Difference between 300 Promax and other blocks?

[CptAJ]

I'm lurking the local shops looking for a replacement cylinder block for my 300 Promax after one rod went rogue and punched a hole through my beautiful engine.

What's the difference between the 250 EFI 3.0 block and the 300 Promax block?

Thanks for any info you guys can provide

[Chaz]

Do you have an early model with twin computer's ( ign and fuel seperate ) at the back of the block ?

Or do you have a late model X with a Motorolla box mounted on the side of the block ?

If you have an early model 300 and the replacement 250 is also an early model , then they are the same .

If yours is an X then a late model 250 block can be used. However it will not make as much power as the original due to differences in porting and exhaust flow.

[CptAJ]

Do you have an early model with twin computer's ( ign and fuel seperate ) at the back of the block ?

Or do you have a late model X with a Motorolla box mounted on the side of the block ?

If you have an early model 300 and the replacement 250 is also an early model , then they are the same .

If yours is an X then a late model 250 block can be used. However it will not make as much power as the original due to differences in porting and exhaust flow.

It's the promax model with the early intake NOT de downdraft one. And yes, two separate computers.

Another question, are the bleed lines all the same? I have the manual for the 250 but I guess there's no manual for the 300.

All good news so far. The early 250 efi block will be MUCH easier to find. Thanks for the info.

[Chaz]

I don't think I have ever seen two running years that the bleed lines were ever the same . They have had some really wierd idea's over the years .
Pretty much on the starboard side :
# 1 reed to # 5 transfer
#3 reed to # 1 transfer
#5 reed to #3 transfer
Do the port side the same way. Thats how they finally settled on the 05 300x . if your block doesn't have transfer bleeds on the port side ( 1/16" NPT ) then tie the three together around the bottom to # 5 reed - # 3 transfer . If you can get your hands on Napa hose # H174 it's fiber reinforced 1/8 x 1/16" F.I. hose. Works perfect for plumbing bleed lines... :thumbsup:

BTW .. All 300's are covered in a separate manual.

https://i.imgur.com/1rV7n01l.jpg

[diegoj220]

Hey Chaz, you may know a bit about this..

I have some questions for you about 3.0L engines,

I have 2 250 efi late models and I would like to do some upgrades to them for higher Hp, If I put some 300x C16 heads on, what are the pros and cons of doing it? and also what else may I need to change from stock rather than carbon fiber reeds in order to made them work fine and "safely". is it worthy? and how much more hp should I get with those heads.
Can I also swap stock ecu's for 300x ones?
Right now both engines are arround 120psi in all cylinders. C15 original heads

[Chaz]

I honestly dont think you'll find a set of 300x heads ready to go, let alone two sets.
Not a big deal, you can cut yours to have 140 - 145 PSI without getting into trouble.

The metal reeds after a while start cutting into the rubber on the cages.
Chris Carson makes a special reed for the cages that have had the rubber removed. Any 3.0L I build I strip the cages and use that reed, only way to go as far as I'm concerned.

It's a good Idea to lighten the flywheel. We cut them to weigh aprox 13 lbs.

If you don't mind taking the mid section apart, cutting the inner can in half will free up the exhaust a bunch.

If your VST canister has the little "chrome dome" regulator. I like to install an adjustable top kit on those , to clean up the tune up on lighter hulls and put a little fuel in for heavier boats that tend to lug the motor .

The motor won't rev any higher because the porting dictates the powerband. What those changes will do is allow you to pull a larger prop to the same RPM as before.
Most guys with single engine bass boats and center consoles have told me the changes first time out are worth between 4 and 9 mph.

[diegoj220]

Chaz! Thanks a lot for the information.
About the heads I already have it. Also the pcm ��
I will think about that cut in the exhaust, cuz everything is already mounted on. But before that i did some minor porting inside the engine block just a clean up, but i know 300x block exhaust are heavely ported. I also did the 2” hole in bottom of the tube assembly exhaust, and porting the exhaust adaptor plate.
Definitely doing the flywheel thing close to that number cause I have a heavy hull.
What kind of regulator kit would you recommend?
here some pics
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