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switch to b-4 or b-3

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Bluecollerboater

Contributing Member
I have a 1994 Volvo penta Gipmda. I had the long block replaced and was having all kind of problems with the fuel injection so I had the ignition system changed and put a carb on it.it was a thunderbolt 4 ignition and 750 cfm carb off a mercruiser 454 it runs great now but my mechanic says it will only get to 4200 rpm with no fuel or water or family junk on board. the gimda spec is 4600-4800. now it Is a carbed 350. so I am assuming it is a 260 horsepower motor not unlike a AQ 260A. what rpm should I get?

Mechanic says I should switch from the B-5 that is on there now to a B-4. It is a DP-C 1.95.
I am not sure what rpm I should get at WOT. And which prop, B-4 or B-3 to achieve it.

Thanks in advance for any help.
 
There is no substitute for reaching the correct WOT RPM. Your engine will be over-burdened at any RPM if not!
If the 5s won't allow for the correct WOT rpm, then drop down to the 4s, and test again!

Make sure that the boat is loaded as you would normally be loaded!



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FYI..... in order to successfully perform a WOT RPM test, the engine must also be in perfect tune.
Not just fuel delivery, but it must be seeing the correct progressive ignition advance.
This will be the advance above and beyond BASE advance!
Have you verified this????

If it's not receiving the correct ignition lead throughout the rpm range, a prop change may not solve your issue.


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On the Thunder Bolt 4 systems there is an identification mark. For example, It might say V8-22

IF so...

Your engine's advance curve would be:

Module Advance: 22 Degrees
Initial Timing: 8 Degrees BTDC
Total Advance: 30 Degrees

Advance starts at 900 RPMs
@ 2400 RPMs should add 17 degrees advance
@ 3400 RPMs all 22 degrees are in

That said... is that the same ( or per current marking) as your original system, i.e., what was the specified timing curve for the engine before you modified it?
 
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I'd want to verify this myself!

You would be surprised at how many so-called Marine mechanics are not up to speed when it comes to checking the progressive and total advance!

When all is said and done, it comes down to LPCP (location of peak cylinder pressure)!
Our ignition systems have the last "say so" in this!

Most engineers have determined that 12° to 14° ATDC is an ideal LPCP.
If the ignition advance is lazy, so is the LPCP.



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I agree with Bill!
Try prop sets until you are comfortably within this range.

If you are not already aware, WOT RPM is a test RPM only.
It is NOT an RPM to be sustained for any duration!


Your "sweet spot" will be when the hull reaches planing attitude, and just a tad more!
Hopefully this will be at/near 3,600 +/- rpm.



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So assuming everything else is good you would probably end up at B3. there arent any prop shops in your area that will let you try before you buy? If not the good news is any prop shop can repitch one size up or down easily.

No one is going to make mention that a 750 CFM carb on a SBC 350 is too much carb????
 
Fish-a-Palooza said:
So assuming everything else is good you would probably end up at B3. there arent any prop shops in your area that will let you try before you buy? If not the good news is any prop shop can repitch one size up or down easily.

No one is going to make mention that a 750 CFM carb on a SBC 350 is too much carb????
Click to expand...

I missed that part, HOLY CRAP, WAY to much carb. The oil company's love you. It doesn't stink like raw fuel around your boat?
 
My '94 5.7GiPMDA says 4000-4600 for WOT. I'm at 4200 (pretty light) with B-6s, let me know if you sell your B-5s. I'd like to get closer to 4600 ...
 
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