Tom @ Fast Forward
03-23-2007, 03:50 PM
If some of you with the BRP/SOT Hotside believe you are making less boost than you should or are running lean, this could be your problem. Awhile back BRP/SOT changed from the cast Dummy Throttle Body (DTB) to a weldment. The new ones have a 1/4" plate welded to a 2.5" (?) tube to connect to the crossover tube. On the side of the tube they welded a fitting and inserted a fitting for the vacuum/Boost pickup. Inside the tube is a very tiny hole feeding the fitting.
http://www.FastForwardSuperchargers.com/Hotside/BoostPickup.JPG
The hole inside the tube is very small and lends itself to Bernouli's principles. The air flow coming in is probably very laminar and creates a negative pressure at the hole inside the tube. We discovered this while trying to get Jim Walton's car running well. We installed the PC-pros and, with normal settings for the main and e-cool cards, were running extremely lean. We noticed the boost seemed to hit ~6PSI almost immediately and then sit there almost all the wat to redline. However, after forcing the cards to add fuel, we were making what seemed to be too much WHP for only 6 PSI boost. We checked the engine (compression and leakdown), checked the bypass valve on the SC to ensure it was closing, checked the belt to ensure it was not slipping (FFS pulleys and BRP/SOT auto-tensioner). Nothing appeared wrong. Then, while the crossover pipe was off, I felt the hole on the inside of the tube for the Vac/Boost line. It is a very small hole. So, we disconnected the cruise hose and connected the VAC/Boost line there. Voila!! Boost is back and the cards, seeing the proper boost, now add the proper fuel.
http://www.fastforwardsuperchargers.com/Hotside/CruiseControlVacuum.JPG
Then, I cogitated on why it used to work with the cast DTB. I believe the answer is that the cavity in the cast DTB that the line connects to is significantly larger in diameter and totally open to the intake.
With the weldment, when you first hit boost, the IM fills with air and fills the small hole. As RPM increases, the amount of air the engine can consume increases significantly causing more flow across that small hole causing an offsetting negative pressure which causes, in turn, the boost gauge to read a constant pressure. The larger hole in the cast version just doesn't react the same. I believe the phenomina that is at work is that between the tube and the fitting is a cavity. It is sealed except for the small hole in the tube. If the hole could be opened up to the size of the cavity, it "might" help. Best is to move the fitting elsewhere. The cruise control location is ideal as it really shouldn't be connected there anyhow as located there and climbing a hill will cause cruise to drop out. Cruise control to work effectively needs to be connected between the TB and the SC where it always sees vacuum and never sees boost. So, you can kill two birds with one stone, if you happen to be hunting bird for dinner?
Anyhow, that is what we found and you should take a look at yours if you are having trouble getting proper fuel control. As boost increases, you need more fuel and, if the cards don't see it, the curve will be all wrong.
http://www.FastForwardSuperchargers.com/Hotside/BoostPickup.JPG
The hole inside the tube is very small and lends itself to Bernouli's principles. The air flow coming in is probably very laminar and creates a negative pressure at the hole inside the tube. We discovered this while trying to get Jim Walton's car running well. We installed the PC-pros and, with normal settings for the main and e-cool cards, were running extremely lean. We noticed the boost seemed to hit ~6PSI almost immediately and then sit there almost all the wat to redline. However, after forcing the cards to add fuel, we were making what seemed to be too much WHP for only 6 PSI boost. We checked the engine (compression and leakdown), checked the bypass valve on the SC to ensure it was closing, checked the belt to ensure it was not slipping (FFS pulleys and BRP/SOT auto-tensioner). Nothing appeared wrong. Then, while the crossover pipe was off, I felt the hole on the inside of the tube for the Vac/Boost line. It is a very small hole. So, we disconnected the cruise hose and connected the VAC/Boost line there. Voila!! Boost is back and the cards, seeing the proper boost, now add the proper fuel.
http://www.fastforwardsuperchargers.com/Hotside/CruiseControlVacuum.JPG
Then, I cogitated on why it used to work with the cast DTB. I believe the answer is that the cavity in the cast DTB that the line connects to is significantly larger in diameter and totally open to the intake.
With the weldment, when you first hit boost, the IM fills with air and fills the small hole. As RPM increases, the amount of air the engine can consume increases significantly causing more flow across that small hole causing an offsetting negative pressure which causes, in turn, the boost gauge to read a constant pressure. The larger hole in the cast version just doesn't react the same. I believe the phenomina that is at work is that between the tube and the fitting is a cavity. It is sealed except for the small hole in the tube. If the hole could be opened up to the size of the cavity, it "might" help. Best is to move the fitting elsewhere. The cruise control location is ideal as it really shouldn't be connected there anyhow as located there and climbing a hill will cause cruise to drop out. Cruise control to work effectively needs to be connected between the TB and the SC where it always sees vacuum and never sees boost. So, you can kill two birds with one stone, if you happen to be hunting bird for dinner?
Anyhow, that is what we found and you should take a look at yours if you are having trouble getting proper fuel control. As boost increases, you need more fuel and, if the cards don't see it, the curve will be all wrong.