Twin Turbo Skylark 350 Progress by sean Buick 76

4 minute(s) of a 265 minute read

I will get you the compression height spec when I get home however it is very close to stock spec. With a Zero deck and 52 CC chambers it would be over 10:1 compression.

In my case with a block that is not milled much just squared the pistons sit 60 thou in hole and the head CC is being enlarged over the stock 58 CC. Shooting for between 8.7 and 9.0 for compression. 

I just noticed a few days ago that from the factory the heads chambers are slightly smaller than the bore and it creates a major "hot spot" around the whole perimeter of the chamber. There is about a 1/8" ring around the head and that with just a 30 thou overbore! I am going to match up the chambers with the cylinders with some grinding then equaling out the CC of the heads. I am making the head chambers smooth as glass and polished right up anyways. Plus I need to gain a few CC in the heads to lower the compression a tiny bit. 

My plan is to put the heads on the engine with the pistons out and with the engine upside down I will use a thin marker to draw on the head chamber edge, then remove the heads and remove material up till the marker line is gone, blend it in and then do a rough smooth on the chambers, CC match them all, and finally polish them all up perfectly. It takes a lot of time but I think this is WELL worth the effort!

Until the other day I had never put a set of heads on the head and then flipped it upside down without the bottom end together... the 350 is an open chamber head design and by its nature it is very efficient however somewhat prone to pre-ignition. I feel this will be a huge prevention in Pre-ignition. 

I will post pics of this ASAP... 

Here is some info on pre-ignition and "hot spots":

Pre-ignition is a technically different phenomenon from engine knocking, and describes the event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires. Pre-ignition is initiated by an ignition source other than the spark, such as hot spots in the combustion chamber, a spark plug that runs too hot for the application, or carbonaceous deposits in the combustion chamber heated to incandescence by previous engine combustion events.

Typical causes of hot spots are sharp edges in the piston head or head chamber. 

Pre-ignition and engine detonation both sharply increase combustion chamber temperatures. Consequently, both effect increases the likelihood of the other effect occurring, and both can produce similar effects from the operator's perspective, such as rough engine operation or loss of performance due to operational intervention by a powertrain-management computer. For reasons like these, a person not familiarized with the distinction might describe one by the name of the other. Given proper combustion chamber design, pre-ignition can generally be eliminated by proper spark plug selection, proper fuel/air mixture adjustment, and periodic cleaning of the combustion chambers.

Causes of pre-ignition include the following:

- Carbon deposits form a heat barrier and can be a contributing factor to pre-ignition. Other causes include: An overheated spark plug (too hot a heat range for the application). 

- A sharp edge in the combustion chamber or on top of a piston (rounding sharp edges with a grinder can eliminate this cause).

-Sharp edges on valves that were reground improperly (not enough margin left on the edges). 

-A lean fuel mixture can also cause this issue.