Greetings, I've searched this site and others for any real data on swapping out only the stock intake manifold for performer/rpm or stealth on a PCM 351w HO motor, but not finding it.  Plenty of intake plus heads or cam or rocker arm combination swaps, but not just intake on the HO config motor.  Wondering what I could expect in way of improvement on the HO config other than general comments I've read saying any of the above intakes are great.

I have a '94 Hydrodyne Grand Sport which came with the Pro Boss TBI, so I come to this site, as it's similar to the CC's of the era (except it's dual exhaust and 1:1 trans).  After replacing the failing TBI with a Quick Fuel 600m back when I bought the boat (thanks to JQuest help) and then later, dropped the ProTec for a DUI dist, it seems to be performing the same as would have in '94, comparing acceleration and top end to the results of Water Ski mags '94 boat tests. 

However, I notice the last 15% or so of throttle opening has no impact (yes, I verified the cable is adjusted and functioning properly, watching the throttle plates move through the full range) and WOT is just over 4500 rpm (vs 4800 max rating).  I'm thinking this is because my airflow is choked downstream of the carb and a better flowing intake could allow more of the carbs capability.  Or, will it make little difference because the heads/cam are really more restrictive?  Or something else??  It has the original GT40 heads/cam and given its acceleration and top speed are same as new and it only has about 600 hrs, I'm fairly confident it's mechanically fine.

Additionally, the original 13x13 Legend prop had a 1/4"x3/4" rat bite out of one blade and some rippled/ragged edges on the other two when I got it and, not surprisingly, was running 43 mph and 4900 rpm WOT until I recently bought a 13x12 OJ 314 Skipro and had OJ repair the Legend for a spare.  Funny thing is, the CNC 13x12 and the old repaired Legend perform pretty much identically, both on accel and top end.  Not at all what I expected, given all the praise the CNC props get.  Swapped the two back and forth a couple times and get the same results.

Ultimately, I was hoping to get top speed up to 46 or better for some head room for 'footing in the 40's.

Thanks for any input.

FWIW, I installed an Edelbrock Performer RPM intake when I moved from busted TBI back to QF carb.  The QF carb, Edelbrock manifold and DUI dizzy was a formidible combo.  I have no scientific data, but the retro-mod FELT stronger than the TBI before it died.  

Tim's advice about the prop is also spot on.  You really need a prop that will allow the engine to spin close to 5,000 RPMs.  If you do decide to go with a Performer RPM intake, they like 2,500 to 5,000 RPMs for peak performance.

JQ

Tell Jonny like you mentioned in the first post, that you already have gt40 heads 

Since the 94 tests said 44.5 ish with the TBI, I'll guess that's about what you have now for speed

The 95 tests with a gt40 engine picked up about 3 tenths of a MPH. (not much power difference between a gt40 and an injected ProTec though)

I doubt that you'd be happy with the results from just adding an intake manifold that runs into a breathing restriction at the cam and doesn't get you the HP or speed you're looking for.

If you're tearing into things, then a cam change would be logical to go along with the intake.

You could read TRB's thread on his engine mods to see how much he had to jack up power to get some extra speed in his 90 SN. (or he could tell you here) You've probably found that thread in your searches.

Yep, 44 mph @ 4500 rpm top end now with either prop.  Realized I forgot that detail after I went to bed.

I'm just questioning if the valves are more or just as restrictive as the intake, I don't know.

I did see TRB's mod post and it was a number of changes to a base 351, right?

Really disappointed in the prop change and repair.  The original damage seemed pretty severe and was expecting a couple mph gain for the repair and one or two with the switch to the 314 on top of that. Only difference is it gets to 44 mph faster than it was getting to 43 mph and the 400 rpm drop at WOT.  Feeling like a waste of boat budget.  I didn't realize there could be much difference in OJ vs ACME, posts/reviews seemed 50/50 on which way to go and I leaned toward the slightly thicker OJ (damage tolerant).  Thinking back to earlier years skiing behind outboards and propping making a big difference.  Also disappointed OJ hasn't gotten back to me after I sent them my results for their thoughts or further recommendations.

Does the "dead" throttle over the final 15-20% opening make sense?

The CNC vs the older hand finished top end results you got are pretty normal when they're both in equally good condition.

The manifold alone won't get you the results you're looking for.

Your exhaust manifolds aren't exactly a set of tuned headers either, like you can use in automotive applications

I bought too much into the CNC prop-aganda.  Could probably drop more in pitch to get rpms up, but won't likely get more top speed and just burn more gas.  Don't feel I need better holeshot.  Maybe the 341 would shine with more power to spin it.

Realizing I'm gonna be limited by the hull design unless I go bigger on the engine mods and right now it's running strong enough to pull adult footers at least 42.  Have to think about where I want (or need) to spend boat budget this off season. going over the entire engine, I fear I may only have a season or two left on the exhaust manifolds given the state of their rust and they're not so cheap to replace anymore.