I just cranked my new engine which has a Weiand blower on it. I am having some carb backfire issues above 2k RPM. To see the details of that problem check out the General Engine board. My question is about hooking up a boost pressure guage. For those experienced with this blower I hooked the boost/vac guage to the port on the #5 intake. All I ever see on the guage is vacuum. Have I hooked up the guage properly?

Thanks,

Glen

I found that I had some intake valves that were not closing all the way. Spark plugs looked bad on 3 cylinders and a compression check confirmed the problem. I reset all the valves and will crank tomorrow. Point being, this should be why I showed no boost but vacuum instead.

Glen

Hope that fixes the problem for you. Hanging an intake valve would usually make it run like crap (especially at idle) but shouldn't prevent you fom making boost. In fact, you're shoving the mix back into the intake, building pressure further.

Regardless, definitely fix the problem. You don't want to know what happens if your motor has an intake "sneeze" with a roots blower on top. The pressure's got nowhere to go, if you get my drift.

BTW- you won't see any boost on the gague until you're up around 80% throttle or more. I have also strapped a vac/boost gague to the windshield and studied the readings while driving. That's the nature of the beast. If you're still getting no boost with the valves set correctly the next thing I'd check is to make sure the throttle is DEFINITELY getting all the way open when you mat the pedal.

The stock pulleys are designed to make 6 PSI on a "stock" 350. If you've got more cubes, bigger cam, higher flowing heads, etc. you will "outrun" the blower. In my first very mild blown 383 I would only get about 3.5 PSI of boost with the stock pulleys- and it still made impressive power gains over running the same engine N/A. My current setup is a slightly more aggressive 383. I swapped the bottom 6" stock pulley (Weiand 142 blower) for a 7" unit and I'm still only getting about 5 PSI. But MAN does it pull hard when it goes into boost, even at only 5 PSI.

I was hoping you would see this post Damon. I reset the valves and did a compression check to confirm and everything was good. Cranked it up and had some blowback through the carb that was consistent - not combustion though. Pulled the valve covers again and an exhaust valve was too loose! I reset all the valves again after looking up how to set them again. (can you tell I'm new to setting valves). This time I must have got it right. Fires up and purrs like a kitten. Tomorrow I will hopefully finish a breakin run. I'm sure my timing is still off. I do see vacuum at idle up to 2500 RPM I only revved it past that once which was by accident and I did see some boost. This engine is on a stand right now not in a car. I can't believe I haven't ruined my new cam with all this starting and stopping and resetting valves.

Don't worry about your cam. People act like they're made out of Play-Doh or something. Can't even count the number of times I had to start and stop a totally fresh motor a buncha times to fix a leak or something during that "magical" 20 minute break-in period from first fire. They're tougher than people give them credit for.

Quick update: The engine cranks and idles easily now. This weekend I need to set the initial timing correctly and run the engine for a while. I have some glowing headers right now and I think it is timing related.

Damon: I know engine size, intake, heads etc. will affect vacuum and boost pressure but can you give some examples of your readings? You mentioned monitoring vac/boost while driving. What did you see at idle/cruising/WOT etc? With the engine stand I built for my engine it revs so fast I really can't get a good idea what boost I am going to make. I sure I won't be able to fine tune this thing until I get it in the car and under load but I am still curious as to what you observed.

Here is the beast I have been working on: Engine Stand

Glen

Depends so much on the cam and displacement it's hard to say. a very mild 350 you'll probably see 6 PSI from the stock pulleys. On a 383 with a cam of about 220-225* duration you'll be down around 3 PSI, jsut to give you a rough idea. Fortunately, in the car it's still very easy to change pulleys- both top and bottom (top being the easiest). If you're way down at 2-3 PSI then change to the larger 7" bottom pulley right off the bat (vs. 6" stock). There's only one smaller pulley for the top beyond the stock one. Stock it's 3.08" or thereabouts. Next smallest pulley is 2.85" and that's as far as you can go.

I have a spreadsheet with all the different pulley sizes and the ratios they give if you want to email me.

Yeah I have already got the 2.85 size pulley but I am not going to mess with it until I get the car running and tuned. I have the extra long neck blower to clear a 3 v-belt water pump pulley and I don't think they make the 7in bottom pulley in that length. For my goal which is an emmissions legal daily driver with 400 hp I don't think I need the 7in pulley anyway. I think my affective compression ratio will end up 12.x:1 according to Holley's charts. Thanks for all the input.