robyw1

Okay now you really got my attention. Why would one want to "step" or texture the exhaust port?? I'm not ribbing you Checkmate, it just seems to me someone is BSing you. Stepping headers is one thing but the exhaust port??

Roby

Vinny P

What do you think? This is how the heads came from AFR. It may be a little hard to see from just a picture, but there are little steps or ridges in the ports.

robyw1

Oh those? That's nothing what I was thinking as steps. Yes as you said those are to promote fuel suspension but I don't see why they're in the exhaust port.

Roby

KAAMA

iTrader: (1)

Those are the porting ridge marks from the CNC machine.

WETTE VETTE

I agree a single plane oval port manifold matched up to rect heads is a good option, but at 540 cubes and 5500 RPM the rect port Merlin is right at home. If we are talking 850 carb and considerably less than 5500 RPM the oval may do better. I imagine the cost of the oval manifold ported to rect heads will be more than the rect manifold for little to no gain.

RLW,
One thing to keep in mind with your combo is the dominator carb with efficient heads has allowed you to make very good average power over your cams operating range. It seems you have a relatively flat torque curve. This is a good thing and trying to move your curve down could create a peakier curve which in my .02 is not as good. The dominator carb may not be a benefit on your motor at 5400 RPM. Just prop accordingly and acceleration and top speed should be great. 5700 RPM is a good RPM for you to run and you will still have great durability.

Craig

PatriYacht

Those are just cutter marks from the cnc machining process. They may aid fuel suspension in the intakes they're not doing much in the exhaust.

Vinny P

Thats what I was told from AFR. They did say that I should not smooth them out by hand and I should leave them alone.

cstraub69@comcast

Intakes runners need to be rough.

Chris

rmbuilder

There is a very simple and accurate way to get wet flow inlet runner sizing for a given displacement and desired RPM at which to reach peak torque. Because torque is an independent variable and HP is a dependent (upon torque production) variable, you will most always focus on torque production in tuning. Peak torque occurs at or near the point of peak VE. Runner length, cross sectional area and plenum volume are the parameters used in intake selection. Runner cross sectional area is most responsible for the placement of where peak torque (and maximum volumetric efficiency) will occur. If you get peak torque where you want it (and keep the curve relatively flat for marine applications) the HP will take care of itself. These cross sectional areas will provide the correct flow velocity relative to displacement and rpm. To determine the correct cross sectional area of the inlet runner use this calculation.
Cross section area = (cylinder volume * peak torque rpm) /88200

Example for a 540 CID engine to make peak torque @ 4200 rpm
(67.5 * 4200)/88200 = 3.214 sq in intake runner cross section
Example for a 540 CID engine to make peak torque @ 5200 rpm
(67.5 * 5200)/88200 = 3.979 sq in intake runner cross section
Example for a 454 CID engine to make peak torque @ 4200 rpm
(56.75 * 4200)/ 88200 = 2.702 sq in intake runner cross section
Example for a 454 CID engine to make peak torque @ 5200 rpm
(56.75 * 5200)/ 88200 = 3.345 sq in intake runner cross section

You see there is a significant difference in the cross sectional requirements.
To build an engine that is not “peaky” for marine applications you want to keep max torque approx @ 75% of the rpm level of peak HP. There are other factors at work to determine the overall optimization. Cam selection, valve timing & size (relative to mean piston speed, max piston velocity, and piston acceleration) cylinder head flow, and exhaust designs. Good heads require less duration and as a rule of thumb durations in excess 236* don’t make more torque, they just raise the rpm torque is produced. That may raise your max HP output past the point of being able to utilize it effectively.
Bob

cstraub69@comcast

Bob,
Always batting "Clean Up". Thanks Bob.

Chris
PS, Kaama want's to talk to you about Morel lifters.