383 heads - any ideas?
 

Hey folks...

I have a brand new built 383 with MOPAR 346 heads, TRW pistons, stock rods,crank and an Edelbrock Performer intake. After some research and many different opinions, I hope to know that the 346 heads were formerly known as "emission heads" with a 88cc chamber...feel free to correct me ;)
The TRW pistons with a 88cc chamber would give me a pretty mild comp ratio of about 9.0:1!
Is it worth it to get some ported heads with a 78cc or even less chamber, maybe with a new intake?

what would the 78cc heads bring the compression up to?
what octane's do you have there?

78 cc heads will bring it up roughly to 9.2:1

what types of pistons are you running?
domed, flat, dish?

It's TRW flat top pistons, .03 over, giving me according to TRW 9.0:1 with 88cc and 9.8:1 with 78.5cc...don't really know if it's worth it and if there's a way to incrase it even more with a smaller chamber. We're running the same fuel as you guys, up to 93 oct premium gas.
I'm looking for a comp ratio of like 10. something ...

9.8 is good to go with, maybe even a little high
to be safe from pinging, 9.5 is ideal

wow i messed up on my math,, whhooops:o

What is your octane computing method? I run an 8.5-1 cr in my Harley. I've set the timing on it to run a minimum of 92 octane on the local gas. I went to Australia with the bike, and their 92 octane made the bike sound like a garbage disposal full of drywall screws! I had to add octane boost to every tank of the best gas I could find. Point is, find this out first!!! No offence to anyone here, but we may be giving you incorrect advice due to the difference in octane computing methods between countries. Try google'ing "octane computing methods" to learn more about this.

Yes the computing method over here is kind of different. I already had this problem with my Mustang as I got a Cold Air Intake with a 93 oct tune. After some research I found out about the difference...93 oct US = 98 oct over here :-)

offence from me im glad you chimed in. i know theres differnt ways of computting octane, didnt noticed he was from germany, wtg scott

Well, I know guys over here safely running 500ci engines with almost 11:1 on premium gas (93oct US/98 over here), so I'm not concerned yet about the octane.
My 346 heads have an open 88cc chamber and small valves...I was just wondering if there's anyone struggeling with the same issues and knows a good setup to crank up the comp ratio and increase the air flow with other heads and maybe intake.

Static compression is just one of the many factors in how an engine runs. Having good quench vs none is important too. A low compression engine with a lousy quench will be more prone to ping than a higher compression engine will with good quench. I found this article at the KB site and was one of the few that was the easiest for me to understand.

What is the most, exact precisely defined occurrence in all piston engines? It isn’t ignition timing, combustion, crank indexing, or valve events. It is Top Dead Center. You can’t build an engine with an error at Top Dead Center because TDC is what everything else is measured from. Spark scatter, crank flex and cam timing can move, but TDC is when the piston is closest to the cylinder head in any one cylinder. The combustion process gets serious at Top Dead Center and about 12 degrees after TDC, most engines want to have maximum cylinder pressure. If maximum cylinder pressure occurs 10 degrees earlier or later, power goes away. Normal ignition timing is adjusted to achieve max cylinder pressure at 12 degrees after TDC. If your timing was set at 36 degrees before TDC that is a 48 degree head start on our 12 degree ATDC target. A lot of things can happen in 48 degrees and since different cylinders burn at different rates and don’t even burn at the same rate cycle to cycle, each cylinder would likely benefit from custom timing for each cylinder and each cycle. Special tailored timing is possible but there is an easier way—“Magnificent Quench”. Take a coffee can ½ full of gasoline burning with slow flicking flame. Strike the can with a baseball bat and you have what I would call a “fast burn”, much like what we want in the combustion chamber. The fast burn idea helps our performance engine by shortening the overall burn time and the amount of spark lead (negative torque) dialed in with the distributor. If you go from 36 degrees total to 32 degrees total and power increases, you either shortened the burn time or just had too much timing dialed-in in the first place. If you have really shortened the burn time, you won’t need so much burning going on before Top Dead Center. Now you can retard timing and increase HP. Did you ever have an engine that didn’t seem to care what timing it had? This is not the usual case with a fast burn combustion but an old style engine with big differences in optimum timing cylinder to cylinder will need 40 degrees of timing on some and others only need 26 degrees. If you set the distributor at 34 degrees, it is likely that 4 cylinders will want more timing and 4 cylinders will want less ( V-8). Moving the timing just changes, which cylinders are doing most of the work. Go too far and some cylinders may take a vacation. Now what does quench really do? First, it kicks the burning flame front across and around the cylinder at exactly TDC in all cylinders. Even with spark scatter, the big fire happens as the tight quench blasts the 32 degree old flame around the chamber. Just as with the coffee can, big flame or small flame, hit it with a baseball bat and they are all big instantly. The need for custom cylinder-to-cylinder timing gets minimized with a good quench. The more air activity in a cylinder you have the less ignition timing you are likely to need. When you add extra head gaskets to lower compression you usually lose enough quench that it is like striking the burning coffee can with a pencil. No fire ball here and that .070-.090 quench distance acts like a shock absorber for flame travel by slowing down any naturally occurring chamber activity. A slow burn means you need more timing and you will have more burn variation cycle-to-cycle and cylinder-to-cylinder, result more ping. Our step and step dish pistons are designed not only to maximize quench but to allow the flame to travel to the opposite side of the cylinder at TDC. The further the flame is driven, the faster the burn rate and the less timing is required. The step design also reduces the piston surface area and helps the piston top stay below 600 degree f (necessary to keep out of detonation). All of our forged pistons that are lower compression than a flat-top are step or step dish design. A nice thing about the step design is that it allows us to make a lighter piston. Our hypereutectic AMC, Buick, Chrysler, Ford, Oldsmobile and Pontiac all offer step designs. We cannot design a 302 Chevy step dish piston at 12:1 compression ratio but a lot of engines can use it to generate good pump gas compression ratio. Supercharging with a quench has always been difficult. A step dish is generally friendly to supercharging because you can have increased dish volume while maintaining a quench and cool top land temperatures. You may want to read our new design article for more information. ".

By John Erb
Chief Engineer
KB Performance Pistons

1970 Challenger,

If you run the street..9.5 compression is max for iron heads....346 are good..same as 906 head..no difference in flow....68--78 they flow mostly all the same....most 346 had hardened seats but make sure.....452 (Later) definately did...I have milled many of these....taking about 060-070 off from unmilled brings them down to 78 ccs or so....those trw pistons you have sit in the hole.....if for the street--you can first try steel head gasket--should be ok.if not go to blue felpros....a 440 with 6 pack pistons that sit .001 in the hole after milling and a 79 cc head will only bring in 10:1...
-if for the strip-port them--they work fine...and if you mill them down--make sure you mill your intake as well .012 for every .010 off heads...

I wouldnt worry about it.....run the 346's--or if you have cash--you can go another point in compression with alum heads..