Flex fuel build - 91 octane stability under high load
Flex fuel build - 91 octane stability under high load Hey all.
I am thinking about making my next engine a flex fuel build. I'll be running it off a Holley HP EFI.
I have access to 91 octane and E85 in California. I'd likely run an E50 mix most of the time.
The engine will be 4.130 x 4.00 LS7 based shortblock.
The camshaft will likely have an IVC of 76-82 setting the DCR.
I would like to target 13:1 with a slight dome on the piston and .040 quench.
If running purely 91 octane I would target 11.7:1.
Under full power I can always pull timing and add fuel until it is stable. My main concern in a street car will be part throttle high load cruise at stoich when I'm running pure 91 octane. I don't want the thing to rattle it's brains out when I tip into the gas on the freeway at 2200 rpm.
I am thinking about making my next engine a flex fuel build. I'll be running it off a Holley HP EFI.
I have access to 91 octane and E85 in California. I'd likely run an E50 mix most of the time.
The engine will be 4.130 x 4.00 LS7 based shortblock.
The camshaft will likely have an IVC of 76-82 setting the DCR.
I would like to target 13:1 with a slight dome on the piston and .040 quench.
If running purely 91 octane I would target 11.7:1.
Under full power I can always pull timing and add fuel until it is stable. My main concern in a street car will be part throttle high load cruise at stoich when I'm running pure 91 octane. I don't want the thing to rattle it's brains out when I tip into the gas on the freeway at 2200 rpm.
I don't know if this will help you but I'm running the same bore and stroke at 12:1 on pure 91. Similar strategy. I am running DCR of 8.3. I have had zero issues even nailing it at 2000 rpm in fourth. You have to be conservative on timing but I think you know that already
That does help. It is nice to know 12:1 is working well for you on 91. Have you dyno'd it on 100+ to see if you're octane limited? 8.3 DCR is a happy spot for 91 octane.
Does Cammotion list their advertised duration specs anywhere? I would think for a 427 I'd have an IVC in the region of 50-52 @ .050. How would you estimate "hot advertised" duration of a solid roller? As a guess that'd put DCR at an estimated 9.1-9.3:1.
For the amount I drive the car I know over time I'd get tired of adding octane booster. I'd have to have a pallet of the stuff in the garage. I would really only want to add octane booster if I was going to drive the car hard. It needs to be DD mode on 91 octane, just with a reduced timing map.
I know this is an odd question...it isn't about power or stability at WOT, it is about being able to simply drive the car and not detonate under high vacuum high load conditions. I *think* I'd be able to detune it enough to run WOT on 91 at 13:1. More worried about the part throttle driving honestly.
If I overbuilt the engine I suppose I could throw a water injection kit on it for running 91.
Does Cammotion list their advertised duration specs anywhere? I would think for a 427 I'd have an IVC in the region of 50-52 @ .050. How would you estimate "hot advertised" duration of a solid roller? As a guess that'd put DCR at an estimated 9.1-9.3:1.
For the amount I drive the car I know over time I'd get tired of adding octane booster. I'd have to have a pallet of the stuff in the garage. I would really only want to add octane booster if I was going to drive the car hard. It needs to be DD mode on 91 octane, just with a reduced timing map.
I know this is an odd question...it isn't about power or stability at WOT, it is about being able to simply drive the car and not detonate under high vacuum high load conditions. I *think* I'd be able to detune it enough to run WOT on 91 at 13:1. More worried about the part throttle driving honestly.
If I overbuilt the engine I suppose I could throw a water injection kit on it for running 91.
Piston Pin Offset = OEM Hi Spanks, I too agree with Jake AND run 12:1 often on Pump 91.
The reason this works is TWO FOLD, the first reason is the LOW R/L specified with a 4.00" stroke crankshaft. (1.53)
The second reason this works for me is the Piston Pin is "offset" as to the OEM side reducing piston rock.
This moves the piston AWAY from TDC combustion FASTER than an "on center"
position.
THUS if BOTH items are understood/used/fitted the engineering will state a HIGHER Knock Resistance.
Lance
The reason this works is TWO FOLD, the first reason is the LOW R/L specified with a 4.00" stroke crankshaft. (1.53)
The second reason this works for me is the Piston Pin is "offset" as to the OEM side reducing piston rock.
This moves the piston AWAY from TDC combustion FASTER than an "on center"
position.
THUS if BOTH items are understood/used/fitted the engineering will state a HIGHER Knock Resistance.
Lance
Thanks Lance! Thats one of those deals where my gut ran counter to what I can usually find.
@Spanks, no dyno yet. Still a new motor and still trying to find weaknesses before I get to a dyno and find them the hard way. As to cam motion ramp rates, I use 54 for quick maths. It usually works out.
You would need to move your IVC out further still to get DCR down. Without my cheat sheets opened, I think you need to be near 60 degrees IVC at 050, and even with all that compression I think you might be asking for trouble. When the engine comes into its own NOT under part throttle, your cylinder pressures will get very high. Even with conservative timing. I would probably draw the line at 12.3 unless I knew I would always be running alcohol. But that's me.
I think you will find you are always at part throttle unless at the track with stickies. Based on my airflow and fueling numbers - and assuming you end up the same - you cannot take full advantage on street tires on the street. Until fifth gear. So, there is my best recommendation for you
@Spanks, no dyno yet. Still a new motor and still trying to find weaknesses before I get to a dyno and find them the hard way. As to cam motion ramp rates, I use 54 for quick maths. It usually works out.
You would need to move your IVC out further still to get DCR down. Without my cheat sheets opened, I think you need to be near 60 degrees IVC at 050, and even with all that compression I think you might be asking for trouble. When the engine comes into its own NOT under part throttle, your cylinder pressures will get very high. Even with conservative timing. I would probably draw the line at 12.3 unless I knew I would always be running alcohol. But that's me.
I think you will find you are always at part throttle unless at the track with stickies. Based on my airflow and fueling numbers - and assuming you end up the same - you cannot take full advantage on street tires on the street. Until fifth gear. So, there is my best recommendation for you
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Hi Spanks, I too agree with Jake AND run 12:1 often on Pump 91.
The reason this works is TWO FOLD, the first reason is the LOW R/L specified with a 4.00" stroke crankshaft. (1.53)
The second reason this works for me is the Piston Pin is "offset" as to the OEM side reducing piston rock.
This moves the piston AWAY from TDC combustion FASTER than an "on center"
position.
THUS if BOTH items are understood/used/fitted the engineering will state a HIGHER Knock Resistance.
Lance
The reason this works is TWO FOLD, the first reason is the LOW R/L specified with a 4.00" stroke crankshaft. (1.53)
The second reason this works for me is the Piston Pin is "offset" as to the OEM side reducing piston rock.
This moves the piston AWAY from TDC combustion FASTER than an "on center"
position.
THUS if BOTH items are understood/used/fitted the engineering will state a HIGHER Knock Resistance.
Lance
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Verdad Gallardo Thanks Lance! Thats one of those deals where my gut ran counter to what I can usually find.
@Spanks, no dyno yet. Still a new motor and still trying to find weaknesses before I get to a dyno and find them the hard way. As to cam motion ramp rates, I use 54 for quick maths. It usually works out.
You would need to move your IVC out further still to get DCR down. Without my cheat sheets opened, I think you need to be near 60 degrees IVC at 050, and even with all that compression I think you might be asking for trouble. When the engine comes into its own NOT under part throttle, your cylinder pressures will get very high. Even with conservative timing. I would probably draw the line at 12.3 unless I knew I would always be running alcohol. But that's me.
I think you will find you are always at part throttle unless at the track with stickies. Based on my airflow and fueling numbers - and assuming you end up the same - you cannot take full advantage on street tires on the street. Until fifth gear. So, there is my best recommendation for you
@Spanks, no dyno yet. Still a new motor and still trying to find weaknesses before I get to a dyno and find them the hard way. As to cam motion ramp rates, I use 54 for quick maths. It usually works out.
You would need to move your IVC out further still to get DCR down. Without my cheat sheets opened, I think you need to be near 60 degrees IVC at 050, and even with all that compression I think you might be asking for trouble. When the engine comes into its own NOT under part throttle, your cylinder pressures will get very high. Even with conservative timing. I would probably draw the line at 12.3 unless I knew I would always be running alcohol. But that's me.
I think you will find you are always at part throttle unless at the track with stickies. Based on my airflow and fueling numbers - and assuming you end up the same - you cannot take full advantage on street tires on the street. Until fifth gear. So, there is my best recommendation for you
I don't like handicapping cam selection simply to run higher compression.
I think if I go down this path I'll plan for having both aux injection and flex fuel. Otherwise I'll keep it at 11.7-12.1:1.
12:1 is nothing to sneeze at...that's quite a bit of squeeze.
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Under full power I can always pull timing and add fuel until it is stable. My main concern in a street car will be part throttle high load cruise at stoich when I'm running pure 91 octane. I don't want the thing to rattle it's brains out when I tip into the gas on the freeway at 2200 rpm.
Just keep in mind, DCR is a very limited formula as it does not take volumetric efficiency into consideration. So, while guys with similar engines can successfully use it as reference, just understand that intake tract cross section, intake runner length and cam timing all effect the volumetric efficiency curve and hence cylinder pressures.
For our standard XA, 55mm hydraulic roller lobes, you can add about 55 degrees to the .050" duration to get the .006 or "advertised" duration.
Hey Spanks13, part throttle can easily be handled with timing and proper fuel calibration also. Part throttle is actually easier because when the throttle blades are restricting flow, the volumetric efficiency is very low. This equates to much lower cylinder pressures. So, if your part throttle calibration is good, you will have no part throttle pre-ignition problems.
Just keep in mind, DCR is a very limited formula as it does not take volumetric efficiency into consideration. So, while guys with similar engines can successfully use it as reference, just understand that intake tract cross section, intake runner length and cam timing all effect the volumetric efficiency curve and hence cylinder pressures.
For our standard XA, 55mm hydraulic roller lobes, you can add about 55 degrees to the .050" duration to get the .006 or "advertised" duration.
Just keep in mind, DCR is a very limited formula as it does not take volumetric efficiency into consideration. So, while guys with similar engines can successfully use it as reference, just understand that intake tract cross section, intake runner length and cam timing all effect the volumetric efficiency curve and hence cylinder pressures.
For our standard XA, 55mm hydraulic roller lobes, you can add about 55 degrees to the .050" duration to get the .006 or "advertised" duration.
Thanks for the feedback!
I've thought about things some more and I think my best bet is just going all in with an aux injection system + flex fuel if I'm looking to push the compression on 91 octane. If there is a really bad window I can always spray some water or water/meth based on MAP and RPM.