I'm running a small roots blower and the car really rips, but I finally decided to hook up a boost gage and discovered that I'm only getting 2.5 lbs of boost at WOT. During light and moderate throttle, I see no boost at all. Now I realized before I purchased the small blower for a 400 plus cube engine, that I just might run into this. The only reason I suspected this would happen is because this particular blower produced 4-5 pounds of boost on a mild 350 CI engine with plenty of power under the curve for street driving. -So - I figure since I have more than 350 cubes, I can only manage 2 lbs of boost. From what I've read, this isn't necessarily a bad thing, and could be an indication that the engine is breathing well and is able to use pretty much use all the boost the blower is producing, so it doesn't "stack up" in the intake if that makes sense... I installed a MSD boost retard box that I have not used yet, because I just don't need it. I have total timing set at 36 degrees with no detonation. I also tried changing up carbs between a 650 DP and a 750 DP just to see if there was any difference, and it actually runs better with the 650 on it. My question is, do you think I would see even bigger gains in power if I changed my current 3.5" drive pulley to a 3" pulley (and a shorter belt)?? That way, I might see 4-5 lbs. of boost - maybe. -Don't get me wrong, I can most definately tell there's plenty of power and the car is traction limited as is (which I need to work on) - but I'm just wondering what most of you guys would do if you were in this predicament - leave well enough alone, or see how much you could get out of it?

Of course switching blower pullies out should result in more boost. It just comes back to blower efficiency and tuning.

I recommend you call the company that makes the blower. They will be able to tell you exactly what to do. Companies can usually tell you approximately how much boost you will make with what pulley given the engine size. They also know when a blower is not the correct size for the application and the smallest pulley size which can be used on the blower.

Going too small of a blower on a big motor you have to spin the blower faster which can build up extra heat. Going too big of a blower on a small motor you have to spin the blower slower which can cause pressure to "bleed back" through the rotors.

Just be careful with your timing if you do go more boost, 36* is probably pretty aggressive when you do start making a moderate amount of boost.

Good advice!
I called Magnuson today, and they have a computer program that simulates how much boost you'll get given a specific engine/drive pulley size. The Tech there told me that if I go down to a 3.0" pulley, I should get 5 pounds of boost. If I go down to 2.5, I'll get 6.5 pounds (+/- .5)

I think to be safe, I'll go with the 3.0" pulley and see how well the timing holds up.

Can you buy a larger bottom pulley? I ask becuase on my Weiand 142 I got belt slippage issues going to the smallest upper pulley with the stock bottom pulley (just not enough pulley contact for the skinny 6-rib belt to get a good grip). I then put on a larger bottom pulley with the stock upper and it works just as well but without slippage/belt-hopping issues.

Amazing how much power you can make with a roots blower even with only a few pounds of boost, isn't it? When you get it up around 5 PSI you'll like it even more. But your rear tires will hate you. It'll hit like a sledgehammer. Low/midrange torque takes big jumps with increased boost.

BTW- you won't see boost on any roots setup until your foot is pretty deep in the throttle. Nature of the beast.

I won't argue about the smaller carb workign better on your combo, but it seems generally a bit small for that combo. Maybe the tune is just better than your 750. I'll merely suggest that you hook up a good (sensitive) vacuum gague to a port under the carb (but above the blower) to see how much of a restriction that carb is at in the upper RPMs at full song. It might surpise you. Remember that roots setups are very sensitive to carb restrictions- much more so than a N/A or blow-through setup. Just one inch more vacuum under the carb can make a noticable difference in the observed boost in the manifold. Probably not much of an issue at 2 lbs. but when you get up in the 5-6 range you might start to notice it more. Just stuff to keep in the back of your brain for later.

Hey Damon!

Can I get a larger crank pulley? Well - it's a good idea, but they don't list any crank pulleys in their catalog. The original crank pulley that comes in the kit is fairly large to begin with. It's a 6.75" diameter. Oddly, they do list 8 rib blower pulleys though, so you'd think they carry the associated 8 rib crank pulleys to match - but again they are not listed....I guess if I have issues with slippage, I'll need to look into it.

It's amazing already how much power I have on the 400 with only 2 lbs. of boost! The tires are already hating life, and traction/wheelhop is an issue I need to work on...I'm hoping a set of better rear shocks might at least help with the wheel hop, and I'll get better tires when these are gone.

When I change pulleys, I'll compare the 650 and 750 carbs again. I just couldn't feel any noticible difference between the two when I tried them as it is now...but the 650 is PV referenced and the 750 isn't, so according to my cheap narrow band A/F gage, the 650 doesn't run so lean.

Hopefully all you guys won't hate me, but I just bought a '09 Challenger RT, and I've been cruisin around in it too lately. It has "refined" power and it's an awesome car...but - my Camaro will still outpower it. lol