RussB

Was wanting to get some advice on whether or not to have head work done on this motor. For more info on my boat it is 23' Baja with a Brovo 1 drive. Currently running 68 to 69 MPH at 5200 rpms with a 23 pitch lab finished prop done by Bronson Hill. And the ECU was reprogrammed by Whipple.

Question
1. What gains have people gotten by just having the original heads port and polished? (HP wise)

2. Would it be better to change out to a different head, and would I gain that much more than just using the stock heads port and polished?

3. What kind of gains would I get by changing out the cam at the same time I do the heads.

4. Does anyone have an idea on what it would cost to have the original heads port a polished?

5. What would new heads cost for this motor port a polished.

6. Besides the prop what else would I need to change out if I had this done.

7. Last ! Does anyone know if there is someone trust worthy in the Dallas area that could do the work????

I also was thing of doing the whipple on it but would come out to be to much work to have it done. Would cost about $10 grand.

Thanks for any info you can give

Russ

Bo Knows

Russ,

Please remember that a lot of this is my own opinion based on previous experiences & research, Some of this info could vary depending on your set up & application. Now that I've put in that disclaimer here it is!

#1 As a rule of thumb a good port & polish job should be worth around 4 - 5 horsepower per cylinder, resulting in 32 - 40 more H.P. for you.

#2 If you have rectangular port heads I would get new ones with a smaller intake port somewear around 260 cc. If you have the oval port heads already, PN 10141280 they are actually a pretty good piece! I would keep them, get them ported, and also I would get the bigger valves ( 2.19" in. & 1.88" ex. ) installed. It has 2.065 in. & 1.72 ex. in it now. The bigger valves will add about 20 horsepower & work well with the porting. Any competent cylinder head shop can do these mods. Have the valves installed first so the porter can blend the new seat area into the valve pocket.

#3 This is a great time to change the cam to a larger one as this will work hand in hand with the increased flow of the head mods. Dont make the " Bigger is better " mistake that a lot of people do when selecting a cam. I would go with a cam that has between .525" - .550" lift and 225 - 235 degrees of duration.

#4 A good port & polish job will cost $600.00 to $800.00

#5 A set of new ported & polished cast iron assembled heads will cost you about $2000.00 Add about $600.00 for aluminum.

#6 Besides a prop change other items to be considered for increased performance would be intake, carb, & exhaust changes. These could be done at a later date depending on how large your budget is.

#7 I don't know of any shops in the Dallas area but I'm sure there are some. Talk to local hot rodders & performance boat people. They can steer you in the right direction.

Good luck!

Bo

JimV

iTrader: (1)

RussB

Bo is right about a 40 hp increase in ported heads. A couple of things you have to look at. A good deal of power is in porting the combustion chamber. The bad news is by porting, it increases the volume, lowering the compression ratio. To compensate you must cut the heads or change to a domed piston. (Usually we do a mild chamber port and cut the
heads.)
The intake port, by todays standards, is a poor design. There is too much volume for the amount of airflow it can produce. The airflow can be increased substantially by increasing the intake valve to 2.25 or larger. The down side is by increasing the valve diameter you will loose available material in the head so you can't cut the deck as much. The 4.250 bore tends to shroud the valves so bore notching the block will help, but lowers the compression as well.
So lets say you send your heads to your favorite head guy. He gets the extra 15% airflow increase out of your heads. By adding the big valve and increasing the port volume to get the big airflow numbers, you have created a port that theoreticaly will be a killer at 68-7200 rpms. This is a problem for anyone with a rev limiter. To offset this and get the power in the rpm range for a boat engine the camshaft selection becomes extremely sensitive. The soulition is to increase the engine displacment,(bore/stroke) or decrease the port volume. This is why the oval ports work so well on the 454 engines.
Another remedy, this is my favorite, is to install an insert in the floor of the port to decrease the volume and at the same time increase the air speed and airflow. This way you can use that snotty hydraulic roller cam (that everyone told you no way it will work) and kick some butt.