With these new higher CR builds to-be and in the works and some already out, I wonder if we'll start seeing more crank failures or worse yet, block failures?
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Compresion ratio ?
- Thread starter TrentNell
- Start date
Oh boy now i am just 
, all the theroys make sence to me and every one makes good points ,and i agree that compression is great...................... if the engine can take it , my problem is I cant afford to spend money on theroy's , a motor will have to last me at least 1 full season of racing and pulling , i have no problem doing re-freshing every year . And if i was going full tilt build with forged pistons and the whole nine yards i might try it , but this is a pretty basic build , rods , cut pistons , key ways , stock -mild cam and heads , and parts needed to reassemble ect .
I will just have to think on it , any other info theroy's or idea's or advice from those who have done it are more than welcome .
oh ya ................... my target boost is going to be 80-90 psi but may be as low as 70 depending on what turbo up-grades happen by spring and how much fuel i decide to give it , so 70-90 .
Edit: target HP is 800 -850 with occasional spikes in the 900 range .
, all the theroys make sence to me and every one makes good points ,and i agree that compression is great...................... if the engine can take it , my problem is I cant afford to spend money on theroy's , a motor will have to last me at least 1 full season of racing and pulling , i have no problem doing re-freshing every year . And if i was going full tilt build with forged pistons and the whole nine yards i might try it , but this is a pretty basic build , rods , cut pistons , key ways , stock -mild cam and heads , and parts needed to reassemble ect .I will just have to think on it , any other info theroy's or idea's or advice from those who have done it are more than welcome .
oh ya ................... my target boost is going to be 80-90 psi but may be as low as 70 depending on what turbo up-grades happen by spring and how much fuel i decide to give it , so 70-90 .
Edit: target HP is 800 -850 with occasional spikes in the 900 range .
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Once you are up on the charger, static compression ratio is no longer a concern. You can increase the dynamic compression ratio by just adding boost. But until you get up on the charger, the higher compression is going to win.
In a drag racing scenerio if all goes right you should be on top of both the chargers with in the first 200 ft ( shorter if you can launch hard enough ) . So i dont see it being a problem aside from maybe not getting the top HP potential out of the motor .
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I would have to agree....
2 psi boost is equal to 1 C/R as far as power output.
Sooo..
1 C/R = 14.7 X 3 = 44.10 psi need to make up for the 3 point C/R drop.
Anyone have any dyno numbers for low compression vs high compression builds with similar setups?
Something doesn't add up
Take 2 engines the same displacement a 8.0:1 engine with 15 psi of boost it will make way more power than a 10:1 NA engine if I understand right it should take 29.4 psi to make it up
In straight terms of compression ratio
a 6.5 Diesel with 21.5to1 compression
a Duramax 17.5to 1
so it should take 58.8 Psi of boost to make up for it thats not even accounting for the boost on a 6.5
Maybe I am not understanding it right and I know the 6.5 doesn't compare to the design of the duramax but it still doesn't make sense
Take 2 engines the same displacement a 8.0:1 engine with 15 psi of boost it will make way more power than a 10:1 NA engine if I understand right it should take 29.4 psi to make it up
In straight terms of compression ratio
a 6.5 Diesel with 21.5to1 compression
a Duramax 17.5to 1
so it should take 58.8 Psi of boost to make up for it thats not even accounting for the boost on a 6.5
Maybe I am not understanding it right and I know the 6.5 doesn't compare to the design of the duramax but it still doesn't make sense
I have to agree Kevin , but the gasser analagy doesnt aply because of all the fuel needed to balance out the AFR to the extra air added .
Compression does not change the amount of air that goes through the engine, boost does.
Compression squeezes the fuel/air mix into a smaller package, so the chemical reaction happens quicker and more completely since the molecules are closer together. The heat generated will be higher, so the charge expands with more force. The gain shows up pretty much as a direct function of RPM/HP. 10% more power at 1500rpm, and 10% more at 8000rpm. So the peak gain is more HP, but close to the same percentage. High compression likes high RPM.
A spark engine at 10:1 and 15psi boost will make more power than a 9:1 at 15psi. But fuel detonation limits what you can run for boost.
Compression squeezes the fuel/air mix into a smaller package, so the chemical reaction happens quicker and more completely since the molecules are closer together. The heat generated will be higher, so the charge expands with more force. The gain shows up pretty much as a direct function of RPM/HP. 10% more power at 1500rpm, and 10% more at 8000rpm. So the peak gain is more HP, but close to the same percentage. High compression likes high RPM.
A spark engine at 10:1 and 15psi boost will make more power than a 9:1 at 15psi. But fuel detonation limits what you can run for boost.
14.7 is 1 atmosphere
so a 16:1 engine would be 16 atmospheres(14.7psi) compressed into one so 16x14.7=235.2psi.
I think that is right but I could be wrong.
so a 16:1 engine would be 16 atmospheres(14.7psi) compressed into one so 16x14.7=235.2psi.
I think that is right but I could be wrong.
Correct.. Whatever your baro reads for that particular elevation is what 1 ATM is equal to.
Sea level is 14.7 whereas Denver is 12.1 psi.
So you multiply the CR x the baro and you have the static compression in PSI for your particular elevation.
Sea level is 14.7 whereas Denver is 12.1 psi.
So you multiply the CR x the baro and you have the static compression in PSI for your particular elevation.
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Your right. 15,000psi injection pressure on a 6.5 vs 26,000psi injection pressure on an LBZ 23000 on an LB7. Also boost doesn't indicate airflow, only a MAF senson will tell you that. Heads aluminum vs steel and design differences, cam and overall designs could move that much more air IMO.
On Edit: Hense why some people say a duramax is not a duramax unless it has Common Rail. Otherwise the 6.6 is a glorified 6.5. (Which is far from true depending on your defination of glorified.)
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When you lower the static compression you have to increase the boost to get the dynamic compression to match what you had or you've lost power.
Example...
A) 17.5:1 with 45psi of boost is a dynamic compression of 20.5:1
B) 15.5:1 with 45psi of boost is a dynamic compression of 18.5:1
C) 15.5:1 with 75psi of boost is a dynamic compression of 20.5:1
D) 19.5:1 with 45psi of boost is a dynamic compression of 22.5:1
Scenario D is what I like the best. We proved it on the CR Cummins that I posted links to earlier.
The old 6.9 IDI's are 21.5:1 static compression. They are non turbocharged but they have a higher dynamic compression than we do on a stock motor at 45 psi.. Food for thought
Question of the day is.. If we can make free power with compression why is the factory decompressing... I'm assuming for NOX reasons.
Probably figured it would help longevity would be my guess. Same reason I figured my 1984 2500 had 160 hp, when a half ton of the same year had 175 hp