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Tell Me If This Speed Sounds Right??!!

bigsbetter

bigsbetter

Active member
Ok This is not about a Checkmate, but a 25' Checkmate WILL be racing it all summer long. My buddy bought a 31' Formula off ebay and is going to pick it up this weekend. It's a 1986 SR31 w/ twin 400 horsepower 454's, with TRS drives. Soppose to be running 23 pitch Mirage plus props at 5500 rpm. The engines are stock with MSD distributors and ignition, aftermarket exhaust and the props were "tweaked". The ebay ad states 80 mph. My buddy knows these boats in stock form run about mid 60's. The guy swears up and down it will run 78mph all day long on a gps! Is this possible? He said when it was stock it ran mid 60's and from the ignition and exhaust and props gained all those mph! What does a TRS drive do? What is their gear ratio? How does a TRS drive work? Is there a transmission or something in there? Are they good?
 
i am sure others will provide more insight but here we go.

will the boat run 78 mph? in a word - no and in 2 words - very no!

The only way that boat will do that speed is on a trailer going downhill!

those engines are seriously overrevving at 5500 rpm - they shouldn't spin anymore than 4800 rpm.

There is no way those simple mods would do that kind of increase without other mods - ie cam, fuel, head work etc.

The TRS drives are very solid, they are heavy and have an external transimisson, i would venture to guess that maybe 300lbs heavier than a comparable bravo setup. The only thing that sounds close to accurate is the props at 23" although i have a feeling that 21's might be a better pick.

As always, you get what you pay for and if it sounds to good to be true then it probably is, especially when you are talking ebay. that boat is a mid 60's boat unless you really power it up - but its a good high quality boat so as long as it's in good shape and priced right it should be a great boat to have.
 
Dont worry, you will beat him!! Even if that boat could spin 23" props to 5500 rpm, that would net it a theoretical speed of 79 mph. Thats with zero slip. As ChrisE said, no way those engines will go to 5500.

Now TRS drives, they are old style drives. However, they are tough drives. They did need a transmission to shift, as the internals of the drive had no provisions for shifting. Eventually, boat builders found that boats needed less weight and more room so the TRS was discontinued by Mercury Marine. It was replaced by the more common Bravo drive. We all know how well Bravos worked out. :sick: :sick: :sick:
 
Formulas are very heavy boats they even so much as brag about it. I looked at getting a 207 or a bit larger but learned that they just wont put up the speed I wanted becouse of there weight. A gut I know has a 302 baja with 496 and it wont come close to 80 im guess mid 60s and lots of fuel. Aaron
 
I ran the same year 31' Formula/motor/drive combo mentioned above. The boat is a mid 60 mph boat and is very heavy. Takes alot of horsepower to make them run. The TRS drives are good but dated drives and very heavy. They require transmissions which also add weight. Many people have removed the TRS drives and replaced them with Bravos.
 
friend of mine had the same twin 400hp trs setup in a ChrisCraft Stinger 312 and the spedo said 72 but the boat was tested in the magazines of the time at 67
 
What is this 400 hp Mercruiser? I've heard of the standard 330hp or the 365Mag. My buddy just got back with the boat tonight. He says it says Mercruiser 400hp on the valve covers, and it has RECTANGLE PORT HEADS ON IT.FACTORY! Is that rare? The guiy says it has different cams in the engines. Lift was about 550 I forget the duration. And he says the engines actually spin to 5500rpm!, with the labbed finnished 23 Mirage Plus Props, he still swears it will do 78! What is the gear ratio of a TRS? Is it 1.5...1.3? My buddy towed it back from Kansas with a Lincoln Navigato and he says it was HEAVY!!!
 
My 496 mag HO's only run 74 on a good day at Lake Travis

They have 425 hp @ 5000 -5200 rpm

Even with low fuel and gps that is it

Might make some money on that bet !:cheers
 
I'm still curious about the rectangle port heads. For Mercruiser to put those on a stock engine along with a factory aluminum intake, they've got to be meaning some business! Maybe this thing really does spin to 5500rpm if it's got rectangle port heads. Why else would it have those type of high performance heads? And also WHAT'S THE DRIVE RATIO OF A TRS??? Vinny how did you figue a mph? What ratio drive did you figure?
 
I used the following formula with a 1.50 ratio. I figured that was the most common ratio.


((pitch X rpm ) / ratio) / 1056 = theoretical speed Then subtract slip.

Getting back to your original post here... You said the only work done to the engines was exhaust and ignition. Also, the props were tweaked. I am not saying this is the case, but we have all seen tweaked props that are now junk.
We all know that while aftermarket exhaust is a great addition, and I swear by and recommend it, it will net you no noticeable increase in speed in a stock engine. In case there is still someone out there who doesnt know, neither will adding MSD ignition. So, as far as I am concerned, this boat has stock power. The question of why Merc put rectangular port intake and rectangular port heads on these engines... Well, I honestly dont know why. I dont have any dyno or internal specs of this combination. I assure you that Merc did their homework on it. My stock '99 454 Magnum MPI engine had rectangular heads and intake on it.
The next question is will those engines or my old 454 rev to 5500 rpm? Of course they will. Will they be making power that high? NO, they wont. Is it safe to run them that high for extended periods of time? NO , again. One more thing to add. If someone doesnt know how to run a boat, they can get all sorts of crazy rpm while claiming to have crazy speed because of the rpm. As I am sure everyone here is aware, if you run the trim too high, the rpm goes to the moon and so will your slip. Too many variables to make a good , educated call here.
Besides, arent you from Missouri? THE SHOW ME STATE!!!
 
400's were a fairly common motor for the mid 80's. i beleive merc used to call them "400 cyclones" they were a 454 block but i have no idea what the mods would be to get that power. a word of warning though, these engines are crankshaft rated and not propshaft rated so that 400 is more likely 325 - 350 at the prop - especially going through the trs drive.
 
I appreciate all the replys. My buddy who bought the boat is a major BBC guy. He recognized the rectangle port heads by the way the intake port came all the way up to the valve cover or something like that. Also says he knew the intake was aluminum because it has a 'w' with like a snow flake emblem around it. Does this make sinse? :confused:
Yes at first I thought the only mods were the exhaust and ignition and props. Now the guy states the cams were changed and Crane Gold roller rockers.
Vinny i guess you're right about the "Show Me" part of it! I'm just excited about having some close compition and I can't wait. Ohhh yea and the guy says he's running Champion Spark plugs and gaps them at .090" WOW What do Ya think of that!!??:eyecrazy:
 
unless the plugs are special high gap plugs, the electrodes will be way out of shape. hold the plug upside down, and look at the outer electrode, should curve 90 degrees and be flat and level with the inner electrode
most plugs are gapped @.035
if you have a good parts guy, he can take the number off of the plug and decifer its meaning. I used to be able to remember most of it, but they keep changing the plug #'s

how long before you get to race the guy?
I would love to see the gps #'s
 
As soon as I'm sure it's not going to freeze anymore we'll be out there to run 'em. Then all will be told. But I'm not riding with him for awhile because the previous owner says it gets real unstable at high speeds. You have to put the tabs down so he says to keep it stable. (thats the last thing I want to be thinking about at full throttle)
 
unstable at high speeds? never heard that about a formula. always heard they were very well behaved.
 
As far as I know, most later model BBC's (especially crate engines, etc) were rectangular port. The only Oval ports I've seen were early 70's back. I'm not sure though when or if they were used after that.
 
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