I love how much you post my name=obsession...

So at what hp level do you feel the 310 or the common AFR 305 heads should be changed to something else and make a nice noticeable bang for the buck improvement in hp on a 540?

I figured I'd at least ask, you never know. We should occasionally try to get back on topic.

The heads will support somewhere in the 800hp range with the right combination. HP really doesn't play into the factor but more RPM and displacement, and where you want to make peak power. The more displacement, the less rpm a given head will handle. I'd say for a 540, if you're trying to make power much above 6000rpm, you would want to at least do an intake valve upgrade to a 310-315cc head, but don't necessarily need a "bigger" head. The port can handle plenty of air but the valve becomes the restriction. A set of 310-315cc heads with a 2.30 intake valve make a nice combination on a 540 running above 6000 rpm. Of course, by the time you blend the bowls and shape the short turn for the larger valve, you'll pick up a few cc's of volume so "technically" they won't be what they started out as, but the cross sectional area of the front of the port will be the same. Just installing a larger valve without doing the follow up work would gain less but still be an improvement. The 2.25 valve is good to about 6000rpm peak power on a 540.

Well let's get more specific. I had a 540 making 660 hp at 5600 with 305 AFRs and a nice mild cam, Vic Jr- 4150. Now that I'm rebuilding and engines are 548, what would be the best bang for the buck improvement? I'm thinking the entire induction package I have is getting marginal for hitting 700 hp at a reasonable boat friendly rpm.

What were the old cam specs ?

And another question would be, how much of a speed increase, will 40hp per side gain in your 38 scarab, and how much will it cost overall

It depends on which 305 to 315 cc head your talking about and what RPM you plan on spinning the motor. Heavy boat vs light boat comes into play with best torque curve for the application. I suggest you give Eddie Young or Scott Foxwell a call to talk over the details of your combination. Its all a compromise and your only as good as weakest link.


Sorry I type too slow. Good info above

I'll get back to you in a minute with cam specs. The heads are AFR 305s. I'd like to keep the max rpm at 5800. It's a light boat and the old engines were over 600 ftlbs for the whole sweep. The speed increase and costs are what I'm currently struggling with. Trying to figure out what parts I should change out while I have the opportunity. The old engines were put together for a much different type of use than what I ended up using them for or I would have made different choices originally * don't seem to have cam specs on the phone. They were roughly .640/ .240

Heavy boat? Keep the TQ my new engines have yet to push my boat faster then old engines that were supposed to be 100hp less a side ....light boats run off HP, heavy boats need the big TQ numbers to carry it

Your induction is plenty for 700 hp @ 5600. The rpm is the issue. HP is air and fuel. Your 548 at 5600 is only moving X amount of air. You can't change that. You just have to use the air/fuel that you have available in the most efficient manner. I'd look at the cam and make sure everything else is spot on. Lot of power to be had in good machining and cylinder finish/ring seal. AFR 305's are an "as cast" head which means there are inconsistencies from port to port. Some of that can be taken care of on the porting bench but you don't need any bigger ports and you really don't need more airflow. The devil is in the details when you start trying to maximize efficiency.