)

If a dome, you will want to find a head with a 76cc chamber.

Most of the 305 heads are 58cc to 60cc.

If the 305 heads are the #601 they can be 53cc. The cr would be very very high with a domed piston on a 350.
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Old 07-27-2010, 11:21 PM
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The dome is flat but it sticks up above the top of the deck @TDC. Don't have a way to measure it...

Sorry this is all new to me so bear with me if I don't understand something...
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Old 07-27-2010, 11:38 PM
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Quote:
Originally Posted by yttaf
The dome is flat but it sticks up above the top of the deck @TDC. Don't have a way to measure it...

Sorry this is all new to me so bear with me if I don't understand something...
It's OK, none of us knew anything at one time in our lives. We all had to learn. Does the piston look like this?
http://www.google.com/imgres?imgurl=...26tbs%3Disch:1
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Old 07-28-2010, 05:47 AM
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Hey Y'all

And hey TI,

This looks like a good thread to follow.
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Old 07-28-2010, 06:13 AM
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I think maybe the piston T.I. is thinking about is what you have,if it is go for 76cc chambers I would also think about maybe sinking the valves a little and using thickest compressed thickness gasket you can get,this will only patch things up for you,Its really hard to get S.B.to run good on domed pistons as quench area changes dramatically when flat tops or dished are not used.(flame front travels best with flat tops)
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Old 07-28-2010, 12:03 PM
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Here is the pistons I have...


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Old 07-28-2010, 05:35 PM
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OK, they're domed pistons, also called pop-ups because the raised portion of the crown "pops up" above the block deck with the piston at top dead center.
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Old 07-28-2010, 06:06 PM
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Detonation will show up on the pistons. Around the outer top edge of the piston, you will see a swiss cheese affect that looks like the inside of an Aero chocolate bar. I don't see that in your pics, but sometimes one has to look very close.

In fact, any sharp edge can suffer, such as the edges for valve reliefs.

As detonation gets worse, more serious piston damage can result, like broken ring lands, etc.

Anyway, perhaps the noise you've been hearing isn't detonation???

Now I'm not sure, but I think detoantion is caused by too far advanced igntion timing, but if combustion is caused by too high a CR due to pop-up or domed pistons, that's pre-ignition. Not sure what the signs are for that, but I'll bet they're bad.

Can anyone add to that?
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Old 07-28-2010, 06:15 PM
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See techinspectors first post . Get rid of the pop-up aka domed pistons . Tech knows what he's taking about .
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Old 07-28-2010, 06:21 PM
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maybe check your mechanical advance? could be worn and staying fully advanced. worth a shot takes a few minutes to check. does it ping/ knock all the time or just under load?
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