Compression VS. Octane

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Duggie

Duggie

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Ref - daily driver '64 Dart /6. Cylinders bored .030 over. Cam unknown, valve job is 2K miles old. 1920 carb being changed out to a Carter BBD with the single barrel manifold cut out mod. Points and condenser may get upgraded to electronic if I don't run out of $. Distributer gives 44 deg. of total vacuum and mechanical advance! I'm doing the poor man's header by cutting the existing header in two and want to shave the head as long as I'm doing intake and exhaust.

First question -> How much dynamic compression can these /6s run on 87 octane gas? I'm reading anything from 8.3-1, to 9.5-1. Any real-world experience out there on compression VS. octane and avoiding the dreaded ping?

Computing dynamic compression -> I have an old dial caliper and ordered a 100cc syringe to cc the combustion chambers with. While waiting for parts, I'm making assumptions and coming up with a dynamic compression of 8.78-1. Seams high for a 'stock' engine... Yes?

*The head base to valve cover surface measures 3.605" ~ 54cc combustion chamber.
*Bore = 3.430", stroke = 4.125"
*TDC to block height = .030", head gasket = 3.520" dia x .040" thick.
*Rod length = 6.699".
*Intake valve closing point (IVC) ~ 42 to 48 deg.

Thanks in advance! I wouldn't even own this car if it weren't for you guys! :steering:
 
Duggie said:
Ref - daily driver '64 Dart /6. Cylinders bored .030 over. Cam unknown, valve job is 2K miles old. 1920 carb being changed out to a Carter BBD with the single barrel manifold cut out mod. Points and condenser may get upgraded to electronic if I don't run out of $. Distributer gives 44 deg. of total vacuum and mechanical advance! I'm doing the poor man's header by cutting the existing header in two and want to shave the head as long as I'm doing intake and exhaust.

First question -> How much dynamic compression can these /6s run on 87 octane gas? I'm reading anything from 8.3-1, to 9.5-1. Any real-world experience out there on compression VS. octane and avoiding the dreaded ping?

Computing dynamic compression -> I have an old dial caliper and ordered a 100cc syringe to cc the combustion chambers with. While waiting for parts, I'm making assumptions and coming up with a dynamic compression of 8.78-1. Seams high for a 'stock' engine... Yes?

*The head base to valve cover surface measures 3.605" ~ 54cc combustion chamber.
*Bore = 3.430", stroke = 4.125"
*TDC to block height = .030", head gasket = 3.520" dia x .040" thick.
*Rod length = 6.699".
*Intake valve closing point (IVC) ~ 42 to 48 deg.

Thanks in advance! I wouldn't even own this car if it weren't for you guys! :steering:
Click to expand...
What dynamic compression calculator are you using?
There is no firm agreement on what intake closing point to use so different calculators provide different numbers.

And the values stated in your “first question” look more like static compression values than dynamic compression values.

Stock slant six heads are closer to 58 cc’s
Why are you using 54

Why do you have a range of 42 to 48 for the intake valve closing point. Dynamic compression calculators ask for specific point.

Lastly, agree 8.78 dynamic compression is high for a stock engine, something is wrong, so again, what calculator are you using.
 
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I'm using this web site. I randomly found another online calculator and it was within a couple tenths of this one.

Static came in at 10.4-1!

I'll plug in 58cc for the compression chamber and see what it looks like. I found the 54cc number drilling through threads here. I won't get the parts to cc my head until maybe Friday.

Cam IVC of 42 and 48 deg were the only references I could find in threads. I don't know what it is for a stock cam. The variance made a difference, but not enough to make the final comp ratio believable.

I appreciate your input. I've read a TON of your stuff on here.
 
Duggie said:
I'm using this web site. I randomly found another online calculator and it was within a couple tenths of this one.

Static came in at 10.4-1!

I'll plug in 58cc for the compression chamber and see what it looks like. I found the 54cc number drilling through threads here. I won't get the parts to cc my head until maybe Friday.

Cam IVC of 42 and 48 deg were the only references I could find in threads. I don't know what it is for a stock cam. The variance made a difference, but not enough to make the final comp ratio believable.

I appreciate your input. I've read a TON of your stuff on here.
Click to expand...
Duggie said:
Ref - daily driver '64 Dart /6. Cylinders bored .030 over. Cam unknown, valve job is 2K miles old. 1920 carb being changed out to a Carter BBD with the single barrel manifold cut out mod. Points and condenser may get upgraded to electronic if I don't run out of $. Distributer gives 44 deg. of total vacuum and mechanical advance! I'm doing the poor man's header by cutting the existing header in two and want to shave the head as long as I'm doing intake and exhaust.

First question -> How much dynamic compression can these /6s run on 87 octane gas? I'm reading anything from 8.3-1, to 9.5-1. Any real-world experience out there on compression VS. octane and avoiding the dreaded ping?

Computing dynamic compression -> I have an old dial caliper and ordered a 100cc syringe to cc the combustion chambers with. While waiting for parts, I'm making assumptions and coming up with a dynamic compression of 8.78-1. Seams high for a 'stock' engine... Yes?

*The head base to valve cover surface measures 3.605" ~ 54cc combustion chamber.
*Bore = 3.430", stroke = 4.125"
*TDC to block height = .030", head gasket = 3.520" dia x .040" thick.
*Rod length = 6.699".
*Intake valve closing point (IVC) ~ 42 to 48 deg.

Thanks in advance! I wouldn't even own this car if it weren't for you guys! :steering:
Click to expand...
what you list as TDC to deck height, most folks call that piston recession. The calculator that you posted is calling that Deck Clearance.
On a stock slant that is typically .160 to .180
Did you actually measure that as .030
As if so either someone walloped a bunch off of the head deck or the motor has longer rods in it.
If the piston recession is .030 the cylinders will have a lot of compression if not correcting that is going to make a big difference in the compression values you are getting.
 
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Mine has a measured 10.2:1. Dynamic is 7.99, but it STILL has over 175 PSI cranking pressure even with a big solid cam. It will run on 87 with no spark knock, but it runs SO much better on super, that's all I use now. Super most places is like a 50 cent increase per gallon. It's just NOT that much more.
 
Most run around on 7.2-7.4 dynamics aka 145-160lbs cranking and miss out on torque below the power band and overall 'snap'
 
MOPAROFFICIAL said:
Most run around on 7.2-7.4 dynamics aka 145-160lbs cranking and miss out on torque below the power band and overall 'snap'
Click to expand...
I can attest to that.....BUT, it's only soft RIGHT OFF idle. Once it's rollin in low gear, it's got balls. With the 3.90 gear I had in it, it was badass. It was just too much on the street with them short tires.
 
Duggie said:
Ref - daily driver '64 Dart /6. Cylinders bored .030 over. Cam unknown, valve job is 2K miles old. 1920 carb being changed out to a Carter BBD with the single barrel manifold cut out mod. Points and condenser may get upgraded to electronic if I don't run out of $. Distributer gives 44 deg. of total vacuum and mechanical advance! I'm doing the poor man's header by cutting the existing header in two and want to shave the head as long as I'm doing intake and exhaust.

First question -> How much dynamic compression can these /6s run on 87 octane gas? I'm reading anything from 8.3-1, to 9.5-1. Any real-world experience out there on compression VS. octane and avoiding the dreaded ping?

Computing dynamic compression -> I have an old dial caliper and ordered a 100cc syringe to cc the combustion chambers with. While waiting for parts, I'm making assumptions and coming up with a dynamic compression of 8.78-1. Seams high for a 'stock' engine... Yes?

*The head base to valve cover surface measures 3.605" ~ 54cc combustion chamber.
*Bore = 3.430", stroke = 4.125"
*TDC to block height = .030", head gasket = 3.520" dia x .040" thick.
*Rod length = 6.699".
*Intake valve closing point (IVC) ~ 42 to 48 deg.

Thanks in advance! I wouldn't even own this car if it weren't for you guys! :steering:
Click to expand...
Yeah, there's no way that is the correct piston to deck distance unless those are forged racing slugs, or some 198 con.-rod/piston combo. That's easily .100"+ too little, even with an aggressive deck job that required a correction. Folks HAVE mowed the deck that much for high squeeze, but not for a rebuild ever, lol!
 
my68barracuda said:
what you list as TDC to deck height, most folks call that piston recession. The calculator that you posted is calling that Deck Clearance.
On a stock slant that is typically .160 to .180
Did you actually measure that as .030
As if so either someone walloped a bunch off of the head deck or the motor has longer rods in it.
If the piston recession is .030 the cylinders will have a lot of compression if not correcting that is going to make a big difference in the compression values you are getting.
Click to expand...
Since pulling the head, I've been doing a two-hour round trip work commute by train. and searching forum threads for info. I want to have my little pointed head wrapped around this so I'm not blindly handing my head to the machine shop. I can ask 'The Wife' to hold the dial calipers over the piston while I turn the crank this Friday. It will be novel to ask her to "hold this" without asking her to talk dirty to me too. :D

Practicing using the online calculator with .170 desk clearance returns a Static ratio of 7.93 and Dynamic of 6.61. Proof again that garbage info in = garbage info out!
 
RustyRatRod said:
Mine has a measured 10.2:1. Dynamic is 7.99, but it STILL has over 175 PSI cranking pressure even with a big solid cam. It will run on 87 with no spark knock, but it runs SO much better on super, that's all I use now. Super most places is like a 50 cent increase per gallon. It's just NOT that much more.
Click to expand...
I opened up my valve clearance to .013 and .022 hot to increase fuel mileage and am getting 165lbs cranking pressure. I also read somewhere that going much over 165 lbs can blow a head gasket. Are you using something special for a head gasket when increasing cylinder pressure?
 
Duggie said:
I opened up my valve clearance to .013 and .022 hot to increase fuel mileage and am getting 165lbs cranking pressure. I also read somewhere that going much over 165 lbs can blow a head gasket. Are you using something special for a head gasket when increasing cylinder pressure?
Click to expand...
No. Just the old style Fel Pro Print O Seal.
 
I had to use the thicker Aussie head gasket in mine, I come up with 0.051compressed. the fire ring dia is a bit bigger than it is on a fel pro.
I had 30 thou shaved off the block, and 70 thou shaved off the head. I have measured my head CCs at 49. Machine shop said this head had 60cc originally.
Punch in all the numbers and I come out at almost dead nuts at 8.4:1. A little lower than I had shot for, but definitely better than it would have originally been.
Oh yeah my engine is only bored 0.020 over, and my pistons are 0.180 in the hole even after the 30 thou deck shave. I was counting on them being closer to the deck surface than that.
This is my real world build.not just a "Bench race session" or pulled out of some book or magazine.
With tolerances as they are/were I was surprised that with all that machine work I still only ended up at "factory blueprint spec" so that tells you how dismal compression really was, on engine after engine down the original assembly line.
 
And turning this engine over on the stand with a crank socket (so I could pre adjust my valves somewhere in the ballpark) it takes some muscle to spin it over vs.cimpression even at that. Much more effort to spin over, than the original engine it is being built to replace that is still in the truck.(that does still run)
 
With mine, it ended up like this by accident. I had the machine shop mill the obligatory .100 off the head. On the last cut, one end of the head slipped off the broach. He tried to clean it up and did, but it milled out crooked. You could actually see the difference from one end to the other in the chamber sizes.

So I took it to another friend who squared it up off the machined valve cover rail and milled it back straight and square in his Rottler. It ended up going from the stock 58cc chamber to 34cc chambers. I had to utilize the head, because it had good guides, seats and valves. Getting any of the rest of these heads I have ready would have cost a lot and we didn't have it. So I pressed forward. It worked out in the end.
 
AJ/FormS said:
That will depend entirely on how far the throttle is opened. Cuz what you engine actually sees is Effective compression ratio and cylinder pressure.
Click to expand...
Being a street driver, it would probably be rare moment the engine sees much more than 4K RPM. I guess...
 
volaredon said:
I had to use the thicker Aussie head gasket in mine, I come up with 0.051compressed. the fire ring dia is a bit bigger than it is on a fel pro.
I had 30 thou shaved off the block, and 70 thou shaved off the head. I have measured my head CCs at 49. Machine shop said this head had 60cc originally.
Punch in all the numbers and I come out at almost dead nuts at 8.4:1. A little lower than I had shot for, but definitely better than it would have originally been.
Oh yeah my engine is only bored 0.020 over, and my pistons are 0.180 in the hole even after the 30 thou deck shave. I was counting on them being closer to the deck surface than that.
This is my real world build.not just a "Bench race session" or pulled out of some book or magazine.
With tolerances as they are/were I was surprised that with all that machine work I still only ended up at "factory blueprint spec" so that tells you how dismal compression really was, on engine after engine down the original assembly line.
Click to expand...
I guess I'll find out when I reassemble, but were you guys able to reuse the old push rods after removing head / deck metal?
 
no
As to detonation;
Think of it this way; at idle, with your throttle nearly closed;
Your Scr could be 9.5,
Your Dcr could be 8.0
But your effective compression ratio might be 5.0 , or less!
because the atmosphere has such a hard time during the allotted intake valve open event, to get into the chamber...... and even after it does, some set-ups with a long-period cam with a late-closing intake, will send a small percentage of it, back up into the intake!

>Most engines will not detonate at low throttle openings and/or low rpms, because of that very low Effective C/R . If it does, it is usually precipitated by pre-ignition.
>Some detonation at higher rpm is easier to get rid of than detonation at low-rpm and Part-Throttle, but if too much timing has to be pulled, the engine rapidly loses power. And the slanty doesn't take that very well, having not much to start with.
>Now, imagine that your carb can ingest a maximum of 200cfm at WOT, at some rpm say near 3600. Still at 9.5 scr/8.0 Dcr and everything is copacetic, and maybe you can run full-Power Timing of say 34*.
How you connect those two data points, namely idle and 3600, will determine if it does or does not detonate, somewhere in-between.
But suppose at 200cfm, the engine just doesn't care, you cannot make it detonate no matter how you try. So you install a 4bbl on your engine that at 4000rpm and WOT, can now pull in 350cfm; and
POW, she's into detonation big time
That's because you have removed the small-carb choke point and
the Effective compression ratio, is now approaching the Dcr; which is just too high for the chosen fuel.

But here's the deal; fuel economy demands hi-pressure. Hi-pressure is power, and if you have it at low rpm, it requires LESS throttle opening to maintain a given speed. That means you can take cruise rpm lower, with gearing, and the lower gearing has the potential to decrease your fuel consumption.
of course, with just a typical TF automatic, your low-speed performance goes right out the window :(
 
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yes, you have plenty of adjuster on the slant rockers for a haircut.

AJ, what did you find? IIRC RRR runs .090 without a pushrod change, barring any geometry issues.
 
Duggie said:
I guess I'll find out when I reassemble, but were you guys able to reuse the old push rods after removing head / deck metal?
Click to expand...
With a total of .100-.125 off the head/deck surface maybe yes, maybe no. It is close.
On a 170 engine at the same specs, probably need shorter push rods.
 
Duggie said:
Since pulling the head, I've been doing a two-hour round trip work commute by train. and searching forum threads for info. I want to have my little pointed head wrapped around this so I'm not blindly handing my head to the machine shop. I can ask 'The Wife' to hold the dial calipers over the piston while I turn the crank this Friday. It will be novel to ask her to "hold this" without asking her to talk dirty to me too. :D

Practicing using the online calculator with .170 desk clearance returns a Static ratio of 7.93 and Dynamic of 6.61. Proof again that garbage info in = garbage info out!
Click to expand...

you really want to find an accurate Top Dead Center.
Easy to do with a garage built fixture.
Get a six inch piece of rigid rectangular steel.
Drill two holes through it that fit bolts that screw into two head bolt holes.
Drill a second through hole and put a bolt and nut in that center hole to act as the piston stop.
Mount the rectangular steel section so it spans across the # 1 piston bore with the piston stop bolt in place.
Turn the crank to the right till the piston is against the piston stop. Mark the damper in line with the 0 degree mark.
Turn the piston in the opposite direction till the piston is against the piston stop.
Mark the damper in line with the 0 degree mark.
Find the middle spot between the two marks placed on the damper.
That is actual TDC.
If that aligns with the TDC mark on the damper that means the damper is accurate and the outer ring has not spun. If if does not align perfectly and you measured correctly that means the outer ring on the damper has slipped.
Once you find accurate TDC on the damper, set the damper to that position and use your calipers or depth micrometer to find the piston recession.
 
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AJ/FormS said:
no
Click to expand...
well ok, that's not exactly true;
Yes maybe you can use the same pushrods and
yes maybe you can make it run with the adjustable valve gear.
But that don't mean it's right. Doing so puts the geometry out, and may shorten the guide life, and will change the Effective rocker-arm ratio. For me, that was year 1994, so I don't recall what I did except that I pumped it up to a true 9.5.
Well ok, that's not exactly true either; I told the machine shop to pump it up to 9.5, and never actually checked it. Then I specified a slightly hotter solid-lifter cam. It made good pressure so I did not question their work.
27 years later, on a day last summer; I sold that car, and that engine still cranked and fired up like new, even a week later, on the day that the buyer came to pick it up.
 
It sounds like a Dynamic compression ratio of 9.0-1 would keep me in the ballpark for pump gas with my shinney new BBD 2 barrel. I could go on a play with the distributor advance curve as free time or 'ping' urgency dictates?

With the needed measurements confirmed on my slant, I could then ask the machine shop to mill the head to a target cc for the desired Dcr.

Yes?
 
Duggie said:
It sounds like a Dynamic compression ratio of 9.0-1 would keep me in the ballpark for pump gas with my shinney new BBD 2 barrel. I could go on a play with the distributor advance curve as free time or 'ping' urgency dictates?

With the needed measurements confirmed on my slant, I could then ask the machine shop to mill the head to a target cc for the desired Dcr.

Yes?
Click to expand...
A static compression of 9:1 may run regular gas in a light vehicle, but you will be leaving ignition advance on the table.
I would shoot for a static compression of 8.5 to 1, that would allow you to run an aggressive ignition curve and have a safety factor for hot weather, big hills, bad gas.
I pay attention to dynamic compression ratios, but I build engines around static compression ratios.
 
I have been promising the little woman that she can have her reliable street manners and I can have a little fun too. A safety factor is just what I need for a win-win!:)
 
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