Comp Ratio someone confirm my numbers please

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Dustedu2

Dustedu2

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I am ready to pick out the head gasket and here is what I know. What I want is best CR and quench without denotation on 93 pump gas.
If I am already to high than best with Octane additive. I want most HP with what I have "street/strip" Build.

Flat Top Ross Piston, Edelbrock Performer Heads
Bore 4.10
Strke 4.00
gasket bore 4.180
Chamber volume 65cc
deck hight .051
Dome volume -7.5cc
with .025 gasket I get 10.71:1 CR with quench at .076

cam is roller XR274R
Roller rockers 1.5
Roller lifters
 
Any ideas from experience? whats top CR with aluminum heads?
 
You might have 10.70C/R as a static compression,but your cam will determine the dynamic compression due to lift and duration.What are the cam specs you have?I would run the steel gaskets for run in and for testing the fuel and timing setting to begin with.At any time in the future you can put the compasion gaskets on and reduce the compression some .3-.5 of a point,mrmopartech
 
I may have made a mistake. But I did it four times. Did you run the numbers?
 
Mrmopartech said:
You might have 10.70C/R as a static compression,but your cam will determine the dynamic compression due to lift and duration.What are the cam specs you have?I would run the steel gaskets for run in and for testing the fuel and timing setting to begin with.At any time in the future you can put the compasion gaskets on and reduce the compression some .3-.5 of a point,mrmopartech
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Mechanical roller tappetBasic Operating RPM Range2,200-6,300Intake Duration at 050 inch Lift236Exhaust Duration at 050 inch Lift242Duration at 050 inch Lift236 int./242 exh.Advertised Intake Duration274Advertised Exhaust Duration280Advertised Duration274 int./280 exh.Intake Valve Lift with Factory Rocker Arm Ratio0.564 in.Exhaust Valve Lift with Factory Rocker Arm Ratio0.570 in.Valve Lift with Factory Rocker Arm Ratio0.564 int./0.570 exh.Lobe Separation (degrees)110Intake Valve Lash0.016 in.Exhaust Valve Lash0.018 in.Computer-Controlled CompatibleYesGrind NumberXR274R
 
StrokerScamp said:
I may have made a mistake. But I did it four times. Did you run the numbers?
Click to expand...

is the piston head volume a positive number or neg. it has a -7.5cc on the spec sheet that came from Ross but does the calculator know it is neg?
 
Dustedu2 said:
I am ready to pick out the head gasket and here is what I know. What I want is best CR and quench without denotation on 93 pump gas.
If I am already to high than best with Octane additive. I want most HP with what I have "street/strip" Build.

Flat Top Ross Piston, Edelbrock Performer Heads
Bore 4.10
Strke 4.00
gasket bore 4.180
Chamber volume 65cc
deck hight .051
Dome volume -7.5cc
with .025 gasket I get 10.71:1 CR with quench at .076

cam is roller XR274R
Roller rockers 1.5
Roller lifters
Click to expand...

I got 10.71 using 7.5cc
I got 12.6 using -7.5cc
Using your specs

If it is a true dome then i think the number is neg, if it is flat I think the number is positive.... Double check if the cc is neg or pos. Don't quote me on it though because I am not really sure
 
Dustedu2 said:
is the piston head volume a positive number or neg. it has a -7.5cc on the spec sheet that came from Ross but does the calculator know it is neg?
Click to expand...

If it's a domed piston, it's negative, because that takes AWAY from the combustion area.

I see you say they are flat tops. You gotta part number? It's possible they could be either way, positive or negative depending on compression height and piston volume. since they are flat tops, I am leaning more toward a positive number, which would put compression at the 10 somethin figure.
 
StrokerScamp said:
If it's a domed piston, it's negative, because that takes AWAY from the combustion area.

I see you say they are flat tops. You gotta part number? It's possible they could be either way, positive or negative depending on compression height and piston volume. since they are flat tops, I am leaning more toward a positive number, which would put compression at the 10 somethin figure.
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heres the cam and piston specs:
 

Attachments

-7.5 cc dome of a flat top piston is pretty typical of the size of the valve reliefs and adds volume above the piston and lowers the compression. Based on the numbers given the static compression ratio is 10.706:1.

NOTE: you will NOT have any quench with this setup. The 0.076 distance is just to large for it to be effective. For ideal quench you need 0.040 to 0.050.

You would be better off with a step dish piston with 15cc dish that you can zero deck and use an 0.040" thick head gasket. That will give you the same 10.7:1 compression and an ideal quench distance of 0.040".

Or, zero deck the pistons you have and use an 0.050" thick head gasket and you will be at 11.3:1 which is still well within reason for the cam you are using and the aluminum heads and still have an effective quench.
 
The compression height form the piston card is 1.449. On a nominal deck height of 9.6" you would have those pistons sitting in the hole 0.028", there is no reason that you can't have the block decked to get those pistons to zero. Is the .051" you mentioned a measured number?
 
dgc333 said:
-7.5 cc dome of a flat top piston is pretty typical of the size of the valve reliefs and adds volume above the piston and lowers the compression. Based on the numbers given the static compression ratio is 10.706:1.

NOTE: you will NOT have any quench with this setup. The 0.076 distance is just to large for it to be effective. For ideal quench you need 0.040 to 0.050.

You would be better off with a step dish piston with 15cc dish that you can zero deck and use an 0.040" thick head gasket. That will give you the same 10.7:1 compression and an ideal quench distance of 0.040".

Or, zero deck the pistons you have and use an 0.050" thick head gasket and you will be at 11.3:1 which is still well within reason for the cam you are using and the aluminum heads and still have an effective quench.
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Dish would be better w/ zero deck but it sounds like he already has the pistons.... how did you get the 10.71 comp? i got that but put in Positive 7.5cc instead of negative. Negative gave it in the 12 range.
About detonation, some say 10:1 some say 11:1 w/ alum heads... Do you really want to have a motor that hot for the street. I have nothing against it and think it is awesome but reliability wise, unless, well it is your choice. all I know is don't waste your time adding octane boost to 93, add a gallon or 2 of 110.... then you can calculate your octane.... Hott Rodder or something did an article on octane boost and found it only helped 87 octane. With 91 octane it made less power than just straight 91. Food for Thought!
 

Flat Top Ross Piston, Edelbrock Performer Heads
Bore 4.10
Strke 4.00
gasket bore 4.180
Chamber volume 65cc
deck hight .051
Dome volume -7.5cc
with .025 gasket I get 10.71:1 CR with quench at .076


Questions for you... These are the deep chamber heads IIRC. What are you calling the deck height? How do you get the ".051" height figure? Dome volume is the valve reliefs, yes? Do you have the part number for the pistons?
 
dgc333 said:
The compression height form the piston card is 1.449. On a nominal deck height of 9.6" you would have those pistons sitting in the hole 0.028", there is no reason that you can't have the block decked to get those pistons to zero. Is the .051" you mentioned a measured number?
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yes, the pistons are down that far.
 
Speedotann said:
Dish would be better w/ zero deck but it sounds like he already has the pistons.... how did you get the 10.71 comp? i got that but put in Positive 7.5cc instead of negative. Negative gave it in the 12 range.
About detonation, some say 10:1 some say 11:1 w/ alum heads... Do you really want to have a motor that hot for the street. I have nothing against it and think it is awesome but reliability wise, unless, well it is your choice. all I know is don't waste your time adding octane boost to 93, add a gallon or 2 of 110.... then you can calculate your octane.... Hott Rodder or something did an article on octane boost and found it only helped 87 octane. With 91 octane it made less power than just straight 91. Food for Thought!
Click to expand...


Depends on how the calculator wants you to enter the dome/dish volume. The one I used says dome volume is entered as a positive and a dish or valve relief volume is entered as a negative number. Since it has already been stated the pistons are flat tops and virtually all perfromance flat tops have a valve relief its pretty easy to conclude the "Dome volume -7.5cc" is referring to the valve relief volume so you enter the number as your calculator instructs. Even if it doesn't tell you you can think it through and realize that a valve relief or dish adds chamber volume which lowers compression and if one way gives a lower number than the other you can conclude its the way that gies the lower number.

As far as compression ratio goes the "some say ....." is a useless statement, there are too many other factors that come into pay when trying to determine the how much compression you can have. With all other things being equal aluminum will allow for a higher compression ratio because it will transfer heat away from the combustion chamber faster. This comes at the cost of less potential power, with all things being equal, including compression ratio an alumium headed engine will make less power than an iron.

FWIW, I have a 360 with 10.6:1 compression with a small cam than has been mentioned here and it runs with no detonation just fine on 89 octane and it I am careful about not loading the engine in the lower rpm ranges it will run on 87 octane without detonation. I get away with this with zero deck flat top pistons and closed chamber magnum heads with a tight 0.040" quench distance. I have a Comp XE268H cam which gives me a cranking pressure of 195+/-5 psi. From a reliability stand point I have put almost 46,000 miles on it since July 05 when I installed it and I do not baby it.

When it comes to Octane Boost when the bottles advertise that they raise the octane "4 points" what they really mean is it will raise 87 to 87.4 (neat little marketing ploy, they haven't lied but they sure make you think a bottle will turn 87 into 91). So, if you have a detonation problem it's not likely that its going to make much difference.
 
dgc333 said:
Depends on how the calculator wants you to enter the dome/dish volume. The one I used says dome volume is entered as a positive and a dish or valve relief volume is entered as a negative number. Since it has already been stated the pistons are flat tops and virtually all perfromance flat tops have a valve relief its pretty easy to conclude the "Dome volume -7.5cc" is referring to the valve relief volume so you enter the number as your calculator instructs. Even if it doesn't tell you you can think it through and realize that a valve relief or dish adds chamber volume which lowers compression and if one way gives a lower number than the other you can conclude its the way that gies the lower number.

As far as compression ratio goes the "some say ....." is a useless statement, there are too many other factors that come into pay when trying to determine the how much compression you can have. With all other things being equal aluminum will allow for a higher compression ratio because it will transfer heat away from the combustion chamber faster. This comes at the cost of less potential power, with all things being equal, including compression ratio an alumium headed engine will make less power than an iron.

FWIW, I have a 360 with 10.6:1 compression with a small cam than has been mentioned here and it runs with no detonation just fine on 89 octane and it I am careful about not loading the engine in the lower rpm ranges it will run on 87 octane without detonation. I get away with this with zero deck flat top pistons and closed chamber magnum heads with a tight 0.040" quench distance. I have a Comp XE268H cam which gives me a cranking pressure of 195+/-5 psi. From a reliability stand point I have put almost 46,000 miles on it since July 05 when I installed it and I do not baby it.

When it comes to Octane Boost when the bottles advertise that they raise the octane "4 points" what they really mean is it will raise 87 to 87.4 (neat little marketing ploy, they haven't lied but they sure make you think a bottle will turn 87 into 91). So, if you have a detonation problem it's not likely that its going to make much difference.
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Wow, lots of good info. I really appreciate you taking the time (everyone) to assist in my build. So Dave, do you think this would be a good combo with zero deck and .050 gasket (.050 quench)? Comp cam tech said max CR should not exceed 11.3:1 with my cam for max performance (which is where I would be to the decimal) if I zero deck the pistons. I am close to an airport to get high octane fuel to mix if needed.

I want a hot build because I will have my 65 coronet vert to cruze in when I don't want to blow away a chevy or ford.
 
If the comp cam tech took all the numbers you posted here and looked at what the dynamic compression is with your cam and said 11.3:1 would be the max static compression I would say that is a pretty safe recomendation. They won't give you a recomendation that is on the ragged edge.

Bigger cams typically close the intake valve further after bottom dead center (ABDC). The latter it closes the shorter the effective stroke is and the lower the compression. i would use the calculator on the Keith Black site and put in the intake closing point numbers of the cam you are using and see what you get for dynamic compression. Moper would have more experience if that number is likely to give you detonation issues.

When I built my engine I did it with a "it should be Ok" mind set but was prepared to swap to a bigger cam if I needed to bleed off some pressure or even to swap back to the LA open chamber heads.
 
dgc333 said:
If the comp cam tech took all the numbers you posted here and looked at what the dynamic compression is with your cam and said 11.3:1 would be the max static compression I would say that is a pretty safe recomendation. They won't give you a recomendation that is on the ragged edge.

Bigger cams typically close the intake valve further after bottom dead center (ABDC). The latter it closes the shorter the effective stroke is and the lower the compression. i would use the calculator on the Keith Black site and put in the intake closing point numbers of the cam you are using and see what you get for dynamic compression. Moper would have more experience if that number is likely to give you detonation issues.

When I built my engine I did it with a "it should be Ok" mind set but was prepared to swap to a bigger cam if I needed to bleed off some pressure or even to swap back to the LA open chamber heads.
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Yes, I filled out Comp's spec sheet when ordering the cam and this is the one they recommended. The Dynamic CR shows 9.2:1 with the 63 ABDC and 6.123 rod lenght.
 
dgc333 said:
-7.5 cc dome of a flat top piston is pretty typical of the size of the valve reliefs and adds volume above the piston and lowers the compression.
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Sorry, but this is incorrect. Look at this calculator.

http://www.kb-silvolite.com/calc.php?action=piston

See where it says "Note: A piston with a positive number is either a flat-top with valve reliefs or a dish. A piston with a negative number is a pop-up."

The reason for this is because of what I stated the first time. A domed piston has a negative cc measurement since it takes away from the combustion area. A flat or dished piston has a positive number because it adds to the combustion area. It's just that simple.
 
Where did the pistons come from? They are custom by the part number I think.. (sorry, just noticed you posted the tech sheet for them...lol) what confuses me are two things.. The valve reliefs are normally around 7 or 7.5cc, so I get that.. But the sheet shows a dish of .029"? Is there any dish in them? They sound like a set for a shorter deck block. On top of that is the deep cut chamber. So you have no quench. If the heads were on the block with no gasket you would be just within the range of any benefit. As far as compression, I ran your numbers and got 10.4:1. But something else... The MP gasket will wear a groove into the aluminum heads over time... You need to run a pre-flattened fire ring such as the Fel Pro Performance 1008. That is .039" compressed, and that will lower the compression to 10.1. The dynamic is estimated but the cam is small. So you will have higher pressures. I'm showing either 8.5:1 dynamic for the 1008 gasket, or 8.78 with your .024. In either case, with no quench, even with the aluminum I think you may be on the verge of detonation. I think I'd say go up one cam size to the XR280R. I think that will drop enough cylinder pressure to keep it safe.
 
moper said:
Where did the pistons come from? They are custom by the part number I think.. (sorry, just noticed you posted the tech sheet for them...lol) what confuses me are two things.. The valve reliefs are normally around 7 or 7.5cc, so I get that.. But the sheet shows a dish of .029"? Is there any dish in them? They sound like a set for a shorter deck block. On top of that is the deep cut chamber. So you have no quench. If the heads were on the block with no gasket you would be just within the range of any benefit. As far as compression, I ran your numbers and got 10.4:1. But something else... The MP gasket will wear a groove into the aluminum heads over time... You need to run a pre-flattened fire ring such as the Fel Pro Performance 1008. That is .039" compressed, and that will lower the compression to 10.1. The dynamic is estimated but the cam is small. So you will have higher pressures. I'm showing either 8.5:1 dynamic for the 1008 gasket, or 8.78 with your .024. In either case, with no quench, even with the aluminum I think you may be on the verge of detonation. I think I'd say go up one cam size to the XR280R. I think that will drop enough cylinder pressure to keep it safe.
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Thanks for chiming in Moper. I called Ross Pistons a little while ago and gave him the job number. He looked them up and they were originally orderd for a .010 deck and a 70cc head volume. I gave him a zero deck, 65cc head and .040 gasket as new build spec's and he said with that info they would run 11.45:1 So, if I run felpro 1008 .039 gasket deck at .010 that should put me at 11.2:1 and a quench at .049 correct? oh yeah, there is a very slight dish.
 
Dustedu2 said:
Wow, lots of good info. I really appreciate you taking the time (everyone) to assist in my build. So Dave, do you think this would be a good combo with zero deck and .050 gasket (.050 quench)? Comp cam tech said max CR should not exceed 11.3:1 with my cam for max performance (which is where I would be to the decimal) if I zero deck the pistons. I am close to an airport to get high octane fuel to mix if needed.

I want a hot build because I will have my 65 coronet vert to cruze in when I don't want to blow away a chevy or ford.
Click to expand...

I see... Av Gas will be great.... its like 5 something a gallon! Good Luck! When I was getting my pilots license, I used to buy a 5 gal jug once a week.
 
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