Discussion Thread: Altitude, Compression and Boost
Thread Starter
Tech Contributor
Joined: Mar 2003
Posts: 12,094
Likes: 197
From: Colorado Springs, CO
St. Jude Donor '08-'09-'10-'11
Discussion Thread: Altitude, Compression and Boost
So I pack up and move to Colorado and know from day one that its the place for me. The mountain views from my house, the relaxed atitude of the people, and the awesome weather in Colorado Springs is always low in humidity with little rainfall/snow. Gas prices are 30 cents lower than NY. My 5000sq ft house has property taxes under 2k a year. Heck, the cost of licing all around is 60% of NY.
So Im here for a whopping 12 days when the enclosed transporter shows up with the rest of my cars. The kids cheer as the corvette was offloaded perfectly detailed by my good friend Craigster05. Sounds like the end to a perfect set-up until i go for a ride.
My car in NY put down 471/435 on motor and 560rwhp/611rwtq on a 75 shot. I get in my car and it moves slower than a bone stock Ford Escape truck. I check air fluel and spark and find its all fine. I take it for a wilder ride and let a nitrous shot go. On a nitrous shot it wasnt as fast as it was in NY on motor alone.
ALTITUDE
When designing a build for real power you have many variables to consider. The effect of the altitude was quite a bit more harsh on my wow factor than I had anticipated. I get over to RMCR (rocky mountain competitive research) and a quickie dual pull on the dynojet 248 shows my HP and TQ to be spot on.....480/433. then the shoe drops. The dyno is at a slightly lower altitude of 6400 feet compared to my house 8 miles away at 6800feet. the correction factor is pointed out to me and its 26%.....ouch. 480 x .74 is 355.2.....my new and true corrected HP figure.
Welcome to Colorado.
On nitrous the car makes about 80 more HP for 430rwhp....not even beating n/a only power in the flatlands. Note that nitrous is immune to elevation hacking because it is its own air supply.
The shop owners tell me what to expect of the locals. Look out for all wheel drive talons with 42lb boost running E85 on 11.5:1 compression motors. They are lighter and make 800whp. Worse than them are the fully built forged 1.8 liter hondas weighng 1800 pounds at 60psi boost and running full race suspensions on the street. They run 9.8's here at a DA of 7000 ft.
I found myself the owner of a slow, heavy, rear wheel drive slug.
What Happened?
Well in short the thinner air at this altitude causes a drop in compression. Yes it affects combustion because it doesnt get air in so you have less fuel in there too to burn.
The result is that just about anything here with a gas burning engine thus uses forced induction. The air is thinner so the s/c or turbo gets less air into it on the inlet side too. Here if you pulley for 20psi you get 16psi into the motor. You also started with less HP before you boosted because the motor's N/A power was down too.
How do you get the power loss to coincide with the lost cylinder pressure?
This calculator shows the effects of all the variables on what it calls dynamic boost compression. The variables are:
1-displacement: so it asks for bore and stroke.
2-dynamic compression: so it asks when the cam closes its intake valve to begin building compression.
3-and finally altitude because less air gives you less to compress and thus less compression
I wanted to discuss the effects of lost compression and how to model the altitude in the form of compression loss and see if the losses coincide with
a)a set amount of compression equal to some level in altitude
b)HP losses in proportion to the amount of lost compression.
Please feel free to comment as this thread is open to any input you want as long as its constructive.
In example 1:
This car has a 228/232 cam on a 114+0 LSA. It is operated in NY at an altitude of 50 feet. The cam's intake valve closing point is 47 degrees and we will use a static compression of 10.9 to reflect the cam being put in a stock compression car for illustration.
Note boost dynamic compression is the same as dynamic compression because this isnt a boosted car so boost=0.
Note that the DCR is 9.71.
The next screen shot is of the same car but it has moved to 6400ft elevation but nothing else was changed.
Note dynamic compression has fallen at 6400feet to 8.44 down 1.3 points. **Dynamic compression isnt the same as static compression.
Here is a discussion point: How much static compression has to be added to make the car have the same dynamic compression at 6400ft?
Answer: 12.4:1 to make the same dynamic compression or about 1.5 full static compression points.
Car number 3 adds 12psi boost to the mix first at 50feet in NY for elevation and its dynamic compression is 9.71, but its dynamic boost compression is now 17.6. Note it isnt an added 12psi to the dynamic compression of 9.7 because the same ratio of loss applies to the boost not building until the intake valve closes.
Car number 4 is 12psi, same cam and static compression, but now at 6400feet elevation.
Here the car makes 8.44 dynamic comp and 15.33 for its dynamic boost compression. The boost dyn comp fell 2.31 so boost gets a bigger whack but there is a hidden loss not refected here.
Can you guess where it is?
Next we drop the compression to 9.8 from 10.9 at 50 feet agan typical of sea level life.
Note the loss in dynamic boost compression went to 15.87 a drop from 17.64 or 1.77 dynamic compression loss.
In our last screen shot, the car has its cam changed from a 228/232 14+2 to a 224/224 115+0. The intake closing point is now at 46 degrees and the loss in dynamic compression is shown to model what to expect in losses for using a different cam with the rest of the variables held constant.
The increase by going to the smaller cam is .1 points in dynamic compression to 15.97 up from 15.87. This cam reflecs a drop in overlap from +2 to -6 so it saves you not just by being slightly better in the low and mid from the modest bump in dyanmic compression but it also isnt allowing boost to be thrown out the exhaust side during overlap.
Feel free to make comments or express your views or guesses on how this relates to power production.
Would the increase in static compression at 6400feet elevation to 12.4:1 have made the corrected number the same as sea level's uncorrected since it matches the cranking compression?
Would the loss is dynamic boost compression on a car lowering the static compression 1 point cause a loss in power up top greater than the loss on an N/A car?
The internet guess number is that each point in compression is equal to about 4% in power. If this is true, then the loss on a 600rwhp car would be about 60HP for a 2 point comp loss.
How does compression relate to cylinder pressure?
So Im here for a whopping 12 days when the enclosed transporter shows up with the rest of my cars. The kids cheer as the corvette was offloaded perfectly detailed by my good friend Craigster05. Sounds like the end to a perfect set-up until i go for a ride.
My car in NY put down 471/435 on motor and 560rwhp/611rwtq on a 75 shot. I get in my car and it moves slower than a bone stock Ford Escape truck. I check air fluel and spark and find its all fine. I take it for a wilder ride and let a nitrous shot go. On a nitrous shot it wasnt as fast as it was in NY on motor alone.
ALTITUDE
When designing a build for real power you have many variables to consider. The effect of the altitude was quite a bit more harsh on my wow factor than I had anticipated. I get over to RMCR (rocky mountain competitive research) and a quickie dual pull on the dynojet 248 shows my HP and TQ to be spot on.....480/433. then the shoe drops. The dyno is at a slightly lower altitude of 6400 feet compared to my house 8 miles away at 6800feet. the correction factor is pointed out to me and its 26%.....ouch. 480 x .74 is 355.2.....my new and true corrected HP figure.
Welcome to Colorado.
On nitrous the car makes about 80 more HP for 430rwhp....not even beating n/a only power in the flatlands. Note that nitrous is immune to elevation hacking because it is its own air supply.
The shop owners tell me what to expect of the locals. Look out for all wheel drive talons with 42lb boost running E85 on 11.5:1 compression motors. They are lighter and make 800whp. Worse than them are the fully built forged 1.8 liter hondas weighng 1800 pounds at 60psi boost and running full race suspensions on the street. They run 9.8's here at a DA of 7000 ft.
I found myself the owner of a slow, heavy, rear wheel drive slug.
What Happened?
Well in short the thinner air at this altitude causes a drop in compression. Yes it affects combustion because it doesnt get air in so you have less fuel in there too to burn.
The result is that just about anything here with a gas burning engine thus uses forced induction. The air is thinner so the s/c or turbo gets less air into it on the inlet side too. Here if you pulley for 20psi you get 16psi into the motor. You also started with less HP before you boosted because the motor's N/A power was down too.
How do you get the power loss to coincide with the lost cylinder pressure?
This calculator shows the effects of all the variables on what it calls dynamic boost compression. The variables are:
1-displacement: so it asks for bore and stroke.
2-dynamic compression: so it asks when the cam closes its intake valve to begin building compression.
3-and finally altitude because less air gives you less to compress and thus less compression
I wanted to discuss the effects of lost compression and how to model the altitude in the form of compression loss and see if the losses coincide with
a)a set amount of compression equal to some level in altitude
b)HP losses in proportion to the amount of lost compression.
Please feel free to comment as this thread is open to any input you want as long as its constructive.
In example 1:
This car has a 228/232 cam on a 114+0 LSA. It is operated in NY at an altitude of 50 feet. The cam's intake valve closing point is 47 degrees and we will use a static compression of 10.9 to reflect the cam being put in a stock compression car for illustration.
Note boost dynamic compression is the same as dynamic compression because this isnt a boosted car so boost=0.
Note that the DCR is 9.71.
The next screen shot is of the same car but it has moved to 6400ft elevation but nothing else was changed.
Note dynamic compression has fallen at 6400feet to 8.44 down 1.3 points. **Dynamic compression isnt the same as static compression.
Here is a discussion point: How much static compression has to be added to make the car have the same dynamic compression at 6400ft?
Answer: 12.4:1 to make the same dynamic compression or about 1.5 full static compression points.
Car number 3 adds 12psi boost to the mix first at 50feet in NY for elevation and its dynamic compression is 9.71, but its dynamic boost compression is now 17.6. Note it isnt an added 12psi to the dynamic compression of 9.7 because the same ratio of loss applies to the boost not building until the intake valve closes.
Car number 4 is 12psi, same cam and static compression, but now at 6400feet elevation.
Here the car makes 8.44 dynamic comp and 15.33 for its dynamic boost compression. The boost dyn comp fell 2.31 so boost gets a bigger whack but there is a hidden loss not refected here.
Can you guess where it is?
Next we drop the compression to 9.8 from 10.9 at 50 feet agan typical of sea level life.
Note the loss in dynamic boost compression went to 15.87 a drop from 17.64 or 1.77 dynamic compression loss.
In our last screen shot, the car has its cam changed from a 228/232 14+2 to a 224/224 115+0. The intake closing point is now at 46 degrees and the loss in dynamic compression is shown to model what to expect in losses for using a different cam with the rest of the variables held constant.
The increase by going to the smaller cam is .1 points in dynamic compression to 15.97 up from 15.87. This cam reflecs a drop in overlap from +2 to -6 so it saves you not just by being slightly better in the low and mid from the modest bump in dyanmic compression but it also isnt allowing boost to be thrown out the exhaust side during overlap.
Feel free to make comments or express your views or guesses on how this relates to power production.
Would the increase in static compression at 6400feet elevation to 12.4:1 have made the corrected number the same as sea level's uncorrected since it matches the cranking compression?
Would the loss is dynamic boost compression on a car lowering the static compression 1 point cause a loss in power up top greater than the loss on an N/A car?
The internet guess number is that each point in compression is equal to about 4% in power. If this is true, then the loss on a 600rwhp car would be about 60HP for a 2 point comp loss.
How does compression relate to cylinder pressure?
Last edited by SpinMonster; Apr 17, 2010 at 09:10 AM.
Thread Starter
Tech Contributor
Joined: Mar 2003
Posts: 12,094
Likes: 197
From: Colorado Springs, CO
St. Jude Donor '08-'09-'10-'11
Oh Yeah, my earlier question: the loss on the s/c boost has a double whammy due to there being less air on the inlet side. A 3.12 on my old 6 liter made 13psi. This would have made 17-18 psi at sea level with a 1500 blower and it was maxxed out just slightly above that. I upgraded to a 2200 on a 418 with 11:1 compression (this compression has the same cranking compression as a car at sea level with 9.55:1 static compression). Its maxxed out too now.
You can spin it to any speed in a vacuum and it will make no boost. It would also never have the impeller hit the speed of sound. Extreme exaggeration but you get the point.
You can spin it to any speed in a vacuum and it will make no boost. It would also never have the impeller hit the speed of sound. Extreme exaggeration but you get the point.
Last edited by SpinMonster; Apr 17, 2010 at 09:04 AM.
Burning Brakes
Joined: Aug 2007
Posts: 950
Likes: 3
From: Chesapeake Virginia
Spin,
Dragon 3.3L Whipple system was breed, born and raise in Colorado at 5390 ft. I am hoping for big numbers here at sea level.
After all isn't that how elite cyclist's train, in the mountains.
Dragon 3.3L Whipple system was breed, born and raise in Colorado at 5390 ft. I am hoping for big numbers here at sea level.
After all isn't that how elite cyclist's train, in the mountains.
Melting Slicks
Joined: Dec 2006
Posts: 2,517
Likes: 50
From: hopatcong nj
Oh Yeah, my earlier question: the loss on the s/c boost has a double whammy due to there being less air on the inlet side. A 3.12 on my old 6 liter made 13psi. This would have made 17-18 psi at sea level with a 1500 blower and it was maxxed out just slightly above that. I upgraded to a 2200 on a 418 with 11:1 compression (this compression has the same cranking compression as a car at sea level with 9.55:1 static compression). Its maxxed out too now.
You can spin it to any speed in a vacuum and it will make no boost. It would also never have the impeller hit the speed of sound. Extreme exaggeration but you get the point.
You can spin it to any speed in a vacuum and it will make no boost. It would also never have the impeller hit the speed of sound. Extreme exaggeration but you get the point.
Melting Slicks
Joined: Apr 2005
Posts: 3,418
Likes: 1
From: Chicagoland Area IL
Oh Yeah, my earlier question: the loss on the s/c boost has a double whammy due to there being less air on the inlet side. A 3.12 on my old 6 liter made 13psi. This would have made 17-18 psi at sea level with a 1500 blower and it was maxxed out just slightly above that. I upgraded to a 2200 on a 418 with 11:1 compression (this compression has the same cranking compression as a car at sea level with 9.55:1 static compression). Its maxxed out too now.
You can spin it to any speed in a vacuum and it will make no boost. It would also never have the impeller hit the speed of sound. Extreme exaggeration but you get the point.
You can spin it to any speed in a vacuum and it will make no boost. It would also never have the impeller hit the speed of sound. Extreme exaggeration but you get the point.
Because you already stepped up to a Novi 2200 you are done in Novi line. A YSI would be next logical step, but likely cost prohibitive. So all you can do now is minimize inlet losses (making your altitude losses worse) and discharge pressure losses (pipes, couplings, turn radius, pipe diameter...). How much is it really worth, hard to say as ECS kit is pretty clean.
As far as supersonic tip speeds... You only worry about inlet (smaller diameter, thus slower) and as diameter grows radially so does pressure, as does the speed of sound. As pressure goes up so does speed of sound, this is how shock losses are avoided... Somewhat generalized - sorry, so lower pressure would make me worry about speed of sound shock losses more.
Mike
