2 bbl to 4 bbl swap or rebuild

[John_S]

Re: 2 bbl to 4 bbl swap or rebuild

Chris,<br /><br />I received the Merc manuals. First off, they are huge! The engine manual is actually two manuals, each larger than the Seloc. The alpha drive manual is one manual, again larger than the Seloc. Now, the bad news is, even with all these pages, there isn't a good description of differences between Magnum, L, and LX engines. It is clear the gen+ has different heads. Also the cam question hasn't been solved. Even the merc manual refers to what lifters you have, not serial or model. Anyway, I'll keep reading, maybe its hiding somewhere in all those pages!

 

[Scaaty]

Re: 2 bbl to 4 bbl swap or rebuild

Just junked my old leaky Quarajunk, and did a Edelbrock 600cfm (with spreadbore to squarebore kit) on my 350 GM. Night a day diff..spend the money for a correct manifold and do the carb. Worth EVERY penny

 

[John_S]

Re: 2 bbl to 4 bbl swap or rebuild

Chris,<br /><br />I'm not making much headway on the cams. But I think I have found another limiting factor, the ECM. From my service manual, the Thunderbolt IV system has different ignition advance profiles for the different engine models. Thunderbolt V added Idle Speed Control, Acceleration Spark Advance, Mean-Best Timing Spark Advance, Rev-limiter, and Knock Retard Spark Control. The manual only gives a "theretical" graph for T5, but gives three "real" example profiles for T4. The 5.7L Alpha curve and 350 mag curve are given. There is a significant difference in the advance curves. The mag curve ramps to 19 degree advance @ 2100rpm, and 24 degrees by WOT. The 5.7 alpha ramps to 12 degrees @ 2000 rpms and 22 degrees @ WOT. Note: the base timing (8 degrees) gets added to the numbers above. If this correlates to T5, this would help explain only a high rpm gain. Given cams are same, a 5.7LX or 350 mag ECM might produce more midrange torque. As far as I can tell, there is not a way to reprogram, or codekey to enable a different profile.

 

[Wart T]

Re: 2 bbl to 4 bbl swap or rebuild

Chris, you have found that your engine didnt really need a 4 bbl. It was already getting nearly all the cfm it needed from the 2. You can't make the engine use more cfm than it is capable of moving. RPM are what decide how much cfm an engine will use. The higher the RPM, the more CFM it needs. If you put a 4 bbl on an engine propped for 4400 rpm on a 2bbl and the engine achieves that 4400 rpm, then it has all the CFM it needs. The whole idea behind adding the extra CFM is so the engine can turn a higher pitched prop. It doesnt sound like yours will. I think you should experiment with props before you start messing with other parts of your engine. It sounds as if your engine is running fine. You now need to adjust the drive/prop to take advantage of what you have bolted on. <br />By the way, nearly all carbs will drip a little fuel down on the manifold. A big puddle, well I guess that could be a problem. You say your hot start was fixed, so I guess that makes it worth what you spent.

 

[Chris611]

Re: 2 bbl to 4 bbl swap or rebuild

Seems like at high RPM's the 2 bbl may have been sufficient. Does that also apply at the lower RPMs, I'm think of getting out of the hole with skiers? Just wondering.<br /><br />Thanks<br />Chris

 

[John_S]

Re: 2 bbl to 4 bbl swap or rebuild

If the rev limiter works as advertised (150 above max WOT spec), it should cut-in at 4750rpms. Won't matter how much power the mod added.

 

[Wart T]

Re: 2 bbl to 4 bbl swap or rebuild

No, the 4 bbls do not help at all with hole shot and in fact can make it worse. If you installed a vacuum secondary the hole shot should remain about the same assuming the previous 2 bbl was operating correctly. If it is a mechanical secondary like the Rochester then you will notice a worse hole shot. You can adjust the restrictor plates on the rochester to compensate for all four plates opening up at once.<br />Remember, RPM demands CFM. At low RPM if you suddenly open all four plates what happens? The air flow slows WAY down and fuel starts to drop out and turn into liquid on the intake manifold. That's why they install "Double Pumps" on those mechanical Holleys, to squirt all kinds of gas down there to make up for what is being lost. <br />Rochester uses restrictor plates to keep the air speed up in the primaries until the engine rpm comes up enough to use the secondaries. At low RPM 4 bbls are useless. High RPM they show improvement.<br />You could try propping down in pitch to get the rpms up. You may actually get higher speed that way. Of course you'd probably exceed the recommended wot rpm, but boy you'd feel the boost those 4 bbls give then.<br />At any rate this changeover doesnt always give good results. At least you say it does run better. Now you know the old carb probably wasnt working as well as it could have been.

 

[John_S]

Re: 2 bbl to 4 bbl swap or rebuild

oldbaldfatguy,<br /><br />According to the article, linked to further up this thread, the carb he is using does have vac secondaries. So at low rpms he should have higher velocity (300cfm vs 425).

 

[Wart T]

Re: 2 bbl to 4 bbl swap or rebuild

Thanks for checking. You should have similar performance with that. Your hole shot may even improve.

 

[fast94impala]

Re: 2 bbl to 4 bbl swap or rebuild

I just completed 5.7L 4 bbl conversion. Installed Edelbrock carb (1409) and dual plane manifold. After dialing it in, the extra power is very noticable. Got another 5 mph on top end AND the boat launches better from dead stop. The key is to dial it in. At first I did not notice much difference and even experienced a bog during holeshots. After advancing timing by 2-3 degrees and regapping plugs to .035 (manual I used has 5.7l spec at .045), the difference was well worth price of admission.

 

[Chris611]

Re: 2 bbl to 4 bbl swap or rebuild

Fast94, what is the total timing them? My boat mech set mine to 8deg BTDC. I wonder if I should go more. I also don't know what the plugs are, I know the mech replaced them when it was the 2bbl at the beginning of the season, so I would imagine they are whatever gap is required for that setup.<br /><br />Thanks<br />Chris

 

[jimi]

Re: 2 bbl to 4 bbl swap or rebuild

Northern Eclipse :<br /><br />Was it hard to set the jets ect?<br /><br />PS Notice you're in TO, where did you buy this stuff from?

 

[fast94impala]

Re: 2 bbl to 4 bbl swap or rebuild

The plastic Mercruiser cover over the flame arrestor contains engine timing and plug specifications. (Of course I didn't notice it until AFTER I replaced the plugs - My manual had .045 for the gap)<br /><br />On my 96 5.7l, gap should actually be set to .035 and timing should be 10 BTDC. Hence regap was required. <br /><br />I have read a few posts where you advance until you hear pinging. Based on my timing gun, my base timing is close to 13BTDC with indicated rpm at 650 (Thunderbolt V), but my tach is probably off. No pinging. I may try to advance a bit more by moving distributer counter clockwise a touch to see what happens, but will mark timing where it is now cause it's running so strong.<br /><br />HTHs

 

[John_S]

Re: 2 bbl to 4 bbl swap or rebuild

FWIW, the Merc service manual say 10 degree BTDC for ThunderboltV and 8 degrees for ThunderboltIV. All EFI and gen+ have 8 degree. <br /><br />Sparkplug gap from my cover says 0.035, but service manual says 0.040, except gen+ has 0.045 gap. <br /><br />Chris or Fast, could you do me a favor, assuming you both have Thunderbolt V systems, look on your distributer and see if you have a anti-knock module, along with the ECM module. Mine does not, and trying to find out if it because it is carbed model. Thanks.

 

[Northern Eclipse]

Re: 2 bbl to 4 bbl swap or rebuild

Jimi, <br /> Just got back from vacation, hence the late reply, I got the intake from Performance Improvements ( best price I could find) The carb I got off E-bay, but you can order one from Canadian Tire for about 500 loonies, Fuel line and flame arrester I purchased from a Merc dealer gaskets from Napa, throttle cable bracket from Merc. I hardly had to touch the Carb I installed it and turned the Key, engine lit up right away, Idle was about 100 rpm to low. The fuel mixture was about right on, Edelbrock does a good job presetting these carbs, Final tuning is simple.

 

[MGallik]

Re: 2 bbl to 4 bbl swap or rebuild

This thread needs to live again.

 

[Don S]

Re: 2 bbl to 4 bbl swap or rebuild

This thread needs to live again.

Why??? <br />It's 3 months old, and I would imagine he has it working fine now since he quite posting.<br />If you have a similar problem, you should really start your own new thread.

 

[Bondo]

Re: 2 bbl to 4 bbl swap or rebuild

This thread needs to live again.

And just Why is that ?????<br /><br />You haven't participated in Any of these Old Threads.......<br />Like Don says,<br />If You have a Question about How Much Horsepower, or How to mount a 4bbl. on Your 350..........<br />Just Ask.....<br />But Please,<br />Start Your Own Thread............

 

[MGallik]

Re: 2 bbl to 4 bbl swap or rebuild

Hmm, Well, I thought it would be best to keep as much information on the same subject together. If you'd prefer to spread it all over the forum, we can do that too.<br /><br />I'm in the proccess of asking more of this thread and adding to it.

 

[Don S]

Re: 2 bbl to 4 bbl swap or rebuild

If you have a question, start a new thread. <br />So far you have done nothing more than bring 2 old posts to the surface for no reason that anyone here can see.