Thread: Boost leak input
07-17-2007, 11:57 AM #1
Boost leak input
OK, I re-shimmed my backup SC (A kit impeller), slip at 108 in lbs with brand new Riva metal washers, spring washers and 1 shim. Installed that, and installed my new Friebe SS-enhanced exhaust manifold and bolted everything up. I am still running 7700 - 7900 rpms, seat on or off, so I pulled out the boost gauge, at 7700 rpms, it was only showing 5-6 lbs of boost. With most other variables ruled out, I am assuming I have a boost leak to find. The silicone couplers are all tight and double checked and I have the metal Riva IC tubes (no BOV), so next week I intend to spray soapy water all over and go for a run and hopefully find my leak.
I was curious of input from others who have had boost leaks, how they found them, and where they were. I did put 10 lbs pressure into the IC and engine and heard no leaks, yet the pressure did not stay up. I should have done the soapy water test but ran out of time.
So any input on finding/fixing boost leaks? If you had one, I am curious where it was ... in my Stage 1 days, my BOV leaked, then I melted the Riva plastic IC tubes, but now its something else, new this year.
07-17-2007, 12:12 PM #2
Are u sure your boost guage is accurate? I believe you need to get those RPMS up to see higher readings 7700 is a bit low, maybe too much prop? When I used glass cleaner on my couplings my readings went back up to 8.5PSI. (Mechanical guage).
108lbs? isnt the S/C slip set around 15lbs/in?
Last edited by Antiguan FZR; 07-17-2007 at 12:16 PM.
07-17-2007, 12:18 PM #3
07-17-2007, 12:18 PM #4
Yes, if you can't find a boost leak, I would be looking at why the RPMs are down. If something is holding your RPMs down, you will not get good boost. It's a little bit of a chicken and egg thing. Is it the lack of boost causing low RPM or is it low RPM causing a lack of boost.
07-17-2007, 12:19 PM #5
Spinning 7700 compared to 8100 is worth 1-2+ lbs. alone.
07-17-2007, 01:19 PM #6
True, at 7700 rpms I would not expect 9 lbs of boost, but I think it is actually a boost leak that is keeping my rpms low. It sometimes runs up to 7900 rpms too, and since I know the SC slip is good (108 inch lbs is good, equals 9 ft lbs slip), my assumption was a boost leak, and low boost = low rpms. The first time out this season I was running 8000 rpms with an 80mm ring and stock 15/20 ... I should have checked boost that day ... oh well.
My boost gauge IS not necessarily accurate ... but I am not running as well as I have in the past. A few things I can do - soapy water and watch for bubbles, drop back to 81mm ring, and make sure my IC is not leaking water internally - not sure if that would cause these symptoms tho. Appreciate the input ...
07-17-2007, 01:24 PM #7
- Join Date
- Oct 2005
Good points about why the boost is down some. I'm sure Vern knows this though...centri's make more boost at higher RPM. They're directly related. But then again if he does have a boost leak wouldn't that cause a loss in RPM also? I think this is his question and idea...I think he's thinking a boost leak is the cause of his low RPM.
EDIT: haha, you were posting as I was typing.
Vern, when was the last time you saw 8100 RPM and what were the weather conditions? I ask because after I've talked to a few other forum members it seems to me that the more heavily modded skis (SC wheels that are moving more air and are supposed to produce more boost) are more sensitive to weather chagnes such as temperature and humidity. I don't know about your side of the state, but it's been very humid in Saginaw lately. Heat soak maybe?
07-17-2007, 01:37 PM #8
All last year I ran 8100 rpms most days, and 77mph. Over the winter I added the better Riva metal IC tubes, Riva rear intake, modded waterbox, pulled my 15/20 back to stock and switched to an 80mm ring. I felt the additonal mods would allow me to run the 15/20 at stock specs and 80mm ring. The first time out this year I ran 8000 rpms ... and was really flying ... but I did not check boost or GPS it. Since then I have been down on rpms a bit and they have fluctuated. I found I had an exhaust leak due to a melted exhaust manifold, and that my new SC setup was slipping at 62 in lbs. I fixed both this past weekend, but still am down, running 7900 rpms some times, 7700 other times ... which leads me to believe I still have one gremlin, likely a boost leak.
If I get the boost leak resolved and switched back to 81mm, I should easily pull 8100 rpm and hit 78.x mph as Dan has. The hot weather does zap me at least 100 rpms at times, but I am used to that, what I have going on now is not that. There were times that Dan and I ran and were dead even ... so I think I just need to get the minor boost leak fixed and I will be there regularly again.
07-17-2007, 01:44 PM #9
What is it like at idle? If you have a boost leak, usually the idle will be erratic. The only place you could have a leak other than the pipe connections is at the stock IC block-off.
I think you probably remember when I had the same issue a couple years ago. I couldn't get over 7650 RPM. I never found that problem. It just went away when I got the b-kit SC.
07-17-2007, 02:07 PM #10
- Join Date
- Oct 2005
He could also have a leak from one of the intake manifold o-rings where it mates up to the block, but I think that's highly doubtful.
EDIT: Vern, are you running a AM front motor mount? If not maybe your prop diameter is wearing down? Just throwing that out there since I've heard about this happening to a couple people lately and they have been down on RPM.
Last edited by RXP-REX; 07-17-2007 at 02:10 PM.
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