Back Pressure

[Speed955]

Has anyone else had this happen. I pulled my mufflers out today just to see if it gained anything in top speed but it actually lost speed. With the mufflers in the boat runs 63.6 mph pretty consistantly (on gps) 32 gal of gas, my son and my big a$$. With the mufflers out it would not go past 61.3 mph same amount of gas same everything just mufflers out. It also didn't feel as quick as it normally does. I know alot of cars need some back pressure to perform well so must be the same with boats?

[cfm]

It can happen but if it does happen it is usually a function of tuning (jetting and/or timing)
What kind of motor and what kind of exhaust ?

[Speed955]

It can happen but if it does happen it is usually a function of tuning (jetting and/or timing)
What kind of motor and what kind of exhaust ?
CFM,
it's a 350 Chev with a Edelbrock Perfomer EPS, Holley 600 with vac sec.(with 4150 metering block kit installed), Comp Cams 268H, GLM aluminum exhaust with 4" thru-hull tips. I think your right, that it is a jetting thing. I have it jetted and tuned with the mufflers in. It seemed rich to me, does that sound right? It will be ran with the mufflers in 99% of the time anyway so I won't re-tune it just to run with them out. I originally took them out to see what it would do, according to CP performance web site you loose two hundred RPM with the mufflers...when I think about it seems like mice nuts anyway. It must be an age thing... when I was younger I liked running my jet boats with very little muffler, but now it gets on my nerves after about 1/2 hour.
P.S Are you the guy's that make the carb spacers? if so, what would you recommend? I am having the interior doen soon and can add the height required to clear a spacer, any input would be apreciated, thanks!

[ghittner]

If it was rich with the mufflers and you removed them it would lean the motor out if anything, because you are moving more air through the motor, resulting in a leaner condition and a result of running faster. What actually happened is the the resulting loss in back pressure hurt the motor, smaller motors especially, like some backpressure, and it helps with power. If you run a smallblock Chevy street dragster with open headers it will likely run faster with an attached 3" exhaust system.....An engine creates the most power when leaner than richer but it is a dangerous balance trying to get lean enough but not too lean to detonate the motor....I prefer a slightly rich condition. What jets are you running? How did you read the plugs when jetting?

[Speed955]

ghittner,
dah, I just thought about it after you brought it up, I should pull a plug and and take a look. Anyway, it's got 63's in the primary and 66's in the secondaries and the plugs were a text book tan color with the mufflers in. One question though, I jetted it in much cooler weather than we had this last Sunday, it was 90 degrees. I jetted it when it was in the mid to upper 60's to 70's. So maybe it wasn't rich? I think I need to pull a plug or two.

[Ryan00TJ]

I usually test when it's hot out to mimic the condition that the engine will see.
Get some new plugs and get the engine up to operating temp and go WOT for at least 30 secs. Bring the boat down from speed and off plane and kill the engine with no idle time and check the plug condition. This is the most accurate way to test. Pulling a plug after cruising or idling on the primary will give you a false reading.

[ghittner]

The hotter it is the less fuel required. ie: if you jet in the cooler weather, you will be richer in summer. Sounds like ya got rich. Ryan is correct but I would give the plugs a little more time for color. I test cruising around to mimic the everyday overall use of the boat. Then, I test WOT for an extended 40 second or so burst and shut down ASAP to get a read as well. That way you know wether to mess with primary's, secondaries or both...

[Speed955]

Thanks for the input guy's,
this weekend is supposed to be hot again, around 88 degrees. Since most of the rest of the year is going to be in this heat range I can jet it for this weather. I had bought one of the Holley box-O-jets that has every jet known to man when I put in the 4150 metering block. I'll bring it with me so I can jet it right there on the water, if I can find some shade;)