nonstop

Copy that on the timing MT. So I'll bump it up grab a few new plugs and take some readings. My guess is I'll be jetting up a bit with 4 more degrees. I have to order the af gauge today. I have the bungs for the egt sensors on my manifolds already but no equipment to read with. I only have about 8.5 to 1 comp.

nonstop

I also ran the calculations for carb size and I need to be in the neighborhood of a 965cfm. If I'm running 4lbs and about 1045 cfm at 6lbs. In order to get to 6 I have to spin the blower to hard (5700rpm) So my carb looks to be close.

ThisIsLivin

I would check the flow of your flame arrestor. I called K&N about their products that are USCG certified and discovered on my NA 524 I had to use a 14x4 filter to flow enough air to keep the power valves open. Most of the stock Merc flame arrestors don't flow enough. I went with a custom venturi style unit with the USCG K&N filter to smooth out the flow to the carb, most arrestors have a sharp right angle into the air horn causing turbulence over the venturis.

MILD THUNDER

iTrader: (3)

What size pulleys do you have top and bottom? That's the deciding factor. The engine at 5700 rpm is one thing, but the blower rpm is another. I'd imagine your blower has to be spinning 12000rpm possibly more depending on pulley size.

In a B&M book I have here ,(vintage), they did a test of air temp in the intake manifold with a 250 blower. At 6000RPM (engine rpm), at 60% overdriven, the temp was 140*. At 112% over, temps were 230*!!

I made the switch from small highly overdriven blowers, to the 420 blower underdriven. Boat is much faster, and I think the engines are much happier.

nonstop

I think after I get the prop gone over and bump the timing up I might pick up a few mph. If I hit 80 that would be great. For now I'm getting time in the seat going kinda fast. The info MT on the air temp is great. I think my chiller is doing its job! I will measure the top and bottom pulleys and let you know. Keep in mind with my 27 I'm only getting 5200.

Griff

Mild Thunder posted a lot of great info for you since I have checked this thread. Everything he said is right on.

MILD THUNDER

iTrader: (3)

Thanks griff

nonstop

Yup, You guys are alot of help! I was doing a Google search for rpm on that blower and came across a few thread's from 2010 from a build that MT was doing very close to mine. Also found an article where a 177 and a 8-71 were run on a dyno. The engine was a budget build, (oval port heads, nothing fancy internally) and the they got 717hp from the 177 at 3psi of boost at 6200 rpm and the 8-71 @ 4.8 lbs was 766 hp at 6200 and @ 6.4 lbs on the 8-71 at 6100 it made 762 hp.

MILD THUNDER

iTrader: (3)

I remember that article on the 177 vs. 8-71. In the past few years, ive learned a ton about roots blowers. To sum it up

Tow Vehicle, street car, low rpm torque=Small blower overdriven.

Race car, offshore boat, high rpm continuous, =Large blower underdriven.

If I was building a street friendly cruiser with a 454/502, I'd have no qualms about using a 250 sized blower. Less low RPM leakage past the rotors, and your just not in the throttle for long sustained periods. It will more than likely make more torque at the same boost level as a 871 under 5000rpm.

Back in the 80's when B&M was making a lot of superchargers, many of the 174's were popular on the production 454 peanut port headed truck engines of the day. They added a nice increase in torque, fit under most hoods, and worked well for that. They soon realized the demand for something larger, but still clear hoods. The 250 blower was introduced. It was longer, but not higher. The 420 mega blower was around, but most guys couldn't fit them unless they had a large scoop or hood cutout. So mainly those were used on serious street cars and drag cars.

In todays modern world of high flow heads, big roller camshafts, free breathing exhausts, the little blowers get easily overworked. Todays 'mild' marine and street engines far exceed most race engines from 25 years ago. Lots of ways to stuff air in the cylinders. A big whipple or procharger pumping 12psi into the intake is still probably cooler than a 174 roots pumping 5psi on a bbc.

In a boat engine, for the most part, on pretty much anything besides a 330HP 454, i'd do a 871, if were talking roots. 540CI and bigger, 10-71, or even 14-71. I'd gladly sacrifice a few FT lbs of torque at low rpm, for the much cooler underdriven air charge, longer blower life, and better fuel distribution than the smaller single carb blowers.