Need help, impeller question

[bigjack]

Was turning Berkeley aluminum A at 5700, changed to B cut stainless with inducer and I'm still turning same RPMS. Really confused.

[shaun]

If your impeller was measured and was a true A and your pump was tight then it would take you around 700 HP to spin that RPM. If you cut it down to a B that should give you a 200 to 300 RPM increase. Add and inducer and that will probably eat up that new RPM. Also, a new impeller is most likely going to bite more than your old one unless your old impeller was nice and straight. A stainless impeller is also heavier than your aluminum one which means it's going to take more HP to spin it.

Do you have a Dyno chart showing your engines power curve?

[bigjack]

No I don't, it's a BBC 525 capable of making big hp.

[shaun]

No I don't, it's a BBC 525 capable of making big hp.

Unfortunately there are a lot of variables here. Big HP to one and means something completely different to another. For example "big HP" (for a jet boat) in my mind is up over 1000 HP. The engine I built (498" BBC) put down 678 HP @ 5700 RPM on the dyno with push rods that were too short. With the correct push rods it's now closer to 700. I pull my "A" stainless impeller to 5600 RPM with a MPD race prepped inducer. Another important factor is where your engine is making it's power. If your making peak at 6500 RPM then your going to have to cut the impeller down to spin that otherwise your just leaving power on the table. When I built my engine I had my cam guy (Chris Straub) profile the cam to make peak power around 5600 RPM and he was spot on! He also knew my mill was going into a jet boat and he has the knowledge and experience to design a cam with a usable power curve in that application.

Based on what you've said so far, it sounds like your making around the same HP at your current RPM.. Again though if your engine is making peak power higher then your just leaving that HP on the table.

[bigjack]

Thank you for all your information, you been a big help.

[shaun]

Np, Just remember that cutting down your impeller means you'll have to put more foot into it when cruising, towing, skiing, etc so you may want to thing twice about cutting it way down. My impeller out of the box was suppose to be an "A" but ended up being closer to an A+ and I was only able to spin 5200-5300 RPM. I had mine cut down to an A to get to the RPM i'm at now.

[bigjack]

Well I'm back with more information, just tried B cut aluminum impeller still turning the same. What I forgot to tell was last year my grandson was turning an A cut aluminum turning 5500 to B cut stainless turning 6000 with added induser, with BBC 496. I guess that's why I'm so confused. Just saying.

[shaun]

Well I'm back with more information, just tried B cut aluminum impeller still turning the same. What I forgot to tell was last year my grandson was turning an A cut aluminum turning 5500 to B cut stainless turning 6000 with added induser, with BBC 496. I guess that's why I'm so confused. Just saying.

Same engine testing all of these?

Are you sure all of these are all measuring correctly? When I purchased my American Turbine A impeller it actually ended up measuring closer to A+ so I wouldn't rely on what the impeller may have stamped on it or if your borrowing one from a friend.

Another key factor is the condition of the impeller. If someone who knows what they are doing has worked the impeller (sharpened the edges, deburred, etc) that will make a big difference.

Depending on your engine's power curve 500 RPM could make a decent difference in HP/TQ too. Looking at my dyno sheet where I made peak HP and then going back 500 RPM I loose almost 40 HP. That doesn't sound like much but my cam was also profiled for a Jet Boat and the power curve is different from what you'd want in a car or prop boat. If i look at the gap between 5000 RPM and 5500 RPM I loose about 60 HP.

What RPM are you trying to turn?