Spark knock on a carb'd 4.3 ? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 07-23-2002, 04:32 AM
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Red face Spark knock on a carb'd 4.3 ?

I just installed a 4.3 Vortec V6 and I'm cursed with a helluva spark knock going into 4th & 5th.

Here's the setup-
'97 block (CPI originally, converted to 4bbl Carb)
'85 head
Edelbrock Performer intake
Holley 600cfm 4bbl
Flat-top pistons
Custom grind CompCam
Big-cap HEI dizzy (4-pin module)
The valves have been checked.
I'm running a Holley fuel-pump, psi checked

I'm posting this for my engine builder. He knows engines- I know computers

Any ideas ?
GonZu

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Old 07-23-2002, 05:14 AM
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Question

What was your final compression ratio...mechanical advance curve in distributor and is the engine now electonically or mechanically controlled?
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Old 07-24-2002, 01:32 PM
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Question

Thanks for the reply KULTULZ.
Unfortunately I don't have the compression ratio numbers yet.
At one time I thought the guy at the shop said it had read 160psi (give or take a few). Recently the owner of the shop said he was looking for around 125. Which number is correct?

The engineswap didn't involve the use of computers so I expect it would be considered mechanically controlled.

Someone else on another forum said it *could* be the dizzy causing the spark-knock problem.
He stated differences in the firing degrees...
Older 4.3s are considered odd firing (108 to 132*) and the newer 4.3s are even firing (120*).
He said if I take the distributor cap and the pickup off of an inline V6, it might fix the problem. Am I on the right path?

I'm not sure what's on there now, but I'll find out soon (as well as getting those other questions answered).

I'm having to track down as much info as I can b/c the owner has lost a few employees in the past couple months.

Any idea is much appreciated

BTW- Forum guys !
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Old 07-26-2002, 05:58 AM
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Angry

Compression is 165-170

Seeing how this thing sees more 4LO time than 2WD, the 125 compression number is much more realistic for my application.

It's either replace the heads with late '90s version (which *could* cause clearance issues) or pull the flat-top pistons and replace them with di*****ype.

Just thought I'd give you the update
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Old 07-26-2002, 06:15 AM
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Static compression ratio is the actual combustion ratio figuring the total swept volume (piston movement) to the combustion cycle, i.e. on most street engines, the compression ratio should be no more than 9.0/9.5 to 1 or 10.0 to one with aluminum heads. This will prevent pre-ignition with todays' premium fuels.

It sounds as though the static compressin ratio is too high or there is too much ignition advance when putting a load on the engine (hi-load/low rpm's).

You may have to either recurve the distributor or limit the total advance at speed, as in a blower motor.

Hope this makes sense. It sounds like pre-ignition under load...which is what you have when going to higher gears without the road speed to support it.
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Old 07-26-2002, 11:45 AM
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Are you saying instead of new pistons and/or head, OR aswell as new pistons

We were hoping for somewhere between 9 & 10 to 1 on compression. I somewhat understand the math, but I know 165 is a high compression number.

It's definitely pre-ignition under load (read: 4th 5th gear).
Rarely do I hear knocking in 1st to 3rd.
Just so you know. We installed a new custom grind CompCam last week. The power is ... , but that didn't solve the spark knock.

Can you give me more info on "recurving the distributor or limiting the total advance at speed" idea. I know I can ask my mechanic but I'd like to sound half-way intelligent when I talk to him

The 4.56 gears (12 bolt) came stock with the truck. I'm just trying to work with what I got so far. IMHO the gearing is perfect.... the 5.38s might require a little adjusting tho.
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Old 07-26-2002, 12:04 PM
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[quote]Originally posted by KULTULZ:
<strong>{snip} It sounds like pre-ignition under load...which is what you have when going to higher gears without the road speed to support it.</strong><hr></blockquote>

...and 33" M/Ts and 6" lift doesn't help the situation

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