kinger
VIP Member
Looking at another build with a apex and curious on peoples thoughts on how far to lower the compression for big boost.
I understand too low will result in a lazy engine and there is always a balance. I will be using a Motec for control so it helps both sides of the CR equation, I can add timing down low to get snap back or even reduce it with stock cr and still be safe.
I am thinking of running my stage 1 MPI, but that being fed with my Impulse race kit (2860R) for a compound boost set up. So the Stage 1 is at 10-11 psi and makes almost 5lbs at clutch engagement. This would help offset a low CR engine.
I'm trying to balance fuel, so far I was dead set against any build over 91 octane limit. Now knowing Torco does work and some safety measures with the Motec, I am thinking of bending the rules a little bit and running Torco on every tank for roughly 100 octane. Not opposed to meth but its just more complexity on what I'm building that will already be overly complex.
I would have the motec control the Turbo boost and run with a ~1psi wastegate spring which would give me a total boost LOW threshold of 13 psi. Then probably have the turbo run 5-7 psi for a compound boost of around 18-25 psi. Using the compression calculator in the sticky you get the following different scenarios:
7:1 CR - 19 psi of boost you could possibly run 91 and make 320hp, this would be the low hp level, max boost would be 27 psi on ~100 octane around 400hp that is possible with the Torco on top of 91
8:1 CR - 16 psi of boost you could possibly run 91 and make 300hp, this would be the low hp level, max boost would be 24 psi on ~100 octane around 375hp that is possible with the Torco on top of 91
9:1 CR - 13 psi of boost (lowest the sled would go) will allow 91 octane and make 270 hp, max boost would be 20psi on ~100 octane around 340hp that is possible with the Torco on top of 91
The lowest I have seen CR reduced to has been around 7.8:1 and possibly a 7:1 on user Mulot30th?. JE makes a 9:1 and 10:1 piston but I would also add a shim to lower it even further.
This is my ultimate trail sled so I need to be able to ride 200-300 miles pop into a station and fill up. I have no issue carrying the bottles of Torco. Also going to look at rigging up a small rugged laptop that is built into the dash and read CAN data right from the Motec ECU to be able to see knock. etc and it will be able to pull all turbo boost away, the 10-11 psi Stage 1 has been flawless on 87 plus Torco so I know it works in a pinch and would be my min (default) running state.
I am wanting to set it up once and not vary the boost after, and clutch it for max performance and just suffer a slower sluggish (but safe motor) sled if I get bad gas or something.
I also know there is a LOT of variables here and that spreadsheet is just a nice tool, so looking for real world engine builders and riders to share their experiences.
Finally the purpose of the twin charge is to get the SC snap which I love, but with the long lake legs of the Turbo, which I also love. I happen to have both kits already and no one wants to buy them, why not build one last beast with the greatest engine ever to be on snow!
I understand too low will result in a lazy engine and there is always a balance. I will be using a Motec for control so it helps both sides of the CR equation, I can add timing down low to get snap back or even reduce it with stock cr and still be safe.
I am thinking of running my stage 1 MPI, but that being fed with my Impulse race kit (2860R) for a compound boost set up. So the Stage 1 is at 10-11 psi and makes almost 5lbs at clutch engagement. This would help offset a low CR engine.
I'm trying to balance fuel, so far I was dead set against any build over 91 octane limit. Now knowing Torco does work and some safety measures with the Motec, I am thinking of bending the rules a little bit and running Torco on every tank for roughly 100 octane. Not opposed to meth but its just more complexity on what I'm building that will already be overly complex.
I would have the motec control the Turbo boost and run with a ~1psi wastegate spring which would give me a total boost LOW threshold of 13 psi. Then probably have the turbo run 5-7 psi for a compound boost of around 18-25 psi. Using the compression calculator in the sticky you get the following different scenarios:
7:1 CR - 19 psi of boost you could possibly run 91 and make 320hp, this would be the low hp level, max boost would be 27 psi on ~100 octane around 400hp that is possible with the Torco on top of 91
8:1 CR - 16 psi of boost you could possibly run 91 and make 300hp, this would be the low hp level, max boost would be 24 psi on ~100 octane around 375hp that is possible with the Torco on top of 91
9:1 CR - 13 psi of boost (lowest the sled would go) will allow 91 octane and make 270 hp, max boost would be 20psi on ~100 octane around 340hp that is possible with the Torco on top of 91
The lowest I have seen CR reduced to has been around 7.8:1 and possibly a 7:1 on user Mulot30th?. JE makes a 9:1 and 10:1 piston but I would also add a shim to lower it even further.
This is my ultimate trail sled so I need to be able to ride 200-300 miles pop into a station and fill up. I have no issue carrying the bottles of Torco. Also going to look at rigging up a small rugged laptop that is built into the dash and read CAN data right from the Motec ECU to be able to see knock. etc and it will be able to pull all turbo boost away, the 10-11 psi Stage 1 has been flawless on 87 plus Torco so I know it works in a pinch and would be my min (default) running state.
I am wanting to set it up once and not vary the boost after, and clutch it for max performance and just suffer a slower sluggish (but safe motor) sled if I get bad gas or something.
I also know there is a LOT of variables here and that spreadsheet is just a nice tool, so looking for real world engine builders and riders to share their experiences.
Finally the purpose of the twin charge is to get the SC snap which I love, but with the long lake legs of the Turbo, which I also love. I happen to have both kits already and no one wants to buy them, why not build one last beast with the greatest engine ever to be on snow!
mysledblows
Lifetime Member
- Joined
- Jan 23, 2007
- Messages
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- Location
- Central MN
- Country
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- Snowmobile
- Sidewinder, mcx phazer, phazer, apex
- LOCATION
- Jenkins, MN
I’m no help on the cr question but sounds like quite a project.
blown apex
VIP Member
stock rocks keep a stock sled
sorry had to say it winter is too short
i have 7 apexes all years 1 supercharged at 12psi
im also not a sidewinder fan yet (power steering apexs are great for long days)
you had to borrow a 600(other post i read)
now compression/boost altitude this works if you now a set up that works that is on the edge
on my car i had a rule change and went from 60 psi at 10.6:1 to 42 psi at 12.8 and the car runs great for the class i had to run
3.70@205 before and 3.78 @200 after not bad for 18 psi less ( .920 60 ft before and after) and upper rod bearings look better after 5 runs than before so i could raise it more or add timing
i used wallace racings calculator too raise compression to match effective compression
Calculate Boost Compression Ratio
keep adjusted altitude at say 2500 ft
and get your boosted compression numbers right by adjusting compression or boost to get were you want or match your good running engine that didnt hurt itself
Wallace Racing BACK to Calculators Todays date is 2/19/2020
example
i use the standard method the second(effective) method seems to raise comp too much for my comfort
Boost Compression Ratio Calculator
Boost Pressure in PSI : 12
Static Compression Ratio :
1) 10:1
Altitude : (Feet) 2500
2 Different Formulas Used For Answers
The first is the standard equation used by many,
The second may be the more reliable one
The Boosted Compression Ratio from your Boost of 12 psi is 17.84 : 1
The Effective Boosted Compression Ratio from your Boost of 12 psi is 13.11 : 1
Boost Compression Ratio Calculator
Boost Pressure in PSI : 18
Static Compression Ratio :1) 8.2
Altitude : (Feet) 2500
2 Different Formulas Used For Answers
The first is the standard equation used by many,
The second may be the more reliable one
The Boosted Compression Ratio from your Boost of 18 psi is 17.74 : 1
The Effective Boosted Compression Ratio from your Boost of 18 psi is 11.73 : 1
sorry had to say it winter is too short
i have 7 apexes all years 1 supercharged at 12psi
im also not a sidewinder fan yet (power steering apexs are great for long days)
you had to borrow a 600(other post i read)
now compression/boost altitude this works if you now a set up that works that is on the edge
on my car i had a rule change and went from 60 psi at 10.6:1 to 42 psi at 12.8 and the car runs great for the class i had to run
3.70@205 before and 3.78 @200 after not bad for 18 psi less ( .920 60 ft before and after) and upper rod bearings look better after 5 runs than before so i could raise it more or add timing
i used wallace racings calculator too raise compression to match effective compression
Calculate Boost Compression Ratio
keep adjusted altitude at say 2500 ft
and get your boosted compression numbers right by adjusting compression or boost to get were you want or match your good running engine that didnt hurt itself
Wallace Racing BACK to Calculators Todays date is 2/19/2020example
i use the standard method the second(effective) method seems to raise comp too much for my comfort
Boost Compression Ratio Calculator
Boost Pressure in PSI : 12
Static Compression Ratio :
1) 10:1Altitude : (Feet) 2500
2 Different Formulas Used For Answers
The first is the standard equation used by many,
The second may be the more reliable one
The Boosted Compression Ratio from your Boost of 12 psi is 17.84 : 1
The Effective Boosted Compression Ratio from your Boost of 12 psi is 13.11 : 1
Boost Compression Ratio Calculator
Boost Pressure in PSI : 18
Static Compression Ratio :
1) 8.2Altitude : (Feet) 2500
2 Different Formulas Used For Answers
The first is the standard equation used by many,
The second may be the more reliable one
The Boosted Compression Ratio from your Boost of 18 psi is 17.74 : 1
The Effective Boosted Compression Ratio from your Boost of 18 psi is 11.73 : 1
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