11/1 compression 8 psi vs 8.4/1 with 15 psi, same dynamic is one more or less safe?
06-12-2014, 12:23 AM
#1
11/1 compression 8 psi vs 8.4/1 with 15 psi, same dynamic is one more or less safe? If you Have two motors that have the same dynamic compression and effective boost compression is one or the other more or less safe, or is there no difference... Assuming iat and fuel are kept the same? I get that people recommend building ls1s to 9.5 or lower in order to run more boost, but is it just as safe to run a higher static compression if the dynamic is kept the same as the lower compression motor when putting less boost in... I am playing with the idea of doing a procharger and am wondering if running 8 psi would be a safe option at my 11.01/1 static compression... The intake duration at .006 is 297 so my cam specs out at 7.81 dynamic with a 78.5 degree ivc 53@50 ABDC and with 8 psi it puts me at 12.06 effective boost compression... this is roughly the same(actually a bit lower) than a 8.4 static compression 408 motor with a 243/263 116+5 cam running 15 psi making 900rwhp... I don't think i would make near as much as that motor, but if i keep the dynamic and effective boost compression the same will my setup be just as safe? I'd like to get to 700rwhp if possible... since i'm at 500 with my current setup i was thinking that a i-1 of f-1 might do the trick? i could lower my static to 10.54 by going from a .040 to a .060 head gasket, but i'm wondering if that would mess up the quench? What are people's thoughts? thanks for any input
06-12-2014, 01:31 PM
#2
11/1 compression 8 psi vs 8.4/1 with 15 psi, same dynamic is one more or less safe?
Bleeding off static compression with high duration cam to achieve lower DCR is a very poor use of power. Whats the point of running higher compression if you bleed off all the benefits?
Drop your compression as much as possible and run more boost to reach your goal. What fuel are you running? What motor? What heads?
06-12-2014, 02:48 PM
#3
all the mods for my setup are in my sig... heads flow a bit better than stock ls7 heads... i'm not looking for a max effort boost setup, i'm really just asking if my setup with 8 psi would be no different in reliability than the 8.4 setup running 15 psi... i'd be happy if i could get close to 700 rwhp... the car is a cadillac and at the power i make now is already kinda ridiculous, plus if i run less boost i wont have to worry as much about iat... the kits procharger spec all come in around 7-8psi so i'm just wondering if i can run than safely with no issues just like they do on stock motors
06-12-2014, 03:06 PM
#5
I understand that and I answered your question. The higher compression lower boost setup will be more prone to detonation and have higher cylinder pressures at the same power levels. 8.4:1 @ 15psi will have less overall cylinder pressure/heat than an 11:1 engine at 8psi. They are not the same thing. Boost is more efficient at making power than compression is.
The reason you care about IAT ultimately is lower CC temperatures. Raising static compression raises CC temperatures much more than a few lbs of boost will. Regardless of what your IAT sensor says, the CC will be happier with more boost and less compression.
The reason you care about IAT ultimately is lower CC temperatures. Raising static compression raises CC temperatures much more than a few lbs of boost will. Regardless of what your IAT sensor says, the CC will be happier with more boost and less compression.
06-12-2014, 03:35 PM
#6
ah ok i understand, thanks for clearing it up for me. I've looked at the cam specs on stock ls3 motors and with their static compression of 10.7 they have effective boost compressions of close to 15/1 running the kit straight from procharger at the 7.5 psi procharger designs the kit for. At 8psi with my cam and static compression i come to just under 12/1. It looks like i will have no problem at at running 8psi safely. What do you think?
06-12-2014, 03:42 PM
#7
I’m not a good person to ask, I go the other route with my setups. Personally wouldn’t run 11:1 on pump gas with any kind of boost.
Lots of guys do it your way too. Motor doesn’t have to be efficient to make a butt-ton of power. Steve Morris usually runs big cams and big timing on his boosted motors. Shoot him an email. http://www.stevemorrisengines.com/
May check around on the yellowbullet forums if your not having luck here.
Good luck!
Lots of guys do it your way too. Motor doesn’t have to be efficient to make a butt-ton of power. Steve Morris usually runs big cams and big timing on his boosted motors. Shoot him an email. http://www.stevemorrisengines.com/
May check around on the yellowbullet forums if your not having luck here.
Good luck!
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06-12-2014, 03:56 PM
#8
ok thanks. I know lots of people are running 10 psi with no problems on the 5.0 coyote motors. a friend of mine has an auto that runs consistent 102-10.4 at 133-135mph. they have 11/1 as well. granted they have better valve geometry than an ls1 but still 10 psi on a 11/1 motor on pump with no meth, race gas or problems seems like an easy thing nowdays
06-12-2014, 04:23 PM
#10
I understand that and I answered your question. The higher compression lower boost setup will be more prone to detonation and have higher cylinder pressures at the same power l evels. 8.4:1 @ 15psi will have less overall cylinder pressure/heat than an 11:1 engine at 8psi. They are not the same thing. Boost is more efficient at making power than compression is.
The reason you care about IAT ultimately is lower CC temperatures. Raising static compression raises CC temperatures much more than a few lbs of boost will. Regardless of what your IAT sensor says, the CC will be happier with more boost and less compression.
The reason you care about IAT ultimately is lower CC temperatures. Raising static compression raises CC temperatures much more than a few lbs of boost will. Regardless of what your IAT sensor says, the CC will be happier with more boost and less compression.
06-12-2014, 04:59 PM
#12
Lower static compression with more boost will GENERALLY be safer and make more power when octane limited and dynamic compression is just one factor. Of course there are tons of factors there but generally speaking...
Just look at what the OEMs do, it's not by accident that they all run lower compression boosted motors compared to their N/A counterparts. If I tried to run my GXP at 10 psi on just our crappy 91 octane our here in AZ with our temperatures it would end up in pieces even with timing in the single digits.
Just look at what the OEMs do, it's not by accident that they all run lower compression boosted motors compared to their N/A counterparts. If I tried to run my GXP at 10 psi on just our crappy 91 octane our here in AZ with our temperatures it would end up in pieces even with timing in the single digits.
06-12-2014, 05:31 PM
#13
but if the two motors have the exact same cylinder temps and pressure, then they are equally safe even though one has a higher static correct... after all is said and done its the pressure and temps inside the combustion chamber that affects detonation and reliability no?
06-12-2014, 05:52 PM
#14
theoretically i should be able to run the same timing as a low compression high boost motor in a high compression low boost motor if both the combustion chambers have the same cylinder pressure and temperature during combustion right? that's why i'm asking this question... with my static compression and valve events i have a 7.8 dynamic compression... with 8 psi of boost and my current setup i will have the exact same cylinder pressure and that 408 8.4 compression motor at 15 lbs making 900rwhp... obviously i will not make 900 but i will be close to to 700 with the same internal cc temps and conditions correct? the 408 has 8.4 compression with a cam with an ivc of 80 degrees has 5.94 dynamic, this gives it an effective boost compression of 12/1 at 15 psi... my motor with my 11.01 static and an ivc of 78.5 degrees has a 7.8 dynamic and at 7.5 psi an effective boost compression of 11.79/1 which is lower than the low compression motor after boost or if i do 8 psi its 12.06 which is very close... this is why i think i should be able to run 8 psi on my setup and have it be just as safe as that 8.4 comp motor running 15 psi. does this make sense?
06-12-2014, 06:13 PM
#15
the stock ls3 motor with 7.5 psi is at 12.63 effective boost compression and this is what procharger spec their system for and sell it at... i think they probably have plenty of r&d on whether or not it is safe on a stock ls3 at 7.5 psi and 10.7 static, the ls3 has around 8.3 dynamic with stock cam. so my setup with 11.01 static and 7.8 dynamic should take 8 psi with no problems right?
06-12-2014, 08:21 PM
#16
I prefer medium to high compression with boost on a street car and a race car. We run 9-12 psi all the time on stock ls3 engines with a cam. Most of them have meth. We keep it around 9 on straight pump. I daily drive a 11 ctsv that we bumped the compression from 9:1 to 10:6. I am timing limited on pump gas at 14 psi but even with the soft timing it still dynos 670whp thru a 6l90. I feel ill make 730whp with post boost meth when we get time to install it. Unless you are trying to set the world on fire with 91 octane I would keep the compression up. I tune a few race cars with 12:1-12.5 that run north of 30psi obviously on race gas.
06-12-2014, 08:44 PM
#17
I prefer medium to high compression with boost on a street car and a race car. We run 9-12 psi all the time on stock ls3 engines with a cam. Most of them have meth. We keep it around 9 on straight pump. I daily drive a 11 ctsv that we bumped the compression from 9:1 to 10:6. I am timing limited on pump gas at 14 psi but even with the soft timing it still dynos 670whp thru a 6l90. I feel ill make 730whp with post boost meth when we get time to install it. Unless you are trying to set the world on fire with 91 octane I would keep the compression up. I tune a few race cars with 12:1-12.5 that run north of 30psi obviously on race gas.
06-13-2014, 08:58 AM
#19
IMO starting out at 11:1 static is too high for a forced induction pump gas setup. A big cam effectively reduces the DCR because it delays the intake valve closing point. The longer you delay this past the optimal point, the more problems you’ll run into. Timing, reversion, poor cylinder filling etc.
Because a cam calculator spit out 7.8:1 DCR does not mean your DCR will be 7.8:1 your entire RPM range. The majority of the DCR drop effect caused by a large cam will happen at low engine speeds. As RPM increases, the DCR will increase as well. I think this will get you into trouble when mixed with boost.
Sounds like your trying to use the cam as a crutch when what you really need to do is drop your static compression. The cam should be matched to your engine and desired power band. What you plan on doing is VERY counterproductive and I don’t think you’ll be happy with the results.
Because a cam calculator spit out 7.8:1 DCR does not mean your DCR will be 7.8:1 your entire RPM range. The majority of the DCR drop effect caused by a large cam will happen at low engine speeds. As RPM increases, the DCR will increase as well. I think this will get you into trouble when mixed with boost.
Sounds like your trying to use the cam as a crutch when what you really need to do is drop your static compression. The cam should be matched to your engine and desired power band. What you plan on doing is VERY counterproductive and I don’t think you’ll be happy with the results.
