Upgraded Engine, No More Power than Before...

[greg82255]

Hello,

I have made several posts about this engine, my '82 Merc 898 (305). Over the winter I put on an Edelbrock Performer intake and switched from a Rochester 2 barrel to a Holley 4160 (600 CFM) 4 barrel carb. I also switched from log manifolds to center-rise exhaust. I was told by several people that I would see a 20-30 HP increase and that I would see a 3-5 miles per hour increase. I haven't seen any increase in speed. The engine is hitting 4400 RPM and 50 mph, which is exactly the same as it did last year before I did anything to it. I'm not a mechanic and don't know how to properly tune a carb and adjust the timing, so it hasn't been done yet. Is this more than likely the reason I am not seeing any increase in speed? Will a properly tuned motor really put out 20-30 more horsepower than one that is not tuned right?

 

[John_S]

Re: Upgraded Engine, No More Power than Before...

You had to pull the distributer to change the intake. It absolutly needs to be timed. If the timing is retarded enough, it is causing power loss. How much? Anyones guess.

New, those carbs are very close to SBC tuning. I wouldn't expect it to be out by enough to cause significant power loss. What did you find about the secondaries and vacuum leaks per the other thread you had? Also, I believe you indicated this was a reman carb, which adds its own set of unknowns.

Even with the "general rule" of 10hp per mph, you are only talking 2-3mph. It takes some practice to do consistant enough runs to see such small changes. Load (gas, people, gear all in same weight/location in boat, trim adjusted, hull bottom same condition, and water and wind currents, etc, etc, as when you ran it last year.

BTW: The 20-30hp is in line with HP differences between 2brl and 4brl Merc 305's.

 

[Philster]

Re: Upgraded Engine, No More Power than Before...

If your new setup doesn't spin the engine any faster (max RPMs) than the old setup, but does produce more HP, the acceleration should be a touch (just a touch) better, and top speed will be spot on what it was before.

If this new setup has 30 new (real.measured) ponies, and she has them in the upper RPM range (such as when at WOT and in max operating range) then we can assume she has some extra grunt there to push a different/bigger prop. That's a big variable.

It's possible that you gave yourself a whole bunch of mid-range efficiency, some middle to slightly upper-range horsepower, but might not have effected any serious kick at the upper RPM echelon. Ergo, you have a little bit more low- to mid-RPM grunt, some better overall efficiency and a steeper power curve, but there is a chance none of it makes peak HP at peak RPM, so the prop ain't gonna spin any faster if the engine ain't.

Still, I would expect more from her.

 

[John_S]

Re: Upgraded Engine, No More Power than Before...

I'm not a mechanic and don't know how to properly tune a carb and adjust the timing, so it hasn't been done yet.

You wouldn't be the first to bite off more than they could chew. But, for others reviewing this thread, if you are unable to time or tune a carb, not an upgrade that should be considered.

Given this was probably your first intake replacement, I highly recommend hooking a vacuum gauge up to test for leaks. Did you use the rubber intake gaskets at the front and back? or throw them away and use just a good rtv?

At this point, nearing the busy july 4th boating season, might just want a mechanic to help speed things along.

 

[greg82255]

Re: Upgraded Engine, No More Power than Before...

What did you find about the secondaries and vacuum leaks per the other thread you had?

I found a vacuum port un-capped, which appeared to be causing the problem. I capped it and they open now. No increase in RPM, which leads me to think they may have already been opening..?

It was my first intake replacement, and I used the recommended gasket set from CPPerformance.com and coated them with permatex gasket sealant like the Merc manual recommends. I also threw out the seals that go on the front and back ends of the intake and used the recommended sealant from the Merc manual.

I have a friend that is a Mechanic, so I will probably just have him tune it and vacuum test it for me. I don't know enough about motors to do it myself.

 

[John_S]

Re: Upgraded Engine, No More Power than Before...

That is good with the discard of the rubber gaskets. Just checking....

A mechanic should take very little time, to time the engine and check for vacuum leaks. Tuning the carb, usually takes a number of boat runs and time on the water. Need to be inspecting the plugs to make sure it isn't running lean, too.

Please report back, once the work is complete. I suspect you will get some additional rpms/speed.

 

[greg82255]

Re: Upgraded Engine, No More Power than Before...

Thanks everything for the help. It's going to the mechanic this week and I will report back once I get on the water again.

 

[WizeOne]

Re: Upgraded Engine, No More Power than Before...

IMHO, that 600 cfm Holley is too big for an otherwise stock 305. They are often used as a generic carb for small block Fords and Chevys but, at minimum, they need to be set up for them and even then they are too big for under 5000 rpm torquers.