Roots Supercharger Boost Comparison

[BUSTI]

First some insight on set up. The engine is a 460" BBC with iron 088 cylinder heads and polished chambers. The bottom end is pretty decent for what it is. Kellogg crank, Manley rods and JE 7.9:1 compression. With a non intercooled 250 B&M supercharger. Dual 750 Holley carbs. When it was built it was considered a 600 HP build. I'm sure it was. The engine runs very strong and is a lot of fun. I'm not looking to increase power but the blower does look a little small. I'd like to upgrade to an 8-71 Blower Shop Billet supercharger. It looks so cool and I'd like to have a little wow factor then I open the engine hatch.

So the question is if I up grade to the 8-71 and stick with the same 6psi of boost what effects will it have on the engine? Will it generate more heat? Will it need to go back on the dyno? Will I loos a lot of power due to the increased supercharger size?

Thanks for all the help.

 

[LargeOrangeFont]

6 lbs of boost on one supercharger is not 6 lbs of boost on another. 6 lbs on a larger supercharger will in theory make more HP and heat. You'll need a retune for sure.

 

[Runs2rch]

6 lbs of boost on one supercharger is not 6 lbs of boost on another. 6 lbs on a larger supercharger will in theory make more HP and heat. You'll need a retune for sure.

Backwards. Boost is just a measure of psi. The larger blower will be creating less heat than the smaller unit at the same psi.

 

[rivrrts429]

I would seriously look into an intercooler regardless of what you do. Blowers make a lot of heat and cooling the charge is the easiest way to gain a few horsepower in a boat with a blower. In a car it's a little more difficult but you're not in a state of constant load either, a boat you are. On the lake you have endless amounts of cool water flowing through it.

I'm not that familiar with the B&M's but I'd imagine it would take a few ponies to turn the bigger blower. Not sure how much you would notice it though. Why not a 1071? You still retain the stock distributor location and have a big enough blower to pretty much do what you want in a build down the road. I always have a problem with going bigger so maybe don't listen to me lol.

 

[wsuwrhr]

I always have a problem with going bigger so maybe don't listen to me lol.

That's too bad.

 

[wsuwrhr]

Lately I have been daydreamin about a PSI blower over a roots style.

Never played with pressurized intakes on my personal toys.

I am pretty sure a PSI is for the big boys, but I was wondering what the benefits were, or was it just plain overkill?

Brian

 

[rivrrts429]

Lately I have been daydreamin about a PSI blower over a roots style.

Never played with pressurized intakes on my personal toys.

I am pretty sure a PSI is for the big boys, but I was wondering what the benefits were, or was it just plain overkill?

Brian

Overkill or just amazingly efficient for its design?

There is a reason they're banned in top fuel. They're waaaayyyy more efficient than a typical roots style blower with their rotor design and produce huge amounts of boost. They can also explode sending parts in all directions, hence the NHRA ban.

 

[wsuwrhr]

Overkill or just amazingly efficient for its design?

There is a reason they're banned in top fuel. They're waaaayyyy more efficient than a typical roots style blower with their rotor design and produce huge amounts of boost. They can also explode sending parts in all directions, hence the NHRA ban.

Yes.

Im not sure Im OK with parts getting sent in all directions.

Brian

 

[rivrrts429]

Yes.

Im not sure Im OK with parts getting sent in all directions.

Brian

No exploding parts in a typical build. Nitromethane maybe lol

 

[wsuwrhr]

No exploding parts in a typical build. Nitromethane maybe lol

Im in. They look even more badass than a roots. IMA thinking my Dart and Hondo Sprint would like one to share.

 

[BUSTI]

I would seriously look into an intercooler regardless of what you do. Blowers make a lot of heat and cooling the charge is the easiest way to gain a few horsepower in a boat with a blower. In a car it's a little more difficult but you're not in a state of constant load either, a boat you are. On the lake you have endless amounts of cool water flowing through it.

The problem with an intercooler on my Bullet is it just doesn't look cool. It's as tall as the 250 blower. I don't want to jump an additional 75HP and risk drive failure. I'm already close to its limits.

 

[rivrrts429]

The problem with an intercooler on my Bullet is it just doesn't look cool. It's as tall as the 250 blower. I don't want to jump an additional 75HP and risk drive failure. I'm already close to its limits.

I hear ya. These boats easily turn into a can of worms. Next thing you know you're adding a whole new outdrive.

You can always run it until she gives up the ghost and then decide which direction you want to go.

 

[SBMech]

I hear ya. These boats easily turn into a can of worms. Next thing you know you're adding a whole new outdrive.

You can always run it until she gives up the ghost and then decide which direction you want to go.

If you are close to the "limit" of what you are told/think that your drive is capable of handling, then you should probably look at just running it as is and cookie jar the upgrade till you break something.

Then go big :thumbsup

 

[BUSTI]

That's the thing. The boat runs really fast. It's not a slouch. I like how fast it is. For an open bow family machine I don't want to go much faster. I want it to look better with the larger blower. I have been changing thing in the boat that IMO should come from the manufacture like all the trim, hatch and drive reservoirs to billet. They never should have come plastic. It's about looking good.

 

[LargeOrangeFont]

Backwards. Boost is just a measure of psi. The larger blower will be creating less heat than the smaller unit at the same psi.

It's actually a measure of restriction. In theory the larger blower will create less heat, but that is not always the case. Think across different blower types.. Twin screw is more efficient than roots.

When i say heat you also must consider general engine heat. Without an intercooler the engine components will just absorb extra heat when making more HP driving air temps back up. An Intercooler stops that in its tracks.

 

[BamBam]

Backwards. Boost is just a measure of psi. The larger blower will be creating less heat than the smaller unit at the same psi.

This. ^^^

However it may rob more HP to make the 6PSI. There are different size blowers for different applications for a reason. If it were linear you could make 6psi with a 14-71 and it would be even cooler than an 8-71, but it takes more HP to turn the 14-71 just as it takes more to turn an 8-71 over the 250 B&M. I don't think you would be worried about an intercooler with 6psi from an 8-71. When you add an intercooler you have to turn the blower harder to get the air through there. It seems counterproductive for 6psi. There is a breaker point where it is more efficient with an intercooler. If you want the cool factor and the look of an 8-71 put an axial Whipple on there. Less HP to run and you can turn up the boost when you build a bigger motor.

 

[BigSteve]

Call Teague
Screw blowers work a hell of a lot better
And they have a complete proven parts package for marine

Roots blowers look good they just don't work that good.
They suck up more hp and produce more heat than a screw

Heat, is the enemy, lowering intake temp reduces the exhaust temp

 

[obnoxious001]

I am a little late to the party here, but currently working on rebuilding a 475 cubic inch (.100" overbore instead of the .060" on your 468) that made 680 HP on the dyno with iron heads and no intercooler, but between the static compression and the tune the dyno operator put on the engine, it only lasted 1 1/2 hours before it burned a hole in a piston.

Without the intercooler, but with the larger (cool looking) 8-71, as mentioned, you should have a cooler charge, and therefore make more power given the same level of boost. Heat increases pressure as air expands, but the hotter charge is not good for making power, as the air is less dense.

I don't think that the power increase itself will destroy your drive, it depends more on how you apply the power, and most important that the engine idles at 800 rpm or less with the Bravo drive. I have guys with engines that I built over 700 hp coupled to Bravo drives with no issues to date.

 

[AzGeo]

a Roots type blower has FUEL going thru it to cool it .

You can tune the cooling, just as you tune the cylinder EGTs . (A/F and spark timing advance)

I too have read a lot of 'theories' about blower sizes and how much power each size takes and I don't believe a word of it . If spinning a 16", 18", 24" long 3 lobed rotor takes much different power levels, it can't be more that 5 to 8 HP across the board . The drive and the gears are mostly the same on all sizes, so the 'rotating mass' and the 'rotor to swept case area' can't be enough for me to ever choose anything but the largest GMC style blower that will fit on the motor .

BOOST is the residual number read on a gauge that tells you how much air the intake and blower wants to flow, and how badly your motor will accept that airflow .

Spreading 'air with fuel in it' over a larger case surface area can't be bad at all, and usually also equates to 'lower overdrive speeds' in relationship to motor size and RPMs .

We are actually talking AIR VOLUME, and boost is the fact that the motor restricts the blower's air flow .

As I understand B&M 250 blowers, they are still a 'two lobe high RPM blower', and hold a place in 'moderate performance applications' .

My company did 'marine application testing' for B&M back in the 1980's and after that, I never sold one . We did work on a few motors and did repairs on owner's units but never sold one to any customer .

Get to the ROOTS of roots . Weiand has a blower that has a 'advertised CFM per revolution', it is generally less than a new or rebuilt GMC/roots type blower . (models 6.71 and 8.71) Other companies do not generally advertise the CFM of each unit, but will give specs as needed .

IF you feel the need to make this change, go with a Blower Shop 8.71, slow it down to your 6 PSI spec, and get the best of many worlds, while saving money .

The new blower will need a new intake manifold, carb top, drive system, and hopefully you run a BILLET CRANK HUB, because stacking shit up on any harmonic balancer is dangerous BS .

Your heads, valve timing, and exhaust will determine the actual 'boost numbers' you will see, but no matter what, you will need to do some 'tune up work' with a change like this .

I'm not selling anything .........

 

[TEAGUE CUSTOM MARINE]

So to answer the question at hand and not open up a big can of worms about engine and induction theory (and practice): if you change to an 8:71 from the 250 and set the overdrive to match the boost at 6 psi, there will be no negative effect on the engine. Depending on what your current carb settings are, you may not even have to change the tune.

Given the current combination you are running, I would like to say that unless you add an intercooler, the change would really be cosmetic. While the roots blower will be more efficient, and capable of much more horsepower/boost/volume, you can't really take advantage of it in a marine application without an intercooler. The larger 3 lobe blower should make the same level of intake pressure with less effort, but the gains wouldn't be worth the price of admission. With the unlimited supply of cooling water in a boat it really is silly to think about performance upgrades and pass on an intercooler.

I would want to know what cam you're running, but given the low compression, if you have good gaskets, fasteners, and exhaust seats in the heads, you should be able to run 8-9 psi with an intercooler safely and really wake the motor up. Since you are going to purchase a NEW blower, it might not be a bad idea to get a 10:71 for future growth in engine or boat configuration. For reference, the TCM 800 was a 10:71 on a 509 at 8.3:1 and 7 psi boost at 1:1 blower drive. It is our experience that the two greatest variables effecting required supercharger overdrive for a given boost level are engine size and compression, unless your cam and heads are too small.

That being said you're happy with the current level of performance and really want the looks factor of the blower over anything else, so maybe don't overthink it.

Also, we have the best prices for all the stuff you will need to do the change.

JT