Big Block Compression Ratios?

[RR3834bbl]

I am looking to build a potent streetable 383 engine for my 69 Road Runner. At this point, I am currently in the process of figuring out what compression ratio (CR) I should shoot for, and how to attain that CR given the limited supply of performance pistons for the 383 engine.

These are the piston choices that I know about as far as off the shelf performance pistons:

1)Keith Black Hypereutectic Pistons, P/N# UEM-KB162030-8- 383 Flat top
O.E. Replacement. 10.2:1 with 72cc Head, and 9.1:1 with 85cc head.

2)Speed Pro Power Forged Pistons, P/N# TRW-L2315F30- 383 Flat Tops. 9.77:1 with 78.5cc, and 8.94:1 with 88cc head.

3)I have not looked into custom pistons and do not know cost and vendor choices.

From the information I’ve seen, the 906 heads that I currently have on my stock 383 will have around an 88cc combustion chamber depending on if they are milled or not, and the head gasket thickness that I use. That appears that between the KB piston and the Speed Pro units I’d get a CR of 9.1:1 or 8.94:1.

Does this mean that if I bolt on a set of 915 closed chamber heads, with one of my piston choices, I could get between 10.2:1, and 9.77:1, at around a full CR point more than the 906 heads?

For a performance street engine, would it be a good choice to run closer to a 10.1:1, or should I shoot for 9.1:1? Will I have bad pinging problems with the 10.1:1? I’m not opposed to running a good fuel octane booster if that will allow me to run a more powerful street engine with the higher CR.

Would it be better to have custom pistons made to get a higher CR ratio with 906 heads? Or should I find a set of 915 heads and run off the shelf pistons?

All you gear heads may want to know additional information before you can answer the questions. I’ll try to fill you in the best I can.

1)A stock looking 1969 383HP for my #s matching 69 Road Runner.

2)Edelbrock DP4B dual plane intake.

3)MP 284/484 purple shaft cam, or CompCams Extreme Energy, or Lunati Voodoo Cam? Want to run a cam of a similar lift design of the old purple shaft 284/484 unit, but perhaps a newer fast rate of lift computer cam of Comp, of Lunati.

4)2600 rpm stall torque converter.

5)3.55:1 rear end

6)Stock HP exhaust manifolds.

7)Dual 2.5 inch exhaust out the back through stock tips. Maybe a tti system with x-pipe?

8)Holley 3310-1 780cfm carburetor.

Please let me know what you think and don’t be afraid to critique my component cho

[dave571]

Compression on a 383 is hard to get.

Opinions will vary on the best match.

First I will say, that quench is a good thing, and makes better power than a non quench engine.

That said, for me, I feel the ideal 383 has a zero deck flattop piston , and a closed chamber head like a 915.

Compression ratio on such a combo with a .040 gasket(which give .040 quench) of 9.5 assunming .030 opver bore and 6cc valve reliefs.. Drops to 8.7 with an 88 cc'd 906, and quench is gone.

9.5 is plenty of compression for a street motor. As long as your compression is reasonably matched to the cam you want (and I think 9.5 goes well with what you are suggesting) the power gaisn from compression increase alone are minimal.

Roughly 4% for a full point of compression gain with no other changes.

You can get a domed piston, and still have the engine built for quench. Just need more clearance checking etc..

[MrChemist]

If you are going to use iron heads don’t go higher than 9.8:1 compression on pump gas to be safe. If you use an aluminum head you can get away with 10.7:1 on pump gas (due to the faster heat dissipation of aluminum). Another thing to consider; with the camshaft selection if you use a bigger bump stick, you can get away with more compression, because big cams bleed off low end cylinder pressure…then again if your definition of “streetable” is fairly conservative (which judging from the exhaust manifolds is a yes), forget the big cam. Good luck with your car.

By the way, if you want custom pistons, be prepared to spend about 600 bucks for the set.

[rumblefish360]

Ops, edit double post, read below.

[rumblefish360]

Iron heads and pump gas equal a ratio of approx. 9.0-1 to 9.5-1 for 91 octane. Pushing past that is a 93 octane useage to 10-1 at best.
The octane requirement just are not there at the pumps anymore.
Like Dave said, the increase from the extra ratio is nominal. However, more can be lost due to low compresion and big cams. IE the 284 in a 7.8-1 ratio engine. Thats a distaser.
If you can get a good closed chamber head on top for a nice quenched set up, you can further push a bit of compression.
For an example, XE274H would be my max for the described engine and IMO would work great. (At 230 - 236 @ .050 duration, similar cams from other manufactures in this area of duration and I believe only about 6* more at max would do well and would be pushing a good balance.)
Pushing it is possible, but not really worth much IMO.

IMO, keep it conservitive or face pinging and retarded timing at a nice power loss. Don't bother with octane booster. It is not a cost effective way to do things. Build it to run on the pump gas without problems of pinging or the need of octane booster and you will be alot happier down the road.
Alot happier.

Mr. C and Dave gave you very good advice. Pay heed.

[JLM440]

Check out KB pistons again, they came out with a newer piston for the 383, its a KB400, i think its a quench dome for open chambered heads.

[RR3834bbl]

Thanks for all the information guys. I knew I could count on the Mopar community to anwser my questions. I just want further make things clear. In my Speed Pro piston appliction list, they say that the closed chamber heads have 78.5cc combustion chambers, while the open chamber head has 88cc. Is this true of non-modified factory heads? BTW, rumblefish360, It must be too early on Sunday morning, I can't figure out what "IMO" means? Oh! and while we are on the subject of pistons, what do you guys like to use? Thanks again... RR3834bbl

[rrunnertexas]

I built my 383 for my '68 RR using the Speed Pro pistons and a "452" head that was CCd to 86CCs. That worked out to a 9.2:1 compression ratio.

With the mild hydraulic cam that I was running 222/234 .467"/.494", I could run 93 pump gas without any problems and full timing (38-40 degrees).

I also use 3.55:1 gears like you are using, but I have a 4 speed.

Having used the MP 284/.484" cam in the past, I would say that with a stock converter, 3.55:1 gears and iron exhaust manifolds, you are going a bit too big for cam selection. I would look for something with a bit less duration. The MP 284 has low vacuum and a big lope at idle. I would recommend the mild Crane cam I used if you are looking for great streetablity and can drive anywhere, anytime reliability. However, I was not pleased with the power output with that mild cam, so I have gone to a larger grind.

Keep in mind that using the iron manifolds will lower output vs. a good set of headers and mandrel bent pipes. I started my car running the manifolds, but swapped to TTI headers after about 2 years. I just wanted to maximize the power potential.

I hope this helps...

[oldguyalso]

IMO: In My Opinion IMHO: add Humble! You're welcome!

[MrChemist]

Originally Posted by RR3834bbl

Thanks for all the information guys. I knew I could count on the Mopar community to anwser my questions. I just want further make things clear. In my Speed Pro piston appliction list, they say that the closed chamber heads have 78.5cc combustion chambers, while the open chamber head has 88cc. Is this true of non-modified factory heads? BTW, rumblefish360, It must be too early on Sunday morning, I can't figure out what "IMO" means? Oh! and while we are on the subject of pistons, what do you guys like to use? Thanks again... RR3834bbl

There are many good piston manufactures out there. I like (and currently use) JE pistons…yes, they are expensive, but you get what you pay for. I never have to worry about breaking one of these bad boys

[ehostler]

78.5cc an 88cc are what was designed. Actual was 2 to 4 cc larger.

As the KB pistons have a thin face, I wouldn't use them. Many have had good luck with them, I just don't like the idea that a tank of bad gas will cause ping and then a good chance of a hole in the piston.

[ehostler]

Originally Posted by oldguyalso

IMO: In My Opinion IMHO: add Humble! You're welcome!

H can also mean Honest

[ehostler]

For question #3, I would go with either a Racer Brown cam or a Hughes cam. Both of them are running up to date cams and both have been serving the MOPAR community for a very long time.

[ehostler]

If you are using stock manifolds, the TTI exhaust system isn't going to do much for you. Buy a Dr. Gas X-pipe and then take the car to your local muffler shop (one with mandrel benders) and have them put together an exhaust system for you.

[RR3834bbl]

Hey MrChemist, I just went to the JE Pistons web site and they do not have any big block pistons listed for the 383, just 400 and 440. Are you running a 383? And if so, what is the JE part number? Thanks for the info.