Where's my boost hiding? (MPI S/C)

[A.Granath]

I only get about 3psi of boost @8000rpm (2000ft), can you guys give me some advice where to start searching?

Specs:
Nytro 2009 xtx. 2" lugs
Brand new Rotrex charger from mpi with 28/28 pulleys
Headshim
Intercooler
MCX gen5 box with 3 extra injectors
AFR reads steady 12.6 @8000rpm

I appreciate all help i can get!

//A.Granath

 

[kinger]

Do you have a blow off valve?

 

[A.Granath]

Do you have a blow off valve?

Yes i do, i have the one with two ports, the membrane and spring looks ok and the valve moves freely.
I can hear a faint "phssss" when i release the throttle but i have never heard of felt the valve bypass anything (if that can be heard)

 

[Srxspec]

The MPI supercharger unit builds boost based on RPM. At 8000 RPM with the 1 to 1 pulley combination at 2000 foot elevation with a head shim you are about right at 3 lbs of boost. If you can get your clutching setup correctly and get your RPM up to 8800 to 9000 RPM you will have 2 to 2.5 psi more boost.

An example, 25/29 pulleys (29 being the gear on the engine) will build approximately 8 psi @ 8000 RPM and 10 psi @ 9000 RPM on a stock motor at just slightly above sea level.

I attached a picture of the boost rise on a supercharger (black line) and also a turbo (red line).

 

[A.Granath]

The MPI supercharger unit builds boost based on RPM. At 8000 RPM with the 1 to 1 pulley combination at 2000 foot elevation with a head shim you are about right at 3 lbs of boost. If you can get your clutching setup correctly and get your RPM up to 8800 to 9000 RPM you will have 2 to 2.5 psi more boost.

An example, 25/29 pulleys (29 being the gear on the engine) will build approximately 8 psi @ 8000 RPM and 10 psi @ 9000 RPM on a stock motor at just slightly above sea level.

I attached a picture of the boost rise on a supercharger (black line) and also a turbo (red line).

A BIG Thank You srxspec! that did clear things up for me!

I have a new pair of pulleys coming in this week. 21 top and 31 bottom, i hope that will get things going!

This is a graph i recorded to my phone from my test run with 28 / 28 pulley combo:

0-80mph WOT

Top White line: Boost pressure (122 = 3.19 psi)
Red line: RPM
Bottom White line: AFR
Bottom yellow line: External injector pulse with

Link to jpg:
https://www.dropbox.com/s/sc9r3x8wea1tij9/readings.jpg


Feel free to comment![web]https://www.dropbox.com/s/sc9r3x8wea1tij9/readings.jpg[/web]

 

[Agman 57]

I am running the 25-29 pulleys. At 750 elevation, I have 1# boost at idle and up to 12# at 9,000 rpm. If I run the bov in, will run 8-9# with it all the way out. Sounds to me like you are going to over correct with the pulley combo you are talking about? just my 2-cents

 

[A.Granath]

I am running the 25-29 pulleys. At 750 elevation, I have 1# boost at idle and up to 12# at 9,000 rpm. If I run the bov in, will run 8-9# with it all the way out. Sounds to me like you are going to over correct with the pulley combo you are talking about? just my 2-cents

Thank's for the reply Agman!

If you look at my graph with 28/28 pulleys i'm not building any boost before 7000 rpm from 7000rpm to 8000rpm it builds about 3psi. So what srxspec said that it builds about 3psi/1000rpm feels about right.

I hope with 23/31 i can start to build boost earlier, right now i have a hard time dialing in my clutches.

If i load my weights in the primary to heavy it doesn't over rev at start but it will not reach the right rpm and the other way around.

 

[Srxspec]

23/31 will give you 13 psi or even more at your elevation. 25/31 at approximately 1000 foot elevation will give you about 13 psi as long as the bypass valve spring is strong enough.

 

[A.Granath]

23/31 will give you 13 psi or even more at your elevation. 25/31 at approximately 1000 foot elevation will give you about 13 psi as long as the bypass valve spring is strong enough.

I have fuel for about 12-13 psi, after that i need to upgrade the stock pump.
If the bov spring doesn't hold up i will make it do!

I can post a new graph from my phone when i get the new pulley combo if you are interested!

//AG

 

[A.Granath]

Well now i found where my boost where hiding.

Here is a reading from my first pull, track is just lifted from the floor... i have no snow here by the coast to do some real tests

MAP = 187.92 = 12.75 PSI @ sea level

//AG

 

[Yamadog]

I would pay close attenetion to the AFR 12.4 at WOT is a bit lean. We are finding that 11.4 is tops on the viper with the same motor - ah ask me how I know

 

[A.Granath]

I would pay close attenetion to the AFR 12.4 at WOT is a bit lean. We are finding that 11.4 is tops on the viper with the same motor - ah ask me how I know

Hi Yamadog!
Do you use head shim, modified ignition or racegas to reach 10psi on your setup?

 

[Yamadog]

I would pay close attenetion to the AFR 12.4 at WOT is a bit lean. We are finding that 11.4 is tops on the viper with the same motor - ah ask me how I know

Hi Yamadog!
Do you use head shim, modified ignition or racegas to reach 10psi on your setup?

Wish it would have been that easy - I blew up at 867 miles the 2nd week I had it. Had to rebuild out of pocket. New block, Carillo rods, Wisco low or no compression pistons (thats the ticket) quick spool waste gate, upgraded fuel pump, boost switch now set at 6 & 9 but had it at 8 & 12 was TOO crazy!

 

[A.Granath]

I would pay close attenetion to the AFR 12.4 at WOT is a bit lean. We are finding that 11.4 is tops on the viper with the same motor - ah ask me how I know

Hi Yamadog!
Do you use head shim, modified ignition or racegas to reach 10psi on your setup?

Wish it would have been that easy - I blew up at 867 miles the 2nd week I had it. Had to rebuild out of pocket. New block, Carillo rods, Wisco low or no compression pistons (thats the ticket) quick spool waste gate, upgraded fuel pump, boost switch now set at 6 & 9 but had it at 8 & 12 was TOO crazy!

I hope you will have better luck this time!
Lowering compression takes away some of that nice character from the engine but without the possibility to get anything better than premium pump gas here in Sweden i had no other option.
I hope that cold air, low compression, and low afr will keep my engine from grenade... better just hold thumbs and pray to the yamagod

 

[Yamadog]

Oh its a whole diff animal even at 6#'s which it was before is much faster than it was prior at 6#'s with the new quick spool waste gate it is stronger out of the hole and pulls hard to over 100. At 9#'s it pulls the skis 12" to 40 mph then they go to almost 3' if Im not carefull and it carries them there till 100 mph. It was originally set up at 7 and 11 on the switch and I backed it down it was not trailable.