Compression ratio concerns

[RRSweden]

I just measured the piston-to-deck-clearance and got .080 on my 383 block. Stock rods and SP ( 366AP30) flat top pistons. I'm using 906 heads (measured 88cc) and the Felpro head gasket ( 8519PT-1 ) wich after some googling is said to be .040 compressed.
Block is .030 bore.

All these parameters gives a compression ratio of 8.13:1

The service manual for 1970 says 9.5:1 compression ratio, and that's without the .030 bore. And as far as I know I'm prette close to stock with this setup...

What's your thoughts, and how could i crank the ratio up?

 

[mmissile]

Closed chamber heads.
Thinner head gaskets.
New pistons.
Deck the block.

 

[RRSweden]

Thanks, I'll se what my budget allows... Is it usual that it differs so much from the factory specs?

 

[IQ52]

If you want, you can get your 350 HP with the 8:1 compression.

http://www.forbbodiesonly.com/moparforum/threads/the-slug-400-on-the-dyno.80843/

 

[RRSweden]

If you want, you can get your 350 HP with the 8:1 compression.

http://www.forbbodiesonly.com/moparforum/threads/the-slug-400-on-the-dyno.80843/

Man! That's some surprising numbers!
Maybe I'll save the cash from decking the block then.

I've been looking at this COMP cam Xtreme Energy 268/280: https://www.summitracing.com/int/parts/cca-cl21-223-4
And coosing between the Edelbrovk performer 383 and Weiand Action Plus (PN8008) intakes..

I have a Holley 650dp and headers

What do you think about the setup?
Any thoughts of the cam and the intakes?

 

[727]

You could use the less desirable 516 closed chamber heads and head shims. Should get you up almost 1 point on compression. Don't worry about the smaller exhaust valve. It will flow plenty for your 383 combo.

 

[Cranky]

If you want, you can get your 350 HP with the 8:1 compression.

http://www.forbbodiesonly.com/moparforum/threads/the-slug-400-on-the-dyno.80843/

What about ping/detonation with sorry quench etc with low compression? I've have a few engines that did pretty good with 8-1 but I usually had some ping issues too....

 

[65-440]

How exactly did you get to that 8.13:1 figure ? .. here is the formula i use...(dia/2)2 x 51.48=4.28/2=2.14 x 2.14=4.5796 x 51.48 = 235.75....now that no. is the costant. Now all you do is multiply the thickness in inches by the constant and it gives you the cc amount..example .019 dk ht....235.75 x .019 = 4.48 cc....figure all the volumes for each item w piston at bdc and divide by vols at tdc = cr

Here's my 383 build ... .030 over, flat tops, similar to your build except my pistons were .019 in the hole. And no, the deck was not milled, was an untouched engine ...

 

[Cranky]

How exactly did you get to that 8.13:1 figure ? .. here is the formula i use...(dia/2)2 x 51.48=4.28/2=2.14 x 2.14=4.5796 x 51.48 = 235.75....now that no. is the costant. Now all you do is multiply the thickness in inches by the constant and it gives you the cc amount..example .019 dk ht....235.75 x .019 = 4.48 cc....figure all the volumes for each item w piston at bdc and divide by vols at tdc = cr

Here's my 383 build ... .030 over, flat tops, similar to your build except my pistons were .019 in the hole. And no, the deck was not milled, was an untouched engine ...

View attachment 373111

Can't you get pretty close numbers using the calculators on the internet?

 

[65-440]

Can't you get pretty close numbers using the calculators on the internet?

Internet calculator gave me 10.35:1 ! That's a big difference !
However, I did use a calculator to get my dynamic ratio of 7.75 ....

 

[RRSweden]

My bad. 7.84:1

 

[IQ52]

What about ping/detonation with sorry quench etc with low compression? I've have a few engines that did pretty good with 8-1 but I usually had some ping issues too....

I've never experienced those problems with any of the low compression combinations (7.5-9.7:1) I have built, from 270-690 horsepower.

 

[Cranky]

I've never experienced those problems with any of the low compression combinations (7.5-9.7:1) I have built, from 270-690 horsepower.

My last LCR engine was a 400 with 452 heads, RV type cam, 383 intake from a 69 with a 650 DP and headers. It was in a 79 3/4 ton pickup with 3.54 gears. It ran great but when the temps hit 185 it would have light ping. At 180 it was fine but just don't let it go over 185! Kinda hard not to here in Texas....

 

[RRSweden]

What does ping mean? (I'm a swede and my english has limits

 

[440+6]

What does ping mean? (I'm a swede and my english has limits

Also called detonation or pre ignition, when the fuel auto ignites to far before top dead center of piston travel and trying to force the piston the wrong direction. makes a pinging sound.

 

[Cranky]

From....http://www.contactmagazine.com/Issue54/EngineBasics.html



Detonation: Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug.

Pre-ignition: Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon.

 

[RRSweden]

From....http://www.contactmagazine.com/Issue54/EngineBasics.html



Detonation: Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug.

Pre-ignition: Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon.

Thanks. So these problems could be caused from low compression ratio along with a to aggressive cam?

 

[Cranky]

Thanks. So these problems could be caused from low compression ratio along with a to aggressive cam?

Aggressive cams....as in high lift and long duration? Generally, they lower cylinder pressures in the lower rpm ranges so they usually require more static compression for them to perform well but if anything, they usually won't create high enough cylinder pressure to cause ping at low rpm. The cam in my 400 was an RV type cam which usually raises cylinder pressure in the lower rpm ranges. They make a low compression engine 'think' it has more compression. Mine was most likely pinging because both the cam AND I ran my timing a bit too advanced. I could retard it a bit but then the low end pep would be less and so long as I kept my water temp below 185 or so, it wasn't really a problem. Did you look at IQ52's link that he posted in #4? He has more experience than I do with low compression engines.....

 

[IQ52]

Every engine is an experiment of one. That is, each engine will be different. But, it has been my personal experience, and in speaking with many people, that there is a greater chance of detonation with an open combustion chamber (like the 906) and raising the compression ratio than there is with a lower compression ratio. Cam timing, hot spots in the combustion chamber, head gasket overhang, spark plug heat range, ignition timing, these things and more will factor in. You just make the best choice you can and see what happens.

 

[RRSweden]

This is tha cam I'm thinking of buying.
https://www.summitracing.com/int/parts/cca-cl21-223-4


Also, I really appreciate your input! I'm learning a lot. As i Said this is my first engine build.