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  #31 (permalink)  
Old 05-08-2019, 07:34 PM
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Tech beat me to the elevation situation you're in and how it will affect your detonation resistance (and he explained it much better than I would've ).

Regardless of your elevation though, this build will have engine bay heat as an enemy. You need fresh air entering the engine bay much more than a standard S10 or Ranger engine bay setup will allow. A cowl hood is required IMO along with wrapped headers. At highway speeds engine bay temps aren't as much of an issue, but in city driving - oofa, it can turn catastrophic quick, and double quick if you aren't in control of your tune. And a heavy vehicle with tight LSA cam and slippery converter aid this issue in stop and go driving.

I've battled this with a turbo under the hood. The intake temps get out of control and force a reduction in timing which also adds to more heat in the headers. Until I was able to get this part of my tune squared away, I had a couple of instances when I had to pull over, open the hood and keep the rpms above 1500 to get it cooled off. Trust me that it's disconcerting to open the hood and see your headers glowing bright enough that they appear that aren't wrapped.

I'd say go with your build and keep your overlap at .050" below 12 degrees if possible, implying use an LSA in the 112-114 range. And don't go visit Bogie at sea level without a 5 gallon jug 100 plus octane in the trunk

Edit : geesh, I'm a little late to the dance on this thread - it popped up on tapatalk on my phone as a trending thread about an hour ago - smh

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Last edited by 64nailhead; 05-08-2019 at 07:52 PM.
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  #32 (permalink)  
Old 05-09-2019, 01:26 AM
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Quote:
Originally Posted by 64nailhead View Post
Tech beat me to the elevation situation you're in and how it will affect your detonation resistance (and he explained it much better than I would've ).

Regardless of your elevation though, this build will have engine bay heat as an enemy. You need fresh air entering the engine bay much more than a standard S10 or Ranger engine bay setup will allow. A cowl hood is required IMO along with wrapped headers. At highway speeds engine bay temps aren't as much of an issue, but in city driving - oofa, it can turn catastrophic quick, and double quick if you aren't in control of your tune. And a heavy vehicle with tight LSA cam and slippery converter aid this issue in stop and go driving.

I've battled this with a turbo under the hood. The intake temps get out of control and force a reduction in timing which also adds to more heat in the headers. Until I was able to get this part of my tune squared away, I had a couple of instances when I had to pull over, open the hood and keep the rpms above 1500 to get it cooled off. Trust me that it's disconcerting to open the hood and see your headers glowing bright enough that they appear that aren't wrapped.

I'd say go with your build and keep your overlap at .050" below 12 degrees if possible, implying use an LSA in the 112-114 range. And don't go visit Bogie at sea level without a 5 gallon jug 100 plus octane in the trunk

Edit : geesh, I'm a little late to the dance on this thread - it popped up on tapatalk on my phone as a trending thread about an hour ago - smh
I'd drink to this but I need the water for my radiator.

Bogie
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  #33 (permalink)  
Old 05-09-2019, 08:35 AM
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Originally Posted by rdobbs77 View Post
You're camshaft has absolutely nothing or dog in the fight from a mathematical standpoint in figuring SCR or DCR. Your cam is the brains of the engine, that's its job. General rule on being able to use pump gas, 9:5 max w/ iron heads, 10:5 max w/alum heads. YOUR DCR will always be a set figure. if you want to figure SCR/DCR, you need to know rod length, intake closing at BDC .050, stroke, combustion chamber, piston volume, bore, deck, head gasket,
What is "intake closing at BDC"?

To calculate DCR, you need top know intake closing angle (After Bottom Dead Center) to determine when compression starts building. So, the camshaft does have a dog in the fight!

Also, not sure it's been mentioned here, but combustion chamber shape and spark advance also have a lot do with how much compression can be tolerated. For an SBC, modern heart-shape chambers don't need as much advance and can handle more compression than D-shaped chambers.

Finally, a large duration camshaft along with high flow intake system and cylinder heads can overfill the cylinders at higher RPMs. This is where you get into >100% volumetric efficiency (VE), which sorta throws SCR and DCR out the window.

Last edited by 55_327; 05-09-2019 at 08:43 AM.
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  #34 (permalink)  
Old 05-09-2019, 09:46 AM
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Originally Posted by 55_327 View Post
What is "intake closing at BDC"?
In a four-cycle motor, there are obviously four cycles.....
Intake
Compression
Power (combustion)
Exhaust
Intake occurs beginning with the piston at roughly top dead center, called the overlap phase because the exhaust valve is just closing as the intake valve is opening, so there is an "overlap" of exhaust and intake going on at the same time. This overlap can be very short or very long, depending on cam timing. As the piston comes down the bore on the intake stroke, with the intake valve opening and with the exhaust valve finally closed, fuel/air mixture will be pushed into the cylinder by atmospheric pressure. The intake valve will reach full-open, with the lifter on the nose of the cam lobe, called maximum lift, at some point just after the piston is halfway down in the bore. At that point, with the piston still decending in the bore, the intake valve will begin closing. The valve will still be open as the piston reaches bottom dead center and reverses itself to travel back up the bore to compress the mixture that has just been pushed into the cylinder by atmospheric pressure. There is weight and there is speed to the slug of fuel/air mixture and if you remember science, you will recall that Newton taught us that "An object in motion tends to remain in motion along a straight line unless acted upon by an outside force." So it is with the slug of air/fuel. It will continue to fill the cylinder until the intake valve closes and stops the filling of the cylinder. Remember that the piston has already reversed itself and is heading back up the bore, so the balancing act for every performance engine builder on the planet is to choose a point where the slug of air/fuel is stopped by the closing valve before the ascending piston begins pushing the slug out the intake valve and back up the intake port. The closing of the intake valve is the most important event in the life of a cam.

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Originally Posted by 55_327 View Post
To calculate DCR, you need top know intake closing angle (After Bottom Dead Center) to determine when compression starts building. So, the camshaft does have a dog in the fight!
Yes it does.

Quote:
Originally Posted by 55_327 View Post
Also, not sure it's been mentioned here, but combustion chamber shape and spark advance also have a lot do with how much compression can be tolerated. For an SBC, modern heart-shape chambers don't need as much advance and can handle more compression than D-shaped chambers.
Yes, some reading on Sir Harry Ricardo may help.

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Originally Posted by 55_327 View Post
Finally, a large duration camshaft along with high flow intake system and cylinder heads can overfill the cylinders at higher RPMs. This is where you get into >100% volumetric efficiency (VE), which sorta throws SCR and DCR out the window.
SCR is a mathematical formula for determining how tightly you squeeze the mixture after it is already in the cylinder and has nothing to do with actual flow into the cylinder in my opinion.

Here is a better explanation of DCR than I can present.....
Why dynamic compression ratio is nearly useless | Matrix Garage
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  #35 (permalink)  
Old 05-09-2019, 01:02 PM
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I'll say this again....Maybe for the last time as I feel like DCR has become a fact when it should be filed in the internet garbage can.

DCR doesn't exist. at least is can't be calcuated like it's explained. Why is this?

The ability of the system working together to create a positive charge(ram air effect) is possible and the Intake Closing point is just one of the factors involved.
So for example, at idle, very little air is entering the chamber. At peak power, it's possible to use the mass of the air flow plus exhaust scavenging to fill the cylinder beyond ambient atmospheric pressure. IE, the same cam, but the amount of air trapped in the cylinder varies alot

So lets say you have this magic number for DCR and you think all is good cause you are running the large cam and "bleeding off" the extra compression(trapped air). What will you do when the engine gets up on the cam and the combination of the Ram Air effect and Exhaust Scavenge starts overfilling the cylinder and it's starts detonating and eating itself up? Nothing that's what. Can't change cams in a running engine!

To add to that, I've never once seem or heard of any cam designer worth his salt ever use DCR to design a cam. EVER. IC yes...As part of the system. There's a reason for that. You can't figure it up like that. The math doesn't work.
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  #36 (permalink)  
Old 05-09-2019, 01:23 PM
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The Dynamic Compression Ratio (DCR) or if you haunt Harley Davidson forums it is called Compensated Compression Ratio (CCR)) not to be confused with Creedance Clearwater Revival. The term CCR is probably more accurate as a true dynamic would take into consideration the throttle position, the actual manifold pressure, RPM, location of the intake valve in its lift profile, rod length, piston speed for each measure step you want for a position in the bore, and tons more of these measures for as small a step in the degrees or their parts of crankshaft rotation. As you can see calculating the true dynamic for a volumetric efficiency (VE) is a daunting task of how to measure these things all at the same time for instances of time and motion. This is not going to be possible outside of super well equipped labs found in major corporations, universities, and government agencies like the military and NASA. This ain't gonna happen in a hot rodders garage or car port.

So what we do at the human species level is search for a simplification that is a reasonably accurate model that includes most of the significant effects. What we know by test is that the single greatest predictor of engine performance is where the intake valve closes in crankshaft degrees the latter being a subset of calculating consumed stroke to that point which puts you squarely into the trigonometry of calculating triangles that at each crank angle so rod length and virtual stroke with these included angles get real important. These are the basic equations that can be used to calculate exact piston location in the bore for any degree of crank rotation, by adding RPM you can calculate the instant velocity of the piston for any point in crank rotation as this velocity is constantly changing as triangles and types of triangles are opened and closed by the crank pin and rod.

The battle with a big cam on the lower RPMs is reverse pumping as the rising piston on the compression stroke creates a greater pressure than found in the inertia of the inflowing mixture, thus pumps what was drawn in on the intake stroke back out again, this is a major element of 'reversion', though not the only contributor. This effect continues until sufficient RPMs are reached such that the mixture velocity in the intake system gains enough inertia to overcome the reversion pressure to force fill the cylinder against the pressures above the rising piston. What happens n the early into the middle RPMs if the cam is very large is there is a great loss of bottom to mid range power because the charge trapped after the intake closes is in terms of density is smaller. So how do you get more power out of less mixture, well you squeeze it harder, a practical application of the Big Bang theory.

We also know that at the other end of the RPM range at high revs the intake system runs out of time to adaquately fill the cylinder so the same reduction in mixture density or mass happens so again how do you get more power out of less mixture, squeeze it harder is again the answer. So paying attention to the DCR/CCR helps carry the top end power as well as improving the bottom end.

The down side is mid RPMs and part throttle can get into detonation problems so you have to compromise this on the street by carefully constructing an ignition curve between the vacuum and centrifugal advances to compromise this zone. This is a difficult place to get right. Right being somewhere that doesn't blow the heads off but still gets decent gas mileage on the street, racing this is just a transition that doesn't need you to knickers all twisted up over it unless this is short track or road course racing.

The 290 horse Goodwrench crate engine is a classic example of getting too much cam for the compression, it's a decently perky street motor with a big appetite for the power it delivers. You have to run so much advance to correct this the motor will almost turn backwards.

By test we know that a DCR/CCR on the street shows best overall performance in a range of 8 to 1 up to 9 to 1. The lower works with SMOG style open chambers of iron where the higher goes well with modern heart shaped chambers of aluminum. Also this gives us ordinary mortals a means of getting to optimum performance across the broad range of operating requirements on the street. For racing where a specific point solution works and especially with higher octane Jelske than you corner gas station has these ratios can be pushed higher.

Bogie
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  #37 (permalink)  
Old 05-09-2019, 02:18 PM
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I'll say this again....Maybe for the last time as I feel like DCR has become a fact when it should be filed in the internet garbage can.

DCR doesn't exist. at least is can't be calcuated like it's explained. Why is this?

The ability of the system working together to create a positive charge(ram air effect) is possible and the Intake Closing point is just one of the factors involved.
So for example, at idle, very little air is entering the chamber. At peak power, it's possible to use the mass of the air flow plus exhaust scavenging to fill the cylinder beyond ambient atmospheric pressure. IE, the same cam, but the amount of air trapped in the cylinder varies alot

So lets say you have this magic number for DCR and you think all is good cause you are running the large cam and "bleeding off" the extra compression(trapped air). What will you do when the engine gets up on the cam and the combination of the Ram Air effect and Exhaust Scavenge starts overfilling the cylinder and it's starts detonating and eating itself up? Nothing that's what. Can't change cams in a running engine!

To add to that, I've never once seem or heard of any cam designer worth his salt ever use DCR to design a cam. EVER. IC yes...As part of the system. There's a reason for that. You can't figure it up like that. The math doesn't work.
Right On!!!Hubba Hubba

Add in TIME, as in amount of time a detonation prone piston-to-head clearance distance(.100-.150" thick) is present during the burn cycle changes with speed(rpm).
DCR is just another calculation tool for the at-home builder...but it should be about 10th on the importance list, and not #! like so many on the internet have raised it up to be.
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  #38 (permalink)  
Old 05-09-2019, 09:44 PM
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Don't build my motor.

No desire to. But, Built mine with 11.75 SCR and safely run 91 octane all day long.

Last edited by Greg T; 05-09-2019 at 09:57 PM.
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  #39 (permalink)  
Old 05-09-2019, 10:15 PM
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No desire to. But, Built mine with 11.75 SCR and safely run 91 octane all day long.
Please share with us how you did that Greg.
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  #40 (permalink)  
Old 05-10-2019, 07:39 AM
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Please share with us how you did that Greg.

with a .037" Quench, aluminum heads, the attached cam installed 2 retarded, 170 thermostat, max 35 total ignition timing all in by 3k rpm, 3.5k rpm converter and 4.56:1 rear. It's not working too hard and doesn't run hot. I've attached a dyno sheet from two years ago as well. This a chassis dyno. I run ethanol-free 91 premium and have not had issues, except for the fuel pressure drops I've discussed last year.
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  #41 (permalink)  
Old 05-10-2019, 12:54 PM
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with a .037" Quench, aluminum heads, the attached cam installed 2 retarded, 170 thermostat, max 35 total ignition timing all in by 3k rpm, 3.5k rpm converter and 4.56:1 rear. It's not working too hard and doesn't run hot. I've attached a dyno sheet from two years ago as well. This a chassis dyno. I run ethanol-free 91 premium and have not had issues, except for the fuel pressure drops I've discussed last year.
That would be how. Wonder how that would do on Banning pass on a 117 degree day?

Bogie
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  #42 (permalink)  
Old 05-10-2019, 01:48 PM
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That would be how. Wonder how that would do on Banning pass on a 117 degree day?

Bogie

Fortunately, I won't have to find out. We usually have a couple of days of high 80s around here in the summer. I think last year they were a Tuesday and Friday. But seriously, This little 388 runs very strong this way and once I get the fuel pressure drop completely fixed I'll be happy camper.
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