Offshoreonly.com - The 39 Rockit with 725 Ilmors and Indy drives hit 115 mph

Offshoreonly.com (https://www.offshoreonly.com/forums/)

- General Boating Discussion (https://www.offshoreonly.com/forums/general-boating-discussion-51/)

- - The 39 Rockit with 725 Ilmors and Indy drives hit 115 mph (https://www.offshoreonly.com/forums/general-boating-discussion/228403-39-rockit-725-ilmors-indy-drives-hit-115-mph.html)

Quote:

Originally Posted by C_Spray (Post 3088164)

Sources elsewhere claim 760 at the crank for 700NXT's..

Boat sounds really, really cool. Beautiful.

Great speed I really like that boat!:drink:

Quote:

Originally Posted by C_Spray (Post 3088164)

Sources elsewhere claim 760 at the crank for 700NXT's..

correct 700 prop hp... ilmore 725 is 725 at the crank.. not prop... correct?

I've seen the video on the 700 boat and now 725 Ilmor boat.,.both beautiful boat...LETS SEE THE GPS FOR VERIFICATION

Quote:

Originally Posted by customryder (Post 3088272)

correct 700 prop hp... ilmore 725 is 725 at the crank.. not prop... correct?

That's my understanding. The Indy transmission and drive are both dry sump and apparently very low drag, so things are probably pretty even at the prop.

The only problem with Ilmore engines that they do not have the same torque as mercs or any other engine in the 700class.
Torque is what pushes the boat.

Quote:

Originally Posted by baja27 (Post 3088513)

The only problem with Ilmore engines that they do not have the same torque as mercs or any other engine in the 700class.
Torque is what pushes the boat.

I am definately not an expert on the Ilmore engines and I am sure someone will chime in on your statment to back me up or back you up here but,

the Ilmore's are 8.4L or 511 cu in. engines while the 700sci's are 8.2L or 502 cu in. While that is pretty close, displacement is a big contributing factor in torque production.

Both are overhead valve engines, and both are pretty close in displacement. The big difference seems to be the Merc uses a supercharger to make similar numbers to the Ilmore and the Ilmore uses 10 cyl vs. 8

So for you to say the Ilmore's are down on torque compared to the 700sci which requires a supercharger to make its power doesn't seem to make sense to me.

A naturally aspirated engine will generally make its torque at a lower rpm and maintain it longer than a super, or turbo charged one. At least that was always my understanding.

Again, could be wrong.

Quote:

Originally Posted by macjazzy (Post 3088643)

A naturally aspirated engine will generally make its torque at a lower rpm and maintain it longer than a super, or turbo charged one. At least that was always my understanding.

This is my understanding as well...I think you are right.

Don't confuse torque at the crankshaft with torque at the propshaft. If the Merc 700 makes it's power at 5200, then it has 707 ft-lb at the crank. If the Ilmor makes 725 at 6300, then that's 604 ft-lb at the crank. If the Ilmor uses a 1.5 drive and the Merc a 1.24, then the Ilmor wins at the prop: 906 ft-lbs at 4,200 versus 877 ft-lb @ 4,194 rpm.

Where the Merc has a potential advantage is under acceleration. If Mercury chooses to up the boost in the mid-range, the power curve can be fattened up as much as they want.

Ain't competition grand? ;)