Mercury MCM165 6CYL 250CID Exhaust Riser for Holley Sniper

[djs42]

Wondering if anyone has installed a Sniper on a Merc MCM165 6CYL 250CID and can share experiences. We've been able to mount the throttle body (TB) by shaving a small amount off of the bottom of the TB linkage. Have heard some people are running open loop without the O2 sensor but would like to run closed loop with the sensor. We did purchase Hardin Marine 620-32430 riser but this does not align with the 6CYL 250CID exhaust.

Does anyone know who might supply a riser for this motor?
Is anyone running a Sniper on this or similar marine application that can share thoughts or tunes?

 

[achris]

There is absolutely nothing to be gained with a TBI over a carb, apart from the hassles involved with installation.

Most marinised 250s are now well over 40 years old and parts support is no longer there. Pretty much everything you need is going to have to be custom made.

The main reason anyone running any form of injection in a marine engine do it open loop is because the only options for the O2 sensor are either in the lower temperature exhaust area (which is full of sea water, which O2 sensors don't like) or higher up in the dry exhaust area which is very hot (which, O2 sensors don't like)...

How that helps.

Chris...

 

[todhunter]

I'll be doing a Sniper on my 5.7 this coming week. I think for the riser you may have to go custom for the 250 - it's a pretty simple design though. I would take it to a local fab/machine shop and have them make one out of stainless for you. Take your stock riser and the Hardin one and tell them you want one that fits the 250 but has the O2 hole like the Hardin one.

I'm planning to run mine in closed loop and see how long the sensor lasts. Another fella i talked to that had a Sniper on his 5.7 said his runs fine in closed loop and hasnt had to change the sensor yet (i think 2 seasons in). If it doesn't last long though, I'll just switch to open loop and plug the port on the riser. Once the fuel map "learns" well enough, the Sniper should be barely trimming any fuel anyhow based on O2 signal, so running open loop shouldn't make a huge difference. If you do run open loop, you can fatten up the fuel map just a hair for some margin if it makes you sleep better at night.

 

[Bt Doctur]

Nothing to gain with a motor that old. Learn how to repair/adjust carbs and you be just fine

 

[Scott Danforth]

There is absolutely nothing to be gained with a TBI over a carb, apart from the hassles involved with installation.

This

 

[todhunter]

Don't worry about the EFI haters...I'll do a writeup on my Sniper installation when I'm done.

 

[Scott Danforth]

OK boomers, lol.

I'll do a writeup on my Sniper installation when I'm done.

I did the EFI thing back in the mid 80's and early 90s. Spent lots of money to understand that plenum volume and intake design has more impact than whether the fuel is atomized by a carb or EFI until you start varying valve timing.

Test peak power and economy before and after.

 

[achris]

Don't worry about the EFI haters...I'll do a writeup on my Sniper installation when I'm done.

Nothing to do with hating. (Look at my boat, MPI engine). Everything to do with practicality and cost effectiveness.

I much prefer injection over carbs and points (just ask @Scott Danforth ) but there's a time and a place. And a 40 year old engine that won't show any benefit is not the place.

Chris...

 

[nola mike]

The snipers are coming down in price to where they're getting competitive with a brand new carb. Especially for a custom build, I think having a/f data is invaluable. Having spent a lot of time at it, you can tune a carb on a boat "good enough", but without a way to get objective data you're guessing. And reading plugs only gets you in the ballpark IMO.

 

[H20Rat]

The main reason anyone running any form of injection in a marine engine do it open loop is because the only options for the O2 sensor are either in the lower temperature exhaust area (which is full of sea water, which O2 sensors don't like) or higher up in the dry exhaust area which is very hot (which, O2 sensors don't like)...

How that helps.

Chris...

Didn't know O2 sensors don't like high temp... I do know they have to be at 650 degrees to even get a reading, and usually heat themselves to around 1300. In my previous heavily tuned car, there was an O2 sensor between the valves and the turbo. EGT in that area was around 1650 degrees. Betting a standard marine SBC probably doesn't get that hot.

 

[Scott Danforth]

Didn't know O2 sensors don't like high temp... I do know they have to be at 650 degrees to even get a reading, and usually heat themselves to around 1300. In my previous heavily tuned car, there was an O2 sensor between the valves and the turbo. EGT in that area was around 1650 degrees. Betting a standard marine SBC probably doesn't get that hot.

at WOT, the exhaust gas is 1300-1500, higher in the center bores with the siamesed exhaust ports. which is much higher than a car just meandering down the road. without water cooled exhaust, boats would burn up from the exhaust heat.