I just repowered my 257 Legend Powerquest. The following are the specs. It's a Gen IV with no computerization, it's all "old school". Drive is a Bravo I with a 1:50 ratio. Exhaust is EMI open through hull. MSD distributor, ignition box. Looks like an aftermarket dual plane intake, but there is nothing stamped on it for identification. My questions are 1. What size carb will this package require? 2. Will I need to switch to a single plane intake? Seems like it runs out of fuel before I get everything out of it (only getting up to about 4600rpm). I have a Demon carb that is not marine (I definitely want to get that changed), without identification on it now (from the old engine), not sure if it's a 750 or what. 3. How do I tell what size carb I have?

HP: 465
TORQUE: 505
Engine Type: Chevy Big Block V8
Displacement (cu.in): 454
Block: 2 Bolt Early Style
Bore / Stoke: 4.280 X 4.000
Crankshaft: OE 2 Piece Seal
Rods: OE
Pistons: Hypereutectic
Rings: Moly
Camshaft Type: HYD. Flat Tappet
Camshaft Lift: .512 IN X .512 EX
Camshaft Duration @.50: 230 IN X 230 EX
Cylinder Head Type: OE Cast Large Intake Port-781 cast
Chamber Size: 118 cc
Valve Size: 2.06 IN X 1.72 EX
Intake Runner Volume: OE
Compression Ratio: 10.1:1
Rocker Arm Type: Stamped Steel
Rocker Arm Ratio: 1.7:1
Sheet Metal: Chrome V/C T/C Black O/P
Recommended Fuel: 92 Octane
Ignition Timing: 36 Degrees Total Advance
Max RPM: 5800

Thanks in advance for any help,

Rick

Rick, Typically a 454 of the specs you are showing would use like a 750 cfm carb with a duel plane intake.No single plane on that motor.The carb you have you will need to find numbers on it or measure the barrels to determine exactly what it is.Also be sure you have a good high volume fuel pump.
I noticed there are a lot of descripiencies in the specs of the engine. For starters there is no way it would be making 465 hp. and 505 ft. lbs. of torque. It's a .030" over 454 with small valves in the heads and a very small camshaft.Compression on that engine would probably be more like 8.75:1 and run fine on 89 octane fuel.Total timing should be more like 32 degrees and max RPM around 4800-5200 rpm. It appears to be more like around 350 hp.
What pitch prop do you have? My quess would be with a 23" pitch prop on that 25' boat that your only going to see 4600 rpm with that engine.

Where did you get the HP/TQ numbers?????

Everything you listed is very similar to 454mag-365hp specs. The CR is higher, but that will only add about 30hp. Cam has almost the same lift and just 10* more duration. That cam is only good to about 5200rpms before it falls off. The heads are smaller and will have better low + mid torque and are probably about the same in max HP as rect ports. I would say its a 400hp engine at best. A 750cfm carb would be correct for it.

Thanks, guys. I've been racking my brain trying to get what ATK said it should run out of it. Go figure, they misadvertised. Anyone reading this, be advised that although their prices are good, their service is crap (I had all kinds of problems with them just to get this engine) and apperantly so is their advertising and their word. Next time I'll definitely pony up the $ and get something from a reputable builder.

I dont know where you guys get your information from, but this is a KB207 hollow dome piston with a 118cc chamber, 256/114cc ports head. This engine does make the power stated and I will post a dyno graph here shortly. If you think a 454 with 781 heads cant make 450+hp you sir are not informed.

Here is a link for you to help verify compression with this piston...granted the chambers on these heads are all over the board...compression would vary between 10.1:1 -10.4:1 or so depending on the decking of the block and this.

The cam spec is right as well as the other information. The engine is dyno tested with a Weiand Stealth Dual Plane Intake and a either a 770cfm or 800cfm carb.

Please, before you bash companys products of which you know nothing about....find out the facts.

And Ricky, we did compensate you for the delay. It took about another 2-3 weeks longer than anticipated, but we like to make sure its built right and not just thrown together. Unfortuantley we are not responsible for the availiblity of the vendors parts/products (pistons were the issue) In this case the pistons were on backorder for quite some time. Till this day we are still waiting to get some as KB slowed down production.

Im sorry to see you would bash us for this, but we are still willing to help you with any technical needs you may have. I know this is an older post, but I had to state the facts.

Brandon
ATK ENGINES.


http://kb-silvolite.com/kb_car/perfo...hp?action=comp

Brandon, I don't think anyone replying to this thread was in any way bashing your companies products. Unfortunately not everyone owns a dyno so all we can do is compare the components of that engine with components of a like engine of known horsepower. Rick never stated that it had domed pistons in it and in my experience with engine building I wouldn't have thought anyone would put domed pistons into a otherwise stock low HP engine. I figured the compression ratio stated was just a mistake. All the parts there are comparable to that of a 365 HP Merc engine (8.75:1 cr) with exception to the pistons. Then Rick stated it would only turn 4600 rpm at WOT and I figured that was about right for a motor of 375-400 hp in that boat.

In 1970 Chevrolet built the LS6 450 hp. It had better heads with bigger valves, a bigger camshaft, higher compression (11:1) and a dual plane with a 800 Holley.

A dyno sheet would be nice if it is for this particular engine we are talking about. Most 454 marine engines I have built have always dynod under or around the 500 HP. Dart oval port heads 225/188 valves, 9.5 to 1, holley 800 cfm, 236-242 dur. / 565-575 lift or so on the camshaft. The last 454 was 485 Hp at around 5,800 rpm.

As far as the engine were talking about goes, who knows. I've been at this for some time now however learn something new almost every day.

IMO, that compression ratio is too high in a marine engine with Hypereutectic pistons, iron heads and pump fuel. Lets see how/if its running in a year.

Took the words right out of my mouth.

I agree a 750 would be right, in my opinion to much compression and timing as well..asking for detonation issues