Max horsepower from a 350
 

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

What does built mean??
 

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.

Well I just heard back from the seller and he said the motor was built by David Dewey, anyone heard of him? It was rebuilt quite a while ago and has sit for the last 8 years with l only a couple hours put on it. He didn't remember the internals that were built but he said there is an aluminum ilstreet dominator intake, Chevy bow tie heads, pistons, roller rocker arms, Holley double pump carb and osco exhaust risers. He didn't remember the specs on the internals replaced but knew at least that much was replaced. I asked him to recheck the compression with a different gauge when he gets a chance and he's still reading 190 to just shy of 200... Thoughts?

It may be camed with a duration/overlap that is building a lot of cylinder pressure. Without having the Cam card from the cam grinder anything is a guess.

When it's running does it have a seriously "lumpy" idle or does it idle out ok at around 800 RPM? I say set the timing at 32 deg total advance and run it...don't beat on it but run it out a few times and see how it responds in your boat. If it has a terrible low end and is a real top end screamer you can easily re-cam it to something more drivable...then again it might turn out to be just right for your use.

Good luck with this...hope it works out for you.

It's has a nice loppy idle, alot more than my stock 350.

If the engine has a fairly loppey idle I would suspect it has a fairly moderate duration camshaft which if normally installed should not generate static cranking compressions like those. Remember this engine was built 8-10 years ago and back then 93 or higher octane fuel was readily available and much cheaper. The GM bow-tie heads with the smaller 64 cc combustion chambers and a flat top piston in this engine could generate some pretty good compression ratios and numbers like these. If the heads are iron heads and this engine has the higher than recommended compression ratio for regular pump gas the engine under load may be a detonation "time bomb" without high octane fuel and very conservative iginition advance. I would not set up an iron small block engine in a marine engine use with compression higher than lets say 9.5 to 1 to run on readily available 89-91 pump gas. I know there are ways to run higher with special timing, aluminum heads and higher octane fuel but these are not really practical builds, and in this case I don't think those thoughts were even on the radar for this particular engine. Hope this puppy has the aluminum Bow-tie heads.

Was this David Dewey an expierenced marine engine builder or just a local high performance street and rod engine builder?

Best Regards,
Ray @ Raylar

According to the shop owner hes a well known guy in the Orlando area and did some great work? He said the timing is at 10* at tdc.

So from what you said, 93 octane may need to be run in the motor to prevent detonation?

When in Doubt-Check it Out!
 

85scorpian:

I can't say here for sure because we really don't have all the facts and parts specs.. The timing you quoted sounds like initial timing advance which is not excessive or really the problem. It will be what the real compression ratio is, whether the heads are iron or aluminum and what the total timing advance will be at lets say 3000 rpms under load.
This engine isn't going to detonate at idle, low rpms or under light loads, it will do it under full loads at midrange and high rpms. If this occurs, timing curves and limits can be moved back and fueling can be somewhat richened and higher octane used to counter these possible effects. There are some options here, but knowing where you are at to begin with is important before you go put'tin the hammer down on this puppy!
Total timing can be measured and plotted from the engine and a timing light degree readout. Its pretty easy to see or know if heads are aluminum or iron, a magnet will tell that quickly if they are painted.
Just take some time now and insure you will be using this engine in the boat in mode as reliable and safe as possible to protect your investment and eliminate a season ending problem.

Best Regards,
Ray @ Raylar

I couldn't ask for better advice, thanks Ray! I may pick your mind again later!


Jason

You can't hear detonation with headers or open exhaust. Detonation happens with the engine under load and not when free reving it.