Dart Iron Eagle 308 Mods
 

I’m looking at using a set of Dart Iron Eagle 308s on a mild pump gas 540 NA fresh water lake motor build. Max rpm 6000 with lots of cruise in the 4000 rpm range. Shooting for 650-700hp. Reliability and durability is most important over big numbers. I’ve read several times that people state in stock form, they don’t flow very well? Anyone have firsthand knowledge? I’m wondering if it’s just cleaning up the bowls, guide boss and transition to the seat or much more in-depth short side and deeper port work? Anyone care to comment ?

IMO, 308's are too small for a 540ci with your power goals. Something more in the 345 range would better

Mostly attracted to the Darts being cast iron (lower corrosion issues) and thought that the smaller port size would preserve low and mid range torque.

Doesn’t answer op’s question but Dart did offer a 330cc CNC’d (intake and exhaust ports) Iron Eagle head.

Also pretty sure JIM V (Valeko) has ported the Dart iron bbc heads.

It takes a really, really good combo for a marine 540 to make 700hp. Most of them dyno around 630-640 from what I have seen. I don't believe you will get there with those small iron eagles unless you really up the compression.

I got 620hp out of a 502, dart 308 heads, 741cam, merlin intake, 850 Aed carb. I very pleased with the outcome of my builds. A friend had 540’s built and they dynoed at 690.

I did a similar build in 2017 but was a 548 with 088 heads that had minor bowl and intake work done. The cam was a little bigger than a 741 but not much. Motor had around 10:25 compression as I normally run at higher elevation.
https://cimg4.ibsrv.net/gimg/www.off...1aedaaf3b7.jpg

. A friend had 540’s built and they dynoed at 690.[/QUOTE]

What heads and compression ratio?

Do you have a heavy, slow hull that drags thru water hard or a zippy , stepped hull?

I believe BenPerfected has a couple sets of Jim's ported 308 Darts and he made 800HP with his 502's. But, remember these engines are a meticulous build. I'd say they are "Above and beyond builds" and raced engines.

What heads and compression ratio?[/QUOTE]
I know he had AFR aluminum head 9.8-10.1 compression.

The Dart 308s are a good head. Will work fine on that build. A good valve job, back cut the intake valves, they will flow well.

741 cam is too small and really not ideal for your build. The right cam, compression, intake carb and exhaust you should be able to meet your goals.

I agree with Mild T.Joe on the Dart 308cc head on a 540cid engine, but maybe with some port work, ect. I do not think it is too small for a 540" engine. I used AFR Cnc ported 315cc heads on a pair of NA 565cid engines for Marine offshore use I had built for me almost 20 years ago. Granted, AFR cnc'd heads are a different design and is a high flow design, but cc/size wise, NO, I do not believe a 308cc head is NOT too small for a 540cid engine destined for Marine hi-perf use---not for YOUR goals as you have mentioned. .

If good mid-range is what you're looking for, then I think you will be just fine. However, I DO believe a 741 cam is NOT too small either. You said you're looking for "reliability" and low Rpm and mid-range Torque and that cam with that Dart 308cc head should be a nice combination---I'm not saying it is the ONLY combination, but it would put you in the ballpark of what you are asking for without going with a "BIGGER" cam. Btw, there is reliability in Cubic Inches.

I run some little baby hydraulic roller cams in my pair of 565cid marine engines (232*/240* @.050 lift on a 115* LSA with Dart single plane intake manifolds and a 4150 Holley 950HP carb---my 565" combination has super throttle response with good signal which relates to Torque...and only about 9.2 comp ratio and excellent idling (700rpm in neutral) and drivability characteristics---according to the dyno the engines stopped making HP at 5100rpm with those baby cams, but in my 32' AT they topped out @5600rpm with 32" 4-bladed props or 30" 5-blades--- I had a very good drive set-up, etc...boat ran like a terror.

When my engines were originally 540" engines with Dart 310cc heads ported by JimV, they made about 650+hp at about 5400rpm or so with a little Holley Spread Bore 830cfm carburetor with Annular Discharge with only about 8.2 comp ratio...yeah, 8.2 cr. I could have just reused those little 830cfm carbs I had for my 565" engines...the 950cfm Holleys showed a 16-hp increase over the 830cfm carbs

Back around 2003, I knew a guy who built a 509cid engine for his single Warlock boat...using the Dart cast iron 308cc heads with some very light clean-up port work by JimV and a Crane 741 cam, Edelbrock RPM Air-Gap dual-plane intake manifold with a Holley 800cfm carb, about 9.0-9.5 comp ratio and it made 560+hp (can't remember at what Rpm though) on Tyler Crocket's dyno----he used tubular headers with a riser on very torquey engine and he was very happy with how his boat ran.

YES, there's a ton of options and combinations and opinions available on here that you can chose from, but if you are going "err" when it comes to head and cam size, then it is better to "err" on the smaller size---and that goes for carburetor sizes also---no Dominator carb (4500) needed. There are a lot of factors involved, hull, length, weight, application and what your own personal goals are. You're going to get a ton of opinions on here, but those are Just my own .02...Anyway, have fun---God bless you all

Mine is a little north of 700 using Dart Pro 1 335s. Torque was all in at 4700 rpm and same HP at 5600. It’s in a Donzi 22 classic. Cruise rpm just under 3000 at 50 mph and my limiter is set at 5400 , just north of 86 mph.

https://cimg0.ibsrv.net/gimg/www.off...22808b924.jpeg

How are you propped currently? Solid numbers!

I am running a 30p Bravo1 labbed by wilson custom marine.

Built 548" with 308's, 525 EFI cam, BMP single plane, Quickfuel 850, made 630hp @ 5200/700ft/lbs @ 3900. I was after low rpm tq/hp

No doubt could have made more with bigger head/cam but didn't want to have to spin the RPM's in a big heavy boat.

Do you happen to remember you cam specs @.050" Lift duration numbers by any chance?...thanks

You need to throw automotive engine building knowledge out the window when building a marine engine. Your never going to turn the high RPM they turn. Yet your going to be above their cruise RPM all the time. A manifold or cylinder head rated to 7000rpm on a 454 in a car will support a 565 @ 5500. You'll make awesome torque with your HP rarely equaling or bettering your torque numbers. In a boat it's all about torque as if your not over 5250rpm torque will always be higher than HP. Those 308 heads are not too small volume wise. But they don't flow great as cast. They need porting to get where you want to be. Using a heads intake port CC as a way to judge it's performance started out as nothing more than a marketing ploy that was spread by the magazines who rely on the advertisement dollars. Flow numbers are what count. If it doesn't flow well the CC of the intake port makes no difference. There's aluminum oval ports out there that flow in excess of 350cfm with only 290cc port size. That'll out perform a poor flowing 330cc port every time. Especially in the rpm range of a reliable marine engine.
have those 308 Heads ported by a reliable well respected head porter like Chad Spier or equivalent and they'll work very well.