carb advice

[jchris1062]

i have a 300hp 5.7 that it i am looking at installing a holley on. would a 750 be too much or should i stay with a 600/650. Thanks

 

[bjcsc]

Re: carb advice

In theory, your CFM requirement = RPM x displacement / 3456. Here's a <a href="http://69grandprix.com/index.php?pageID=car_stuff&calculator=carb_size"target="_blank">link</a> to an online calculator.

 

[Don S]

Re: carb advice

Where did you get your 300 hp rating from? The ratings now used on boat are prop shaft HP, and I haven't seen any 350 carbed engine make it past 260hp. Only the MPI engines have that rating, and going from MPI to a carb will lower your HP rating, not increase it.

 

[jimmbo]

Re: carb advice

The Holley Marine 650 spreadbore part #0-80552, would be a good choice. I put one on my 5.7 and gained several hundred rpm which required an increase in prop pitch. The small primairies on this carb helped the low end torque enough that holeshot did not suffer due to the increase in pitch.
To further increase the power of a carbed engine you can remove the stock rocker arms (1.5 :1 ratio) and replace them with the ones the EFI engines use (1.6:1).
Putting the 4bbl and the rocker arms will take the 260hp 5.7 past 300hp

 

[John_S]

Re: carb advice

I am not sure which 750 Holley you are loking at, but you don't want any double pumper mechanical secondary carb on it. You want a vacuum secondary marine 4brl carb. If you have a spreadbore manifold, the carb jimmbo suggests is good. If you have squarebore go with 600 edelbrock 1409. There is a edelbrock 750 squarebore, but you don't need it. I think holley has a marine 600cfm squarebore vac secondary, too.

Don's comment on hp ratings is true for production Mercuisers. When Merc went to vortec heads on 5.7s, they haven't offered one with 4brl carbs. You could get a 2brl 250hp, only. They did have a TBI vortec which was rated at 280hp at prop shaft. That would be a reasonable hp rating for a 4brl marine vortec 350.

I am guessing you are talking about one of the many marine longblocks that claim 300-315 hp. These are probably rated at the crank, and you will loose some power through the drive. i think it is about 25hp in an alpha drive. also, they may have a stronger cam than a stock mercruiser, so could be very close to 300hp at prop. Anyway, doesn't matter for your question. A marine 600-650cfm vac secondary carb is all you need.

 

[John_S]

Re: carb advice

jimmbo, I did some online part number look-up at a couple of different sites, on the rocker arms. As far as I can tell, the MPI rockers are the same as the carbed, and same as the prior non-vortec head rockers. Part number 35329. Do you have a different part number for the MPI rockers?

 

[jimmbo]

Re: carb advice

I was looking in the volvo parts book at the dealer and found there was a difference. I don't have the numbers, but I guess I can get them in a couple of days. The cams were the same between the carbed and the MPI models, well at least they were in 2002 for the volvo 5.7s I was just going to go to the local speed shop and buy a set of stamped aftermarket 1.6 rockers.

 

[John_S]

Re: carb advice

jimmbo,

the extra lift, about 0.030, per valve, should provide a small hp increase. I was just surprised that it was used in a production marine motor. I would have to look to see if GM makes 1.6 ratio rocker.

GM rates the marine 5700 at 292hp at 4800rpms. That is with their MPI and HEI. Merc and Volvo put on their own or modify/adjust to their taste, so there could be some small hp increases from that. They need to be in the range of 325 hp at the crank to get to a true 300 at prop. I believe they only need to be within 10% of stated hp rating. The stock GM rating gets you very close to 300hp at the prop with that 10%. 292(hp) + 29(10%) = 321 - 25(drive losses) = 296.

Anyway, if you find those particular part numbers for Volvo 1.6 rockers, let me know. I will file it away in my books. Thanks.