[72LuxuryLeMansLa.]

Originally Posted by tom s

I did a 9.5 CR 455 with a summit 2801 and it pumped 200 and had to use race gas.It was put in straight up.Tom

Thanks Tom. I can look up the numbers on that cam. Was this a 4.21" stroke with stock rod length?

Karl

[tom s]

yes,stock build with dish pistons.16 heads,stock intake and QJ.Tom

[zeebo]

there is a table in an old book i have that shows psi vs. cranking compression. i know that this is really for low perf cams, as explained above, larger cams with overlap will affect cranking compression psi vs a small cam.

[Schurkey]

As soon as you start asking The Multitudes for cranking compression results, you're going to discover that plenty of folks have inaccurate compression gauges; their faulty information will make applesauce of the data points and overall accuracy/reliability of your conclusions.

[aktattoo]

maybe something in here http://www.wallaceracing.com/Calculators.htm

[72LuxuryLeMansLa.]

Originally Posted by Schurkey

As soon as you start asking The Multitudes for cranking compression results, you're going to discover that plenty of folks have inaccurate compression gauges; their faulty information will make applesauce of the data points and overall accuracy/reliability of your conclusions.

This is not just aimed at you Schurkey; you just provided an opportunity for clarification. I don't seem to be very good at getting my point across:

This is not a scientific research project for me to get a doctoral degree in mechanical engineering! Let me try this again.

I know these numbers are not the end all/be all in building an engine. Even in well built engines it is possible to have different amounts of ring and valve leakage that WILL EFFECT the numbers! This is just a way of learning about cam selection. I don't think that I will be able to produce a magic chart to say what SCR lines up with what DCR and in turn produces XYZ for a cranking compression so you should use cam X........

This is to LEARN. If you do not believe that compression pressure is relevant , fine, don't post any numbers...............
I, on the other hand, believe that the amount of cranking pressure that an engine has, will have a direct effect on whether or not it will tolerate pump gas. There is also the desire to have as efficient an engine as possible within my budget. Higher combustion chamber pressure will help with faster burn rates, cooling the chamber between burns(along with quench) and in turn this improves fuel mileage. It all fits together, and the one place that I have seen lacking in discussions on this and other boards is, how much ACTUAL pressure can be run on the street. Everyone discusses Static Compression Ratios, but the actual pressure in the cylinder is what should be talked about. As stated earlier, 9.5:1 with two different cams will give you 6.5:1dcr or 8.5:1 dcr and these will have two VERY different cranking compressions........

Karl

[72LuxuryLeMansLa.]

Originally Posted by tom s

yes,stock build with dish pistons.16 heads,stock intake and QJ.Tom

Tom,

I ran the numbers on a 455 (stock bore and stroke) with 83cc chambers and 15cc dishes to get an scr of 9.52:1. With the Summit 2801 it shows a Dynamic Compression Ratio of only 7.20:1 . 200psi seems very high in that case according to other numbers I have seen. Did you have some of those gapless rings that people complain about with high pressures? You also said that you installed it "straight up"; was this degreed to the ICL from the card (my understanding of "straight up" ) or was it the "dot to dot" definition of straight up?

Thanks,

Karl

[tom s]

Just speed pro pistons dished by Pontiac Dude.If I remember the pistons had a 20 CC dish and the 16s were smaller.We did not degree it just put it in dot to dot.We were blown away when it pinged like a bitch on 91 gas.I wanted to pull the front cover and retard it a couple but he was OK with mixing race gas with it.Ran like a raped ape so he like the power.He sold the car last summer.Tom

[paul s.]

Originally Posted by 72LuxuryLeMansLa.

This is not just aimed at you Schurkey; you just provided an opportunity for clarification. I don't seem to be very good at getting my point across:

This is not a scientific research project for me to get a doctoral degree in mechanical engineering! Let me try this again.

I know these numbers are not the end all/be all in building an engine. Even in well built engines it is possible to have different amounts of ring and valve leakage that WILL EFFECT the numbers! This is just a way of learning about cam selection. I don't think that I will be able to produce a magic chart to say what SCR lines up with what DCR and in turn produces XYZ for a cranking compression so you should use cam X........

This is to LEARN. If you do not believe that compression pressure is relevant , fine, don't post any numbers...............
I, on the other hand, believe that the amount of cranking pressure that an engine has, will have a direct effect on whether or not it will tolerate pump gas. There is also the desire to have as efficient an engine as possible within my budget. Higher combustion chamber pressure will help with faster burn rates, cooling the chamber between burns(along with quench) and in turn this improves fuel mileage. It all fits together, and the one place that I have seen lacking in discussions on this and other boards is, how much ACTUAL pressure can be run on the street. Everyone discusses Static Compression Ratios, but the actual pressure in the cylinder is what should be talked about. As stated earlier, 9.5:1 with two different cams will give you 6.5:1dcr or 8.5:1 dcr and these will have two VERY different cranking compressions........

Karl

And this is why some guys can make a combination run while others scream that it can't be done.

There is always more than one way to skin a cat but the common thread is looking at cylinder pressure and what is happening in the cylinder. The actual dcr isn't the be all end all, but it's another way of looking at it.

[quicksilver97ta]

I used the pkelley calculator,
10.5x:1 static (measured, not advertised)
8.2x:1 dynamic w/ IRON heads
Pump gas (premium)

I think it’s close to the edge but safe. In the past I’ve heard some pinging with different carb set ups, timing, and distributors I’ve tried but right now I’m running 34-36* and never hear a hint of it. I’ve read where others have ran a lot of timing with their closed chamber heads but mine doesn’t tolerate it. Cooling is well under control and the car breaths outside cold air (ram air) vs underhood air. I used the pkelley calculator on my LT1 build as well at 8.8x:1 dynamic but it’s aluminum headed and reverse cooled. I have not measured the cranking compression on either but keeping control of engine temps and using the pkelley calculator has served me well so far.

[72LuxuryLeMansLa.]

Originally Posted by quicksilver97ta

I used the pkelley calculator,
10.5x:1 static (measured, not advertised)
8.2x:1 dynamic w/ IRON heads
Pump gas (premium)

I think it’s close to the edge but safe. In the past I’ve heard some pinging with different carb set ups, timing, and distributors I’ve tried but right now I’m running 34-36* and never hear a hint of it. I’ve read where others have ran a lot of timing with their closed chamber heads but mine doesn’t tolerate it. Cooling is well under control and the car breaths outside cold air (ram air) vs underhood air. I used the pkelley calculator on my LT1 build as well at 8.8x:1 dynamic but it’s aluminum headed and reverse cooled. I have not measured the cranking compression on either but keeping control of engine temps and using the pkelley calculator has served me well so far.

If you get a chance, I would love to hear what the cranking compression is on that Iron head. Is that the 434?

Karl

[paul s.]

Remember, cranking compression can be skewed by the methods by which the testing is performed as well as the cranking speed. Was the throttle wide open or closed? What temp was the engine at? Etc.

I think if YOU have a set method for YOUR engine combinations using the same gauge then you have SOME consistency.

[72LuxuryLeMansLa.]

Originally Posted by paul s.

Remember, cranking compression can be skewed by the methods by which the testing is performed as well as the cranking speed. Was the throttle wide open or closed? What temp was the engine at? Etc.

I think if YOU have a set method for YOUR engine combinations using the same gauge then you have SOME consistency.

There is also varying amounts of leakage from engine to engine and cylinder to cylinder....... I assume everything has variables and treat all information accordingly.....

Karl

[72LuxuryLeMansLa.]

Originally Posted by paul s.

It's funny you say that; I look at static and cranking compression and don't think twice about dynamic.... too many variables in my opinion. Not saying I am right and anyone else is wrong.... this works for me. I shoot for 180 cranking compression for a safe number for my customers.

My shop car pumped 240psi.

Paul,

You say that you "shoot for" 180 cranking compression. How do you design for that? Do you use software to help solve for a certain PSI with cam specs? This is one of the main things I would like to learn how to do on paper......I say on paper because I just do not have the resources for tons of trial and error to gain the "experience" ....... I know a bunch of guys say they do things like this from "experience" but there have to be certain cam specs/ valve timing events that are used in conjunction with chamber size and bore and stroke no?

Thanks for the help.....

Karl

[dmac]

I know what you are getting at. I would like to see a chart developed for cylinder pressure vs octane required, but there are likely just too many variable.

Maybe if cylinder pressure was measured in a consistent manner, such a chart could be developed. Something like:
engine fully warmed
all plugs removed
minimum rpm to test(400 rpm?)

but even then, the actual guages would vary. But a general range could be developed maybe like

120-150 lbs- 86 octane
150-170 lbs- 89 octane
170-190 lbs- 91 octane

I am not saying this chart is close- but something similar, and variable lifters like Rhoads would not follow this general chart.

With that info, you could 'aim' for a certain cyl pressure, and be pretty confident as to what fuel you could run. And if your gauge is off by 5-10%, at least you have some idea, and you can use other gauges to verify, or maybe have your gauge tested.

I had a 400 that had consistent 220+ lbs of cyl pressure, and I had to run at least 110 octane, back in the late seventies, just about the time 104 octane at the gas station was being phased out.

[ta man]

Combustion chamber design,aluminum heads versus iron can make a huge difference in what is required for octane...

[LATECH]

Here is a chart for engine temp/octane/dynamic compression ratio.
Kinda kool.
It is for an Iron headed beast

[72LuxuryLeMansLa.]

Originally Posted by LATECH

Here is a chart for engine temp/octane/dynamic compression ratio.
Kinda kool.
It is for an Iron headed beast

Did you find that here: http://www.popularhotrodding.com/tec...h/viewall.html

Karl

[paul s.]

Originally Posted by 72LuxuryLeMansLa.

Paul,

You say that you "shoot for" 180 cranking compression. How do you design for that? Do you use software to help solve for a certain PSI with cam specs? This is one of the main things I would like to learn how to do on paper......I say on paper because I just do not have the resources for tons of trial and error to gain the "experience" ....... I know a bunch of guys say they do things like this from "experience" but there have to be certain cam specs/ valve timing events that are used in conjunction with chamber size and bore and stroke no?

Thanks for the help.....

Karl

Sorry, just experience.

[72LuxuryLeMansLa.]

Originally Posted by dmac

I know what you are getting at. I would like to see a chart developed for cylinder pressure vs octane required, but there are likely just too many variable.

Maybe if cylinder pressure was measured in a consistent manner, such a chart could be developed. Something like:
engine fully warmed
all plugs removed
minimum rpm to test(400 rpm?)

but even then, the actual guages would vary. But a general range could be developed maybe like

120-150 lbs- 86 octane
150-170 lbs- 89 octane
170-190 lbs- 91 octane

I am not saying this chart is close- but something similar, and variable lifters like Rhoads would not follow this general chart.

With that info, you could 'aim' for a certain cyl pressure, and be pretty confident as to what fuel you could run. And if your gauge is off by 5-10%, at least you have some idea, and you can use other gauges to verify, or maybe have your gauge tested.

I had a 400 that had consistent 220+ lbs of cyl pressure, and I had to run at least 110 octane, back in the late seventies, just about the time 104 octane at the gas station was being phased out.

Dmac,

Check this out: Compression Pressure
Assuming your engine has good ring and valve seal, a simple way to determine if your engine has enough compression for the cam being used is to check cylinder compression pressures. For engines of my own utilizing a near-zero leakage ring package and intended for use with 93 octane fuel, I tend to set 190 psi as a lower limit with a preferable 200 - 210 psi target. For every octane number less than 93, the compression pressure needs to be about 5 psi less to avoid detonation under normal circumstances.

I ran an engine design in Engine Analyzer Plus v 3.4 . This is a 4.15" bore x 4" stroke with 6X-4 heads milled to 81cc and zero decked with 8cc flat tops for a SCR of 10.02:1 with the Stump Puller with Solid Rollers and a 7.7:1 DCR . This engine has a Predicted Cranking Compression of 195 PSI . Going by Vizard's personal targeted cranking compression this would show an octane tolerance between 91 and 93 octane. Using his lower limit of 190 PSI @ 93 octane, 91 octane would require 180 PSI max. , 170 PSI max. for 89 octane, and 160 PSI max. for 87 octane if my math is correct.
In reality my heads have an 87cc chamber so my SCR is 9.50:1 and the DCR is 7.3:1 and the estimated cranking compression is 181 PSI. If this estimation plays out I will be leaving efficiency and power on the table probably very little safety margin to show for it. Now I wish I would have had SD Performance mill my heads down to at least 83cc for a SCR of 9.8:1 or so. But.....I will stand pat and see how this plays out. I can always remove the heads later and mill them but I can't have more deck added. My main concern here is efficiency and power is the added bonus.

If you have not read this article yet, it is very informative!

Read more: http://www.popularhotrodding.com/tec...#ixzz2TBfKBXwq

Karl