Drive pressure on twins; stock HE351/S400

M

mq105

Thinkin'
Joined
Sep 1, 2009
Messages
131
I did a test on an '07 5.9 with MPI's stock over S400 twins to check DP. I used a 0-60 turbo boost gauge for total boost and a 0-100psi turbo boost for DP. It ran 1:1 all the way to 62psi, where boost maxed and DP went to 65psi. I used the data logging feature of Isspro's Performax gauges to check pressures as my boost gauge tops out at 60. It was maxed, and you can't tell if you have 62 (as in this case) or 80. I temporarily took the sensor out of the cold pipe to use for the DP.
I was pleasantly surprised that the DP is... ideal. I think it is good info for those contemplating twins using the stocker.

DP1.jpg

DP2.jpg

DP3.jpg

DP4.jpg

DP6.jpg
 
Do you know the specs on the S400? That is pretty good to know! Twins are definately the answer to good spoolup with huge airflow.
 
S400 is 75mm ind/102 exd compressor, 96 ind/88 exd turbine, 1.32 A/R

That would be a 75/88 (something)...
 
Internal gate is stock. I had reservations, but they have proven to be unfounded. I may, at some point, enlarge the internal gate to 1" just to see what difference it makes. The stock opening measures .840" so it wouldn't be a huge change, but I would think it should be noticeable.

Yes, spring wastegate actuator. Actually, the spring holds the gate closed... You can see it in one of the pics.
 
Last edited:
it DOES make a difference. they can be opened to 1.1" no problem. give it a try and let us know.
 
quick math.. a hole about 31% larger in open area....quite a bit of reduction in DP.. i would say
 
Dont be too sure that opening the hole will drop drive pressure significantly. If he can control the boost from the top turbo now then the wastegate is big enough to do it's job. The turbine side of the stock turbo is what causes the drive pressure not the flow of the wastegate (which is enough for the 650hp the kit is rated for).
 
but he wont be in danger of over spinning the small turbo. with greater control..
 
24vriviera said:
but he wont be in danger of over spinning the small turbo. with greater control..
Click to expand...

How do you know he is now?? Do you have a turbo speed sensor?

If boost is where he wants it, and drive pressure is in check, the setup is efficient enough, and he's not spinning either charger off it's MAP.

Nathan (IdahoCTD) has done his homework;)

Chris
 
If he was overspinning the small turbo now the drive pressure and egt's would be much higher. The controller allows for a much greater range of motion than the stock actuator therefore it allows it to flow more without having to enlarge the hole. He can turn the controller back and dump more from the small turbo but the drive pressure and egt's will go up if he does. The same thing happens if he tightens it down. That's why you have to tune twins to their "sweet" spot.
 
mq105 said:
S400 is 75mm ind/102 exd compressor, 96 ind/88 exd turbine, 1.32 A/R

That would be a 75/88 (something)...
Click to expand...



I thought the original stocker over S400 twins came with a 71mm compressor S400. Has MPI upped the ante on their new kits to 75mm?
 
That is good to know. I am considering moving to this setup myself since I tow alot. Tough decision though, i already have a HTT 62/71/13ss.
 
It tows very well. It is claimed to spool 100 rpm higher/later than stock. I believe that, but can't tell. It definitely ramps up faster, though, when you put your foot in it. If you drive it easy as a DD the only noticeable difference from stock is the whistle/siren-sound from the 400, which I don't like... :rolleyes:
 
The highest boost I've seen is about 65psi. With total boost at 64-65psi, the primary was at 32-33psi. Higher math puts the 351 at 31-33, which I think is pretty efficient. Getting to that point the primary is making less than half total boost. The lower the total boost, the higher the ratio of total to primary. For example, at 10psi total, the primary reads 1-2psi.



Signature600 said:
...If boost is where he wants it, and drive pressure is in check, the setup is efficient enough, and he's not spinning either charger off it's MAP.

Nathan (IdahoCTD) has done his homework;)

Chris
Click to expand...
 
You must log in or register to reply here.
Back
Top