420 Merc's Budget Build
 

I have a Formula 357 with freshly rebuild 420's / trs drive set up. The motors were rebuilt to stock specs, and currrently have 90 hours each on them. Boat is currently running 58-60 mph on GPS spinning standard 21 P mirage props.

I am toying around with the idea of some bolt on performance upgrades and possibly a cam and cylinder head changes to increase top speed to 65 mph or so. I was hoping that other members, have done mild build up with these motors and can tell me what works, and what doesn't (is it woth it to change intake manifiolds?). Would it be cheaper to build a 525 sc clone? Which combo will use less fuel, etc. etc.

Just trying to put together the budget and build list.

Thanks,

Jeremy

mines are roughly 480 hp each. best I've seen (full of gas and 6 people) was 63 gps, going down the st-lawrence river...

had to rebuild them completly, stock, except cam and victor jr intake.

You will need to add about 100hp per engine to meet your 65mph goal. Nearly anytime you increase HP, you will increase fuel consumption. If you are even remotely worried about how much fuel it will use, leave it alone. With any significant hp increase, you will also have to run higher octane fuel.

Do you have enough hatch clearance for superchargers??? There ar ebetter options than building a 525SC clone. The 525SC actually had 7.5 CR pistons also.

I think a friend of mine still has 2 great SC packages for sale. I don't think he sold them, but not sure.

Here is a link to his old ad in the swap shop http://www.offshoreonly.com/forums/s...nickerson.html

I dont know about cheaper, but supercharging will get you there. Do a search for Hawk 525sc motors. They're pretty much your stock 420 with a 177 blower, bigger carb, and timing backed off to 26 degrees.

How about some 420's, with aluminum dart heads, 741 crane roller cams, weiand 177 superchargers, and good exhaust. Made enough power to push my 402 Formula to 71-72mph on GPS..

Like Griff says, 250 series blowers would be a better pick, but the 177's work well also, they were stock on the 525SC engines, and those were great engines. Even just bolting one on to your stock 420 will wake it up.

Your 420 has all the good stuff to SC with except maybe the exhaust and you are probably running 8.8:1. 177's will work 250 will get you more but you may have to change from your Holley 850 to a bigger carb.
You should be able to bolt a 177 on, run a NGK5673-8 heat range plug, drill 3 1/8 holes in your 140' thermostat, up your jetting if you have an 850cfm Holley and add a MSD or Mercury V6 module for enough initial timing and a larger fuel pump. Oh and fuel and boost guages keep your boost below 5lb's. You already have Rect heads with better valves, roller rockers and HD rods. This set up with the 177 w/o intercoolers should get you around 520PSHP. This is pushing the limit of a stock thru hull exhaust.

The boat does have newer style Gill exhaust manifolds on it, I don't know if this is what you are refering to as stock.

Gils will work fine for what you are doing.