Hello everyone,
I'm sure the subject of dynamic CR is wearing on some of you. It's been covered a lot but my issue should be quick and easy compared to others. I'm re-doing a 383 build after timing isues not compression ruined a hyper piston due to detonation. :o to high initial and to small limit bushing. I was never quite satisfied with the original build anyway and would like to gain some performance from this re-work.

The original failed build was as follows:

Block
bored .030 over to 4.030
decked for pistons .005 in the bore

64cc alum heads

speed pro hyper pistons
12cc dish
5.7" rods
3.75 stroke

camshaft
lobe seperation of 114
Crane cam, heres the card http://www.cranecams.com/?show=brows...73&lvl=2&prt=5
(I'm this far into things and not totaly against replacing this cam)

not settled on a head gasket yet but been using .040 compressed with a bore of 4.06 bore for calculations

I've been using the dynamic compression ratio calculators found at K&B pistons and the much more complex download suggested in other threads here by Patrick Kelly

My delima is that I always thought the compression seemed low on the engine as described above that failed, and now want to replace the pistons with 5cc dish (or some other head volume) instead of the 12cc used before.

Initially using the k&b calculator and the suggested
"add 15 to the adv @.05 intake closing ABDC"
so using 41+15= 61
the dynamic seemed way to high. and not at all corect for my experiance with the engine 10.99 static/ 9.04 dynamic

I nearly ordered the 12cc pistons before I started trying the new calculator. The Patrick kelly program "timming and overlap calculator" portion adjust the intake closing point to 79 deg. If I'm entering the cam specification corectly. This setup seems pretty close to optimum. 10.99 static/7.76 Dynamic. I'm undecided and unsure I'm entering the cam information correctly and would greatly apreciate a couple of people looking at my work so I don't build another expensive goof!

Thanks for any replies

 

The KB calculator is wrong so do not use it.

The Pat Kelly calculator works. I have built a spreadsheet and it agrees with it so I trust it.

The cam card you posted does not have opening and closing points on it so you are "guessing" - not good.

Your static compression was around 10.2:1 based on my calculations.

I would not try to guess at the intake closing point on the current cam. I did grab a Comp 282S cam card and plugged it in and it looked good at just a gnats hair over 8:1 DCR. I guess it depends on what you were trying to do with the engine but what ever cam you choose, the Intake Closing Angle should be close to 67 with the 12cc dish pistons.

With the right heads and a sharp tune it will be 500 HP

-Mark.

 

Stinger69,
Thanks for the reply. This is exactly the source of my confusion.
I didn't think I was guessing. I thought the center section of the Pat Kelly program was calculating the closing point based on my input from the cam card.

The instruction say to use the center section of the program first. Then the stroke calculator then the compression section. When I enter in the center section of the program:

the advertised intake duration(300)
advertised exhaust duration(310)
lobe seperation(114)
intake center line(109)

the program calculates a intake closing point of 77. I'm not sure of my results because 77 is so far from the 46 @ .050 and the K&B estimates a 15 deg increase. And I wasn't sure that 109 was indeed the intake center line because the crane list it as the max lift.
Incedentially the K&B seems to give identicle results if you enter the same number at the intake closing point.

I guess my problem basically is I don't know how to determine the valve closing point with the information I have and am not sure of the calculation derived at with the center section of the Kelly program. If 77 is correct then a 5cc piston volume seem better. If 67 is correct I'm sure a 12cc just like I had is better.
I'm thinking now I should go bolt the heads back on find a degree wheel and see if I can measure it. Might require a whole new thread though!

Thanks again for any help

 

The total flow into the cylinder really determines DCR. I was able to run 10.7 with iron heads and the crane 278 power max h - flat cam. I ran that motor for 10 years. When I changed to 210 cc aluminum heads and a solid roller it had detonation problem. Even though the solid roller cam was something like 8 degrees larger on the intake. The heads and lobe profile were packing the cylinders with much more A/F pressure.

If you use a wimpy H-flat cam you can get away with higher static compression

 

Darin,
Sorry if I confused you. The 67 degree intake closing point is for the Comp 282S that I was trying to use as an example. I know it works well in a streetable 383 running a SCR close to yours so I popped it in there.

The Pat Kelly calculator is giving you 77 degrees and that is based on information you put in. It is giving you the right answer based on the info provided. I would feel a LOT better if they gave you actual opening/closing points rather than assume the cam card duration is seat to seat. But that is all you were given.

As for the lobe centers being the high point on a lobe, that is comon for aftermarklet cams but not for GM designs. The GM lobes are asymetrical.

If I input the Crane cam card data and make the assumption that the advertized duration is true seat to seat then the DCR comes up 7.57 which is a bit on the lazy side for my taste. If I change to a 5cc dish it comes out 8.16 which is much better.

Is DCR 7.57 worth changing pistons over? Personaly I would rather look for a cam card you can trust with a near 67 degree intake closing point and keep the 12cc dish just for the costs involved but getting the DCR up a bit one way or another is the goal.

-Mark.

 

Yes, there have been a number of discussions on this topic, and I've taken part in some of them. The biggest problem with the term "Dynamic Compression Ratio" or "DCR" is that it has been used to describe two different, tho related meanings relative to compression.

While I understand and can accept the often used meaning of DCR as being the theoretical compression ratio calculated from the piston's sweep from IVC to TDC, the "dynamic" that makes that meaning "theoretical" and not "actual" is volumetric efficiency. VE% varies with conditions and RPM, and it is this variable factor which truly determines the compression ratios actually occurring within an engine, and thus actual DCR = VE% x SCR. This latter meaning of DCR is that which Mr. Vizard and Mr. Yunick were really talking about.

Of course, you can only project VE% until you build a given engine and test it, so knowing the theoretical DCR is still somewhat valid info to have on hand when making decisions. Just keep in mind that 1) theoretical DCR is only accurate at the precise point where VE% = the percentage of swept volume remaining at IVC, and 2) engines that operate at high VE%'s produce higher peak cylinder pressures, which necessitates the theoretical DCR leave some margin in such builds.

Confusing, no? Clearly separate, universally accepted terms for identifying these two values would certainly have helped, but those who started publicizing "theoretical" DCR calculations muddled things up, IMHO...

 

What heads are you using?

I'd for sure not be married to that cam either.

Trans? Gearing? Intake? Carb or EFI?


And please tell me you learned your lesson and will be getting a set of *real* pistons this time around and won't be using cast anythings.

It was mentioned above and I'll echo it. The better a motor breathes...the touchier this becomes. You can cruise around all day on 87 octane with 13.0 compression as long as you keep your foot out of it. When you open the throttle, the cylinders fill and you get issues. Same thing with your 10/1 motor. If the heads, intake, cam and exhaust all start working together at some given RPM...your cylinders will get very full and you could have issues. If any of them are not matched so well...you will either get detonation earlier than intended...or never at all. A perfect setup will run on that edge and not have any issues......an improper one might not have detonation issues either..but won't make the power the better one will.

Busch motors have 9.0 compression..yet when those incredible heads start their magic...the cylinders get VERY full and you better have good race gas in it or you'll have shrapnel soon.

The concept of Dynamic Compression is a REAL thing and it's what everyone is after...but the calculators I've played with to me are just neat toys. There is no way they can allow for the variations in a motor build. I.E...good heads with bad intake, poor ex vs good, good chamber design vs poor, huge quench vs tight etc etc.

I'd suggest getting some specific recommendations from some proven builds, then run them through the calculators to see what they say. But don't bank on that either. The cam in my motor suggests I can run well over 12.0 compression on 93 octane. I've been doing this long enough to know that's not happening in the real world on the street in TX heat. But at 11.06 I race, dyno and drive on 93 octane with never an issue.


JIM

 

Mark,
I really apreciate your input on this. You're not confusing me at all. The cam info and how to use it with the calcuator is what's confusing but now we're getting somewhere.
I have to buy pistons. Two of the 12cc originals are destroyed. The decision is what cc to buy. I feel better now about my results with the calculator and my inputs since you were able to come up with similar results. Do you think the calculations are close enough to go for the 5cc or should I try to get some better information on the cam first? Another option I've considered is trying to find a piston around 7 or 8cc just to build in a little safety margin.

427 hot rod,
Thanks for the interest.
Yes I am going to forged pistons for sure, but I think the hypers would have been fine had I corrected the timing advance limit. I also learned I need a better distributor clamp!!! The heads are pro-comp (I've already had the chinese knock-off lecture and hesitated mentioning that in fear of getting the thread on that subject)

To the others with input about the whole subject of DCR, thanks for your advice. I think I have a fair understanding and know the pitfalls in putting to much weight on DCR. I need to order pistons soon and want to make sure I don't get carried away with the compression and don't leave a lot of power on the table. My instincts are that the first time around the 12cc's were not optimum. I'm trying to do my homework and make sure I get what I need using the DCR as guidance. I welcome anyones view on the piston volume you think I should go with.

As of now I'm leaning strongly toward a forged piston around 5-8cc and re-using the cam I have. Does anyone have ideas to more accurately get my intake closing number. Summit has a total of 1 piston that fits the bill in less than 12ccA any suggestion on pistons and sources would be helpful also.

This forum is awesome
Darin

 

A cam with 290 in and 300 ex advertised duration with 114 lobe separation and 106 intake centerline ( guessing on this one ) will have a intake closing angle of 71 deg ABDC

 

Hyper's *can* run well...but they are very unforgiving to having anything go wrong. Heck I've sprayed N20 on many a stock cast piston and got by with it...but I watched things closely. Forged just gives you a nice measure of insurance and allows for a much lighter piston too if you shop around some. Check with SRP, Probe, Mahle etc for pistons. Bound to be someone with what you need.

The ProComps are what you have...so we'll use them. They don't have the newer *quick burn* style chambers...so they may be a little more prone to detonation. But hey....none of them had that stuff a few years ago...so no biggee.

Quick calculations have you at just over 11.0 compression with 5cc dishes. I'm getting roughly 10.2'ish with the 12cc ones. That is fine and you have tight quench...but I'd sure be looking for cam with some more duration for that much compression. A better cam will add a lot more power than a point of compression will. Did that cam have the manners you liked?

You can always open the head chambers up to the bore size to get a couple of CC's and help breathing at same time.




JIM

 

Jim/427 hotrod,
I can't say for sure if the cam had the manners I liked. The idle had a very slight lope I set the idle pretty low or it was smooth. The overal package seemed a little weak so it's hard to report on the cam. My goal in the first round was to concentrate on low end torque and horsepower. I didn't use a forged crank and set rpm limit at 5200. I don't race and really just want impressive acceleration without excessive gearing. That may explain the distate several have with the cam. The gearing is high 400 turbo to I believe 3.07 rear. I choose to keep the high gear because I really like to cruise long distance once in a while and have had other vehicles with low final ratio's and always hated the drone of high revs at around 70mph. I also think someday I may install one of those under/overdrive systems. I wish I had the skills and knowledge for opening up the chambers. I have a guy who's gonna dissasemble the heads and make sure the seats are ok and look for any dammage. I will look into him working the heads over further. I'm sure these heads arn't the greatest but they seemed to offer a lot for the money at the time. They are vortec I think,,they used a vortec intake anyway. Like I mentioned earlier ditching the cam is a possibility but I don't know where to start on that selection and for now I need to get the pistons in hand so I can get the block to the machine shop for a hone and fit.

Motorhead,
Thanks for another set of eyes on the calculator. The intake centerline is 109 I think, listed as max lift on the card.

 

Knowing the gearing and trans helps a lot. I'm assuming tight stock type converter too?

Headers? Exhaust?

For what you describe, I wouldn't push compression to the 11.0 range unless you live at real high altitude. I'd much rather see you with less compression and the ability to time/tune it as it needs to be and not have to retard timing to make it driveable. The type of cam that will *like* 11.0 compressions isn't going to idle smooth and be comfortable sitting gear at a red light with a 600 rpm idle speed.

Check into the heads and what chambers they copied into them. Maybe I mispoke and they DO have newer style chambers? Honestly haven't kept up with those heads, but it wouldn't surprise me. If they do, you could still slightly open them on the outer walls to help breathing and not screw up flame travel.

I wouldn't be scared of 5500-6000 rpm with your parts. You'll be fine there.

So would you like a smoother overall operation? Was it too lopy for you? it appears to have a lot of advertised duration and not a lot of .050. You could drastically improve on power with similar manners or improve manners and still make more power with the right cam.


JIM

 

Go with the 12 cc dish and the smallish cam. Your build was probably not the problem with it seeming to lack power, probably your gearing. Your gearing will not be real responsive and will not like a monster cam. Need a lower gear or different trans to take full advantage of a bigger cam.

 

427 hot rod,
No it wasn't to lopy for me. Changing the sound isn't a priority but I would want a rougher idle if I make a change. I'm not @ high elev.
I nearly ordered 5cc's last night. Glad I paused!
Hooker headers and sidepipes. The converter is a stall. I sent all the info to fuddle racing and let them do there thang. I'll have to varify the stall speed but remember it seemed high 3400 I think. Fuddle will exchange it for anything I want and credit the price of what I have. I should probably have mentioned 1.6 ratio alum. roller rockers with stud girdles. The crank, rods, pistons and flex plate were purchased togeather as an internally ballanced package. Hardened two length special order pushrods (if that mattered)
You obviously know your stuff.. I'm setting on the edge of my seat wondering what piston and cam combo you'll come up with. I debated a retrofit roller cam in the orginal build but it just didn't seem worth much horsepower under 6000rpm and was pretty spendy.


63Mako,
I hear you. I wanted to get some improvement with all this work but getting used to the idea of "I had all I could get without changing gearing."