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View Full Version : Preliminary numbers ! a little help ?



Sam
04-06-2002, 04:03 PM
Not exactly boating weather here in Michigan but I couldn't help myself (39 degrees). I took the sixteen out today for it's first run with the upgrades I added this winter. I wanted to make sure all was well under the hatch as well as put some time on and do some preliminary prop testing. The water conditions were very light nothing more than a six inch chop or so.

I started off with an out of the box 21" Mirage Plus.I must say that the low and mid range power was incredible. I wanted to be very conseritive due to the fact that the motor only has shop time on it up until now. Here are the numbers for the 21" Mirage. All the numbers below are with a full tank of gas.

37.5mph @3000
45@mph @3400
50mph@ @4000
54.5@ @4400
61.5 @4900( Hit rev limiter)

Now this is where it gets interesting I put the 21" Stiletto on and it starts bumping at 4900 rpm, almost like a small miss, obviously hitting the rev limiter. So I put on an out of the box 23" Mirage Plus, with the following numbers.

38mph @3000
48mph @3400
53mph @3800
56mph @4000
62mph @4800

I think that there is at least another 200 rpm to be had with the 21" Mirage. Why ? because the Stilleto definitely hit the rev limiter and I believ the 21" Mirage did as well. The 23" never made it to 4900 which leads me to believe that this number is true. So what does this mean, it means that when you do the pitch vs RPM math it appears that I have left RPM's on the table based on the 21" Mirage numbers and the rev limiter.

So what to do ? that's where you come in. Jamie Root informed me that the Thundebolt V ignition on my boat does indeed have a rev limiter pre set at 5000 rpm or so. Jamie didn't you say that you switched your V for a IV and by doing so eliminated the factory limiter. I need some sugestions on what I might do. Do I switch to a Thunderbolt IV or do I look at something from the after market. Keep in mind that I would like to do this as cost effectively as posible.

I think the 21" Mirage looks the most promising at this point. I know there is more speed there based on the rpm numbers I just have to find it.

THX
Sam

Rootsy
04-06-2002, 10:17 PM
Yes Sam, my TBolt V is now a TBold IV... all started with a little scare when the V module took a big dump... came off of a good high rpm run and it was running on about 6 cylinders. so i got out my book checked out the TBolt IV ignition module curves, found the one i wanted (350 Mag) and stopped at the local marina and bought it and the plug/harness to go with it... the IV module will replace the TBolt V module and you'll have to remove the existing plug and wire harness for the TBolt V since it isn't the same as the IV. took me bout an hour... and i think 40 minutes of that was cussing and diggin through the 3 rolls of electrical tape some over achiever at Mercruiser wrapped around the harness. total cost for both pieces (module and plug) was somewhere in the 250 dollar range. add a soft touch rev limiter and that is another 100.

the difference between the IV and the V is that the TBolt V makes up it's own ignition curve depending upon what the motor is doing. it'll advance the curve as long as the motor rpm is increasing. it does have an upper limit built into it... something like 20 or 22 degrees. the TBolt IV on the other hand has a set curve in the computer and it applies it just like a regular distributor with weights and springs would... in most performance apps the Vortec motors seem to run best @ 32 degrees total advance whereas non swirl/fastburn heads like more... in the neighborhood of 34 - 38 depending upon what's in the motor.

you have other options though... a complete MSD setup will run you 600 - 800 bucks... that includes a distributor, ignition box and soft touch rev limiter... tack another 80 on for plug wires and 40 on for a coil. you don't get much better though...

you can also look at the mallory unilites... all self contained with springs and weights for ignition advance and a two wire hookup. you'll have to get plug wires though since the cap is a points style. If you can find a mallory marine dual point you can purchase the kit for about 80 bucks and turn it into a unilite.

that is a bit strange also... my 21 Mirage GPS'd @ 60 flat at 4500 rpm last summer and it was 75 and muggy that evening too... the boat was trimmed til it began to chinewalk then i took it back in til it was stable... that's the fastest to date... generally i am in the 59 - 59.5 range.

AND IT'S THE GREAT PUMPKIN CHARLIE BROWN!!!!

http://www.tdi.net/jaroot/pictures/donzi/dsc00005.jpg

smokediver
04-07-2002, 06:00 PM
sam, the mirage plus is a 1/4 larger in diameter than the stilleto so you will turn more rpms.I think a 1/4 inch is equal to a degree in pitch rpm wise . you have to figure out how much blade area you need like coming out of the hole for example versus how fast you want to go.some where there is a happy median. probably a lab finished 23 mirage plus if you change you rev. limiter. I have been trying props myself on my 16 and have went from a 21 laser 2 that was way under to where I needed to be to a 25 mirage plus i could hardly get on plane with. i figure that the 23 mirage plus labbed is my best bet. of course ted told me that from the get go but i am a little hard headed but he was right. the turbo may be a little faster due to smaller diameter but then the trade off begins so good luck to you

Sam
04-07-2002, 09:22 PM
Smokediver, If I was a betting man I would agree with your 23" lab thought. I will talk to my engine builder tomorrow and see what he has to say. He may be abel to hook me up with a Thunderbolt IV setup. Once I know for sure what the true R's are I will look into getting the prop labed. Thanks for your reply.

Sam

Rootsy
04-07-2002, 11:04 PM
Sam,

did you aquire those speed readings via GPS or Speedo?