Help with decking or not?

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MoparMark91

MoparMark91

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So im in the process of building a 340 bored 30 over and a 3.58 stroke. Purchased on summit.
Using scat rotating assembly
Stroke-3.58
Rod length 6.125
Piston compression height if im not mistaken -1.675
My deck height
LF-9.606 RF-9.600
LR-9.601 RR-9.610
I understand my deck height is not squared up, so my question is if i decked the block to square it up to 9.600 what would the set my Compression ratio at? My goal is to not exceed 10.5 to 1. Hoping i could get some input guys my block is at the machine shop.
 
U show the end to end but the decks could also be sloped and or twisted
and it looks like 9.590 would be 0 deck ( unless your rods have been rebuilt- had the caps cut so shorter
closed chamber heads
you can try 9.6 and use the .028 gasket or if you have to go less and use a thicker gasket
remember some SBM cam with the piston above deck
you should be able to make it work
tight quench is better
 
Wyrmrider said:
U show the end to end but the decks could also be sloped and or twisted
and it looks like 9.590 would be 0 deck ( unless your rods have been rebuilt- had the caps cut so shorter
closed chamber heads
you can try 9.6 and use the .028 gasket or if you have to go less and use a thicker gasket
remember some SBM cam with the piston above deck
you should be able to make it work
tight quench is better
Click to expand...
The rods are new from the kit.
I was thinking have him square up the block. Decking to 9.6. My pistons have valve reliefs 5cc. They are icon 741. Just want to make sure i dont deck it too much. I plan on giving him the call tomorow to ler him know but trying to get all inputs in and do some research since this motor is bores and stroked.
 
What's your chamber volume? 9.588 is zero deck with that piston, rod, stroke, you've got .012 to play with before you have piston sticking out of deck. Not that, that is a bad thing, just something you need to keep track of. Gasket and chamber volume are your biggest variables.
 
Head is unknown so this can only be purely guessed at until you give us more info, as above.

Guess #1: With 9.600" deck, stock 340 heads unmilled, and Felpro 1008 head gaskets, SCR is around 9.4.

Guess #2: With the same deck and head gasket, and Edelbrock chambers at 63 cc, the SCR is 10.3.

Guess #3: If decked down to 9.588" for true zero-deck, and with Edelbrock heads and 1008 gasket, SCR is 10.6.

So what are those heads again??
 
nm9stheham said:
Head is unknown so this can only be purely guessed at until you give us more info, as above.

Guess #1: With 9.600" deck, stock 340 heads unmilled, and Felpro 1008 head gaskets, SCR is around 9.4.

Guess #2: With the same deck and head gasket, and Edelbrock chambers at 63 cc, the SCR is 10.3.

Guess #3: If decked down to 9.588" for true zero-deck, and with Edelbrock heads and 1008 gasket, SCR is 10.6.

So what are those heads again??
Click to expand...
My apologies i didnt realize i didn't post the heads
 
Scr is a means to an end. The cam will decide the running pressure at the chosen Scr. If the pressure is too high for available gas then you get to start over.If the pressure gets to be too low, the car gets doggie until you start fudging with higher stalls and higher number rear gears.
At Identical Scrs, small cams make more low-rpm pressure than big cams, while big cams may make more pressure at higher rpms.
There is such a thing as too much low-rpm pressure which just makes finding traction difficult while making showing off easy.

Teeners had low Scrs but made good pressure for their intended applications.
Early 340s had hi Scrs , and made hi pressures ; and as DDs make too much for our modern EFI-gas.
360s are very adaptable with a generous swept volume that can be made to pump out the right pressure for just about any application.

IMO, target a pressure,that fits your application, and let the Scr be what it will be.
With iron heads and Quench over .080, 160psi is gonna be close to the limit,for pumpgas, depending on the application. I wouldn't spend much time on decking for compression gain, unless you are struggling at 4000 ft elevation. .012 is only 2.5cc. But at 4000 ft you really need a ton of pressure, so wouldn't be using iron heads anyway. In a tight-Q design with closed chambers, the piston to head clearance becomes much more important.
Chose the cam for the intended application.
I don't see much sense in running a 7000 rpm cam with 3.55 gears, in a 2-gear streeter. The power peak in second is way past the speed-limit, like 90 mph. Meanwhile the neighbors teener correctly geared and cammed for 60 mph, and with the right pressure, gets to spank you.
The 3.55s pretty much dictate not much more than 222*cam, and each higher cam size (in 7* steps) is gonna want about 6 or 7% more gear, so by the time you get 4 sizes up, to 250@.050, the chassis is gonna want something like 4.56s, depending on the stall.
If you wanna stomp a certain somebody who has iron heads, go aluminum, crank the pressure up and then you can run a lil less cam than him. There are a few guys here on FABO who have reported running a tic over 200psi, still on pump gas, using aluminum heads. Quite a number of us are between 175 and 185 psi. I myself run 183psi on 87E10, with full timing and a minimum coolant temp of 205*F. Awesome engine with every cam I have tried, always maintaining the pressure.
Happy HotRodding
 
MoparMark91 said:
I have stock 340 2.02s
Click to expand...
There ya go.. tnx. So you will be around 9.4 for SCR IF the heads have never been milled down. But beware..... There was a recent thread where that was not true.. the OP in that thread had heads that had been milled .050-.060" and the chamber sized had dropped 9 cc's from typical stock, and he did not know it for sure. CC'ing the heads revealed this. So you don't know for sure until you CC the head chambers.

As a rough check on if the heads have been milled, the small flat area by the plug hole in the chamber is supposed to be .720+/-.007" down from the head surface.
 
AJ/FormS said:
Scr is a means to an end. The cam will decide the running pressure at the chosen Scr. If the pressure is too high for available gas then you get to start over.If the pressure gets to be too low, the car gets doggie until you start fudging with higher stalls and higher number rear gears.
At Identical Scrs, small cams make more low-rpm pressure than big cams, while big cams may make more pressure at higher rpms.
There is such a thing as too much low-rpm pressure which just makes finding traction difficult while making showing off easy.

Teeners had low Scrs but made good pressure for their intended applications.
Early 340s had hi Scrs , and made hi pressures ; and as DDs make too much for our modern EFI-gas.
360s are very adaptable with a generous swept volume that can be made to pump out the right pressure for just about any application.

IMO, target a pressure,that fits your application, and let the Scr be what it will be.
With iron heads and Quench over .080, 160psi is gonna be close to the limit,for pumpgas, depending on the application. I wouldn't spend much time on decking for compression gain, unless you are struggling at 4000 ft elevation. .012 is only 2.5cc. But at 4000 ft you really need a ton of pressure, so wouldn't be using iron heads anyway. In a tight-Q design with closed chambers, the piston to head clearance becomes much more important.
Chose the cam for the intended application.
I don't see much sense in running a 7000 rpm cam with 3.55 gears, in a 2-gear streeter. The power peak in second is way past the speed-limit, like 90 mph. Meanwhile the neighbors teener correctly geared and cammed for 60 mph, and with the right pressure, gets to spank you.
The 3.55s pretty much dictate not much more than 222*cam, and each higher cam size (in 7* steps) is gonna want about 6 or 7% more gear, so by the time you get 4 sizes up, to 250@.050, the chassis is gonna want something like 4.56s, depending on the stall.
If you wanna stomp a certain somebody who has iron heads, go aluminum, crank the pressure up and then you can run a lil less cam than him. There are a few guys here on FABO who have reported running a tic over 200psi, still on pump gas, using aluminum heads. Quite a number of us are between 175 and 185 psi. I myself run 183psi on 87E10, with full timing and a minimum coolant temp of 205*F. Awesome engine with every cam I have tried, always maintaining the pressure.
Happy HotRodding
Click to expand...
Thank you for your in
nm9stheham said:
There ya go.. tnx. So you will be around 9.4 for SCR IF the heads have never been milled down. But beware..... There was a recent thread where that was not true.. the OP in that thread had heads that had been milled .050-.060" and the chamber sized had dropped 9 cc's from typical stock, and he did not know it for sure. CC'ing the heads revealed this. So you don't know for sure until you CC the head chambers.

As a rough check on if the heads have been milled, the small flat area by the plug hole in the chamber is supposed to be .720+/-.007" down from the head surface.
Click to expand...
So if i dont deck the block and heads never been milled it would set me at a 9.4? I plan on calling the machinist today and let him know whether to deck it or not. Thank you for your help and input also
 
I would go with a 6.588" and juggle the head gasket thickness to get the desired c.r.. Being mindful that decking may effect bolt alignment .
 
You've got one corner at 9.600, and an opposite corner at 9.601......have your machinist could pick up the corner at 9.600, make a sweep to bring the rest of the deck down, likewise on the opposite corner, start at 9.600 and make a sweep. Weather or not the block would square with those cuts is a different story, to square the block could take more of a cut, depends on how far out of square it is. If I had a choice between just even deck height or even deck and square, but even deck and square throws me off someplace else, I'd just go with even deck. It's not the end of the world if the block isn't square.

OR, go ahead and take minimum cut to deck and square and use chamber volume and gasket to adjust for desired comp ratio.
 
MoparMark91 said:
So if i dont deck the block and heads never been milled it would set me at a 9.4? I plan on calling the machinist today and let him know whether to deck it or not. Thank you for your help and input also
Click to expand...
Yes and no... Decking to equalize the deck higher on all pistons equalizes the CR between cylinders. For the number in post #11, I assumed you decked the block to get 9.600 on all the cylinders.

If you do not deck and keep the uneven deck heights in the holes, then you would have:
  • Approximately 9.4 for the cylinders at 9.600.
  • Approximately 9.2 for the cylinders at 9.610.
  • And these numbers all depend on the the gasket chosen... I assumed the Felpro 1008... you can bump up or down from there about 0.2 points in CR.
  • And this again assumes a 'typical' unmilled head like the type you have. (340 'open' style chamber) If you don't cc them, then no guarantees.....
 
since he has open chamber heads so no worries about quench clearance I square up the block is it not off about 010 might as well do it right and adjust with gasket thickness .028 to over .050 are easy to find
then think cam if it takes .590 to get it square that's ok
 
So i was able to get my hands on af ome eddy 60779 heads so i was thinking of going eith those on my build. After doing all the math my piston would be .012 in the hole. With a felpro .039 gasket added to that would a .051 quench be bad? Still trying to grasp concept of quench. Should i possible go with thinner gasket?
 
.051 is at the top of the zone, but with aluminum heads, it will be fine. It might be finer with the .028s .. Aluminum heads like 11/1 Scr or better depending on the cam. Cylinder pressure can easily go to 185 or better. I run that with 87E10. I have successfully run Q at .034 since year2000.
My 360 at .034Q and 10.9Scr is now making a lil less pressure with the bigger cam (230/237/110) with a slightly later Ica of 64* . I seem to recall 178psi
At PT (Part Throttle), aluminum heads are sucking so much heat out of the chambers that IMO, they need a full point higher in compression just to break even with iron heads, in terms of efficiency. This leads to the pressure needing to be at least 10 to 15 psi higher than what can be used with iron. So if you had a tight-Q iron design that successfully runs pumpgas at 165 psi, a straight swap to aluminum heads will yield a net power loss at the same pressure. So you have to jump it up to to 175/180 to break even. This tight-Q and high pressure is a 1-2 kick in the rear for PT throttle response and efficiency. So you jump the Scr up to ~11/1 to equal the 10/1 iron headed combo in terms of effective Scr.
At WOT, the Al heads are still sucking the heat out, but the time factor is diminishing. So; IMO, the rising chamber temperature increases the Effective compression ratio and together with the rising efficiency of the bigger hi-overlap cam,now the effective cr is coming closer to the design Scr, and that is why the horsepower rises. In a good design the Effective Cr may exceed the design Scr, and then the VE goes greater than zero, and that is now making power.These are my opinions. In an sffort to keep the chamber-temp up, I run a minimum cooling system temp of 205*F. The car hit 93 in the Eighth on it's maiden voyage, so until someone tells me his 230-cammed,iron-headed 9.5 or 10.0 360-beast can do that all day long at 930 ft and 3467 pounds with 3.55s..... my opinion is likely gonna stay.
But I tell you what, when you hit the torque peak in third gear, and feel that power surge to shift-rpm for the first time, you will ask yourself; "what the heck just happened?" This ain't a VVT.
Iron-headed 9.5Scr SBMs don't usually do that with small 110LSA cams.
Enjoy
 
Last edited:
If the deck is at the nominal 9.600" deck height, then your SCR will be right around 10.5:1 with those .039" head gaskets, and the Icon pistons given before. I agree with the above that .051" gap has some use, though not optimum, but the AL heads help with detonation avoidance anyway. But can't agree that AL heads 'like' a particular CR.....

One you get things together, measure the rod length and CH on one piston/rod set, and use that to move around to each cylinder and check the deck height, to be sure you have what you think. (You'll have to assume the stroke is the same on each crank throw unless you measure that too.) Deck might be high or might be low.... I have a factory 273 block that is low all around and is .008-.009" low on one corner. So you never know 'til you measure.

What is the planned use for this engine? Any idea on the cam?
 
what aj and nm said above
you have checked the deck with the engine mocked up not just a paper calculation?
"So you never know 'til you measure." I mentioned odd block deck angles above
so measure all four end holes
if .51 I'd sure use the .028 gasket as aj said you are on the downhill side of quench
sure- tell us about chassis, gears, convertr, intake, exhaust etc and most of all usage and you will get more cam suggestions which can be sorted out for what you need
or just pick the bottom cam from the catalog page- like that works
 
AJ/FormS said:
.051 is at the top of the zone, but with aluminum heads, it will be fine. It might be finer with the .028s .. Aluminum heads like 11/1 Scr or better depending on the cam. Cylinder pressure can easily go to 185 or better. I run that with 87E10. I have successfully run Q at .034 since year2000.
My 360 at .034Q and 10.9Scr is now making a lil less pressure with the bigger cam (230/237/110) with a slightly later Ica of 64* . I seem to recall 178psi
At PT (Part Throttle), aluminum heads are sucking so much heat out of the chambers that IMO, they need a full point higher in compression just to break even with iron heads, in terms of efficiency. This leads to the pressure needing to be at least 10 to 15 psi higher than what can be used with iron. So if you had a tight-Q iron design that successfully runs pumpgas at 165 psi, a straight swap to aluminum heads will yield a net power loss at the same pressure. So you have to jump it up to to 175/180 to break even. This tight-Q and high pressure is a 1-2 kick in the rear for PT throttle response and efficiency. So you jump the Scr up to ~11/1 to equal the 10/1 iron headed combo in terms of effective Scr.
At WOT, the Al heads are still sucking the heat out, but the time factor is diminishing. So; IMO, the rising chamber temperature increases the Effective compression ratio and together with the rising efficiency of the bigger hi-overlap cam,now the effective cr is coming closer to the design Scr, and that is why the horsepower rises. In a good design the Effective Cr may exceed the design Scr, and then the VE goes greater than zero, and that is now making power.These are my opinions. In an sffort to keep the chamber-temp up, I run a minimum cooling system temp of 205*F. The car hit 93 in the Eighth on it's maiden voyage, so until someone tells me his 230-cammed,iron-headed 9.5 or 10.0 360-beast can do that all day long at 930 ft and 3467 pounds with 3.55s..... my opinion is likely gonna stay.
But I tell you what, when you hit the torque peak in third gear, and feel that power surge to shift-rpm for the first time, you will ask yourself; "what the heck just happened?" This ain't a VVT.
Iron-headed 9.5Scr SBMs don't usually do that with small 110LSA cams.
Enjoy
Click to expand...
Thank you AJ always very informative on explaining it and breaking it down greatly appreciated. Still undecided on cam to go with. As this is my first performance build
 
nm9stheham said:
If the deck is at the nominal 9.600" deck height, then your SCR will be right around 10.5:1 with those .039" head gaskets, and the Icon pistons given before. I agree with the above that .051" gap has some use, though not optimum, but the AL heads help with detonation avoidance anyway. But can't agree that AL heads 'like' a particular CR.....

One you get things together, measure the rod length and CH on one piston/rod set, and use that to move around to each cylinder and check the deck height, to be sure you have what you think. (You'll have to assume the stroke is the same on each crank throw unless you measure that too.) Deck might be high or might be low.... I have a factory 273 block that is low all around and is .008-.009" low on one corner. So you never know 'til you measure.

What is the planned use for this engine? Any idea on the cam?
Click to expand...
I habe not decided on cam yet and intended use is street beast with being able to go to track few times a year
 
Wyrmrider said:
what aj and nm said above
you have checked the deck with the engine mocked up not just a paper calculation?
"So you never know 'til you measure." I mentioned odd block deck angles above
so measure all four end holes
if .51 I'd sure use the .028 gasket as aj said you are on the downhill side of quench
sure- tell us about chassis, gears, convertr, intake, exhaust etc and most of all usage and you will get more cam suggestions which can be sorted out for what you need
or just pick the bottom cam from the catalog page- like that works
Click to expand...
Paper calculation as im getting ready to do the mock up soon. As for the gasket thickness are refering to the mr. Gasket or cometic.028? Couldnt find any felpro with that size
 
MoparMark91 said:
So im in the process of building a 340 bored 30 over and a 3.58 stroke. Purchased on summit.
Using scat rotating assembly
Stroke-3.58
Rod length 6.125
Piston compression height if im not mistaken -1.675
My deck height
LF-9.606 RF-9.600
LR-9.601 RR-9.610
I understand my deck height is not squared up, so my question is if i decked the block to square it up to 9.600 what would the set my Compression ratio at? My goal is to not exceed 10.5 to 1. Hoping i could get some input guys my block is at the machine shop.
Click to expand...

You will have to end up measuring to find your true compression. If a piston for example is advertised at "10:1", that means all things measured and AS ADVERTISED from the factory. So, if you just sling it together without decking for instance, your compression will be lower, because Chrysler always had higher than spec decks. If it was mine, I would deck it to spec, making SURE the shop knows how to parallel deck. That means the deck is supposed to be flat and parallel to the crank center line. If the shop cannot do that or they act like they don't know what it is, find that shop that does. The block is the foundation for everything and if it is off, you will just build an out of spec engine.
 
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Agree with RRR
we used the popular BHJ tooling which locates the cam and crank and makes it easier to get everything square, ( and do the lifter bores dead nuts on if necessary
AS RRR intimates shops without the proper tooling have been known to use stepped reamers to do the lifter bores for bronze bushings and they end up all over the place
and are a ***** to straighten out- have to go to oversize bushings
I was thinking of the Mr gasket .028 also less expensive
Cometic needs a finer deck and head finish- Mr gasket could use also but Mr Gasket can use the more common finish
Do the Mock up and measure as I suggested and then calculate compression if the pistons have a dome or dish you can do a pour of cc with the cylinder vertical
and really nail the compression but then again the piston vendors cc numbers and the deck height should get you close since most likely u do not have unknown cc custom pistons
measure all four corners - you may or may not have to deck your block
I've had blocks that I wanted to 0 deck based on the pistons and rods (440 six pack pistons, KB in one case) but ended up with them .015 out just to get the block squared
Then I had to use a .050 gasket to end up with .035 quench on one .030 on the other
hope you get lucky- most do
get back to us with the measurements
glad you are doing a mock up- only way to get it right
last few years doing mopars I'd deck the blocks minimum square when doing the bore procedure
then mock it up and cut it again if necessary
 
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