Any Guesses??

[ck7684]

Just curious to see what what you all think my speed is. I havent had a speedo or GPS in yet to find out...
My boat is a 16ft 1979 GMC CobraJet, Berk "F" pump with an "A" impeller. Pump was just rebuilt using an Ultimate wear ring from HTP. Engine is a fairly stout 468 BBC...
I have touched 5300 RPMs briefly, but for discussion sake, let's say @ 5000 RPM.
Years ago, with the stock engine, worn pump (not sure how worn, but used condition) and 3 people on board, a handheld GPS said 70 MPH as top speed (not sure what RPM)
Any guesses???:idea:

[Rondane]

65

[ck7684]

Any serious guesses?

[Jetmugg]

Here's a dead serious "guess" -
62 mph.

[Jetmugg]

62mph @ 5000 rpm's would be moving pretty well.
Get a GPS, then share the info with us.

[ck7684]

Sure will, but it sure feels like it's much faster than it was before...

[ck7684]

Finally got it out on the water with a GPS. One passenger and full tank of gas, I was doing 64 mph at 4800 RPM, so the guesses were pretty close!! However, I cant seem to get past 4800 now. One change I made was that I put on a scoop that might be a bit more restrictive than my 4" tall flame arrestor. Anybody have any experience with this? I'd hate to lose the scoop because it looks killer, but I dont like the idea of giving up any performance. I going to try switching back and see what happens. Trimming didnt seem to have much affect on top speed...
:idea:

[Wet Dream]

You have to take into consideration that your old worn out pump is not efficient, allowing the motor to rev higher. 4800 isn't bad, but probably not far from peak with a few tweaks.

[ck7684]

This is all with a new engine a freshly overhauled pump & UWR. I have hit 5300 RPM, and although I didnt have a GPS with me, it definately felt faster than it ever has been. I put the scoop on the engine and I seem to have lost 500 RPM and it feels a tad bit slower (I just recently got the GPS). With the stock engine and worn pump I had a GPS reading of 70 (I dont recall the RPM, and as the pump is now rebuilt it wouldnt help with comparison anyhow)

[ck7684]

I seem to be having some issues with my throttle cable, and it was only opening about 3/4 of the way...
Any guesses at what my speed might be ay 5300 RPM?? Hopefully I will find out this weekend...

[ck7684]

I've got full travel on my cable now, but still cant seem to get past 4800 RPM. Playing with the trim I was able to get to 66 MPH.
BTW, when crusing at a steady 3500 RPM, my GPS seemed to take forever to get up to speed. It seemed to update faster in my car (different day). Anybody have their GPS update really slowly? I'm wondering if I didnt have a good signal. I was running out of lake pretty fast also. (I only have about 1/4 mile of usable water on the lake I was on, which is close to home) I'm sure I was maxed out by then though...
Any input?

[79cobrajet]

Just curious to see what what you all think my speed is. I havent had a speedo or GPS in yet to find out...
My boat is a 16ft 1979 GMC CobraJet, Berk "F" pump with an "A" impeller. Pump was just rebuilt using an Ultimate wear ring from HTP. Engine is a fairly stout 468 BBC...
I have touched 5300 RPMs briefly, but for discussion sake, let's say @ 5000 RPM.
Years ago, with the stock engine, worn pump (not sure how worn, but used condition) and 3 people on board, a handheld GPS said 70 MPH as top speed (not sure what RPM)
Any guesses???:idea:

my speedo doesnt work either but I has similar case , first day out , low gas and basically empty for a wet test and she ripped that little like it was a puddle 4200 rpm and quickly too, took her out to river and it seemed to get more and more sluggish (although the engine was revving up fast) it just never quite hit 4k on rpm but was hitting 3800... I later foumd water logging in the lowest level of the boat, just enough to make her fat , heavy and lazy!
Probably doesnt have much to do with yours but just thought it is good to mention... By the way ...
I was wondering if you had ever gotten any useful info on the GMC CobraJet boat... I have a 1979 , 16 1/2 ft. that I just got and I am having one heck of a time locating info on this desighn.