So I am porting and polishing the heads for my 383 and have a set of world torquer heads. Long story short, the chambers need to be 82cc and they are currently between 77 and 79 after polishing. These heads should be able to accomodate 5cc of material removal in the chamber, but I am not going to just pick a spot and grind a big chunk out - I want to be informed about what I am doing to the shape of the chamber.

So all that being said, where do these wedge style combustion chambers benefit most from chamber mods? should I unshroud one of the valves, or both? Should I try to generate more swirl? Should I leave the spark plug bump alone?

What should I do?

Vizard says in one of his books that unshrouding valves (intakes) only gets you more flow to a point. But he as well as many other sources also talk quite extensively about static flow not being the only thing. Some even go so far as to say that swirl is more important than raw flow.

So here I sit. Trying to decide how to educate myself on how best not to butcher my combustion chambers.

 

Just take a look at the chambers on the newer heads (AFR eliminator, Dart Pro1, LS1, etc) and you should have a good place to start guessing on the black art of swirl.

 

Ha! I never thought of doing that.

The chambers themselves are pretty open as is (76cc) but maybe there are some gross chamber features I could emulate on the designs you speak of.

 

I'd try to enlarge the area around the valves moreso than any other area. But towards the spark plug, not in a 360* circle around the valves.

Out of curiosity, where did you come up with *needing* 82cc of chamber size?

 

Thanks for the suggestion - on the LS6 and Dart Pro1 heads you can see this long side swirl (favoring unshrouding the intake valve toward the spark plug) pretty well.

Good question about the 82. To be specific, its a 383 with flattop pistons. I originally had the 76cc chambers as cast on the world torquers. This required a .041 fel pro (p/n1003) gasket to arrive at ~9.2:1 SCR and ~7.8 DCR which is the most I feel comfortable with with 91/93 octane.

Problem was I had very reliable knock above 3000 rpm that wouldent go away with less timing or octane boost (even used avgas). So then I read up and with my virgin deck block height of 9.025 and that .041 gasket I was at .066 quench height. EVERYTHING I have read in the past few weeks that I have had the heads off, points me to that quench distance as being a major contributor to my detonation problem. So I am going with a .015 shim gasket to reduce my quench to .040. That would bump my compression to 9.75:1 SCR and 8.2 DCR which is just WAY to much for pump gas (specifically the DCR). So 82cc's brings me right back down to 9.2/7.8 that I was originally shooting for. Also I didnt think 6cc's of chamber shaping would be that hard, just had to get some good idea of what to modify.

Of course I am going to measure my piston heights below deck individually to make sure that I know where they all are so I dont have any piston to head meetings! Then Ill probably mark the heads as drivers and passengers and tailor the chamber volumes to each piston height.

Isnt building engines fun?

 

The plan:

Here is a chamber that I polished to a level I am satisfied with.


You can see the red area I shaded is where I will try to open up the chamber.

I think I will be able to get 4-5 cc out of this area without making it too thin but I will check the thickness if I can and not be too agressive.

Lemme know what you think.

Thanks.
----------
sorry about the ginormous picture!

 

i would recommend not removing to much from that area... no telling where the water jacket is... focus more towards the 10-11 o'clock position... and try and get rid of the steps around that valves

 

6cc is a lot of material to remove in any one area. Setting the valves deeper would gain some, so long as the heads will take it. Of course, that changes the valve train geometry and you'll have to adjust accordingly. The other side of that coin is that it lowers the valve bowl volume (and port volume) without making the runners smaller. That may be a benefit in velocity.

Obviously, I don't have the measurements, but it appears that you could stand to unshroud the chamber wall side of the intake valve a bit more. There is a bit of material around the top side of the spark plug boss (in the photo) which could be taken out to finish a complete semicircle.

 

Well I just got done polishing ALL 8 chambers and cc'ing them. They are all nice and shiny but come in between 77 and 78.5. So in reality the most I should have to take out for a cylinder is 5 cc. That is provided the pistons are all .025 in the hole which they may not be, most likely they will be higher which means I can take out even less.

I wanted to finish only the intake side of the spark plug semicircle to promote more swirl but if I need more then I will probably just take the intake side of the chamber out more like vader suggests.

 

With only 9.2:1 you should have ad no problems on 91 octane and certainly not on 93.
Something else was causing it to ping at part throttle cruise.
Too hot of a spark plug heat range. Sharp glowing edge in the combustion chamber,
Intake manifold/carb vacuum leak or carb fuel circuit fault causing 1 cylinder to run leaner than the rest.
Excessive vacuum advance at cruise. I see lots of sharp edges in your combustion chamber than need smoothing.
The fact that av gas would not suppress the pinging tell me you had a major fault in the motor like above causing one or more cylinders to run incorrectly at cruise. quench or no quench.

Possible inaccurate spark timing tab location. possible distributor rotor phase alignment error that gets worse with additional vacuum advance at cruise. ) {spark scatter} Both, are common ignition faults that will cause pesky engine pinging at cruise.
What cam, manifold carb jetting etc??? The area you want to carve on is not a big issue on the open chamber SBC heads You're not going to gain much if any there on that particular head (other than make the chamber bigger). If you want stronger swirl, work on the intake port bias offset. (bowl offset under the valve) These heads can flow pretty good with good velocity and swirl if ported correctly. Stall off the raised spark plug boss in the chamber. All you'll achieve is exposing spark plug threads to heat in the chamber and make it ping worse.
Use a cooler non projected tip plug Autolite 144, champion RV8C.
If you just smooth off the sharp edges you have now, your finished chamber will be very close to 82cc.
Post some picks of the ports and bowls.


This is the basic port/bowl form your looking for to promote high swirl with good flow right where you can use it. (street cam).
See the strong port bias to the common wall side of the port.
Makes the flow swirl in towards the spark plug as the valves lifts off seat.
This is a 305 head but the job is the same.

 

was not really pinging at cruise so much as WOT at 3k rpm and up.

I have put a semicircle on the tops of the chambers above the spark plug boss and lowered the roof of the chamber around the valves to the top of the valve height. All this and polishing basically got me to 80cc. I dont have my camera at the moment but I will post pics when I can.

Cam is 260/264 seat to seat hydr. roller. 383 with a non-cc qjet. Jetting is 76 primary with 48 rods, secondary rods are AY I think... anyway the mixture on my wideband is perfect 14.0:1 at cruise and 12.5:1 at WOT that gradually gets to 12:1 over 3500 rpm. Distrubutor seems ok, its a newer HEI and the balancer has been verified TDC matching the mark on the tab.

I also got around to checking piston to deck height today. Most were right around .025 with about 3 of them being deeper. I should be able to use a .015 shim with no problems. Now looking back on it those pistons that were deeper below deck were most likely running .070 quench distance.