RPM vs Speed |
Post Reply |
Author | |
gt40KS
Gold Member Joined: August-05-2017 Location: Wichita Kansas Status: Offline Points: 943 |
Post Options
Thanks(0)
Posted: September-30-2017 at 6:03pm |
Got the boat out for the first time this morning since most of the new stuff ( 95 SN, GT40) - prop shaft, prop, gauges, etc. All but the speed could be checked at home of course, but I felt I needed to check the gauges on the lake, specifically the GPS speedometer to make sure all was good. GPS checks out perfect, but I was curious about the speed vs the RPM as my tach registered about 2500 @ 32mph. Didn't go WOT yet as they said the engine only had abut 3-4 hours on it when I bought it, but I did push it up to about 3500 - 3600 and was seeing around 42. This seemed a bit fast for 3500, so I was then wondering about the tach, if I had it set properly. Thoughts?
|
|
JCCI
1995 Ski Nautique GT40 |
|
8122pbrainard
Grand Poobah Joined: September-14-2006 Location: Three Lakes Wi. Status: Offline Points: 41040 |
Post Options
Thanks(0)
|
Joe,
Get a shop tach on it and see how it compares. |
|
KENO
Grand Poobah Joined: June-06-2004 Location: United States Status: Offline Points: 10696 |
Post Options
Thanks(0)
|
If you Google "correctcraftfan 95 ski nautique speed vs rpm" you'll keep yourself busy reading for quite a while.
Or you can leave off the correctcraftfan part and get numbers from that "other" website and from ball of spray also. You'll even find your old post titled "correct prop size" Most all of the info would suggest roughly a 100 to 1 relationship meaning 3600 rpm would be about 36 mph |
|
KENO
Grand Poobah Joined: June-06-2004 Location: United States Status: Offline Points: 10696 |
Post Options
Thanks(0)
|
A couple of other things
You mentioned tach settings, I don't know what you have, maybe a 4-6-8 selector but it must be set for 8 right now otherwise if it was on 4 or 6 it would be reading higher than actual RPM which shouldn't pertain to your situation You have either a low reading tach or a high reading GPS or some combination of both. I don't think you ever said what you had for a prop now in any of your posts, I just see a picture of an ACME but no numbers in an old post, but I figure there's probably nothing that would give numbers like you're seeing for speed vs rpm. So..............what's the new prop? |
|
Orlando76
Grand Poobah Joined: May-21-2013 Location: Mount Dora, FL Status: Offline Points: 3108 |
Post Options
Thanks(0)
|
Just curious as to what gps speedo you have?
|
|
Please support The Cystic Fibrosis Foundation
1976 Ski Nautique 351 Escort 1993 Ski Nautique purple and black 351 HO PCM |
|
KENO
Grand Poobah Joined: June-06-2004 Location: United States Status: Offline Points: 10696 |
Post Options
Thanks(0)
|
Here's a Faria troubleshooting guide
link Maybe page 12 will come in handy.............or maybe it won't. I have a phone GPS that I use when I want to feel "speedy". It reads 5 mph high. Hope your phone GPS that you're comparing the installed one to is better than mine |
|
gt40KS
Gold Member Joined: August-05-2017 Location: Wichita Kansas Status: Offline Points: 943 |
Post Options
Thanks(0)
|
Sorry Ken, I thought I had posted the prop previously. It's a 422 that I picked up from a buddy, virtually new with only about a half hour test time and looks pristine. As for the GPS speedo, I confirmed it's accuracy with an app on my phone, which was in turn confirmed using the GPS nav system in my truck. I should have snapped pics of the pair while driving, didn't think of that at the time. It's possible I've got the wrong #'s I suppose, I'll check it again next time out. The new tach though has a lot more settings that I remember from the old days and they don't necessarily correspond to the # of cylinders. One even spec'd "mercruiser v8 only"
I'll have to pick up a reliable meter or shop tach to check it further. I have a cheap tach meter that I used to set the idle on a set of SeaDoo's but I doubt it will be accurate enough for this. |
|
JCCI
1995 Ski Nautique GT40 |
|
gt40KS
Gold Member Joined: August-05-2017 Location: Wichita Kansas Status: Offline Points: 943 |
Post Options
Thanks(0)
|
Faria 4" |
|
JCCI
1995 Ski Nautique GT40 |
|
baitkiller
Platinum Member Joined: October-11-2011 Location: SW Florida Status: Offline Points: 1693 |
Post Options
Thanks(0)
|
Pete is right. Listen to the Professor $11 at Amazon. i use one every day and its dead on balls accurate.
|
|
Jesus was a bare-footer.............
|
|
Gary S
Grand Poobah Joined: November-30-2006 Location: Illinois Status: Offline Points: 14096 |
Post Options
Thanks(0)
|
Last paragraph on page 11 is helpful too...... |
|
KENO
Grand Poobah Joined: June-06-2004 Location: United States Status: Offline Points: 10696 |
Post Options
Thanks(0)
|
when you get right down to it, page 12 sends ya' back to page 11 in the reading high and reading low sections.
Like Gary said that paragraph is real helpful |
|
gt40KS
Gold Member Joined: August-05-2017 Location: Wichita Kansas Status: Offline Points: 943 |
Post Options
Thanks(0)
|
Well, go figure - didn't read the part about the gauge having a fine tune adjustment.
However, for the most part the mystery is solved. It seems I had the setting wrong anyway. I found one of my old Bosch competition tachs and set it up as the "shop" device for a first test. As Ken figured, there was quite a marked difference - 1550 on the Bosch, 1200+ on the Faria. For whatever reason, this gauge had 2 different setting labels on it ... mistake? I don't know, but I had set it according to the label on the side of the gauge just under the adjustment dial which said 8 cyl - position 4. It's also the one that had the special Mercruiser setting. The label on the back has 8 cyl as position 3 and 10 cyl on position 4. So reset to #3 and tested again - Bosch, 1500+ and Faria 1450. Still a bit low, but now I know there's a fine adjustment ( thanks Gary ) so I'll be doing that tomorrow. |
|
JCCI
1995 Ski Nautique GT40 |
|
KENO
Grand Poobah Joined: June-06-2004 Location: United States Status: Offline Points: 10696 |
Post Options
Thanks(0)
|
I think that position 3 is right which leads to the question of what's more accurate, your old Bosch or your new Faria?
I'm not too sure I'd adjust the Faria to match the Bosch I am sure I'd head out on the lake and get some numbers before thinking about doing any adjusting, but that's just me |
|
gt40KS
Gold Member Joined: August-05-2017 Location: Wichita Kansas Status: Offline Points: 943 |
Post Options
Thanks(0)
|
Perhaps you're right Ken. I think I'll get out that cheap digital induction meter I used for the skis today .... If the $11 Amazon model is good enough for Baitkiller to use every day I guess the EZtach should be close enough to see if I even need to worry about it.
|
|
JCCI
1995 Ski Nautique GT40 |
|
Post Reply | |
Tweet
|
Forum Jump | Forum Permissions You cannot post new topics in this forum You cannot reply to topics in this forum You cannot delete your posts in this forum You cannot edit your posts in this forum You cannot create polls in this forum You cannot vote in polls in this forum |