Why no boost?
 

Went to the barn today to hopefully finish my winter project,installing a 250 blower on a Gen. 5 502. Bought a used blower,looked ok,but what do I know. Finished all new plumbing etc. , but when engine was started I had no boost psi, actually running a vac. of up to18 psi. at wot. Tried hooking a separate vac. guage to confirm dash guage reading,the same. Unhooked blower belt , no differance. My thought is seals on blower are shot.looking for second opinion,anyone??

Were you running the engine out of the water? It won't make boost without a load on the engine. Mine will run up to 50mph with 0 boost. You need to test it with a load on the engone first. If the blower was in good condition it may jus be something simple.

Yes running on garden hose. Checking for any leaks, setting timing,etc.. Didn't Know about needing a load,May have to splash her tomorrow to check:p Thanks for the info.,maybe I can put the blower rebuild off till next season,it's always something:rolleyes:

I'll bet that once you get it on the water and hit wot you'll see that boost gauge rise. You do have a gauge I assume.

BTW not all blowers will need a load to create boost. The need for a bypass valve depends on the amount of boost. But, most blowers will have some sort of bypass valve so that when the engine is in vacuum (part throttle) the boost is routed out of the intake tract. Now, if you were testing your engine and you are showing 0 vacuum (wot) and it's still not making boost you have a problem. I'll be interested to see what happens.


Brent

Can't measure blower boost unless throttle is wide open. If the throttle plates are closed, there's no air for the blower to compress..

Ummmmm, WOT on the hose???

most important question,what size pulley are you running??:confused:

The weather is not looking too good for Sun..I'm running a 3" pulley and the wot was only for a short burst. Air intake was the first thing that we checked along with the throttle plates,everything seemed to be alright as far as we could tell .Just heared weather update ,basically they don't know what it will do :rolleyes: Anymore suggestions i'm all ears, may just have to get wet tomorrow :)

If the output volumn of the blower is the same size or smaller than the displacement volumn of the engine ... the blower will NEVER make boost... unless you drive it faster than crankshaft speed... Good Luck...;)

I thought the same thing. with a side of:eek:

Roger,
It would be a good idea to find out how many hours since last rebuild on the 250. The bearings in back need replaced every so often and if they go bad the rotors can contact the case wiping out the seals. That can be much more costly than a freshen up.

You will not see any boost until around 3500 or so depending upon cam, valves, heads so on. Did you swap out the exhaust valves for full Inconel? What is your water pressure? There is a whole laundry list of things that should be done if you want a blower motor to be reliable. Several other board members are running B&M 250's. What all have you done in preparation for this mod?

Bad dog, You may be somewhat familier with this motor,bought from US-1 Fountain last year. Wanted to take the motor back to it's happy state of being blown:D . Had the heads milled a bit ,not perfectly flat, ck'd block,cometic head gaskets. Springs,guides,and valves checked by local shop. Upgraded fuel pump & lines, installed mercs wp relief valve to hedge off any over wp problems.running crossovers with 143' t-stat with 3 by-pass holes drilled through & restictor in the connection between the the crossovers. Had carb. sent to Dean Nickerson ( very knowledgable ) for complete overhaul. Started everything up with no problems:) just no boost psi :confused:

The engine needs to be under a load.

It's calling! :D :D :D If only I had known you wanted a blower motor, would have saved me some time changing over. :)

BTW, Motor is an HP, so the valves are the inconel. I would have looked REAL close at the springs though if you did not replace them. (you know the issue there) The retainers are titanium. Also fuel pump was already the high volume and only 1 yr old. I'm sure you noticed the fuel separator and fuel pump ports are larger than the regular Merc stuff. The separator was up graded from the Merc one. The fuel line itself was put back to stock size using reducers when blower was removed.
You install a V6 ignition module?

27" hydro=82mph.
Also, check the gas gauge. It seems to drop a whole lot faster with the blower. :confused:

Have full

Roger,
Sounds like you will be fine on the laundry list. Just be sure to check into getting the blower serviced. Cheaper to do up front. From the previous posts you can see that you will not have any boost until you get it wound up. That engine should run great for you! Let us know how the merc pressure relief valve works out. I think most have had trouble with wp and crossovers. From what I have read the merc valve is supposed to work very well. I am running a 120 t-stat and an oil thermostat. Oil temp comes right up and no milk problems at all.

Jerry, I thought I read where you are having high pressure on the new boat. Give Brace a call if you want a pair of those valves. I am sure he would make you a good deal. Maybe see you around Cumberland this year. The boys have a new (to them) ski boat this year. 302 OMC and they love it. I am looking forward to doing some skiing myself.

Tim T.

Hey, you can only assess boost at WOT. If somebody wants to try to do it on the hose, then that's their prerogative.

I've done some real quickie blips to 4 grand just to watch the boost gauges react, but obviously you can't do what you need to do to check "real" boost. Or you can't do it more than once without picking up the pieces afterwards...

Under load you can obtain boost without WOT, but as stated you will not see what max boost is unless WOT & under load. Forget the hose & hit the water