Originally Posted by sprink58 (Post 3786400)

+1...especially with aluminum heads in our environment.:evilb:

Originally Posted by 01scarab29 (Post 3786241)

Sitting here reading this thread and learning a lot. I too am going through this tough time with engines. I have a 29ft. Scarab, twin Vortec 5.7's, MPI fuel delivery, advertised at 300HP, 1.5 Bravo 1's, 23p Mirage Plus... One engine shattered an exhaust valve. Did tons of checking, asking, reading, and pretty sure the cause was from a riser gasket leaking.... To fix the problem ..... Going back as 383's .. parts are: Eagle rotating 400 steel crank, forged 5.7" rods., dished hyper .030 over pistons, vortec heads with match port(machineing valve guide down), holley MPI single plane intake (2000-7000), full roller rockers, comp cam XM270HR (int-.495, exh-.503, @.050 int-218 exh-224, int centerline 110, lobe seperation 112, RPM range 1500-5500. Will use stainless marine or emi manifolds. The boat ran 68 on GPS @4900, 100 gals. of fuel 2 people.... dont know what my results will be... I am going to get them dyno , one shop can do prop dyno, the other is engine dyno. I am more concerned with being dialed in right (fuel, timing), but will like to know HP/T ...... I would really like anyones honest input on this.... first engine is about half complete....

Originally Posted by Diego9040 (Post 3786450)

John ,

It was a pleasure talking to you this afternoon, I felt like we could talk for hours non stop. Nice to share our experiences,
By the way,as I said earlier,need to find a dyno around here,if anybody knows someone reliable, please let me know.

Originally Posted by sprink58 (Post 3786455)

Same here my friend. I will keep my ear to the ground on the Dyno...I know Tommy over at Powerboat Nation/Chief in 'Lauderdale has one and I think Cobra in North Miami has one as well. I'll make some calls next week...how close are you to a run?

Originally Posted by sprink58 (Post 3786398)

I have the exact cams in 357 Vortecs w/ Vortec Heads,1.5 Comp Roller Rockers, Dual Plane Edelbrock w/ 750 DP Holley's. Mine made 400 horse at 5200 and 435 Ft. Lbs. @3500...TQ Curve is 400 + from 1800 to 4000. Dyno runs were with automotive headers and I will be running new style MerCruiser Exhaust....so subtract 25 HP at the top and keep the TQ high.

Based on my results I'm throwing a guess that you will make 450 HP in the low to mid 5K RPM range with TQ around 450~475 Ft. Lbs.

The 383's with the same props with 900 HP in the water will run a solid 75+ predictably...biggest pick up will be in the snappiness of the boat as well as a stronger cruise with a load.

I talked to Diego this afternoon and told him if I had it to do over again I would build a pair 6.0/6.2 LQ9 based motors and run reliable N/A 500 HP....but that's the next boat.

Originally Posted by 01scarab29 (Post 3786584)

Hey Sprink58 ,,,, did you have any reversion problems ,, what boat are they in ,, how deep does it sit in the water

Originally Posted by Diego9040 (Post 3786621)

Question

Do reversion and/or water ingestion problems get resolved by closed cooling(half syst) ??


http://www.perfprotech.com/store/App...-half-flow.jpg

Originally Posted by sprink58 (Post 3786805)

Makes no difference...the factor is the overlap between the point where the exhaust valve is still open while the intake valve is beginning to open....creating a negative pull on the exhaust system. I think any thing beyond 275* of duration w/112* lobe centers becomes a problem. If it's marginal then it can be overcome with longer risers.

Reversion happens usually when you back-down suddenly and the following wave pushes sea water into the exhaust system. It can also happen with a sudden reverse in a marina. Even though it's not performance oriented...the stock Y pipe set up with the flappers does a great job of preventing this. FYI...according to Dennis Moore in his SBC Chevy Marine book...the stock thru prop system does a superior job of exhaust scavenging at low and mid RPMs as opposed to thru hull. This is due to the negative pressure created by the prop spinning thru the water...the tipping point where the thru hull gains an advantage is beyond 4000 RPM. Interesting point to consider when dialing in a Cam/Exhaust combo. You have to ask yourself...what RPM is the point where you run most of the time?