L6 250/292 ?

[achris]

Re: L6 250/292 ?

What's your 4.3 get for GPH at 2800? curious?

Not sure at 2800, but at 3000rpm (22knots) I'm using 6.3 US gallons per hour. That works out at 3.5 nautical miles per US gallon. At full noise, 5100rpm (40knots) I'm using 16 US gallons per hour, that's 2.5 nautical miles per US gallon. I distinguish between US gallons and imperial gallons because there is quite a difference (1 gallon = 4.55 litres, 1 US gallon = 3.78 litres).

They is no way I can guess on the 292, or even the 250 for that matter. We don't know what size prop you'll be using or the final top speed of the boat.. If I had to pull a rabbit out of my a... I'd say .... "I have no idea"...

Cheers................

PS. I use one of these to measure consumption...

 

[starsnstripers]

Re: L6 250/292 ?

I think i have 19p prop not sure of dia. but I have no idea either we'll have to see in the spring. right now i'm in about 2' of snow. I think i'll be using the 250 this season anyway the 292 is a back up or for the other boat i'm looking at. I'm watching people run 292 L6 motors on "you tube" videos internet. thanks for the info i'm sure i'll be back

 

[starsnstripers]

Re: L6 250/292 ?

Oh and it's -1 degree out BRRRR!

 

[achris]

Re: L6 250/292 ?

Oh and it's -1 degree out BRRRR!

Same as here... Did 10 runs on the ski slope today... Will try for 12 tomorrow.... And the snow is over 2 metres deep here....

cheers.

 

[starsnstripers]

Re: L6 250/292 ?

The H*ll with this weather i wanna go boating/fishing! Oh well gives me a chance to bug Don here on IBOATS ! haha! Gives me time to get everything done and ready for the season coming. I'm going to cover my cuddy cabin cushings on cold days. Everything i read say the 292 has good low end torque and thats what boats need ahey? Hard to get a happy medium with enough power and economical to run, i think I'm in the ball park we're gonna try it. I dont have alot of funds so I'm counting on the ole stovebolt in an ole Slickcraft to get me out to the striper fishing grounds. "no worry i'll have a kicker motor" haha !

 

[Lakester]

Re: L6 250/292 ?

Stop picking on my Stovebolts! There are some nice L6s out there still running with less issues than alot of the new junks! haha! There's a mercruiser L6 292 on ebay right now that has a holley 4brl for 900.00 but it's about 700 miles away. Atleast this 292 has the mounting ear/tab to recieve the merc front eng. mount,unlike the 250 i have. Thats a plus! Well I don't know I'm going to run the 250 this coming season anyway and prob rebuild the 292 for next yr. It will be fun to try and see the difference, who knows i might go back to a 250 or do up the 292 for the SS 235 i'm looking at with the outdated obsolete OMC - V8 in it. Thanks for the input and i got my answer: 200hp w/4brl carb.

ahh, heck! if it dont work out, just use the inlines to go inline drag racing and bolt in a V8. should be close but for the exh manifolds...

varrom, v-room!

regards
lakester

 

[dirtyoldman]

Re: L6 250/292 ?

The stock carb on the 160/165 is a rather small rochester 2 barrel. It performs very well for the 250, but I think I would step it up a size for a 292. I would not run a four barrel at all, but the rochester 2gv came in a number of different configurations all the way to 500 CFM.

I don't know what the options are for a marine version, but they all appear to be in the top cover (and the port for the PCV valve) to me.

 

[achris]

Re: L6 250/292 ?

The stock carb on the 160/165 is a rather small rochester 2 barrel. It performs very well for the 250, but I think I would step it up a size for a 292. I would not run a four barrel at all, but the rochester 2gv came in a number of different configurations all the way to 500 CFM.

I don't know what the options are for a marine version, but they all appear to be in the top cover (and the port for the PCV valve) to me.

No-nah-no.... The 292 needs a 4bbl. A 2 bbl with 500cfm would be a pretty crappy idler. Not enough air flow at low engine speed to pick up fuel consistently.

sns,

Been doing a bit of looking on the web and found you just the right carb... An Edelbrock 600cfm marine. The Edelbrock number is 1409. Even seen a couple on e-bay. It's a Carter AFB knock-off, but a damn good carb.... Can't go wrong with that one.

Chris.......

 

[starsnstripers]

Re: L6 250/292 ?

Hey thanks for the carb info. Why wouldn't a 2brl with like 400cfi work? If it takes 422cfm @ 5000 rpm and it takes 337cfm @4000 rpm and i think a 292 long stroke motor would be happy wot about 42-4300 then 400cfm + 2brl should work too, no?

 

[achris]

Re: L6 250/292 ?

Hey thanks for the carb info. Why wouldn't a 2brl with like 400cfi work? If it takes 422cfm @ 5000 rpm and it takes 337cfm @4000 rpm and i think a 292 long stroke motor would be happy wot about 42-4300 then 400cfm + 2brl should work too, no?

See the first paragraph of my previous post.....

Double post..... could one of the mods 'zap' this one please...

 

[achris]

Re: L6 250/292 ?

Hey thanks for the carb info. Why wouldn't a 2brl with like 400cfi work? If it takes 422cfm @ 5000 rpm and it takes 337cfm @4000 rpm and i think a 292 long stroke motor would be happy wot about 42-4300 then 400cfm + 2brl should work too, no?

See the first paragraph of my previous post.....

Basically the more air you make available at the top end, the slower the air flow at the low end. A carb works by using air speed through a venturi to create a low pressure area for a calibrated volume of fuel to be drawn into. If the air speed is too low, then the volume of fuel drawn is very difficult to control or make consistent. A 4bbl carb works best because the 2 larger bbls are closed at all but high engine speeds, thus combining the advantages of both high volume (high engine speed) and low volume (low engine speed) systems.

C.....................

 

[starsnstripers]

Re: L6 250/292 ?

The 1969 292 w/4brl I just found out calls for 3900-4300 wot. I see your point for having a 4brl thats why that one year 1969 mercruiser put a 4brl on it. They must have made the manifold at that time to recieve a 4brl by removing the plate mine has with the 2brl on the 250. So that carb you mentioned sounds like a good prospect. i'll check em out. P.S. going to unbolt the ole stovebolt 292 tomorrow>

 

[DJ]

Re: L6 250/292 ?

I'm in "dirtyoldman's" camp on the correct sized two barrel for that application.

Less hassle and less chance of just throwing $3.00/gallon go juice down an engine that won't use it all.

The 250/292 do not like lots of RPM's. 4300 is tops and they'll complain about that.

 

[starsnstripers]

Re: L6 250/292 ?

Well the one year that merc had a 292 L6 It Had a 4brl on it and a 2brl on all the 165 250s. Ive had trucks that are better on fuel with a four brl than a 2brl because of what Achris said, And the primarys are smaller on a four brl than on a 2brl carb so you'd only use more fuel if you used high rpms alot to open the secondarys.

 

[starsnstripers]

Re: L6 250/292 ?

Don't know if there is a marine application 2brl with enough cfms? I agree about maybe less hassel as far as linkage i'm not sure.

 

[achris]

Re: L6 250/292 ?

The other thing I forgot to mention in my previous diatribe was that a large 2 bbl will produce less torque at low revs than a 4bbl. The reason is quite simple. Slower air/fuel speed means less bang for your buck. Means less torque. Have a look at the bigger 2 stroke outboards. Using the same engine block they manage to get more horsepower by just putting carbs with bigger venturis on. No such thing as a free lunch... the price of more horsepower at the top end is less torque at the bottom end... why? because the bigger venturis have lower air speed at low revs... et al..... Enter the spread bore 4 bbl carb... Small venturis in the primaries so higher air speed at low revs, producing more torque and two ginormous secondaries.. more than enough air flow at high rpm to allow the engine to rev without suffocating.

Chris...........

 

[dirtyoldman]

Re: L6 250/292 ?

The best running carbureted vehicle I ever had was a 1970 impala with a small block 400 and a rochester 2gv carb on top. That was as big as chevy used it, and ford put them in all the 400's and a few of the 429 industrial engines made back then as well.

I do not agree that you lose any idle capability. The rochesters have a fine idle and off idle circuit and are more than capable of handling bigger cube engines. Some of the biggest engines required drilling the throttle plate to allow the engine to idle without using the off idle circuit (too much vacuum). However, a 292 is not nearly big enough to have that problem.

What you do give up with a 2bbl is volumetric efficiency. You want maximum air flow through the venturi to make it efficient. In a marine engine, I think a 2 bbl would do a better job of this because it is consistently at 50% or more throttle. On an automobile, a 4bbl can actually give better mileage than a 2bbl, and that is because at low throttle conditions, the velocity of the incoming air is higher through the two open venturis than it would be if you had one bigger 2bbl. I think that since marine engines are utilized more fully in normal conditions and rarely run in low throttle conditions like that, a 2bbl would be as good if not better.

As mentioned, the stock carb on the 250 is like 300 CFM. A small 4bbl is about 500 CFM and would be very much oversized on a 292. The 1409 you mention is 600 CFM, which is better sized for a 454 or high HP 350. If I were going to run a 4bbl that would be the one I would pick. It is based on the carter AFB, which was a superior carb in many ways.

 

[DJ]

Re: L6 250/292 ?

Don't know if there is a marine application 2brl with enough cfms? I agree about maybe less hassel as far as linkage i'm not sure.

As mentioned previously, the Rochester 2 bbl.carbs were all very similar. The only real difference in the automotive and marine versions was the float bowl vent tubes. In the auto version, they overflow to atmosphere, in the marine version they overflow to the carb. throat (J tubes).

I do not disagree on the air velocity, etc. We all have to realize that the 250/292 are not high performance engines in standard trim. The 292 is going to pull more air simply by nature of its larger displacement and very long stroke.

We also have to remember that HP rating systems have changed. I'm not talking about crank vs. prop HP, I'm talking about Brake vs. Net HP. In the 1972/3 timeframe engine ratings were changed from brake to net HP ratings.

I have no idea what a 292 would do on a dyno. today.

Don't get me wrong, I love the simplicity of those old engines. They were great and would run forever with reasonable care. I'll bet not many people know that the original Toyota Land Cruiser engine was basically a carbon copy of the 250/292 built under license from GM.

If it were me, I'd go the simple route, get it running good and then start tinkering.

 

[achris]

Re: L6 250/292 ?

As mentioned, the stock carb on the 250 is like 300 CFM. A small 4bbl is about 500 CFM and would be very much oversized on a 292. The 1409 you mention is 600 CFM, which is better sized for a 454 or high HP 350. If I were going to run a 4bbl that would be the one I would pick. It is based on the carter AFB, which was a superior carb in many ways.

Check Edelbrock's website ("http://www.edelbrock.com/automotive_new/mc/carbs_acc/performer.shtml Scroll down to near the bottom of the page, under 'Marine') and look at the specs on this carb and their recommended application. 4.3l and SBC. The 4.3l is 262 cubes. A full 30 cubic inches smaller then the engine sns is talking about. They recommend the 750cfm for the 454.

Running a 2 bbl, of any size, on this engine would just strangle it. Sns is looking for horsepower and fuel economy. A 2 bbl on the SBC will only deliver 198 horsepower so how do you expect it to deliver 200 on a smaller engine?

Chris.............

 

[dirtyoldman]

Re: L6 250/292 ?

Check Edelbrock's website ("http://www.edelbrock.com/automotive_new/mc/carbs_acc/performer.shtml Scroll down to near the bottom of the page, under 'Marine') and look at the specs on this carb and their recommended application. 4.3l and SBC. The 4.3l is 262 cubes. A full 30 cubic inches smaller then the engine sns is talking about. They recommend the 750cfm for the 454.

Running a 2 bbl, of any size, on this engine would just strangle it. Sns is looking for horsepower and fuel economy. A 2 bbl on the SBC will only deliver 198 horsepower so how do you expect it to deliver 200 on a smaller engine?

Chris.............

The 2bbl I had on my 400 Chevy was able to pull 265 flywheel HP out from that engine. You want a larger model than the one that we use, probably by the ratio of the increase in engine sizes. I think that's about 350 CFM.

Also, a carb really doesn't "strangle" an engine. Running too small a carb is really no big deal, it just can't perform to it's fullest ability. I actually ran the carb we have on our 250 engine on a 460 ford for a little while. It was like driving a 6 cylinder, but it still had lots of torque. I had no problems with leaning out too far or needing to rejet, like I was told by so many people. It was well jetted for what I was using it for, and it just didn't let me open up the throttle like I wanted to.

I think that overdoing the carb is much worse than underdoing it (even more so on a boat). Too big a carb will make economy go through the floor and too small a carb only hurts HP. I would error on the side of being too small.