DCR and Octane
06-05-2008, 02:48 AM
#41
Staging Lane
My .02 is that SCR , camshaft events and cranking compression test are really the only way to really tell how much you can run with pump gas. I have had engines with 11.7 -1 with big cams (245 @ .050" and up) run just fine. These would pump below 195 psi on the cranking compression test. Now other engines that may not be as high SCR but a much smaller, more efficient camshaft thats pumps higher (220 psi and up) will have a harder time utilizing 91 octane fuel. In other words it's going to want much less timing. I have personally seen a small cam gen 1 SBC with 11.5-1 pump 285 psi!!! Torquey as hell made great power but could only take about 25 degrees of total timing. The engine would damn near light off without the ignition turned on!
I guess what I am saying is that using SCR, Cam size, Valve timing events and cranking compression are a better way of seeing how a engine will take pump gas and most of that comes with experience. IMO there are too many variables in "calculating" DCR. Head flow, intake tract flow characteristics, exhaust system (to name a few) will all effect "DCR".
According to this link http://members.uia.net/pkelley2/DynamicCR.html given above, DCR is calculated using valve closing events, crank stroke, rod length so on a so forth. This is fine when the engine is on the stand in your garage but when it's running those events are happening SO fast even when at cruising speed that there is no way those numbers will apply to the engine in real world conditions. The air flowing through the intake tract is moving so fast that it will not fill the cylinder that same as is does sitting on the stand, therefore rendering the "DCR" calculations useless.
The only way to actually measure real DCR is with sensors in the combustion chamber to measure actual cylinder pressure. So unless you have Indy or F1 racing technology at your disposal you probrobley would not be able to accurately measure DCR.
So using SCR, Cranking compression test, Valve timing events and some experiance will give you the best guesstimate to how far you can push pump gas.
I guess what I am saying is that using SCR, Cam size, Valve timing events and cranking compression are a better way of seeing how a engine will take pump gas and most of that comes with experience. IMO there are too many variables in "calculating" DCR. Head flow, intake tract flow characteristics, exhaust system (to name a few) will all effect "DCR".
According to this link http://members.uia.net/pkelley2/DynamicCR.html given above, DCR is calculated using valve closing events, crank stroke, rod length so on a so forth. This is fine when the engine is on the stand in your garage but when it's running those events are happening SO fast even when at cruising speed that there is no way those numbers will apply to the engine in real world conditions. The air flowing through the intake tract is moving so fast that it will not fill the cylinder that same as is does sitting on the stand, therefore rendering the "DCR" calculations useless.
The only way to actually measure real DCR is with sensors in the combustion chamber to measure actual cylinder pressure. So unless you have Indy or F1 racing technology at your disposal you probrobley would not be able to accurately measure DCR.
So using SCR, Cranking compression test, Valve timing events and some experiance will give you the best guesstimate to how far you can push pump gas.
07-04-2008, 12:31 AM
#42
I don't think that anybody's disputing that there's more to this than a simple DCR calculation. For the average street engine builder however, it will get you a good idea of what kind of characteristics you're looking at.
Once I've selected the cam that will give the power range I'm after, I figure out what static compression ratio I need to build to in order for it to work. I do use a DCR calculator to determine what SCR I need. Now keep in mind this is for NA gasonline applications and as stated earlier with increased VE the DCR is not going to give enough information.
Combined with a reasonable quench, it should give you an idea if it will run on pump gas or not. I don't dispute what's been posted here, but a person trying to figure out if they're car is going to run on 91 octane gas probably isn't following the same plan of a builder who's building prostock engines and for their purposes I feel a DCR calculator will give them the information they need to order the right parts the first time.
Of course this IS the advanced tech section so maybe I'm just thinking a bit too simple at the moment.
Once I've selected the cam that will give the power range I'm after, I figure out what static compression ratio I need to build to in order for it to work. I do use a DCR calculator to determine what SCR I need. Now keep in mind this is for NA gasonline applications and as stated earlier with increased VE the DCR is not going to give enough information.
Combined with a reasonable quench, it should give you an idea if it will run on pump gas or not. I don't dispute what's been posted here, but a person trying to figure out if they're car is going to run on 91 octane gas probably isn't following the same plan of a builder who's building prostock engines and for their purposes I feel a DCR calculator will give them the information they need to order the right parts the first time.
Of course this IS the advanced tech section so maybe I'm just thinking a bit too simple at the moment.
Last edited by itchygomey98; 07-04-2008 at 01:07 AM.
11-09-2008, 01:58 PM
#43
Sorry to bring this thread back from the grave. Everyone on here would tell me to search if I started a new thread, so here is my proof at least that I searched!
I read through this whole thread looking for a 'rule of thumb' approach to deciding what is the lowest octane I can get away with, without worries. I've tried to set this motor up to run on the shittiest gas possible since I'm often out needing gas where only 5-ers are available that have had gas sitting in them for months in the back shed. A fresh supply of 91-93 is not always available to me.
I'm still in school for my mechanical engineering degree so I know my way around formulas and plenty of math. I've been taught to take a systematic approach to problems such as these throughout my time in school so I'm wondering if there is a safe way to say something like this:
(By the way I'm pulling these numbers out of my ***)
7.0-7.5 DCR = 85-87 octane
7.8-8.0 DCR = 87 octane
8.0-8.2 DCR = 89 octane
8.2-8.5 DCR = 91 octane
8.5-X.X DCR = 93 octane or race gas...ect ect
I have no idea if these numbers are correct, I'm shooting from the hip based on the few threads I've read on "dynamic compression", even though I am in agreement that nothing about it is really dynamic other than what we put into the motor (IAT, grade/quality of gasoline and timing).
For those wondering here is my build:
Tahoe 5.3L bored to 5.7L
3.898 stock 99mm bore
3.622 stock 92mm stroke
6.098 stock rods
241 heads milled to 66cc (from 66.66cc), stock valvetrain
.052 stock GM replacement head gasket
Speed pro flat top LS1 replacement pistons (no valve reliefs, perfectly flat)
10.3:1 SCR
Truck intake and all accessories.
Currently tuned on stock 5.3L timing, more to come later if allowable.
CompCams XR259 (206/212, .515/.522, 112 LSA+2, 110 ICL) Duration 259 intake, 265 exhaust, IVO 20*, IVC 59*, EVO 67*, EVC 18*.
Cam card, http://www.compcams.com/Cam_Specs/Ca...csid=1084&sb=2
The 5.7L is built for brute low end torque and comfortable daily transportation. It's a DD truck motor, nothing else. Using the available DCR calculators online, I come up with a "dynamic stroke" of 2.94 and a DCR of 8.56. I'd prefer if I could fill up on 87, think it's possible? It is my understanding that timing could be pulled to make it run on 87 which if that is true, I could tune the low octane spark tables to save the motor and any time I am the one filling it up on 89-91 it'll revert back to the high octane spark table, correct?
I read through this whole thread looking for a 'rule of thumb' approach to deciding what is the lowest octane I can get away with, without worries. I've tried to set this motor up to run on the shittiest gas possible since I'm often out needing gas where only 5-ers are available that have had gas sitting in them for months in the back shed. A fresh supply of 91-93 is not always available to me.
I'm still in school for my mechanical engineering degree so I know my way around formulas and plenty of math. I've been taught to take a systematic approach to problems such as these throughout my time in school so I'm wondering if there is a safe way to say something like this:
(By the way I'm pulling these numbers out of my ***)
7.0-7.5 DCR = 85-87 octane
7.8-8.0 DCR = 87 octane
8.0-8.2 DCR = 89 octane
8.2-8.5 DCR = 91 octane
8.5-X.X DCR = 93 octane or race gas...ect ect
I have no idea if these numbers are correct, I'm shooting from the hip based on the few threads I've read on "dynamic compression", even though I am in agreement that nothing about it is really dynamic other than what we put into the motor (IAT, grade/quality of gasoline and timing).
For those wondering here is my build:
Tahoe 5.3L bored to 5.7L
3.898 stock 99mm bore
3.622 stock 92mm stroke
6.098 stock rods
241 heads milled to 66cc (from 66.66cc), stock valvetrain
.052 stock GM replacement head gasket
Speed pro flat top LS1 replacement pistons (no valve reliefs, perfectly flat)
10.3:1 SCR
Truck intake and all accessories.
Currently tuned on stock 5.3L timing, more to come later if allowable.
CompCams XR259 (206/212, .515/.522, 112 LSA+2, 110 ICL) Duration 259 intake, 265 exhaust, IVO 20*, IVC 59*, EVO 67*, EVC 18*.
Cam card, http://www.compcams.com/Cam_Specs/Ca...csid=1084&sb=2
The 5.7L is built for brute low end torque and comfortable daily transportation. It's a DD truck motor, nothing else. Using the available DCR calculators online, I come up with a "dynamic stroke" of 2.94 and a DCR of 8.56. I'd prefer if I could fill up on 87, think it's possible? It is my understanding that timing could be pulled to make it run on 87 which if that is true, I could tune the low octane spark tables to save the motor and any time I am the one filling it up on 89-91 it'll revert back to the high octane spark table, correct?
11-09-2008, 02:18 PM
#44
Pinning down DCR is like pinning down drivetrain percentage loss on a chassis dyno...you cant. There are too many variables to consider.
Being an ME, you know about safety factors. Always consider them and build for the existing conditions along with future ones.
In your situation make SURE the knock sensor is functional.
DCR as stated above is what the cylinder actually SEES.
Density altitude, carbon buildup, seasonal fuels in gas station tanks are a few to consider.
To run on the ragged edge with DCR being hi IMO its imperative you find a good fuel such as SHell 93 along with a station that the fuel doesnt stay in the ground long in other words a busy station.
In your case I see no need to have that much DCR, you want torque right? Timing is a big factor in producing bottom end torque. You will not be able to run much initial timing with that setup.
WHat is your deck height and quench?
Being an ME, you know about safety factors. Always consider them and build for the existing conditions along with future ones.
In your situation make SURE the knock sensor is functional.
DCR as stated above is what the cylinder actually SEES.
Density altitude, carbon buildup, seasonal fuels in gas station tanks are a few to consider.
To run on the ragged edge with DCR being hi IMO its imperative you find a good fuel such as SHell 93 along with a station that the fuel doesnt stay in the ground long in other words a busy station.
In your case I see no need to have that much DCR, you want torque right? Timing is a big factor in producing bottom end torque. You will not be able to run much initial timing with that setup.
WHat is your deck height and quench?
Last edited by FASTFATBOY; 11-09-2008 at 02:24 PM.
11-09-2008, 02:42 PM
#45
Thanks for the quick reply.
Stock deck height, thankfully the block was square during the machine work, no decking needed. Quench is stock since the Speed Pro pistons are OEM replacements and I'm using the stock LS1 head gasket. I did not measure piston out of the hole, but if all things are stock I assume .006" out of the hole, that's a guess. 0.052 gasket - .006 out of the hole...= .046 quench (**educated guess**).
The motor is already built and I can't change anything, so I'm willing to do what is necessary to make it work. When available, I will fill up with 89-91/92 good gas but it will not always be available to me. Highest around here in MN I've seen is 92 but only at a handful of stations. I have BP, Shell, Holiday and Super America stations around me.
And to take care of a few more variables...my cooling system is overkill. To say the least, it's about twice that of an ordinary Tahoe but in a lightweight Sonoma. It is overkill in terms of radiator volume, CFM through the radiator and quality parts within the system. The motor is brand new, so no carbon build up, MN altitude is around 800ft.
Stock deck height, thankfully the block was square during the machine work, no decking needed. Quench is stock since the Speed Pro pistons are OEM replacements and I'm using the stock LS1 head gasket. I did not measure piston out of the hole, but if all things are stock I assume .006" out of the hole, that's a guess. 0.052 gasket - .006 out of the hole...= .046 quench (**educated guess**).
The motor is already built and I can't change anything, so I'm willing to do what is necessary to make it work. When available, I will fill up with 89-91/92 good gas but it will not always be available to me. Highest around here in MN I've seen is 92 but only at a handful of stations. I have BP, Shell, Holiday and Super America stations around me.
And to take care of a few more variables...my cooling system is overkill. To say the least, it's about twice that of an ordinary Tahoe but in a lightweight Sonoma. It is overkill in terms of radiator volume, CFM through the radiator and quality parts within the system. The motor is brand new, so no carbon build up, MN altitude is around 800ft.
