Reply With QuoteI think you are backwards on the vacuum thing. At wide open throttle there is tons of vacuum under the carbs on a blown deal.
Can anyone tell me the difference between blower carbs and regular carbs? I've been told that blower carbs need to be mechanical seconday due to the blowers having generating little to no vacuum. other than that, how hard is it to modify my Holley 850 marine mechanical secondary carbs to work on an 871?
I think you are backwards on the vacuum thing. At wide open throttle there is tons of vacuum under the carbs on a blown deal.
You may need to richen the jetting up a bit, but other than that..... I believe they're the same.
Paul
Here are a couple FAQ's from Holley that describe the differences in their supercharger carbs "boost referenceing" system. From the looks of it, if you run power-valved holley's I would be a good idea to have blower carbs or modified setups.
QUESTION Do I need to run a blower calibrated carb with a supercharger?
ANSWER A lot is going to depend on what the setup is and what you are going to do with it. If it is strictly a race setup with no street use then usually you can get away with a standard carb with the power valves plugged and the carb jetted up to compensate. This does not work well on an application that will get mostly street time. For those applications we do offer out of the box Holley carbs with Manifold Referenced Power Valves which will work correctly on the blower. These carbs will allow the use of the power valves which will give better idle quality and street driveability with a blower.
QUESTION What is a Manifold Referenced Power Valve?
ANSWER Nothing will kill a blower or Nitrous engine quicker that a lean condition. You want plenty of fuel available for the engine to use .There is a thing you need to know about the power valves on a roots style blower engine. The power valve is installed to keep the engine from loading up and running rich at an idle. On a normally aspirated engine the engine vacuum at idle will hold the power valve closed. When you step on the gas the throttle plates open and the engine vacuum drops as you accelerate. When the vacuum drops below the rating of the power valve , it snaps open and richens up the main system. On a blower with the carb mounted above the rotors there is constant vacuum all the time even under wide open throttle. The power valve will never open and you will have a lean condition. To remedy this there is a modification you can have done that is called manifold referencing the power valve. You plug the vacuum feed hole in the baseplate for the power valve. Then you drill a hole in the side of the main body into the hollowed out vacuum chamber for the power valve. You then insert a vacuum nipple in this hole. You will run a vacuum line to the lower intake manifold from the new vacuum nipple. Now you will have vacuum on the power valve at an idle, and when you hit the gas as the boost builds, it will force the power valve to open and richen up the main system. This can be done by most carb modifiers or even yourself. We offer quite a few different size blower carbs with this already done. Consult you local Holley dealer or our Techline for the correct application.
Well here we go again,
Before going through the added time or expense of boost referencing the carbs it's real easy to check and see if you need it.
Check the vaccuum AT THE CARB BASE at wide open throttle.
If the vaccuum is below the rating on the power valve then boost referencing will not make a difference. The power valves are opening fine already.
I've checked this before and I would like to hear the results of others who have checked carb base vaccuum at WOT.
A few years ago when I was running 830's, I blocked the power valves, changed the linkage to 1:1, put in bigger shooters and jets,and ran just fine.
These were carbs that I bought from a friend of mine that had them gone through by a "pro". They even had a cute little sticker on them from the builder. He couldn't get them to work on his deal which was much more CI and boost at the time. They did work with those couple of changes.
I upped my setup and bought some Dominators, 1050's, from a friend for what was then a 500ci Mark IV with an 871 at 6 to 7 #'s boost. I sent them to a "big time " pro. He ended up buying them from me when he completely fckd them up.
George at Clay Smith said to buy some 1150's, they worked perfect to 12 #'s boost with just seasonal jet changes. They came with blocked power valves.
This year I'm trying a hat.
See May issue of ***boat Magazine! but for now.... on a low boost situation(7-8 lbs or less) the main thing is fuel economy, when the power valves are blocked, the carb runs full rich all of the time. This causes about 12-14% extra fuel to be used, if thats not an issue, block them. On a higher boost deal the engine experiences boost at most throttle settings and the referencing just usually cleans up the bottom end cruise. The main problem I see at my shop is that most of the plugged power valve carbs do NOT have the proper jets installed. The usual "add 6-8 jet sizes" doesn't get it. On Dominator carbs the jets must be increased by 12-16 sizes to compensate for plugged power valves, depending on the list #. In that case the carb is running LEANER than stock! To figure out the required amount to richen the carb, pull the front metering block, and measure the power valve channel restriction, calculate the area, and add that to the area of the stock jet. Then calculate the area of the jet size that corresponds to the new area. This is your new required jet size, it will be larger than you think, but you should still add 2 or 3 more sizes to be on the safe side. You can lean it out a little later if the plug reading warrents it. The formula for area is 3.14 x radius squared. The other problem with leaving the power valve in and NOT boost referencing it, is whenever you back down on the throttle under boost, the throttle plates cause a big airflow restriction, and that sucks the power valve shut! The boost may fall from 10 lbs to 7, but now the power valve leans the mixture back to cruise jetting. In a boat this will stabilize to the new rpm and the power valve will reopen, after a few seconds or so, but during that time the engine will experience detonation. Every time you back off of the throttle! This is why many people believe that blower motors need to be freshened more often than unblown motors, the destructive hammering to the bearings does not occur for long enough to kill the bearings immediatly, but the effects of this detonation are cumulative, requiring a freshen up more often. The air bleeds should be replaced with bleeds about .002-.004" smaller too. This will richen the upper end of the rpm range without having much effect at cruise speeds. TIMINATOR
See May issue of ***boat Magazine! but for now.... on a low boost situation(7-8 lbs or less) the main thing is fuel economy, when the power valves are blocked, the carb runs full rich all of the time. This causes about 12-14% extra fuel to be used, if thats not an issue, block them. On a higher boost deal the engine experiences boost at most throttle settings and the referencing just usually cleans up the bottom end cruise. The main problem I see at my shop is that most of the plugged power valve carbs do NOT have the proper jets installed. The usual "add 6-8 jet sizes" doesn't get it. On Dominator carbs the jets must be increased by 12-16 sizes to compensate for plugged power valves, depending on the list #. In that case the carb is running LEANER than stock! To figure out the required amount to richen the carb, pull the front metering block, and measure the power valve channel restriction, calculate the area, and add that to the area of the stock jet. Then calculate the area of the jet size that corresponds to the new area. This is your new required jet size, it will be larger than you think, but you should still add 2 or 3 more sizes to be on the safe side. You can lean it out a little later if the plug reading warrents it. The formula for area is 3.14 x radius squared. The other problem with leaving the power valve in and NOT boost referencing it, is whenever you back down on the throttle under boost, the throttle plates cause a big airflow restriction, and that sucks the power valve shut! The boost may fall from 10 lbs to 7, but now the power valve leans the mixture back to cruise jetting. In a boat this will stabilize to the new rpm and the power valve will reopen, after a few seconds or so, but during that time the engine will experience detonation. Every time you back off of the throttle! This is why many people believe that blower motors need to be freshened more often than unblown motors, the destructive hammering to the bearings does not occur for long enough to kill the bearings immediatly, but the effects of this detonation are cumulative, requiring a freshen up more often. The air bleeds should be replaced with bleeds about .002-.004" smaller too. This will richen the upper end of the rpm range without having much effect at cruise speeds. TIMINATOR
Good post timm! Thats alot of info you put down. I am learning blower motors and Im glad that you took the time to post that much info.
Thx,396
Great post Timinator, I knew a pro would post some real info.
I was running 104 jets on the 1150's and closely watching the plugs.
The 104's were fat as hell in the summer.
The 1150's came blocked from Holly and I would assume Holly allowed for this in their original jetting ( I think they were 94's), air bleeds etc. although the first three letters blah blah blah.
Can you be getting enough detonation to wipe the bearings with no indication on the plug or piston?
I just ran a set of #8082s 1050 Dominators on my12-71 at 10lbs. of boost on a dyno.Ran power valves front and back.NO boost referencing.Worked great.The guys at the dyno said you only boost reference if carbs are too small.I dont know if that is true.#8082s worked great out of the box.I think we ran 10.5 power valves and 94 jets.
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