CR's boosted and N/A
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Teching In
Joined: Mar 2017
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CR's boosted and N/A Alright guys I don't claim to know everything and I'm here to learn so please don't eat me alive haha. I've done some searching and I've learned some stuff but nothing that explains my exact question. Any how, I'm building an ls1 from a bored out 5.3 iron block. I only wanted to run 11:1 maybe 11.5:1 CR and was planning on boosting. I was doing some research and figured out that to run more boost (10psi) I should lower the lower the compression ratio to let's say about 9.1:1. I would achieve this by using 317 heads and slightly dished Pistons. Well my understanding is if I have high compression pistons and go the n/a route ending up with 11.5:1 CR I'll make "x" hp. If I lower the the CR to 9.1:1 then boost (10psi) and tune to end up with 11.5:1 CR I'll make more power then the n/a high comp Pistons 11.5:1 CR. Am I understanding this right? And if I am, why? If they both have the same 11.5:1 CR in the end, meaning your still only taking 11.5 units and squeezing it into one, why would the boosted route make more power? Just trying to get all the knowledge possible.
-Cheers
Dillon
-Cheers
Dillon
TECH Resident
Joined: Apr 2014
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Higher compression just means you have less room for tuning error. But all things being equal, a higher compression motor when boosted will make more power. But you need to do a couple of things to keep it safe:
-On 93, you need less timing with more CR, so the lower boosted motor can run more timing which equals more HP per PSI; you may be able to turn the boost up higher with the lower CR motor too before you have issues on pump gas, and that means more power
-With E85 or methanol injection or any kind of high octane race fuel, you can run timing, compression, and boost sky high. This makes the most power.
-I'd recommend as much compression as you can get, running 100% methanol injection, a good intercooler setup, and 93 octane on a street turbo. You may limit yourself a bit, but the efficiency of the motor goes up. The timing advantage diminishes between the setups when you have enough octane. And enough octane for optimized timing and more compression makes for the most efficient setup. And I'd rather make 30+HP per pound of boost than 25.
Of course, a poor tune can destroy the motor. But that goes with either a low or high CR motor. Just read the plugs and back off the timing as warrantied.
If you can't run methanol, I'd go with the lower CR engine as you can make more power with it on pump gas.
-On 93, you need less timing with more CR, so the lower boosted motor can run more timing which equals more HP per PSI; you may be able to turn the boost up higher with the lower CR motor too before you have issues on pump gas, and that means more power
-With E85 or methanol injection or any kind of high octane race fuel, you can run timing, compression, and boost sky high. This makes the most power.
-I'd recommend as much compression as you can get, running 100% methanol injection, a good intercooler setup, and 93 octane on a street turbo. You may limit yourself a bit, but the efficiency of the motor goes up. The timing advantage diminishes between the setups when you have enough octane. And enough octane for optimized timing and more compression makes for the most efficient setup. And I'd rather make 30+HP per pound of boost than 25.
Of course, a poor tune can destroy the motor. But that goes with either a low or high CR motor. Just read the plugs and back off the timing as warrantied.
If you can't run methanol, I'd go with the lower CR engine as you can make more power with it on pump gas.
(Above info is good)
To me, the short/simplified answer is like this: high CR(to a certain point) allows you to produce more power with the same air charge. Boost, at any compression ratio, gets more air in there. Mix with more fuel, more power. High CR won't do anything to get more air in there from the atmosphere, though it will help make more power with whatever you do get into the cylinder.
If normal atmospheric pressure is ~14.7PSI(sea level), and you have 10 PSI positive pressure in the intake tract, in theory you have 10/14.7 more air available to fill the cylinder, or ~68%. Nothing is 100% efficient so there is heat and other losses, but this gives you greater cylinder fill before CR even really comes into play.
CR(static) is only a measurement and speaks to ratio between cylinder volume at BDC and TDC. In the boosted scenario, you have up to 1.68 times more air in there at BDC to start with.
To me, the short/simplified answer is like this: high CR(to a certain point) allows you to produce more power with the same air charge. Boost, at any compression ratio, gets more air in there. Mix with more fuel, more power. High CR won't do anything to get more air in there from the atmosphere, though it will help make more power with whatever you do get into the cylinder.
If normal atmospheric pressure is ~14.7PSI(sea level), and you have 10 PSI positive pressure in the intake tract, in theory you have 10/14.7 more air available to fill the cylinder, or ~68%. Nothing is 100% efficient so there is heat and other losses, but this gives you greater cylinder fill before CR even really comes into play.
CR(static) is only a measurement and speaks to ratio between cylinder volume at BDC and TDC. In the boosted scenario, you have up to 1.68 times more air in there at BDC to start with.
Last edited by Mercier; Mar 4, 2017 at 08:31 AM.
Teching In
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Well i am going to give it a try i have a 12.5 CR 438 (454 block w/427 rolling assembly bored 30 o) engine steel crank Canfield heads. Induction is a tunnel ram with 2 -750 quick fuels. P600B pro charger into a water to air intercooler. I am going to start out with a 4" pulley slow down my compressor speed, bump up my jets to 80 square hope that i get a EGT at 1350. headers start glowing i know to back the timing down and richer up the carbs. Backyard mechanic but it does work. reading the spark plugs is old school it is the best without a computer. I have been on Youtube and other forums i have not seen a boosted tunnel ram with 2 carbs yet... anybody got this set up i would like to check it out.
Well i am going to give it a try i have a 12.5 CR 438 (454 block w/427 rolling assembly bored 30 o) engine steel crank Canfield heads. Induction is a tunnel ram with 2 -750 quick fuels. P600B pro charger into a water to air intercooler. I am going to start out with a 4" pulley slow down my compressor speed, bump up my jets to 80 square hope that i get a EGT at 1350. headers start glowing i know to back the timing down and richer up the carbs. Backyard mechanic but it does work. reading the spark plugs is old school it is the best without a computer. I have been on Youtube and other forums i have not seen a boosted tunnel ram with 2 carbs yet... anybody got this set up i would like to check it out.
