Compression ratio with Summit K6400 Cam

[EngineerDoug]

Hello,

It looks like I will have to include pistons & rings in my 383 rebuild, so I now have some latitude in choosing my compression ratio. If nothing else I need to decide whether I will use a steel 0.020" or an 0.038" composition head gasket. I would appreciate some guidance as to how high I should go, given the following parameters:

'68 383, 0.030" over
906 heads - don't know the chamber volume yet but will cc them
Summit K6400 cam - a mild street cam with 214/224 degrees duration @ 0.050"
3.23 gears
3400 lb Coronet
Torqueflite, probably stock converter (2BBL engine)
KB flat top hypereutectic pistons - these should be about 0.026" below the deck

I would like to use pump gas (California) without pinging issues. This will not be a long term engine for the car, but rather an interim powerplant that may see 1 or 2 years of service.

I am thinking maybe 9.5:1 would be the max for this combo, but welcome your thoughts.

Thanks!

 

[IQ52]

Run it like you have planned. With the thin head gasket you'll most likely be around 8.5:1 compression. If you jack the compression to 9.5:1 the most you would gain is 10 HP and you'll be on the edge of detonation.

 

[Banzaiii67]

First step..you need to deck your block if you are using uncut 906 heads, otherwise you'll have less than 9:1 even with .020 head gaskets.. Ask me how I know. Also 9.5:1 with iron heads is on the edge of detonation. 9.2-9.3:1 is a safer bet.

If i go back in time and not spent $5000 that i have into my 383, I would stroke it with 440 source parts and be done with it. 438 or 450 ci is worth the extra $$

If you're dead set on keeping the stock cubic inches, then the speed pro 2315 with the deck surface at 0.010 below deck since they are flat tops or the diamond pistons 4cc valve relief at zero deck are much better choices than the KB pistons. KB 162 have a huge 8cc valve relief and the KB 400 dome shape is not a efficient combustion chamber design.


Speed Pro
https://www.summitracing.com/parts/slp-l2315nf30/overview/make/chrysler

Diamond pistons
http://www.manciniracing.com/di94bo3st.html

 

[EngineerDoug]

Thanks for the tips - i will err on the side of caution and keep the compression ratio low. It sounds like my piston choice will be between the KB with the valve reliefs, or a flat top with no relief. As far as the latter choice, does anybody have experience with the K6400 cam? I would like to know if there is any chance of piston to valve interference with this mild cam. Obviously it would be best to mock it up, but i don't have that luxury.

 

[khryslerkid]

"Two cam profiles were used in most 440's.
These are listed from my 1971 Factory Service Manual:

The Standard 350 HP cam specs:
260/268 Duration, 0.425"/0.435" lift, 38 degrees overlap (113 LSA), installed at 112 Intake CL.

The HP 375 HP cam specs:
268/284 Duration, 0.450"/0.458" lift, 46 degrees overlap (115 LSA), installed at 113 Intake CL."

http://www.moparchat.com/forums/showthread.php?t=111404

Summit 6400 cam

Advertised Duration:288 int./298 exh.
Intake Valve Lift with Factory Rocker Arm Ratio:0.446 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio:0.466 in.
Valve Lift with Factory Rocker Arm Ratio:0.444 int./0.466 exh.
Lobe Separation (degrees):112

https://m.summitracing.com/parts/sum-6400

 

[dieseldazzle]

If you are going to replace Pistons and bore the block and balance the rotating assembly etc then I've already done the math for you.

I just did this on an original 1967 383 4 barrel charger.

I got h beam rods, forged Pistons with 1/16 ring pack and 438 cubic inches for 780 dollars more than I would have into the shortblock just putting a decent piston in it and keeping it stock stroke.

It would have been even cheaper but I went with the arp2000 rod bolt upgrade for 75 bucks more.

Can you tell it's not a stock 383?

Thing runs so good.

Stock 516 heads with 1.74 exhaust valves, comp springs, purpleshaft 455 272 cam Eddie carb. Petronix and flowmasters.

Everything else stock

 

[lewtot184]

basically the 6400 cam is a 112lsa version of a stock magnum cam that used a 115lsa. there is a speed-pro flat top, L2315F, that will give 8.8:1 with a .039" gasket and 88cc heads (906), or about 9.25:1 with a stock .020" steel shim. with no quench i'd keep the compression at 9:1 or a little less for pump gas. with some carbon build up compression will be well over 9:1 with L2315F or the KB162 piston. 383's don't have generous piston to valve clearance and this gives the KB162 a little bit of an edge.

 

[EngineerDoug]

Thanks for the tips - I just cc'ed my heads and they come in around 90cc. Flatness is within 0.002". The block has been bored 40 over, and I will go with the KB162 pistons. Better to have lots of valve clearance and not push the compression ratio.

After closely examining the crank with no oil on it, I did see some scratches that I could not feel with a fingernail. I bit the bullet and decided to try polishing them out myself. I was surprised to see that a little time with a fine sanding strip took care of them, but the paper left its own less-than perfect finish. Took a gamble and got some bore paste out of the gun cleaning supplies - this stuff is used to clean your rifle bore but is aggressive. You only want to use it infrequently or you will start to erode your barrel. I put a bit on the backing of the sanding strip and easily brought out a nice sheen to the journals and mains. Not an expert here, but it looks so much better than when I started.

The last hurdle will be the valves. The valves and the seats look good to the untrained eye, but how would you decide whether to replace the valves and/or do a valve job? Budget build here, and I could always do the heads later if I decide to. Note the valve guides feel tight; I can't wobble the stems side to side at all.

Thanks.

 

[Garys1969RR]

If there is no side to side movement when you move the valve head back and forth, you may be in luck. The stock guides seem to wear fairly quickly. Most of us put in bronze guides, or just replace the heads with aftermarket ones. How many miles are on the heads? Like you said, you can always swap heads later. There is so much to be considered with heads, volumes can be written about them. Myself, I like to build a motor with.035 to .040 quench. This allows higher C/R without the pinging issues, and improves throttle response. With your pistons being at .026 below deck, a closed chamber head would get you .046 quench with a closed chamber head. With your 906 heads, you're looking at .160" of quench or more. So basically none. They will work OK, just keep the C/R down.

 

[miller]

The last hurdle will be the valves. The valves and the seats look good to the untrained eye, but how would you decide whether to replace the valves and/or do a valve job? Budget build here, and I could always do the heads later if I decide to. Note the valve guides feel tight; I can't wobble the stems side to side at all.

Going by 'feel' is fine...as long as...your starting with parts that are clean, including the insides of the guides. Keep in mind, through the engine's life, those guides do take a little oiling. Oil can turn to carbon. Are your valve clearances off carbon, or guide bore?
Not saying they can't be good. If they are, go for it.
I'll say it again...usual reason for replacing valves, is wear on the valve stems, not the face. Measure your stems, and if within limits, can be okay. Polish the stems. I like to use 600 grit cloth, or better. Just need the stems smoothed, and polished.
If your starting with cleaned parts, and guide clearances are good, you can do a liquid leak check for the valve seating. Use something like kerosene, head surface up, drop the valves in, and fill the 'bowl' with fluid. It will show any leaks. Just a get by, but valves can be lapped in using fine compound.
Hint...once a valve is lapped in...should be the hole THAT valve goes into. Anyway, you get the idea.