Help with CR solution

[JJRJR]

OK, as many of you know, my engine came off the dyno last week with disappointing results. As I pushed for more info; knowing the results had to be wrong, the dyno shop agreed to measure the actual CR and do a leak down test on the heads. (New Eddy Performer RPM's)

Although my builder assured me he got 9.8:1 CR, I didn't really believe him. Turns out the dyno shop is coming up with more like 8.75:1 using a tool called a whistler and by adding the known CC volumes. A major "unknown" is the volume of the relief valves on the pistons. He used 6cc in his calcs, but thinks they are bigger. Despite my continued requests to get the part number of the pistons so I can find this out, builder is stalling me and has been for months. In addition, dyno shop found a 15% leak in one of the exhaust valves and some other leaks less than 5%, from what I hear that is acceptable??

OK. Moving on; the engine is being torn down again to see what can be done about getting a REAL 10:1 CR. I'm told the pistons are .010 in the hole, the Eddy heads have an 84CC volume, 4.28 bore, 3.375 stroke, using .040 gasket thickness (not sure of this). I think the builder's solution is going to be zero decking the block and milling the heads, but I'm thinking (I know almost nothing about this) that they will have to be milled quite a bit to get the numbers we need.

What would you guys do? Is it better to find different pistons (not sure what kind)? I was told these pistons have valve reliefs for a reason so not sure anything else can be used.

I've decided that while it's torn apart to add a bigger cam (Comp XE285HL) and an 850 Holley Double Pumper and stick with the dual plane intake. I'm told that's easy stuff to change and that the heads will be no issue to fix the leaks. So, my big problem is the CR. Can you guys throw me some ideas?

Sorry for the long post.

John

 

[A31PKG]

Just remember that milling the heads &/or deck may result in the need to machine the intake manifold for proper fit. That said, if your engine builder "sold" you a certain build and it turns out it is not what you paid for, then it appears that you have an issue with the builder that definitely should be addressed. If on the other hand you got what you paid for, I'm sure this engine would likely fry the tires just fine as-is!

 

[metalstorm]

is 20 hp from more cr going to mean that much to you?you may be opening a can of worms milling heads,block and that may cause you to need to cut the valve reliefs deeper depending on the piston to valve you have now.[.100 on both int/exh clearence]

 

[JJRJR]

Well, yes 20HP is important to me. I want this to be all in can be on my budget. I am having a hard time understanding why getting 10:1 CR is so hard when the original 383 came with it??

 

[metalstorm]

the engine builder should have measured all aspects of the engine before ordering parts!

 

[JJRJR]

Obviously the builder didn't do what he was supposed to do. I'm just trying to make it right.

 

[sixgunrunner68]

Well, yes 20HP is important to me. I want this to be all in can be on my budget. I am having a hard time understanding why getting 10:1 CR is so hard when the original 383 came with it??

I hear ya, but consider this. I don't know what your budget is, nor am I asking. From the sound of it though it would seem that you're going down the road of machining, replacing pistons , reassembling and running it on the dyno all over. If I were to go through all this extra expense and aggravation, I'd aim a little higher than another twenty ponies.
:cents:
Rework flushes any budget down the toilet.

 

[JJRJR]

I appreciate the advice. It all makes sense. Having never gone down this road before, now in hindsight when I first started this is September; I know how important the planning stage is, etc. Let's wait for the builder to see what's up with the CR and the leaking valves. He claims it won't be a big deal to fix. I may settle for as close to 10:1 as he can get without changing pistons, etc. A little mill work on the heads and a thinner gasket maybe all we need to get close. He owes me that for no money; I paid for 10:1.

Interestingly, CompCams has a pretty cool CamQuest software on their website. Once you pick the cam, you can then add that to your build and get a simulated dyno result. While I realize now that the software dynos are not accurate; changing from the XE275 and 750 Holley to the XE285 and 850 Holley netted about 5 more HP at higher RPM. TQ numbers are actually a bit better with the smaller cam and carb. Again, I have no clue how accurate that is, but it sounds like spending another $1,500 in cam, lifters and carb just isn't worth it.

At the end of the day, once the CR is closer to 10:1 and the heads are fixed; if I can get north of 410HP, I'll be pretty darned happy. I'll keep you all posted.

Thanks again,

John

 

[metalstorm]

sometimes a valve does not seat all the way when leak checking them and a tap on the valve without the rocker gear can seat the valve and improve leakage.

 

[moparchris]

A leaky valve will gain nothing but a better idle quality. As far as the compression goes. It sounds like you have found a inept and not very consciences engine builder. Zero decking the block is different than just shaving the heads. The reason is quench. When you cut the head you will still have, we will use a generic number to keep it easy, .050 of clearance between the piston and the closed chamber portion of the head. When you cut the block .010 then you have a quench clearance number of .040. There is a sweet spot most engine builders shoot for, less than .050 to be exact. A steel rod motor can get as close as .020 without hitting at max RPM (rod stretch!) where as aluminum rod motors need closer to .050 to keep the piston from hitting the head. When you have the quench right the piston squeezes the air fuel mixture and sprays it into the chamber creating a nice flame front which is what you want for max power. Without this effect you are more prone to detonation. That is one of the reasons that the old stock 915 closed chamber heads make so much more power than the open chamber 906. An Eddy head is open chamber design. By my calculations from the info you have supplied I came in at 8.499. This is with a 10cc valve pockets which is what most custom pistons have for big block chrysler. If you cut the block .010 then it would result in 8.670. If you cut the block .010 and put a .015 head gasket in it (which is what a stock 383 came with) then you will yield 9.142. You would have to do all that and cut the head to 74cc will yield 10.077. Okay so you have to ask what does all this work mean to me? Well, from what I gather from reading the book Thermal Dynamics, moving from 9.00 to 10.00 will yield you 12% hp increase and moving from 8.00 to 9.00 will yield a 16% hp increase. Of course these are theoretical situations and the actual results will vary greatly. In your case this would be an increase of 40+ hp and probably a similar increase in tq. The heads would require a head cut in the .040 range. Yes you will have to cut the intake. I did this very exercise with my small block 360 after being frustrated with the performance. It had a static compression of 7.823 and I increased it to 11.14 and saw a 97 hp increase at the rear wheels. This was on a 350 hp motor to begin with. Some will say that its too much hassle. What has happened to me was I picked an intake to cut and stuck with it like your planning to do. If you need to change intakes then a good machinist will know what to do as long as you remember what you cut off your heads. Another thing to remember is that the heads have an open chamber design and the piston will have tons of clearance so dont worry about that. Most engine builders charge $500 to assemble an engine. I would want to see inside my $8000 motor that was built by a guy who didn't sweat any details. The cut on the heads will be $100 and the head gaskets will be a little more expensive, ($100 ) you will have to use a MLS (multi layer steel) gasket from cometic or fel pro. These gaskets will require a special finish on the heads and block but your already cutting both surfaces. So there you go. So you will have to spend another grand all said and done for 50 ish hp and a secure feeling that it will hang. Very worth it in my opinion.

 

[JJRJR]

Chris, thank you so much for taking the time to give me that great info. I truly appreciate it. I will pass it along to my engine builder. The only thing is that the Eddy Performer RPM are a "closed" chamber head, not open. Does that change anything regarding the safety of cutting them?

Thanks again,

John

 

[bigmanjbmopar]

Here is my set up. I waned to have 9.5:1 CR for gas reasons. In order to get my stroked 440 to 9.5:1 I had to order my Diamond pistons dished 24cc. If I had left them flat it would have been around 11:1. You need to find out what pistons he used and what if anything was done. In the end I ended up with 9.6:1 and run on 91 pump gas with fire under my tires ;)

 

[moparchris]

Chris, thank you so much for taking the time to give me that great info. I truly appreciate it. I will pass it along to my engine builder. The only thing is that the Eddy Performer RPM are a "closed" chamber head, not open. Does that change anything regarding the safety of cutting them?

Thanks again,

John

A closed chamber head will take more material removal to bring the chamber down. 84cc is big for a closed chamber head. A 915 is 80cc's typically and a 906 is usually 88-90cc's.

 

[JJRJR]

Interesting site http://www.bgsoflex.com/crchange.html which calculates hypothetical HP gain for a change in CR. Pretty cool. I've read that in general each point of CR increase is worth about a 4% power gain. This seems to be correct based on the site calculator. In my case, if I'm at 8.499 now and I go to 10:1, I gain 19 HP or about a 5% gain.

 

[bigmanjbmopar]

I am wondering why you are so stuck on 10:1? There are several ways to achieve higher horsepower. Like others have mentioned you should try to get the original builder to fix the motor with the correct / adjusted pistons. Or build it your self it is not really that hard just get an engine stand and a good torque wrench. I would get him to admit the wrong ones were installed maybe per a kit he bought but at least give you a cash credit towards a new set even if you have some one else install them. Rings too of course. re use the pins and connecting rods.

 

[moparchris]

Interesting site http://www.bgsoflex.com/crchange.html which calculates hypothetical HP gain for a change in CR. Pretty cool. I've read that in general each point of CR increase is worth about a 4% power gain. This seems to be correct based on the site calculator. In my case, if I'm at 8.499 now and I go to 10:1, I gain 19 HP or about a 5% gain.

That is a gross assumption. If you would like some light reading, thermal dynamics is a very informative book but very dry. Like most things involved with these engines nothing is cut and dry across the board. There are many factors that influence the amount of return you will get from a point increase.

 

[69hemibeep]

I am wondering why you are so stuck on 10:1? There are several ways to achieve higher horsepower. Like others have mentioned you should try to get the original builder to fix the motor with the correct / adjusted pistons. Or build it your self it is not really that hard just get an engine stand and a good torque wrench. I would get him to admit the wrong ones were installed maybe per a kit he bought but at least give you a cash credit towards a new set even if you have some one else install them. Rings too of course. re use the pins and connecting rods.

In John's absence I will do my best impression... Early in my build plans I went with aluminum heads to allow higher compression but it seems my builder let me down so I'm still trying to achieve 10.1 CR... How did I do John :jester:

 

[JJRJR]

LOL. Spoke to the engine builder yesterday; he admitted that he must have made a mistake in his math. They dyno shop gave him the milling necessary on the block and heads to get me to 10:1. He's also going to re-seat the leaking exhaust valve. "Should be" done this week sometime.

John

 

[zupanj]

Maybe I'm missing something here but does it make sense to cut a block and heads and intake manifold rather than look at a new set of custom pistons? I don't know which pistons were chosen for this build but when I was looking to rebuild my 383 the only off the shelf pistons were KBs advertised as 9:1. There were several extensive posts on Moparts about 6-7 years ago that indicated the real CR was more in the low 8's to even high 7's. The conclusion to all this was to spend the money for a custom set of pistons from someone like Diamond.

From the posts it sounds like some of you guys know a lot more about this than me and I would love to hear you opinions.

 

[JJRJR]

If that's the best way, then I'll ask him but he hasn't mentioned anything. I'll let you know. Thanks.

John