Hey guys so this is a 2 part question. I am repowering my Chris Craft with a built 350. I went and checked the motor out today and the compression is great, 195-205 across all 8. It has been rebuilt and only has a couple hours put on it since. This is coming from a boat servicing company and the guy I have been talking with says that he expects it to have an easy 400hp, I feel like this is far too optimistic. Is it possible or what would it take to get a merc 350 to 400hp. And if in fact it is possible, what ballpark mph would that get a Chris Craft Scorpion 210 to? thanks for any input!


Jason

Define what "Built" means with specs and info. 400HP from a properly built 350 does not seem unreasonable but the build will determine that as well as exhaust upgrades and such.
Give us some good input and we'll try to give you some good output.

Best Regards,
Ray @ Raylar

He is emailing the exact specs right now. I will have them shortly

How much HP from a 350? My friend has 1900+HP. Well, 383ci. That work well in your CC.

I believe 195-205 psi of cranking compression will not like pump gas in a boat.

i beleive you might be right.

Very good observation Grasshopper!

That's one of the reasons I asked for 85 scorpian to get us the build specs.
That compression would be high for pump gas, possibly up around 10.5 to 11.5 to 1 and that could be a problem. I hope maybe their compression guage is a happy one (out of calibration on the high side)

Best Regards,
Ray @ Raylar

Really? Never thought of that. It was running on 93 when they did the test and when I saw it running today. I didn't hear any detonation or anything... What kind of symptoms would i see if it didn't agree w pump gas?

It's tough to hear in a boat like you will in a car but when the engine is under a load you will hear a loud knocking sound. In a boat you will probably see very high cylinder head temps until something breaks or melts.

I agree that those numbers do seem high ...normal cylinder cranking PSI falls in around 135~145 PSI in a SBC. I would re-check the readings with a different gauge before I became too concerned.

The 400 HP number is reasonable with camshaft (roller) and cylinder head (Vortec & aftermarket) technology being what it is today. In a boat the exhaust system has a big effect on the ultimate "power to the prop" number you achieve. Marine exhaust systems however can quickly cost as much as the engine that they are on. I have been told by several engine builders that it's possible to loose 10% between dyno numbers and installed numbers with a "stock" marine exhaust system. You could end up with 350~360 actual horsepower after install from a 400 horse dyno engine.

Keep in mind that the real number that you feel is torque...especially in a marine application. You want to build it fast at a low RPM and keep the curve as flat as possible throughout the engines RPM range.

i have a 10 to i motor with 175 cranking compression.you could lower that compression with a cam change.