Improving 500 EFI heads?
 

How much power can be gained by porting the stock 500 efi cylinder heads? I am thinking about also adding a 525 efi or equivalent cam.

Would it be advisable to increase the compression ratio at this time or not?

Would aftermarket cylinder heads be a better alternative?

Thanks, John B

I have 2 great sets of Merlin 310cc rect. port heads for sale. Lots of HP improvements.

A carbureted engine needs intake ports sized perfectly for the rpm range that the engine will run in. This gives the intake port the maximum pull (vacuum) on the carburetor venturi for the given rpm. The higher the velocity of the intake port, the better the engine will run.
An MPI EFI engine (with individual port injectors) doesn't need high velocity intake ports because the injectors push the fuel into the combustion chamber. Vacuum in the intake manifold is almost a non issue with MPI.
The stock, large rectangle port heads, can be ported to your hearts content without affecting low speed performance. Install larger intake valves (2.25 inch) and pocket port the heads for a small increase in high rpm performance.
Dennis Moore

Thanks Dennis,

Are we looking at 20 or 40 horsepower? What would you expect from good aftermarket heads?

Mr. Yacht,

My engine program shows your heads (stock) aren't any better than the stock GM square ports. Is this right?

Anybody have any test results? Dyno tests would be great!

Thanks, John B

What about Valenko's wedges?

I would expect them to be better. They have the 2.25 intakes and the exaust port looks much better, however I don't have any data to back that up. They have also been pocket ported and cleaned up to the gaskets.

The exhaust ports are a problem on the stock heads. There is not enough short turn radius to make the ports flow. With extensive porting the flow can be increased dramatically, but not as good as ported aftermarket heads. A ported small block exhaust with a 1.6 valve will out flow a stock big block port by 5%.

Fully hand ported Dart exhaust ports will flow 300 cfm @ .700 lift where a stock head ported will flow about 270. The intakes 400 cfm vs. 360 for the gm heads. Less on the left hand port.

As far as hp increase it's depends on the combination. Our head/cam upgrade makes 650hp on a 502. No way with the stock EFI manifold.

Jim V,
If the EFI is used with a modified program, what could be expected on your combination? I think the max desirable RPM range would be in the 5400-5700 range.
Do you do head work?
Why do you like Dart heads?
Thanks, John B

BOY does Jim do head work.
he did my heads.
I am running stock heads with the wedges.
Real good workmanship.
I run 502's that are carbed, Dart intake, wedged heads,Lightning headers, Holley HP 950's ( a little too big - but plan on adding more cubes later ) and a solid ULTRADYNE cam.
Boat runs 77 gps at 5100 rpm with 28 pitch bravo 1's.
Estimate of HP - 600

What are wedges? How much can be gained with work on the stock heads?

John,

I don't know how much gain using the stock efi intake. Years ago we dynoed the Arizona Speed manifold against a single plane modified for injectors. The single plane made 80hp more so I don't do much work with the stock manifold.

I prefer the Dart heads because the short turn radius in the exhaust port is better for marine engines. I can machine the port to peak at .650 lift @ 300 cfm. Most other heads don't have the material in the port floor. They flow great at .800 lift but have a hole at lower lifts.

Wedges are inserts that are single bolted and epoxied in the floor of the intake ports. They will add dyno proven 35 hp gain plus whatever port work has been done. They increase velocity in the port by reducing the area in the port without any airflow loss in the usable camshaft lift range. Airflow losses are at .700 lift and higher. This allows you to use a larger intake valve to increase airflow. The work the best in small cubic inch motors although we have used them in 540 cid. Also for the skeptics I have them in our 510 race motor.

I have a set of them in my 468 , seat of the pants - They work period.

Heads
 

Jim,
Have you done anything with the BB Dart Iron heads?
Sprague
213 575 2367

JIm,

Maybe this is why the new 525 EFI from Mercury only makes 540+- horsepower on the dyno. How close is your cam to theirs? What do you sell, parts or complete motors? Have any of your customers dropped the EFI in favor of intakes and carbs? What do you recommend here?

Thanks, John B

Hey Sprague, how you been?

Dart claims the Iron and aluminum castings are the same. Iv'e been doing a ton of the aluminum castings. I have a couple sets of iron comming up, I could let you know the results.


John B

Our 540 cam is considerably bigger than the merc primarily because the heads will support it at higher lifts. We use hydraulic rollers for durabilty and lack of maintance. Our base cam lift is just under .650 lift. but may change according to boat weight and hull design. The heads are ported to achive max flow at the same number as max lift.

I believe an after market programable EFI tunnel ram set up is superior but pricey.

I have a pair of JimV's fully ported Dart Pro-1's 310cc heads on my naturally aspirated 9.6 comp ratio 540's. On Tom Earhart's dyno they made:

682hp/651tq @5500rpm

699hp/639tq @5750rpm

Peak torque was at 4800rpm---704 ft. Pounds.
The boat is a 32' A/T and I have it proped out with 32" Bravo Ones and turns 5600rpm @WOT.

Not too shaby for N/A 540's with hydraulic roller cams

Thread from a long time ago....