Fwiw.

Icdedppl swapped out 871's, for 1071's, on his 540s. What it took to make 6-7lbs of boost with the 871, was 3% over. What it took to make 6-7lbs of boost with the 1071, was 3% under. Roughly a 6% decrease in blower speed. It reduced the blower speed, from 6,180, to 5,820 RPM at 6000 Engine RPM. So, about 360 RPM of the rotors.

Going from a 10-71, to a 14-71, all things being equal, (no hi helix or anything ), you're looking at about a 10% change in blower overdrive, to make the same boost/air movement. So, if you're running a 1071, at 20% over making 8psi, the 14-71, will probably get it done at 10% over.

Those changes, will net a change of about 600rpm reduction in blower speed, at 6000 engine rpm. A 16-71, slightly more. Figure the 16-71 will get it done, at about 5% over. Again, all things being equal.

From what I have seen, on the dyno doing some testing with a 10-71 Littlefield with a blower shop intercooler, was that, the increased overdrive, didn't have much if any affect, on the intake air temperatures. At 6psi, 870hp, 8psi, 930hp, and 10psi, 1000hp, the intake temps stayed in the 110-115 degree range during the pulls. I believe to make 10lbs, the 1071 was at or around, 20% overdrive, I don't remember the exact ratio. But the temps stayed good, and the power kept going up, but we figured enough is enough.

On my personal engines, I saw a slight increase in intake air temperature, going from 1:1, to 8% over. About 10 degrees increase. Temps were still below 110 degrees at 8% over. At straight up, they barely hit 100 deg during the pulls.

Out in the boat, held at wot, air intake temps at 8% over, stayed in the 110-115* range, even on hot humid days. thats with a 420 B&M, and Teague superchiller intercooler.

The non intercooled 10-71, at 3% under, runs about 135-140* intake temps, at wot in the boat. The only time it gets higher, is AFTER a run, and you come back to idle. She will climb to 150ish idling , then cool back down after a bit.

From my limited experience with roots blowers, is that, the power gains, are not so much, from 8-71 to 1071. Its just not a HUGE change. However, the power gains from those baby blowers, like going from a 177, to an 871, or a 250 to an 871, can be substantial. Makes sense, when you're reducing rotor speed, by in some cases, over 100%. In otherwords, a baby blower turning 12,000 rpm, to a blower turning 6,000rpm , at WOT.

just thought I'd mention it. I think billy boats has you covered. Run what you have, and enjoy it, or, switch to a screw, but skip the 16-71 .

Also, as Billy Boats said, going too big with a roots, or even a screw for that matter, can hurt torque. Screws like to turn some RPM, and Roots dont. Downside with slowing either of them down, is the efficiency at lower speeds can drop, just like it does with a roots at high speeds.

If you look at some dyno data, on engines with small , highly overdriven blowers, you'll many times see, boost start out say, at 7lbs, and by 6000rpm, they may drop to 5lbs or so. Say, with a 177 or 250 blower. But, you bolt an 871 on it, and the boost starts out maybe at 5lbs, and by 6000rpm, its up to 7lbs, which is the opposite. If I had to pick, i'd rather have a blower that maintains , within reason, within a pound or so of its max boost from 3000 to peak rpm. Of course cam timing, exhaust flow, etc, can affect those readings as well. (too small a cam will cause boost to stack up at high rpm)

For the most part, in my opinion, an offshore boat, should have a blower, that is efficient at higher engine speeds, rather than low engine speeds. If I was building a street 454, I'd opt for a 250 blower. For an offshore 454, I'd opt for the 871. An 871 lugging around at 1500-2000 engine rpm, is going to have a good amount of "carry back", aka rotor leakage, at low speeds. Basically, its inefficient there. A smaller blower, like a 250, will be much more efficient and responsive at those rpms, and you're not holding a street engine at 5500 rpm for 30 minutes.

good information joe,let me add by saying that the engine making 1000 with the proper sized roots will see a big gain in power with the properly sized screw blower due to less parasitic loss.

Just to throw this in and everyone can think I'm nuts but my builds are gonna be 555 871's with Valako promaxx 317/340 heads and only want to run enough boost to achieve 900 hp. I don't run all day long wot but do have a tendency to get on the buds and leave it pinned for unknown periods. Is what it is. Lol. my personnel goal is just 8.25:1 comp and like 5-6 pds to hit 900. Im not worried about it but drives are gonna be hating it regardless what I do unless I flipped and went back to NA. Even with SC the magic in the heads work miracles. Jim didn't even have a concern to achieve 1k plus hp even with an 871. Either way the 12 meter will be loosening up and free.

john,what cam did you decide on for these engines and what carbs?also will they be innercooled.

Has anyone dealt with steve morisson or nelson racing engines? Both these guys build some serious power. They both can make custom cams to your spec or will design one for you. It seems like they have it figured out .

Thats why i asked if you knew were they were dynoed at and who it was who had them for sale. I crunched the numbers a little bit at those bsfc's , would take almost 5 gallons a minute at those bsfc's to run them at 6000 rpms!

Steve morris is a big procharger guy. Not sure how much roots stuff he plays with. He does do some big power stuff !

[ATTACH=CONFIG]565243[/ATTACH]Here's an intercooled roots 8-71 547ci at 8.1 comp, running 10 pounds of boost on pump gas w/ Brodix 345cc intake runners setup this Winter by YPM. Dyno was performed w/ accessories and wet CMI E-Tops.

[QUOTE=SecondWind;4534391][ATTACH=CONFIG]565243[/ATTACH]

Here's an intercooled roots 8-71 547ci at 8.1 comp, running 10 pounds of boost on pump gas w/ Brodix 345cc intake runners setup this Winter by YPM. Dyno was performed w/ accessories and wet CMI E-Tops.[/QUOTE

What cam is in this? Off the shelf or a custom grind, solid or hydraulic?