Rebuilt SN89 low boost

Last year I rebuilt and installed an SN89 on a carbureted 306. The motor is fairly built with windsor jr heads, rpm air gap intake, long tube headers, etc. The cam is a comp cams 270H magnum. It is pullied to 40k rpm at 6k engine rpm. I am using the new high output impeller and rebuilt to specs. It has about 40 inch pounds of resistance. I'm only seeing 3 or maybe 4 psi max in the intake. I've tested for leaks and it's only leaking through the throttle shafts a little - could I be losing much boost through there or do you think my cam is bleeding off boost from too much overlap? I'm also at 5200 feet elevation. Could that be killing boost as well?

I was thinking this would make 6-8 psi on my small 306. Any help or ideas would be great. Thanks.

You're right in saying that a properly rebuilt SN should see something in the neighbourhood of 6-8psi of boost, especially with the HO impeller. And from everything you've stated above, your rebuild was done correctly, so I doubt there's any issues with the rebuild itself. Even taking into account the small amount of bleed-off through the throttle shafts, that shouldn't result in a 3-4psi drop (maybe 1 or 1.5psi, depending on how bad the leak really is).

Where are you measuring the boost pressure? At the blower outlet?

If anything is causing your low-boost condition, it's more than likely a combination of the cam you're running and the amount of airflow your heads/intake are capable of flowing. I'm not overly familiar with the Windsor Jr. heads you're running, but if they're flowing good numbers (more than 200cfm), coupled with the cam and high-flow intake then the combination could be a contributing factor to the low boost you're seeing. The SN is good for about 750cfm at just under 40,000rpm. If you're engine is capable of flowing more than that, then the boost you're see will be lower than the expected 6-8psi.

I'm also assuming you're not running a lot of piping or an intercooler between the blower to the carb either. If you are, that will play a role as well. The greater the volume the blower has to fill between it's own outlet and the engine's intake, the lower the boost pressure you'll see.

Do you have a port on your compressor housing? If so measure boost there, at the charge pipe, and at the manifold. If you are loosing boost pressure it will be lower after the loss than before. If you have low pressure at the compressor housing you either have the wrong drive ratio, a leak from the compressor housing at the seam, or too much clearance between compressor housing and wheel therefore it is not compressing just blowing. At least that's how every supercharger or turbo I have ever dealt with works. I am just learning about the SN series but would imagine aside from the ball drive the theory of operation has to be the same as any other.

Thanks for the input guys. I broke a rocker stud in the fall, and just getting sound to changing it now. I am measuring boost from the intake itself, below the carb. I have another boost gauge, I'll rig up something to see boost at the carb hat to see what the difference is.

As far as the piping, it is maybe 1 ft total with a bov in the middle.

What should the impeller clearance be? I should double check that anyway.

Impeller to scroll cover clearance should be .016 - .020 inches. That is what Paradise Wheels recommends.

Quote:

---

Originally Posted by JPJP

Impeller to scroll cover clearance should be .016 - .020 inches. That is what Paradise Wheels recommends.

---

Where did you get that info from? I have not been able to find anything on this site except for some torque numbers.

I was also told the same thing from Paradise Wheels. No less than .010 on the back of the impeller and .016 - .020 on the curved side.

The real question is how the heck do i measure the clearance on the curved side between the impeller and scroll cover?

I checked the impeller clearance and removed .005 in the shims to put me at .015 on the scroll cover and .010 on the rear. I machined my throttle shafts to accept o-rings and fixed all the other little boost leaks. I even tightened the belt. It's still only hitting around 3 psi. I'm going a little rich under boost, but was still hoping for 2-4 more psi. Must be my camshaft that is bleeding off the extra boost unless anyone can think of anything else?

I also swapped out the synthetic b&m I had in there for some original b&m and saw no change.

Ok, definitely getting belt slip. I thought my belt was plenty tight, but it was getting worn pretty bad after only maybe 1000 miles. I might be to the point where going to a larger SC pulley or decreasing the ball tension would actually give me MORE boost. Currently running a 5" crank pulley and 3.25" SC pulley.

A 3.25 pulley is pretty small. That may be causing it? I was told besides over spinning it a 3.75 SC pulley was as small as you can go due to belt slippage.
I just want to reiterate, I was told this. No idea if its BS or not.
Are you running a 6 groove belt? I'm running a 3.75 but its on the stock 6.25 crank pulley. I get no noticeable belt slip & Im over spinning it a bit too.
SN92 w/upgraded impeller on a 93 cobra. Dyno recorded 6.9# when i got it tuned. Their dated as hell but they work.

Yes, I would like to run bigger pulleys, however since I am using custom brackets, I am somewhat limited in pulley selection. My crankshaft pulley is only 5" in diameter, so the 3.25" SC pulley gives me about a 40,000 impeller RPM at redline - which should be optimized. The belt is a 6 rib belt. 6.9 lbs is very impressive - I'm getting about 3 - 3.5 with the HO impeller. I'm going to loosen the SC case bolts a hair to decrease the pressure on the ball drive and see if that decreases belt slip any. If not, I might have to eventually build a new (larger) crankshaft pulley and invest in a larger SC pulley ($$).

The car runs great and drives really well and has a ton of power - I just know there should be about twice the boost, so it's somewhat frustrating!

I am running a Ø3.75 S/C pulley on my '86 Mustang. I have the Ø6.00 crankshaft pulley and am getting no slip. I too am running the HO impeller. I highly recommend a Gatorback belt. They slip less and the serrations allow a tighter bend radius than other belts.

A way to check for belt slip is to look for evidence of the slippage (belt dust) on your cooling fan blades and other accessories.

Quote:

---

Originally Posted by JPJP

I am running a Ø3.75 S/C pulley on my '86 Mustang. I have the Ø6.00 crankshaft pulley and am getting no slip. I too am running the HO impeller. I highly recommend a Gatorback belt. They slip less and the serrations allow a tighter bend radius than other belts.

A way to check for belt slip is to look for evidence of the slippage (belt dust) on your cooling fan blades and other accessories.

---

I am getting a little dust after I take it out and do a couple pulls. I'll look into getting a gatorback belt. Thanks for the recommendation.

Quote:

---

Originally Posted by Michael

You're right in saying that a properly rebuilt SN should see something in the neighbourhood of 6-8psi of boost, especially with the HO impeller. And from everything you've stated above, your rebuild was done correctly, so I doubt there's any issues with the rebuild itself. Even taking into account the small amount of bleed-off through the throttle shafts, that shouldn't result in a 3-4psi drop (maybe 1 or 1.5psi, depending on how bad the leak really is).

Where are you measuring the boost pressure? At the blower outlet?

If anything is causing your low-boost condition, it's more than likely a combination of the cam you're running and the amount of airflow your heads/intake are capable of flowing. I'm not overly familiar with the Windsor Jr. heads you're running, but if they're flowing good numbers (more than 200cfm), coupled with the cam and high-flow intake then the combination could be a contributing factor to the low boost you're seeing. The SN is good for about 750cfm at just under 40,000rpm. If you're engine is capable of flowing more than that, then the boost you're see will be lower than the expected 6-8psi.

I'm also assuming you're not running a lot of piping or an intercooler between the blower to the carb either. If you are, that will play a role as well. The greater the volume the blower has to fill between it's own outlet and the engine's intake, the lower the boost pressure you'll see.

---

Little update, I changed pulleys to a 6.5 lower and 4.11 upper, machined and o-ringed the throttle shafts, and played around with fluid types. I didn't see much of a difference, but finally got around to putting the other boost gauge in the carb hat and it showed around 6-7psi and climbed much more linear than the other gauge. I then put the first gauge into the carb hat, and it still showed 2-3 psi less than the new gauge. So I'm thinking that gauge is just stingy. Where are you guys connecting your boost gauge and what psi are you seeing? The car runs really strong, so I think I'm going to buy a new boost-only gauge (not boost/vacuum) that will fit my console and just run it off the carb hat. I have a cooler hooked up, but it still seems to get almost too hot to touch, so I am going to buy a larger cooler with it's own fan, and run the intake piping into the fender well instead of sucking the super-headed air above the headers.

What size engine is that on?
My gauge is connected to the vacuum tree on the firewall. I have an auto meter boost gauge that does not "0" when I shut down the car. I always wondered if it was off a bit. I just had the car dyno tuned for the second time due to a conservative tune because I was having spark blow out and cooling issues during the first. It was only off about 1#.
I'm running a cooler as well and the head unit is very hot to the touch. Though my intake temps weren't bad. It was a 92 degree day & it never went over 180. I'm toying with getting a larger cooler. What fluid do you use? I use B&M trickshift

Quote:

---

Originally Posted by Winchester73

What size engine is that on?
My gauge is connected to the vacuum tree on the firewall. I have an auto meter boost gauge that does not "0" when I shut down the car. I always wondered if it was off a bit. I just had the car dyno tuned for the second time due to a conservative tune because I was having spark blow out and cooling issues during the first. It was only off about 1#.
I'm running a cooler as well and the head unit is very hot to the touch. Though my intake temps weren't bad. It was a 92 degree day & it never went over 180. I'm toying with getting a larger cooler. What fluid do you use? I use B&M trickshift

---

It is a 306 SBF. My current sunpro gauge sits at -1 with the engine off too. I bought a VDO boost-only gauge that should be here today. I currently have type F ATF in the unit as recommended by Craig Conley and Cundn Racing. I have had both synthetic and non-synthetic B&M in it previously, and will probably replace the ATF with the B&M synthetic again here shortly. I really need to get the intake into the fender. Right now it's right above the headers, which I'm sure is not helping with my intake temps.

How does the synthetic B&M work? Have you seen in apart after using synthetic? Does it run cooler? Originally I figured synthetic was the way to go.
But I have always read on this forum not to use synthetic but there was a guy on here that swore by it and even said Craig at PW recommended it. There's some contradicting information here when you start reading the older posts.
I'm running a 4" power pipe but I have no idea if it helped since it was always on their. To me the stock air box works better then most of the CAI kits sold. I don't like the filter just sitting in the engine bay uncovered. Imo its grabbing all that hot air. But then again, I see them all the time on other cars.

1). Go back to the synthetic B&M Trick Shift. None better .

2) oBoost gauge probe should be right at the blower case exit where the boost reference line is going to the fuel pump. You will T off of the same line.

3) you must have a cam with a wide lobe separation angle (LSA). A very good cam with the vintage ball drive Paxton, is the old 289 HiPo K code cam. Compcam sells a good replica of them. The so called power cams all have narrow LSA's and will contribute to the low boost numbers you are seeing

Z

Interestingly, without changing anything else, I replaced the type F with synthetic B&M this weekend and lost 1.5 PSI. I was getting 6PSI with the type F, and only 4.5PSI with the synthetic B&M.

B&M Trick Shift is not synthetic. In fact you don't want to run synthetic in a ball-drive blower. The drive mechanism relies on some friction being present - eliminating it can have a negative effect on the amount of boost the blower can produce.

Stick with either regular Type F fluid or original B&M Trick Shirt. Do not use a synthetic fluid.

n

did you try T'ing the stingy gage and the better reading gage to see if they show the same amounts of boost from the same source??
2different brands of gages will have different calibration tolerances +/-% [1%, 2%, 5%, & even 10%]
any progress on this issue??

I haven't done much with this car in the last little while as I have been building another N/A Mustang, but the VDO gauge in the hat (obviously) read higher than the gauge in the intake, but It still doesn't feel like it's giving me all it should (I'm not convinced it's any faster than when it was N/A). I'm still getting a lot of belt slippage even with my larger pulleys. I'm thinking the ball tension is setup too tight eating up too much hp to turn and causing a lot of belt slippage. I'm going to pull it apart and set it up at a lower rotational torque and see if that helps reduce belt slippage and free up some horsepower and boost.