7.4 and Imco Powerflows

I just upgraded from stock exhaust with Y pipe and out the drive to Imco Powerflows and through transom exhaust. My 7.4 is mostly stock except for a larger Holley carb and K&N filter.
When I asked some of the local shops, they all suggested starting with upgrading the exhaust. I have to say that I am dissappointed with the result as the cost was about $4k.
Anyone else have similar results?
Part II:
The motor was recently apart due to overheating. Heads were sent out for a valve job. After idleing through a long no-wake zone the boat really hesitated once I hit the throttle. It seems like it "loaded-up". I have also noticed a lot of black "soot???" on the transom. I expected that there would be more due to the through transom exhaust, but not this much. Any suggestions?
I thinking timing may need to be advanced a bit.
Thanks

I did the exact same thing 2 years ago. Not much in changes :confused: . So after that investment I told the wife the engine needed a tune up. Well several thousands of dollars later I am still messing around and still not happy. Pulled the engine with 20 hours on it this weekend to see a spun bearning :cry: :cry: I don't think you will see any gains until you get inside the engine and really make it breath :) Put a blower on it and tell the wife its a muffler!

I just upgraded from stock exhaust with Y pipe and out the drive to Imco Powerflows and through transom exhaust. My 7.4 is mostly stock except for a larger Holley carb and K&N filter.
When I asked some of the local shops, they all suggested starting with upgrading the exhaust. I have to say that I am dissappointed with the result as the cost was about $4k.
Anyone else have similar results?
Part II:
The motor was recently apart due to overheating. Heads were sent out for a valve job. After idleing through a long no-wake zone the boat really hesitated once I hit the throttle. It seems like it "loaded-up". I have also noticed a lot of black "soot???" on the transom. I expected that there would be more due to the through transom exhaust, but not this much. Any suggestions?
I thinking timing may need to be advanced a bit.
Thanks
Sounds like your getting too much fuel...You may need to jet down the new carb. The next step is a high flowing intake. A stock intake is the bottle neck in your system as of now. If you can do the intake the carb may work better as is. Remember a engine is a air pump....

Which 7.4 do you have? I've got the mag which is supposedly the 365 hp one. the motor is 15 years old and I just put the Imcos on about a year ago. I needed new exhaust cause the stockers were starting to leak. The gains I noticed were about 2 mph on the big end. Not much but now it will top out will a full boat. Comes out of the hole better also. I gained some rpms also so I'm gonna go with a little bigger prop. and try that.

On a stock 454 an 850 is to big. (yes you can go to big) Just rebuilt my friends mild 454, all of the engine builders and carb manufactures said no more than 750cfm. When my 502 motor was naturally aspirated I ran a 800cfm holley dp, aluminum heads that had been flowed, roller valve train, 560 horse on the dyno. :)
Merc's carbed HP 500 had a 800cfm holley. :wink:
Clint

I don't think the carb is an 800 or higher. I'm guessing somewhere in the 700 series. I guess my frustration is that the carb was not touched during the upgrades and now I have some hesitation/loading up occurring.
I've had the boat for 3 years and have never had a problem like this before.
It would seem that the new exhaust would actually let the fuel flow in and out of the motor quicker, thus letting a bigger carb perform a little better then before.
I may try the intake. Teague recommended that also as a low price upgrade.
Next would probably be the MSD ignition.
A friend of mine has a 502 and also had problems with a lot of black residue on the transom. Once he installed the MSD and a fuel pressure regulator, the residue reduced drastically.

I don't think the carb is an 800 or higher. I'm guessing somewhere in the 700 series. I guess my frustration is that the carb was not touched during the upgrades and now I have some hesitation/loading up occurring.
I've had the boat for 3 years and have never had a problem like this before.
It would seem that the new exhaust would actually let the fuel flow in and out of the motor quicker, thus letting a bigger carb perform a little better then before.
I may try the intake. Teague recommended that also as a low price upgrade.
Next would probably be the MSD ignition.
A friend of mine has a 502 and also had problems with a lot of black residue on the transom. Once he installed the MSD and a fuel pressure regulator, the residue reduced drastically.
Your exhaust is a little too wazuu for what your doin with a stock 454. You could have got some used cmi's for $1500 but it's too late now. You'll need the intake and a carb rebuild with the correct jetting. Don't waist your money on the MSD untill you put in a biggger cam. Just make sure your current ing. system is up to snuff. Talk to Al Fonse at www.fonseperformance.com He's a great guy and will go over your set up and help you out. He's pretty good on the prices and his knowledge is priceless.. Tell him corey sent you. Good luck.

Most of the time opening up the exhaust will have to be matched by leaning out the metering in the carb a little. The extra airflow actually does help pull the fuel thru, which richens the mixture. The carb may have been on the rich side to begin with, and you didn't notice it due to the under water exhaust and the extra backpressure. Some of the soot may also be from the headers pulling unburned fuel through the cylinder, and burning it in the headers, called over-scavenging. :hammer2:
I built a Pontiac wagon with a stout 400, and it would just barely idle with open headers (wouldn't idle in gear), Perfect example of over-scavenging. It smoothed out some with headpipes and mufflers, but ran best after the tailpipes were installed. Lost a little top-end, but the low end torque and drivability were much better. Some combinations need more backpressure than others to run best. :idea:
The other posts are right, you need to open up the rest of the motor to really take advantage of the exhaust, but on a good note, you can make the rest of the changes and not have to worry about having to do anything els to the exhaust now!! ;)

The only way to get more horsepower out of the motor is to design the motor with horespower intensions. You can not just choose parts and expect higher results. The bottom end needs to be reinforced then your pistons, rods, valve train, intake cooling, etc....
Unless you like working on your motor and have lots of $$$ for dynos just be happy that it starts.
Somewhat disgruntled again, ran her lean this weekend again and popped and crapped. Guaranteed, valves tuliped.
540 blown big block.

Pulled the carb and found out it is a Holley 750. The primary jets are 72. Does anyone know if that is standard for this model (Holley PART # 0-3310C). The 6.5 Powervalve is also blown which I'm told can allow the carb to leak fuel through at lower demands such as idle or slow speed. We're going to change the powervalve, reinstall the carb and see if there is any improvement. Will probably take the boat out along with an assortment of jets and she if we can get er dialed in.
Thanks everyone for all the tips.