Carb Size

[Riv]

Have 1982 Mercruiser model 260 (350 Chev) with quadrajet carb # 17080561. Have tried to find out what cfm it flows. Can anyone help. Riv

 

[jimmbo]

Re: Carb Size

Most q-jets have the potenial to flow 750 cfm, and a few from big block buicks in the early 70s were able to do 800 cfm. However a 350 turning 5000 rpm is only going to need about 500 cfm even at 100% Volumetric Efficeincy. At a more likely figure of 80 - 85% VE, 400 to 425 cfm is more realistic.

 

[Mkos1980]

Re: Carb Size

Stock sbc was 630 and the bbc's were 750 for the mercs.

 

[Mischief Managed]

Re: Carb Size

CFM spec. on carbs is a bit misleading because it does not specify a pressure differential between the inlet and outlet (vacuum), but the amount of vacuum makes a huge difference in the amount of air that will flow through a carb (or any other orifice). In other words, a big engine can easily draw much more than 600 CFM through a "600 CFM" carb and a small engine would pull much less through the same carb. The amount of HP an engine has potential to make (or the amount of HP one wishes to limit an engine to) should determine the carburetor size. If you calculate the carb size based solely on the carb's CFM rating, the engine's displacement, the max RPM, and a given VE, the carb will pretty much always be too small.

 

[John_S]

Re: Carb Size

All Merc SB Q-jets were 750 cfm.

When they changed to webers, in general:
- 5.7LX and 350Mags used 750cfm
- 5.0LX and 5.7L 4brls used the 600/625cfm

All of the above have vacuum secondaries and will only flow for what is needed.

If you are looking for a replacement marine carb, your choices will be between a marine q-jet rebuild or a Holley marine spreadbore 650cfm, otherwise you would probably have to change intake manifold or use a dreaded adapter. (I am assuming you do not have the high-rise dual plane intake manifold - which can use squarebore or spreadbore carbs)

 

[jimmbo]

Re: Carb Size

4 bbl carbs are flow rated at 1.5 inches pressure drop. 2 bbl carbs are flow rated at 3 inches pressure drop. The carb can flow more than its rated CFM at WOT but the pressure drop will be greater.
Case in point. My 5.7 VP came with a 2 bbl 500cfm Holley, which when flowed rated like a 4 bbl becomes a 370 - 380 cfm. When run at WOT @ 4900 The engine was showing 5 inches of vaccumn on a gauge. I purchased Holleys 650 marine spreadbore carb. Initial runs showed the prop being too small. After increasing the pitch the revs were back at 4800 and the vacumn gauge was reading 1.25 inches vacumn. The secondaires were observed not fully open because the engine wasn't able to use all the capacity of the 650.
Boat did gain about 5 mph in top end and the smaller primairies on the spreadbore maintained the low end torque even with the higher pitch prop.

 

[Scaaty]

Re: Carb Size

My 2 cents..sell it on Ebay, pick up a Edelbrock 600CFM Marine carb, with the adapter kit for the spreadbore Man..I did, aint looked back in years..probably never will..no need too..

 

[Mischief Managed]

Re: Carb Size

4 bbl carbs are flow rated at 1.5 inches pressure drop. 2 bbl carbs are flow rated at 3 inches pressure drop. The carb can flow more than its rated CFM at WOT but the pressure drop will be greater.
Case in point. My 5.7 VP came with a 2 bbl 500cfm Holley, which when flowed rated like a 4 bbl becomes a 370 - 380 cfm. When run at WOT @ 4900 The engine was showing 5 inches of vaccumn on a gauge. I purchased Holleys 650 marine spreadbore carb. Initial runs showed the prop being too small. After increasing the pitch the revs were back at 4800 and the vacumn gauge was reading 1.25 inches vacumn. The secondaires were observed not fully open because the engine wasn't able to use all the capacity of the 650.
Boat did gain about 5 mph in top end and the smaller primairies on the spreadbore maintained the low end torque even with the higher pitch prop.

This is a perfect example of how VP limits the HP of the engine by putting a small carburetor on it. If they put the correct sized carb on, it would make practically the same power as the fuel injected version of the same engine. Customers would have less incentive to spend a couple grand or more on the fuel injection upgrade if the HP difference is small.

 

[180shabah]

Re: Carb Size

This is a perfect example of how VP limits the HP of the engine by putting a small carburetor on it.

Merc did the same thing, and I'm sure OMC would have if they had managed to stick around. There in this thing to make money.

EFI doesn't produce power, airflow(with fuel) does.

That said - an EFI engine can typically make max power safer than the same engine with a carb. Less chance of a single lean cylinder, ignition timing can be a little more aggressive...........

BUT

We are talking about 250-300hp 5.7's, not exactly what I would consider high performance territory.

 

[Mischief Managed]

Re: Carb Size

Merc did the same thing, and I'm sure OMC would have if they had managed to stick around. There in this thing to make money.

Didn't mean to imply only VP did this. Basically, if you have more than perhaps 2" of vacuum at WOT, and your flame arrestor is not choking the engine, a larger carb would make more power.

Anyone know what a 220HP Merc 5.0 pulls for vacuum at WOT? I used to have one and was going to put a 4bbl on it but sold it before I got around to it.

 

[jimmbo]

Re: Carb Size

Anyone know what a 220HP Merc 5.0 pulls for vacuum at WOT? I used to have one and was going to put a 4bbl on it but sold it before I got around to it.

I can't comment on the crusier but I do know that VP uses the same 500 2bbl holley on the 5.0 litre as they used on my 5.7. This carb is almost perfectly sized for a 5 litre running 5000 rpm. There should be no gain switching to a 4bbl.

One thing I forgot to mention earlier. I didn't have to install a 4bbl manifold when I did the carb swap. VP uses a very nice 4 bbl manifold along with an adaptor to mount the 2bbl.