2000 7.4L MPI 310hp T.B. gasket mismatch
 

I brought home my boat from storage and was doing some checking around to see how things held up over winter.

I've had the boat a year now but never really looked into the throttle-body before. With the motor not running I opened the throttle to wide open. That's when I see an obstruction in the lower part of the opening. :confused: The gasket that goes in between the T.B. and the cast elbow that runs to the intake plenum is in the airpath. The gasket is not matched for the opening and is blocking airflow. I couldn't believe it!:mad:

Has anyone ever seen this before?

I just talked to a mercruiser mechanic that looked up my motor on a microfische and found that a D-shaped gasket is correctly placed in between the T.B. and the intake elbow. Why would Mercruiser not use a gasket with a round hole for this T.B. assembly? Any other guys out there with a 7.4L MPI motor look into their throttle bodies lately? :confused:

Just got off the phone with Crocket marine about the miss mach they know little about it. They seemed to think the throttle body might be a bit large and this is a way to compensate. I also did some calculations on the size of the flame arrestor and thought it is too small to properly flow around 700 + cfm. One fellow stated he removed his and felt no differance. Maybe the gasket is the restrictive part. Might have to try opeaning up the gasket and see what happens. It woud be nice if it ended the soot on the transom condition.

I have the same soot problem on my transom too. There must be something to this.

Hey EX20 just read yor post. I am going to look tomorrow at my 2000 model 7.4's. I'll let you know what i find. Keep us posted on what you do to remedy the situation.

Sounds like a good place for fuel to puddle then run into the runners instead of being atomized.

the gasket is about 1" behind the butterfly plate in the throttle body, (no fuel is added yet) I have no idea why they used that gasket, mine has it to. maybe it is used for idle quality or tuning, but it takes up about 1/4 of the space in the throttle body. anyone what to test this with it and then trim it and see what happens?

Guessing that merc put it there because if removed the 7.4 out performs the 502. Going to purchase a new gasket this weekend but wont be running the boat till the first weekend in May. Also have been trying to purchase a K&N filter but they keep sending me the wrong catalog and it takes 3 to 8 months to get it.

I dont think the tech surport at k&n knows the differnace between the efi and mpi I believe it needs a adapter to fit

The 7.4 dosent need an adapter because the conection is round the mag and above is rectangular

I was told by a mechanic to go ahead and trim the gasket the ics is what controls the idle speed and the only change if any should be top end mph

Was under the hatch last night and checked out the gasket. Went ahead and trimmed it. True it wont effect idle. It might change throttle position a bit a cruse speeds. Noticed fuel looked like it puddled up behind it in the elbow. This might cause some of the soot. I dont think removing it will cause a top end leanout. Be careful if you cut it in place it is a paper gasket with a metal /foil core. I can think of several reasons why the gasket is like that one is just to eliminate intake air noise at the cost of a very small amount of performance.

I'm still waiting for my mechanic to call me back. He hasn't heard from the Merc rep yet. Big surprise.

He thought it might be there to prevent gas that puddles in the elbow from getting to the flame arrester. I guess it might leak out. Why is gas puddling there in the first place with a multi-port fuel injection?

I will probably trim it too, and just keep an eye out for any gas drips from the flame arrestor.

Sorry my bad puddle was from fogging engine. I need some more sleep to many projects. Any way it dosent really cover much of the opeaning. But it would cause turbulance and a bit of air flow reversal possibly. I would like to get that elbow on my flow bench and see if filling in that area were the puddle formed would increase flow.

Im waiting to the first day I take the boat out this year, Ill make a wot run trim the gasket and make another and see if there is any change in mph

Any one run there boat yet?

Still waiting for it to warm up.

Got the boat in the water this week end. Only changes were to the throttle body gasket (trimmed it out) and changing to Redline oil in the out drive. Looked to be a lot less soot on the transom. Performance seems to be up. Some of the soot maybe from fogging oil burining off. No full throttle runs yet had it up to an easy 60 for a few. Mid range seems a bit better dont expect to see top end gains with the rpm limiter and not changeing the prop

I trimmed the gasket and ran it last weekend but the water was too rough to do top speed runs.

A loud whistling sound is present now when the throttle blade starts to open and when running between 2200 and about 3200 RPM. Any of you guys that trimmed your gaskets get that now? I wonder if that's why Mercruiser has the gasket shaped that way. I have to decide if it bothers me too much. I'll get a new one and put it in again if it starts annoying the crap out of me.

Yes I got the whistle too. Some manifolds I have designed "fingers"were incorperated in the design to reduce harmonics but the boat seems more responsive in mid range. Also have added a Jacobs omni with restrike not enough testing yet but it seems to have helped alot too.

did mine had it ran fine then it wouldnt ggggget over 2900 rpms found the fuel pump went bad, and yes mine makes the noise but not badIm running thru hull, k&n and the trimed gasket what mph and rpms did anyone notice and did you check your plugs to see if your lean now?

After a few outings ... ran out stored fuel.... installed the Jacobs Omni pack from my other boat and just regapped the old plugs ..... they looked rich if anything. I am going to see if they clean up.

SO guys?

Finally... What's the issue ?

Let's continue that debate... Is it a good thing to cut that gasket??? I am very curious....

NO BODY?????

It doesn't seem to make a difference one can feel in the real world of varying conditions. It does make a difference that can be heard. Others might think you have some boost..... As I stated before the mismatch is probably there just to reduce the whistle.
I design intake manifolds as well as any other part on an engine and what we call "fingers" in the inlet are incorperated in some designs to reduce NVH.

Thanks.

So You guys who own the 7.4 MPI as I do... (Mine is a 2001)

Mine is stock-stock no gadgets installed- no K&N no blablabla...

The only thing I have is direct Tru-hull exaust (no mufflers)

Is there any little things I can do to increase anything?

I've read that it's not worth the money to put tousands of dollars on those 7.4.

Any suggestions, tips that have been tested with little succes?

I tought about taking the air outside of the engine compartment.
With some kind of fan. (The air is very hot inside the engine compartment !) It's like a stove in there. Shure, most be carfull for water. Maybe it would be good to take the air in the sides near the bolster seats (there is no water going there). Lets say that the air is 25 degrees cooler, then it would be a 2% increase in power. 310 * 2.5% = 7.75 HP for under 100 $... It could be a beginning...

Also...

Considering that an engine is a air pump air considering that it sucks the air : If I install a powerfull fan, let's say a 700-800 CFM fan with reducers that would push a powerfull blow directly in front of the flame arrestor... (At wot only).

My thingking is if you are sucking air with a straw and someone blows at the other end of that straw... It would be easier for you sucking more air....

What do you think about that?

I brought this up again because I trimmed my gasket and it did seem like it spanks the rev limiter a little harder. Has anyone tried this with any MPH gains? Just curious

Roby

Hey Roby,
What is the rev limiter set at?
I normally run a 26p 4400rpm at WOT.
I tried a 24p and got 4600rpm.
-2mph with the smaller prop.

Mine starts poping out at 4800. However it is supposed to be 4600 I believe. My tach is probably a little high. I'm running a 25p Mirage plus at 4800. according to my tach

Roby,
My engine is a Volvo Penta. I don't have the elbow between the throttle body and the intake. I also don't believe I have the "D" gasket issue, but I have never been able to spin more than 4600rpm.

I have heard some rev. limits are set at 4650, some at 4800. My Volvo Manual says 200 rpm above max. recomended rpm. I don't think I have ever heard my engine popping or loosing spark on a rev. limiter.

I also have a 25p Mirage plus, never got it over 4500rpm. The Mirage is faster than the Bravo. Maybe I need to barrow someones 23, might turn up more than the 24 Bravo.

Do you know how much air the throttle body on one of these will flow? I was thinking, mine would not go over 4600 because it ran out of air.

I'm not too sure about the Volvo throttlebody however you can figure the needed CFM by the following method:

cubic-inches X Max RPM divided by 3456 =

Roby

I have another complete 7.4MPI system from a Merc. sitting in my basement. Same throttle body as my Volvo. The only difference I see between the two systems is the elbow between the throttle body and the intake.

Using your formula: 5000rpm would only need about 650cfm...

Roby, I now that you have experience with fuel injection systems. Is there a way to figure how much the throttle body should flow? I believe it is 80mm round...

The CFM rating of any given 80mm throttle body can vary by a wide margin. However it should be plenty of CFM to run your engine. You are wanting to run your engine past 4600 RPMs and without knowing the exact Volvo Penta engine you have I can’t determine if you can.. or should.

The rev-limiter on the 7.4-MPI is supposed to be 4600 from what I’m told. Because of the limited flow of the Vortec heads and cam specifications, this engine isn’t going to make any power above that anyway. You might want to check with one of the prop experts here to determine the correct application for your hull & engine. You may get a couple hundred more RPM out of it by reducing pitch, but you may not be gaining any speed.

Best regards,
Roby

Hey Roby,
The Volvo engine I have is the same Vortec 7400 (L-29) base engine from GM that Merc. used. It's just painted gray rather than black.
I don't plan to run over 4600rpm for this year, but with off season mods., I am hoping for 4800-5000 for next year.
My plan is already in motion. I have a set of heads being ported, and new valves and springs will be installed. I have purchased Stainless Marine exhaust manifolds. I will be changing the cam to something with more lift and duration. Then I will need the ECU reprogrammed. I also have a set of larger injectors if needed.
My question is if the throttle body will flow enough for 4800-5000 RPM, or if I need to add to my shopping list.

Oh no for 5K RPM it should be more than adequate