blown 83

iTrader: (1)

I have a couple question for guys running blowers. It's about lobe separation and blower carbs. First I will give a little info on the engine.
-350 chevy, 4blt main, .030 over, decked
-4340 eagle non-twist forged crank, w/double keyways
-4340 eagle H-beam rods
-J&E forged 24cc dished blower pistons,spirol locks, and flycut
-AFR 210cc heads w/2.02 and 1.60 valves
-shaft mount roller rockers
-8-71 supercharger w/8 pounds to 14 pounds depening on pulley
-two 650 holley double pumpers
-pete jackson gear drive
-Isky solid roller cam with 266/276 @.050, lift is .656/.672 with the 1.6 ratio shaft roller rockers, with a 110 deg lobe separation

The first question is about the lobe separation. This is the first blower motor I have built,but not the first serious engine. It has taken me a year to gather all the engine parts with a lot of time spent on each and every part I have chosen. Every thing I have ever read or every one I have talked to says 112 or 114 deg of lobe separation for a blower cam.
Now when I was speaking with Blower Drive Service about a cam they said they put 110 deg lobe separation in all thier motors. I questioned that and they said it helps with cooling and flatter touque curve. They said they have set up thousands of engines over 30 somthing years. So I bought the recomended cam and now I question the 110 deg LS.
do most agree with the 110 blower drive service recomends or with the 112 and 114 that most every one says is correct.
By the way you all probably realize this is not a engine used on the street too often but will see some ocational street duty in the local area.

Second question is about carbs. Who has set up a few blower carbs. once again this is my first go at it. Biggest question is about vacum referancing the power valves to the manifold. I know some like it and some don't. happens that I do and would like to see a couple of pictures or good explanation of it.

If anyone has run somthing simialar and sees me making a mistake some where or just a few tips, I'm all for it. It's not fully assymbled yet so I'm open to ideas. I'm shooting for low tens with about a 4000rpm stall +/- 500 and 4.11

Damon

I'd be wary of a 110* LSA on a blower cam. You can squeeze it down tighter if it's a short duration cam becuase there will still be very little overlap even with the tight LSA.

However, once you start building some duration into the cam (230* @ .050 or more) you better step up the LSA or you'll be blowing a lot of boost right out the exhaust valve during the overlap period. Run some desktop dynos and see what the increased LSA does to the power curve. Don't look at the specific numbers- they're not reliable. But look at what happens as you take the same cam and open it from 110* to 116* LSA relative to eachother. You may find the results elightening.

Can't help you with the carb power valve referencing. I'm one of the guys who doesn't like doing that.

blown 83

iTrader: (1)

Thanks Dammon,
I ran it through the Desktop Dyno (not that I put a lot of faith in it) and it gave me a little improvement but nothing to write home about. I expected more than it showed, thininking more boost would be lost out the exhaust. It showed about 10-15 hp down low and about 8-10 hp on top with less improvement in torque.
If a lot of boost was being lost out the exhaust I wonder if Desktop Dyno could even determin that. It is just that every one is preached that if you run tighter than 112,114,or 116 deg LS that you WILL HAVE DEVISTATING POWER LOSS. and Desktop Dyno (if correct) is not showing much difference.
I do believe Blower Drive Service in one aspect of mabey keeping blown motor cooler with a 110* LS. For one reason that it should be bleeding off a little cylinder pressure, don't really think that would be the best way of doing it though.

RB83L69

I doubt it will have much effect at all in the setup you're proposing... a 8-71 blower will move enough air for 2 of those engines, so bleeding a little bit of boost off isn't the end of the world. With the smaller centrifugal ones I can see where every molecule counts. Even with a 6-71 which would be the more normal choice for a sub-500 CI motor such as your 350, LSA isn't near the issue that it is with the street units. Frankly I would suspect that the LSA changes would have essentially the same effect in a setup like yours as it would in a NA motor: i.e. tighter LSA will make the motor produce more power at all RPMs.

Whenever you put a blower on a motor, the relationship between intake "vacuum" and engine load becomes meaningless. A power valve attempts to correct the mixture to compensate for the engine's different fuel mixture requirements between light and heavy load, as measured by vacuum. When that signal is no longer reliable as a measure of engine load, it makes no sense to use a power valve at all. That's why most of the time people plug them off and re-jet the carb.

BDS knows what they're doing with those professional racing blowers. I'd believe what they say a long time before I'd let somebody with only street blower experience change my mind.