stevesxm

i don't believe for an instant that the alleged exhaust scavanging has increased the airflow thru the motor enough to make it lean. there have been about a zillion posts on here from people swapping out exhausts on the dyno and seeing absolutely zero difference in af ratio not to mention my own experience which would be the same. what this is going to be is, as has been suggested above, that when they did the service they changed something.... adjusted something, or did something on BOTH motors that they now don't like. to lose 5 mph at 60 mph means these motors lost close to 50 hp EACH .... that could be firing order which in certain cases will be almost un noticeable... or a spark plug or two that had the gaps closed up when they were being fumbled into place or if they screwed with the timing without doing the jumper deal that some of these needed etc etc etc...

the bottom line.... its going to be something that was changed...just as you suspect... and its not going to be the exhaust.... unless the gaskets occlude the port or something...

DKerns

I won't argue, I bought my boat cheap with NEW exhaust!!! The only change the previous owner made was the exhaust. Motor leaned out and had the head of a valve in the oil pan!!

stevesxm

uh huh. im sure thats just what happend

RonS

you said you just had the motors serviced, did they adjust the initial timing, if they retarded it it could cost you the power, they may not have set it correctly. I know the computer should adjust it but it may not be able to adjust it enough.

Just a guess.

stevesxm

and its a good guess.... and exactly what i suspect, personally.

tomas_wallin

Yes, but the new exhaust probably increased the rpms as well, so the 200rpms might have been 500rpm.

VetteLT193

Another idea that would do this is wrong spark plugs and/or plugs gapped wrong.

If the mechanic used the wrong heat range or wrong plug all together you could see a big change in power and it would be the same over both engines.