Turbo Compression Calculator

[Buckeye]

Here is a link to turbo calculators. I am looking at the one that calculates turbo engine compression and recommended octane for different boost levels. How do we figure the engines VE? I would like to know the different octanes depending on boost level.

http://www.turbofast.com.au/TFcompB.html

The bore is 2.91"
Stroke= 2.28"
Stock compression is 11.8, low comp pistons 10.0, .057 head gasket is 11.0
Low comp pistons and .057 gasket is 9.1
Boost anywhere from 7.5 to 20

VE=?????

Maybe Jeff knows? Or Viper34?

 

[spray25]

Here is how you figure it out:

http://www.installuniversity.com/instal ... 012000.htm

I don't have the means to figure it out. :?

 

[Jonny Rocket]

I hope that isn't correct. It stated a 4 valve per cylinder engine should have a VE of 75-90%. We have 5 valves per cylinder so I put in 82% just to be in the middle, with a 0.057 gasket (11:1 ratio) at 5 #'s boost you are suppose to use 101.5 octane. With 10 #'s of boost 110 octane. I must be missing something here. Anyone?

J

 

[Buckeye]

I think the VE decreases as the rpm's increase. If the VE is 82 at say 6000 it must be less at 10,300? All the turbos seem to run on 93 oct at 8#'s so maybe we need to work backwards until someone who knows comes forward.

 

[spray25]

I hope that isn't correct. It stated a 4 valve per cylinder engine should have a VE of 75-90%. We have 5 valves per cylinder so I put in 82% just to be in the middle, with a 0.057 gasket (11:1 ratio) at 5 #'s boost you are suppose to use 101.5 octane. With 10 #'s of boost 110 octane. I must be missing something here. Anyone?

J

I did a similar calculation and we are obviously doing something wrong. MC allows 10 lbs with just the gasket if I remember correctly. That is with pump fuel.

 

[Jonny Rocket]

Spray, I agree there must be some part of the equation we are missing. It did sound like the higher the RPM the lower the VE so maybe Buckeye is correct. It would allow the equation to make some sense.

J

 

[Boondawgin]

RX1 VE is about 53 @ 10500RPM

Calculation for Volumetric efficiency or how well fuel flows through the ports of an engine given it's displacement at a constant RPM.

(5600*projected Horsepower*100)/(RPM*((number of cylinders*Enginesize)/16.4))

(5600*240*100)/(10500*((4*999)/16.4))=53

Please don't ask me to define all the constants, but if you plug in the four variables it work for both 2 and 4 strokes.

If you plug that into the proram of Ray's it comes out about right.
I wrote some engine design software and this calculation comes from the software.

(Note that Ray's program doesn't address altitude. Compression and fuel requirements decrease dramatically with elevation.)

 

[Boondawgin]

You can expect to lose roughly 2 octane points per 3000 feet in elevation. If ray's calc says 95 and you are at 6000 feet, 91 will work just fine.

 

[spray25]

Cool formula. I plugged it into excel so that I could lay with it.

Does the octane loss apply to a turboed motor at elevation or only naturally asperated?

 

[spray25]

I think we are still missing something. MC says 10 lbs on pump fuel with thicker headgasket.

When I plug the numbers into the calculator though it says that you should be running 95.2 octane.

Even at 10 to 1 with LC pistons it still says 92.5 octane.

 

[Buckeye]

It works for my front mount. At 11.0 compression and 7.5 lbs of boost, it calls for 92.5 octane. The 93 prem gas covers that. Now at 9.1 compression I can run 13 lbs on 93 octane gas and 20 lbs on Sunoco 100 octane race gas, which is available at the local station.

 

[spray25]

Oh, I thought MC was 10 lbs with a gasket. Is 7.5 what MC says for the rear mounts also?

I was hoping to run 10lbs with pump gas and my LC pistons. That requires 92.54 octane. I need to check and see what the exact octane rating of my gas is.

Let's assume it is 93. It's a little scarey to think I might only have .5 of a point for error. What if the gas is old??? What if my gas is only 92??

AAAAHHHHHHH!!!!!!

Of course then there are the guys that you here about that ran 8 and 10 lbs on pump gas with 11.8 compression. That should require 95 and 97.3 octane.

For some reason my head hurts. I need a beer.

 

[Buckeye]

I think rear mounts can run more boost on pump gas than the front. It has to do with the size of the turbo. Out here most premium pump gas is 93. I have to believe you would be good with 93 at 10 lbs. Plug in the boost bender says you can run with the 11.8, is it 7 lbs? then change the VE until you get to 93 octane on the calculator. Then use that VE and plug in 10 lbs boost and see what octane it tells you.
I put in 7 lbs and VE of 51 and got 92.9 octane. At 10.0 to 1 and a VE of 51 you can run 11.5 lbs of boost at 93 Octane.

 

[spray25]

Unfortunately, Terry is admitidly conservative with his boost/octane numbers. He suggests 4lbs on pump gas and 8lbs on 50/50 race fuel (100.5 octane). I ran 10lbs last year at about 98.3.

Here is what the calculator says the octane should have been:
4lbs 90.34
8lbs 95
10lbs 97.3

 

[Buckeye]

Unfortunately, Terry is admitidly conservative with his boost/octane numbers. He suggests 4lbs on pump gas and 8lbs on 50/50 race fuel (100.5 octane). I ran 10lbs last year at about 98.3.

Here is what the calculator says the octane should have been:
4lbs 90.34
8lbs 95
10lbs 97.3

Conservative is an understatement! Just about every other Turbo seller lets you run at least 7 to 8 lbs. on pump gas. :ORC