nauticaL ILLUSIONS

I have SBC's that are pushing around 420HP each. I run really strong and I'm wondering if there is a gain in getting the heads done up. I run on GPS around 76MPH prime conditions at 5400 RPM. Now thats in 2 footers and 65 degrees. Bottom line is if I were to take the heads in and have them staged, changed the cam to something more aggresive (not sure whats in it) and put a 1 inch spacer in the carb and a stainless exhaust what would I see for MPH increase or HP increase. I'd like to see 80+. What do you guys think.

mcollinstn

Rule of thumb is:

Great heads plus mild cam will give the same peak power as stock heads and wild cam. Guess which motor will be easier on parts, behave around the dock, last longer, be more fuel efficient, and have more all around torque...

The kicker is, the heads need to be done by an expert. Not your local dragracer. Their prices are probably closer together than you'd think.

euby

heads are the key....but as mcollinstn said. have them done by a pro and not a redneck with a die grinder(like me) If your getting 420hp out of small blocks then it sounds like your already got a stiff cam....i'd go with the heads!

nauticaL ILLUSIONS

The engines were built by a marine engine shop. He went with a combo which his books said makes 420HP. I'm not questioning that one bit, I've got lots. The heads are Dart (not sure which ones). The cam I'm not sure either, it idles very smooth doesn't sound like its built at all. I wasn't planning on having any rednecks touch my motors but its good advice. I would have probably taken the heads to a speed shop though. Will porting make a difference though. I now speed gains above 70 take HP.

mcollinstn

Darts flow well out of the box. There's always room for improvement but you will net "less" of an increase porting the darts than you would by porting the stockers.

Porting (properly done) will always net an improvement.
In your case, though, you are at a point (420 hp) where there are no "silver bullets" to gain a huge amount with a single change.

A 4mph increase from where you are will require a host of complimentary changes (compression increase, head porting, cam change, and spinning well over 6,000 rpm most likely).

Additional cubic inches (via a stroker kit) or a blower would be two feasible options for you. Be aware, though, that if you decide to go with a blower, you may need to lower your compression ratio depending on where it currently is (I'd take a wild stab at 10:1 with 420 hp).

Changes in weight distribution, shorty lower units, extension boxes, and labbed props are NON MOTOR alternatives for more speed.

Good luck.

turbo2256

Heads are what make the power. Good port work will increase the power better than any cam carb exhaust combo. The heads have to be ported to your cam and RPM limitations. What most dont realize is that bigger is not always better. If your cam has only a .5 lift or less after market heads are to big out of the box for lower RPM boat motors. At .5 lift stock SBC heads can be ported to flow in the 250 CFM range. Stock heads can also be biased to flow close to 250 at .4 lift. Most aftermarket heads are lucky to flow somewere around 200 at these lift figures. If you figure up total flow the after market heads realy loose because they flow even smaller amounts at .1 ,.2 compaired to ported stockers. Yea in most cases the after market heads show an improvement in HP say 50 HP but properly done stockers might get you 100.

Dennis Moore

Working over the intake ports won't help unless you have the best exhaust system available. Individual tube headers are needed.
There is a point where intake flow isn't helped unless you open the valve higher and longer with a better camshaft. A roller lifter cam is needed and it needs to match the intake manifold/intake port/exhaust port/exhaust header combo.
You will probably have to increase your maximum rpm for a performance increase at this point. Call a camshaft toll free line and ask them.
Sincerely
Dennis Moore

nauticaL ILLUSIONS

So my take on this is......there isn't really much gain to be had for marine applications. Sure a car will feel 25 HP but a boat over 70MPH most likely will not show much. I was considering an alternative to supercharging for the moment. The engine has been built for a blower. Forged pistons and 9.1-1 compression. I guess that might be the answer. Could I get away without an intercooler at 5psi or would I detnonate.