We have simular troubles here with my Cig Bullet exhaust thru prop...! Twin 600hp on dyno with 5300 rpm and only 70 mph on fresh water. :-( (Bravo 1 - no notch - 3 blade props 27 labbed or 4 blade 26 - 5000 rpm) X-dimension is 7" below pad. Boat weights 4 tons - 31 feet.

Looking into dart heads and the cyclon header
Where did you buy your heads from

Buddy had a 31 bullet with 600s. Turned 26p bravos to 5500-5700rpm. Had CMI headers

[QUOTE=TELLefant;4515634]We have simular troubles here with my Cig Bullet exhaust thru prop...! Twin 600hp on dyno with 5300 rpm and only 70 mph on fresh water. :

You are trying to get 600hp worth of exhaust through the prop hole?
How many CI engines?

[QUOTE=14 apache;4515670]Yes, we have to go thru prop - that's the law. :-( We have custom lightening headers which fit with the y-pipe. They are 454 mags with Edelbrock Viktor 24 (7748) aluminum heads. We know that we loose some hp thru prop but even silent choice is not allowed here.

[QUOTE=TELLefant;4515684][QUOTE=14 apache;4515670]

Yes, we have to go thru prop

Wow that is killing you for sure.
Wonder if you could do some porting on the drive. LOL


Smaller camshaft could help make more speed.

You're right - that's killing us since years! Terrible issue, and yes, cam is changed.

The thru prop exhaust law,there is part of a conspiracy by volvo penta lol....
.....but i have heard their drives offer less backpressure through the prop ..

[QUOTE=TELLefant;4515684]just curious, does the law state "thru prop" or below waterline? if you have speed masters or arnesons there is no way to do that. if it states below water line could you not just run down pipes that exit at the bottom of the transom. would be better than trying to put it all thru a 2 1/2 inch hole in the gimble.

prob has 500 hp or less at the prop - maybe something like this - Dyno shop a higher correction factor than believed as its way in their favor, Not uncommon,

Auto headers to marine non true headers loss of 40 to 50 hp., Dynoed with no or min accessories bolted up front to engine - 10 to 20 hp loss depending, also on the dyno for peak numbers maybe timing was or could be have advanced more so than currently. Next is, Hp loss thru the drive. IMO - this engine 475 to 500 hp at the prop max.