Transum thickness

jeffswav · 10-09-2009, 02:48 PM

What would be the downside to increasing the transum thickness by 1/2". How far does the driveshaft extend past the coupler? I would like to raise the drive height by 3.5 or 4". I got a idea when I saw a OMC to Merc adaptor plate. A friend of mine with a machine shop has access to a CNC laser cutter up to 1". He thinks I could make a plate out of 1/2" alluminum to raise the outdrive (to cover the hole left at the bottom). Unless you make a series of 2 plates it would increase the transum thickness. A single plate design would be much easier to make and install. Anyone tried this before?

fireboatpilot · 10-09-2009, 11:05 PM

Sounds like Afro engineering to me. I wouldn't want to take a chance of something very bad happening back there. Just my 2cts sorry.

Griff · 10-10-2009, 12:21 AM

Get a -2 shorty lower drive and call it good. Going up 4" is a ton.

Irishtornado · 10-10-2009, 12:37 AM

Or go with the 3" raised extension box from Imco and put a -1 shorty on be alot simpler and less afro tech going on...There's definately too many variables with using that plate like your describing..

getrdunn · 10-10-2009, 12:37 AM

both my dad and i had 24 challenger back in the late eighties and his was built after mine with only 5/8" higher and the boat handled no wheres near as good. it didn't increase top end speed but it threw a nice rooster however it chine walked pretty bad also at higher trim. i would check with the manufacturer or somebody that raised theres. once you go past the fine line it's toooo far. like griff said i would at least even see if you can try a shorty out first. if you ever see an old challenger with or without power for a good buy be sure and let me know. they made them in my home town and had a pretty fast bottom. you didn't sacrifice in the rough water much either.

fireboatpilot · 10-10-2009, 02:24 AM

Besides all this, the engines would also have to be raised the same amount. Ton of work for something that won't work? Lots of easier ways to raise the prop.

jeffswav · 10-10-2009, 05:42 AM

Thanks for all the input. In a ideal world I would put a raised box and a 2" shortie on. Do you know how much that cost? Just looking at other alternatives to my 8" deep outdrive. I have another post started on "What is your outdrive depth?". Feel free to add input there if you know what yours is. I found a article from merc that says a max thickness of 2 1/4". That may make the machining more difficult. A two piece design and more cutting would be required. If shorties were less money I would already have one, 2 to 3K for 2-3 MPH is not a very good return. "Man of War" over on Speedwake raised his 3.25" and gained about 10 MPH from a transum recut. Just weighing out all my options before trying anything. The wife is not exacally on board for spending more money right now. The 2K I spent this spring on the RMbuilder cam and valvetrain got me 5 MPH. I would like to be done with the engine for a while but changing the setup is expensive for the return.

jeffswav · 10-10-2009, 05:49 AM

Here is where I got the idea from. http://cgi.ebay.com/ebaymotors/ws/eB...K%3AMEWAX%3AIT

jdub · 10-10-2009, 08:09 AM

There is nothing wrong with making the adapter plate like you want to do. It's just as easy to fill and recut the transom though. But as mentioned you will have to raise the motor either way...which is easy, but make sure you have enough hatch clearance to go up that much. But as far as your original question goes, if you go with making the plate, your transom assembly studs are only so long. If your plate is to thick you will not have enough stud going through the transom, your plate, and the innner transom plate to bolt everything up. So measure twice and laser cut once.......

Griff · 10-10-2009, 01:20 PM

2-3k is not too bad to gain 2-3mph. You can also sell your old lower to get some $$ back.