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View Full Version : Changing Exhaust On My 1987 Z21



Greg Andrew
10-22-2001, 01:23 AM
My Z21 has a 454 Mag with stock exh. manifolds,4 inch round ss pipes that exit through the rear side corners above waterline. The thru-hulls are oblong and appear to be quite restrictive. I am changing to Gil manifolds and want to re-route the exhaust straight back through the transom under the step in the middle of the swim platform. Has anyone out there ever done this mod, and if so is there anything I should be careful of before doing so? Thankyou for any light that can be shed on this topic

EricG
10-22-2001, 01:13 PM
First of all, welcome to the board!!

I'm curious as to why you think the Side exits are restrictive? I've got the same exhaust configuration on my Z21 and I guess it never occured to me that they wouldn't flow enough.....they seem to be at least equivelent to a 3" round outlet.

Anyway, from all of the Z21's I've seen that didn't have the factory exhaust like ours, they have run the exhaust over the swim step. I'm guessing because there isn't room to put it under the step and above the water-line, but that's just a guess.

Keep us posted on your project, I for one will be curious to see if you gain anything by getting rid of the side exits...you know, always looking for that little boost :D

Good luck!

EG

Jim
10-22-2001, 02:52 PM
Eric,
I have a 1988 Z21 with the standard through
the stern drive exhaust. I had looked at a few Z21's before I purchased this one and one of them actually had the 4inch chromed
exhaust tips on each side of the swim platform. If you ever went to climb up with the thing running you would get a face full. The funny thing was that when I looked in the engine comparment you could see the original holes plugged that went out towards the center of the transom, which would of come out in the cavity just under the swim platform.
I plan on upgrading my exhaust to a silent choice system instead of straight dry exhaust which should keep me from having to remove the engine. I am curious on what you find out becuase I had assumed the main reason they didn't run it out under the platfrom was because it would be so baffled since that cut out under the platfrom is pretty much sealed by water when your not
on plain. Besides worrying about back preasure I can't believe any back wash could
make it up to the exhaust logs since the exhaust tips would be so much lower then your logs and the flaps should keep that from happening anyway. It would be a shame
to run the exhaust out the sides of the boat since you would loose some of your storage.
Let me know what you find out and I will keep you posted on what I end up doing.
Thanks Jim

Greg Andrew
10-23-2001, 12:02 AM
Hey EricG,
Thanks for the reply! I'll let you know the outcome on my exhaust change.I plan to weld up the inside of the side thru-hulls and plumb in deck drains.
Off topic, I noticed you live in Edmonds, my ex-girlfriend lives a few miles south of you in Ravenna. She's also a Donzi owner (1988 blue Donzi ZF33 w/twin 300HP V8 outboards).She keeps it @ the Seattle Yacht Club.
By the way, do you ever make it up to Roche Harbor or the San Juans?

EricG
10-23-2001, 11:10 AM
I've seen your GF's boat - I really like that series ZF boat - it must scream with the 2 300's on it!!

I've done the San Juan's in several different boats, but never actually my Donzi, I finally dipped it in Salt at the end of this season, so there will definitely be an Island run next year....maybe we'll have to try for the San Juan Donzi meet....hmmm.

EG