Searching for a good compression ratio and quench...

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Setup for my '66 Chrysler 383.

So let's start with the compression ratio. I'll be using closed chamber heads. Either 915 or 516. I have one of each so if I can find another 915 I'll use those. If not I'll have to settle for the 516s. Anyway..

Iron, closed chamber heads. KB flat tops with 5CC valve reliefs. .040 head gaskets. I'm going with the a full edelbrock performer induction system (cam, carb, intake, timing set) for good torque on the low end. Cam specs:

Right now, as is, no milling of the heads (78.5CC) .020 below deck height, I'm looking at 9:1 CR. Now, not really sure what I should be aiming for (i'll be using premium fuel...) I'm thinking between 9.5:1 and 10.1:1. Help me out here and lets think over what CR I should be aiming for. Then we can talk quench, which currently, with a .040 head gasket and .020 below deck height, I'm looking at a quench of .060. That really seems ineffective to me.

So lets talk this over. Many thanks.

posted by  DodgeRida67


The best engine I ever built was a bored and stroked truck 454 to 502 CID for a DAX Cobra replica. The CR was 10:1 which was on the high end. Large battery needed to be fully charged to be able to turn the bastard, even with a torker starter.

I would stick to 9,8:1 to be on the safe side and to avoid heating problems: the higher the CR the bigger the termal load on the engine parts (valves, seats, head..).

I built a 383 as well with 9.7:1 and do not remember that quench was a problem. I used TRW forged pistons, but do not remember the measurements, nor the heads used. In general, the larger the better (safety), altough it effects the squeeze effect negatively...

Good luck and keep us posted!


posted by  SamJaarsma

The mild cam should give only a very slight lope, squish (quench) height of 60 thous is poor = get the piston top flush with deck and you shoould get about 35 thous with gasket crush. Get a piece of graph paper and plot the valve openings versus piston deck clearance = try to maintain 100 thous minimum clearance; its better to flycut if in doubt.

With cam events 28/62 & 77/23 your intake is advanced 5° so is designed to give you a better torque on the lower end of the power band.

Running CR 10, you need to look at maybe multi seating the valves, bigger bore dual exhaust, tuned extractors (headers) and recurved ignition advance.

With standard rod length of 6.768" and stroke of 3.75" your dynamic stroke length will be 2.95" and DCR 8.1. Which means you are heading into the PULP region and will need strong internals as well.

Did any of that concur with your thinking? :wink2:

posted by  Wally

Just keep in mind a 383 is a B not an RB. B has a rod length of 6.358 and RB engines have a rod length of 6.75. B engines have a stroke of 3.375.

Anyway, exhaust will be 2 1/2 inch dual and I will be running with headers. Also keep in mind you could get a 383 with somewhere around 10:1 CR. I think a little above. Stronger "internals" aren't needed. This beast has a forged crank as did almost all 383s and forged steel rods that are stronger than most and are probably stronger than 4340 forged H-beam rods from eagle (as I was told by mopar guys). Also the engine skirts on these engines run 3 inches below the crank centerline which adds strength to the mains.

Did it concur with my thinking? Well, the quench being ineffective at .060. We seem to agree on that I think. If I shave the deck down to 0 deck heigh (.020 off) then my quench height will be right at .040. TO me that seems good. Also it would raise my CR to 9.5:1 MAYBE that will be a winner, I'll have to get more opinions. If the quench is good that way, I could shave some off the heads from 78.5CC down to 72 CC to raise the CR to 10:1. 10:1 and .040 quench sounds good to me but, I don't know. We'll see.

Anyway, not arguing with you at all. Thanks for the input I appreciate it. I'd love to see ChrisV and Hobo's input.

posted by  DodgeRida67

Sorry I just swiped the rod/stroke specs from a net site.

I know I shouldn't, but Im running 289 rods on old style 302 pistons (smaller compression height) that gives me 5 thous over deck. I'm not unused to 25 thous squish height on Jap engines.

The CR by itself means zip really , the inlet closing angle is the key. Even with those revised rod and stroke lengths your DCR is still close to 8.1 (8.085) with CR 10, which still doesn't put you in the earth shattering league. When you get up around DCR 8.5 or more then you have to start thinking seriously and getting good ECUs (Autronic or Motec) on board. Of course if you dropped a couple of points you could think turbocharging with that cam. :wink2:

posted by  Wally

Hm. Well look at this...

Hm. Looks like 10:1 is going to be the magic number since we know it works OK in this engine. I just have to worry about the quench, now.

posted by  DodgeRida67

OK well I think I'm set with 10:1.
0 deck clearance, mill about .021 off the heads to bring them down to 73CC, .040 gaskets for .040 quench height and 10:1 CR. That'll be a total of about .047 milled which I may have to use rockshaft shims for. On top of that I'll have to worry about piston-to-valve clearance which I'm sure with the 5CC valve reliefs in the piston, will be fine. I'll clay it up and check. We'll see.

posted by  DodgeRida67

Ain't it fun walking on the wild side :mrgreen: You should get a car with a turbocharged engine and mod it to accept 30 pounds plus, slap in a dogbox and give it a good thrashing:- now that is trepidation. :wink2:

posted by  Wally

I will. :wink2:

posted by  DodgeRida67

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