Compression Calculator

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plumkrazee70

plumkrazee70

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Hey Everyone.

I am moving forward with a set of edelbrock aluminum heads for my 340. Since I am going to be in there, I am going to have my hydraulic cam reground.

Ken suggested a custom grind with the following info:

224/[email protected]
480 lift
112 Lobe (due to EFI)

He did say determining compression ratio will help him make a better choice, so I plugged my numbers(screen shot attached) into the wallace calculator and got 9.84. I wasn't sure what to put the deck height at (is this how far the piston sits in or out of the hole)?

I just wanted to be sure that the numbers I plugged in looked correct. I got the headgasket thickness and the bore diameter from the edelbrock listing on Summit.

Wallace Calc info.png
 
If you put nothing in the space, it assumes zero. What pistons does you 340 have?
 
RustyRatRod said:
That's probably have a positive deck height. Go on the KB web site and use their deck clearance calculator. That'll tell you. Just use the stock deck height. That'll get it fairly close. It's gonna be more than you figured though.
Click to expand...

I am on their site and I can't find a calculator at all. I did find these specs, which are pretty in depth. UEM Pistons
 
I have KB 243's in my 340, they measured .018" over the deck (which is the spec with the stock deck height). The compression calculators I was using, including the Wallace calculator, had some issue dealing with the deck height and the piston volumes because of the way that piston is shaped. It looks flat but it actually has a .030" protrusion, so the shoulder of the piston sits .012" below the deck but the top is .018" over. I'm not entirely sure where the 6cc volume comes from that they advertise, but the tech I spoke to at KB straight up told me not to use it. Instead, he told me to use a deck height of 0 and a 7.3cc volume, with that volume accounting for the protrusion and the valve reliefs.

IMG_0654.jpeg


From the tech...
Hi,
The protrusion has to be treated like it is a dome so the piston is down in
the hole .012 and it has a .030 thousands high dome of 2.08cc's.This all
equates to positive 7.3cc's.Use this to calculate your compression ratio.
The 6cc's figure is incorrect and the 7.3cc is the one to use. Your ratio at
4.100 bore is 9.95 to 1.
Tech
Click to expand...

I ended up running a different head gasket than the one I gave the Tech and my head chambers ended up 1cc larger, so it calculated a little differently when I updated the gasket
compression.png


Also, with the KB 243's if you do end up with the tops of the pistons .018" over the deck like they're supposed to be you'll need to check your piston to head clearance if you run closed chamber heads. You may need to adjust that clearance with a thicker head gasket. I run open chamber 308's on mine.
 
72bluNblu said:
I have KB 243's in my 340, they measured .018" over the deck (which is the spec with the stock deck height). The compression calculators I was using, including the Wallace calculator, had some issue dealing with the deck height and the piston volumes because of the way that piston is shaped. It looks flat but it actually has a .030" protrusion, so the shoulder of the piston sits .012" below the deck but the top is .018" over. I'm not entirely sure where the 6cc volume comes from that they advertise, but the tech I spoke to at KB straight up told me not to use it. Instead, he told me to use a deck height of 0 and a 7.3cc volume, with that volume accounting for the protrusion and the valve reliefs.

View attachment 1715876349

From the tech...


I ended up running a different head gasket than the one I gave the Tech and my head chambers ended up 1cc larger, so it calculated a little differently when I updated the gasket
View attachment 1715876343

Also, with the KB 243's if you do end up with the tops of the pistons .018" over the deck like they're supposed to be you'll need to check your piston to head clearance if you run closed chamber heads. You may need to adjust that clearance with a thicker head gasket. I run open chamber 308's on mine.
Click to expand...

Thanks, Blu. Using the 7.3cc on the Wallace Calculator is still came out to 9.7 so overall its really close to yours. You are using a .039 head gasket where mine is .050, which is what Edelbrock recommends with the 63cc heads. I think this gets me close. I'll give Ken a call back and see if what he recommended for a cam still is a go.

What cam do you have @72bluNblu?
 
340 Deck height 9.60, rod length 6.123, piston compression. Height 1.840. 1/2 crank stroke 1.6551 added up...9.618 ...your proud of the deck +180 thou. Maybe.....add 6cc to your chamber volume and add compressed head gasket thickness. Plug those values in. These are blueprint specs: may get you close. The edel 340 heads are open chamber, the other Edels are closed chamber
 
plumkrazee70 said:
Thanks, Blu. Using the 7.3cc on the Wallace Calculator is still came out to 9.7 so overall its really close to yours. You are using a .039 head gasket where mine is .050, which is what Edelbrock recommends with the 63cc heads. I think this gets me close. I'll give Ken a call back and see if what he recommended for a cam still is a go.

What cam do you have @72bluNblu?
Click to expand...

Yeah it's not a crazy difference, only a few tenths anyway between the published and the info I got from the tech. I built that engine for my Challenger like 12 years ago now, I couldn't even tell you how I decided I wasn't getting the right numbers with them being so close. It ended up in my Duster because I bought it before I tore down my Challenger for rust repair.

The cam is a Lunati 60404, I think the new number is 10200704.
Lunati60404camcard.png
 
Last edited:
72bluNblu said:
Yeah it's not a crazy difference, only a few tenths anyway between the published and the info I got from the tech. I built that engine for my Challenger like 12 years ago now, I couldn't even tell you how I decided I wasn't getting the right numbers with them being so close. It ended up in my Duster because I bought it before I tore down my Challenger for rust repair.

The cam is a Lunati 70404, I think the new number is 10200704.
View attachment 1715876360
Click to expand...

Thank you sir. A little more aggressive than what I want. (a nice torque street car). Ill take the compression numbers back to Ken and go from there. Thank you all. I wanted to be sure I was on the right track.
 
plumkrazee70 said:
Thank you sir. A little more aggressive than what I want. (a nice torque street car). Ill take the compression numbers back to Ken and go from there. Thank you all. I wanted to be sure I was on the right track.
Click to expand...

Yeah to be complete honest it’s a little aggressive for how I use my car too. It was too much cam for the 3.55’s I was running behind my 4 speed, and was definitely one of the motivating factors for the T56 so I could run 4.30’s out back. It’s a ton of fun, but it’s more than a little hot for a “commuter”!
 
72bluNblu said:
Yeah to be complete honest it’s a little aggressive for how I use my car too. It was too much cam for the 3.55’s I was running behind my 4 speed, and was definitely one of the motivating factors for the T56 so I could run 4.30’s out back. It’s a ton of fun, but it’s more than a little hot for a “commuter”!
Click to expand...

Lol. This will be a 4.10 car with a 4 speed with Passon OD gearset.
 
72bluNblu said:
I have KB 243's in my 340, they measured .018" over the deck (which is the spec with the stock deck height). The compression calculators I was using, including the Wallace calculator, had some issue dealing with the deck height and the piston volumes because of the way that piston is shaped. It looks flat but it actually has a .030" protrusion, so the shoulder of the piston sits .012" below the deck but the top is .018" over. I'm not entirely sure where the 6cc volume comes from that they advertise, but the tech I spoke to at KB straight up told me not to use it. Instead, he told me to use a deck height of 0 and a 7.3cc volume, with that volume accounting for the protrusion and the valve reliefs.

View attachment 1715876349

From the tech...


I ended up running a different head gasket than the one I gave the Tech and my head chambers ended up 1cc larger, so it calculated a little differently when I updated the gasket
View attachment 1715876343

Also, with the KB 243's if you do end up with the tops of the pistons .018" over the deck like they're supposed to be you'll need to check your piston to head clearance if you run closed chamber heads. You may need to adjust that clearance with a thicker head gasket. I run open chamber 308's on mine.
Click to expand...

In the picture of post number 7, the deck surface appears to show markings from previous decking. Do you know for sure if yours was previously decked?

As for the numbers, I think I agree with KB on this one, or at least they are pretty close. If we treat that as a flat top piston, which typically has around 5 or 6 cc valve pockets, you would add the volume of that little step around the piston to the valve notch volume, and that is how they get 7.3 cc. But, the numbers say that "flat top" would sit about .018" above the deck, assuming 9.6" deck height, and the numbers measured from 72bluNblu confirm this on his engine. With those numbers in the calculator on the diamondracing.net website, it comes out to 9.99 to 1, and that is including .305" top ring land height.

plumkrazee70 said:
Thank you sir. A little more aggressive than what I want. (a nice torque street car). Ill take the compression numbers back to Ken and go from there. Thank you all. I wanted to be sure I was on the right track.
Click to expand...
Is this engine already assembled? If not, maybe you can test the half inch down fill volume and that will give you exact numbers. You would "travel" the piston down half and inch from true TDC, not just measure the piston to the top of the deck directly. Subtract measured half inch down volume from a calculated perfect half inch tall cylinder with your 4.08" in bore and that will give you the true cc of that piston. Then we know how close to 6cc or 7.3 cc KB was on their estimate.
 
Just so you know;
I ran 11.0 Scr with those Eddies and a Hughes 223/230/110 cam, and got about 180psi CCp; and it burned 87E10 at 34* PowerTiming, no problem. I loved that combo and would still be driving it today, if the lobes hadna started falling off the cam.
That cam also made funtastic fuel economy when geared 85=2100.
 
Bobzilla said:
In the picture of post number 7, the deck surface appears to show markings from previous decking. Do you know for sure if yours was previously decked?

As for the numbers, I think I agree with KB on this one, or at least they are pretty close. If we treat that as a flat top piston, which typically has around 5 or 6 cc valve pockets, you would add the volume of that little step around the piston to the valve notch volume, and that is how they get 7.3 cc. But, the numbers say that "flat top" would sit about .018" above the deck, assuming 9.6" deck height, and the numbers measured from 72bluNblu confirm this on his engine. With those numbers in the calculator on the diamondracing.net website, it comes out to 9.99 to 1, and that is including .305" top ring land height.
Click to expand...

As far as I know my 340 was not previously decked. But, it came to me as a disassembled short block that was .030” over already and in need of a rebuild. The story I got was that it was only bored out previously, but beyond the piston measurements being on spec when I rebuilt it I can’t say 100% that it wasn’t. I didn’t have it decked when I took it out to .060”, my machinist said it was fine and I was a little light on funds at the time so I let it ride.
 
I called and talked to Ken and he wouldn't stray from his original recommendation. I was talking to my dad and he has a cam new in box that never got used for a small block. What are everyone's thoughts on this one?

Again here are the engine specs:
68 340
KB 243
Edelbrock 63cc heads
273 adj rockers
4 Speed
4.10 gears
25.7"
Screenshot_20220224-055101.png
tire
 
Bobzilla said:
In the picture of post number 7, the deck surface appears to show markings from previous decking. Do you know for sure if yours was previously decked?

As for the numbers, I think I agree with KB on this one, or at least they are pretty close. If we treat that as a flat top piston, which typically has around 5 or 6 cc valve pockets, you would add the volume of that little step around the piston to the valve notch volume, and that is how they get 7.3 cc. But, the numbers say that "flat top" would sit about .018" above the deck, assuming 9.6" deck height, and the numbers measured from 72bluNblu confirm this on his engine. With those numbers in the calculator on the diamondracing.net website, it comes out to 9.99 to 1, and that is including .305" top ring land height.


Is this engine already assembled? If not, maybe you can test the half inch down fill volume and that will give you exact numbers. You would "travel" the piston down half and inch from true TDC, not just measure the piston to the top of the deck directly. Subtract measured half inch down volume from a calculated perfect half inch tall cylinder with your 4.08" in bore and that will give you the true cc of that piston. Then we know how close to 6cc or 7.3 cc KB was on their estimate.
Click to expand...

The motor is assembled, but will be coming apart so I can put the heads on. I could probably do that measurement. I am going to order the heads today and drop them off at th machine shop for a QC check on Monday or Tuesday. He said give a few days to check em over. Just trying to get th cam nailed down now.
 
Geezus frikkin **** man. You've talked to a cam expert TWICE and he recommended the same thing both times. How many here grind cams for a living? uhh......none.
 
RustyRatRod said:
Geezus frikkin **** man. You've talked to a cam expert TWICE and he recommended the same thing both times. How many here grind cams for a living? uhh......none.
Click to expand...
He's not 100% sure what his compression ratio is yet. Sounds like he has access to a cam his dad has that may be in the running also. He has options.
 
RustyRatRod said:
Geezus frikkin **** man. You've talked to a cam expert TWICE and he recommended the same thing both times. How many here grind cams for a living? uhh......none.
Click to expand...

Lol. Settle down. I am not disagreeing with Ken at all. But I have access to a brand new cam that I can install literally in a few days, versus the 3+ weeks Ken is out due to material. So I was just getting opinions.
 
92b said:
He's not 100% sure what his compression ratio is yet. Sounds like he has access to a cam his dad has that may be in the running also. He has options.
Click to expand...

This ^^ I am not an expert, but I am sure the compression is going to come in right around 9.5-9.8. but if my dads cam is still well suited and is accessible right now, why not go with that and not have to wait.
 
plumkrazee70 said:
This ^^ I am not an expert, but I am sure the compression is going to come in right around 9.5-9.8. but if my dads cam is still well suited and is accessible right now, why not go with that and not have to wait.
Click to expand...


The only way to actually know for sure your actual compression ratio, especially considering the shape and style of that piston is to do a .500 down fill on a cylinder and then you will know exactly what the dome volume is and not be guessing at it.
 
There is another way to calculate compression. You could have it whistled while the head is still on it. It's kind of a long shot but if you have a circle track near you with a compression rule the tech guy might have a whistler. You may be able to get the tech to check your motor. Looks like this.

16457461314841672488790678136872.jpg
 
plumkrazee70 said:
Lol. Settle down. I am not disagreeing with Ken at all. But I have access to a brand new cam that I can install literally in a few days, versus the 3+ weeks Ken is out due to material. So I was just getting opinions.
Click to expand...

I don't think it's a terrible idea. Remember that what the cam grinder is recommending is going off of what you told him you wanted. The engine itself can support more cam that what was recommended by a decent amount. Even if you're at the lower end of that compression number you'll have enough compression to run a hotter cam. And with 4.10's you'll have the gearing too. So you won't turn the engine into a dog with the bigger cam.

I'm going to compare with what I know, which is the Lunati Voodoo series. The recommended cam is light on lift compared to all the Lunati cams, it has much slower ramps. Duration wise it's about halfway between a Lunati 702 and 703. Your Dad's cam has a bit more duration than a Lunati 703, but less lift. And it's closer to the 703 than it is the 704 IMO. Biggest thing is the lobe separation, the 112 that your grinder is recommending will give you a little more vacuum and a little more lower end compared to the 110 that all the lunati's and that comp have. It's not a massive difference but it is a difference.

I run the 704, which I already said is a bit hot for what I do. But honestly, your engine compression can support a 704 and so can your gearing. And although the 704 is a little wild for commuting, I've put over 30k miles on my car as a daily driver with that cam. It's a little rowdy when you get stuck in bumper to bumper traffic but it does work! And it's a lot easier to deal with now that I have 4.30's instead of 3.55's, 4.10's would be fine. The 702 and 703 are pretty well regarded as good street cams. The Comp cam that your Dad has is definitely hotter than what is being recommended to you, but it's still within what your engine can support and it's less than the 704 I run. It would have been easier for me to downgrade my cam than install a T56 and 4.30's, but I kept my cam and did all that other work anyway. It's a fun engine.



I wouldn't have any hesitation running the Comp cam your dad has. It really comes down to how you use the car, if this is just a fun weekend car I don't see how it's going to be a problem. If it's going to be a daily driver you may want to stick with the grinder's recommendation. 3 weeks may seem like a long time now, but once the car is together you're not going to want to tear it apart to swap the cam, so 3 weeks in the grand scheme of things isn't that long.
 
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