1988 sr1 272 prop question
 

88 272sr1 with the twin special mpi 320 hp 5.7 l. "Factory high performance" and bravo 1.5 ratio. I bought it a year ago and love it. Im wondering if anyone KNOWS what props it came with when new?
Currently i have 23p at 4800rpm 64..gps. Also with the 23's it snaps on plane no problem. But at 4800 its lugging the motors because..........
These motors are supposed to run at 5400 rpm. i have 21 p to try should get me up to that.

ok here i am answering my own question so those of you who are curios.
I called Formula tech line
206.724.1413 and gave them my HIN and he looked at the build sheet and sent me a copy. he said my boat went out with 23's then we discussed the logic of it and he said at the time formula was more concerned with safety and reliability and prop tuning was up to owner/dealer. He agreed with me that possibly going to the 21's would make more sense.

They would certainly get you up closer to the 5400 mark.

yeah I hope so. maybe more speed if that gets me more HP.... well see

If you get 5400 and your slip stays at 8 the calculator says 66

I had the same boat also 1988 but with 350 MPIs and acceleration was great with 23 mirages but maxed out 45-4600. I always felt the boat hit 60 but didn't want to get beyond it. The engines had no issues getting to top speed quickly. I question if the hull design is set up to run much over 60? Then again you have a little more power.

Heres the deal, try the 21s, but you might find they spin higher rpms for the same or less speed. And then you lose cruising speed with the small pitch. I bet on a dyno those motors are done a 48-5k

whay would you say that on a dyno done at 4800-5000?

Meaning their peak power pretty much flat lines above 5k

mine says 5400. i thought that meant peak output is 5400 and below or above that generally means less than peak, like 4800 is 600 rpms less than 5400

The 260 horse 350 has a specified range of 4200-4600 but peaks at about 4400 on a dyno. What is the specified range for the 320efi, 5000-5400? I just meant that in real life on a dyno there probably isnt much gain from 5k to 5400 with that motor, maybe just a couple horse.
Be interesting to see what you find with the 21s

ok ill let you know maybe 2 weeks, boat is uncovered but still needs preseason, before going to the crooked. You on sebago?

I run mostly the kennebec and midcoast but have been to sebago

oh ok cool.

I have an 87 272 with the same 320efi engines but I am also running the AlphaSS drives so it's not a direct comparison. I am currently running a pair of 23 mirage and have no trouble spinning the motors above 5000 rpm. I'm actually looking for a pair of the mirage pluses to bring the rims down a bit and hopefully improve my slip some.

Have you checked your tachs to verify they are accurate? I have seen them off by as much as a few hundred rpm in the past. The reason I ask is your slip seems very low (which is great if it's accurate) but I wouldn't be surprised if your tachs are a bit off and your actually closer or slightly above 5000 rpm.

Oh. No
What would you recommend a method of checking the tachs?

Compare to tach on timing light.

Same method glenamy posted above. Get yourself a nice digital timing light with a tach output. Very useful tool.

Howdy, I thought you had 21's if I recall.. when you bought that boat last year? also didnt you junk the EFI? and go with carbs?

I have two tachs for each motor, I have the digital synchronizer and the VDO factory tachs they read the same... I just remembered
I will test with 21's soon a week from next sat. I did the full preseason yesterday finally.

You have a bottom rocker at the stern of the boat. Check with a straight edge. Also you might want to try some Hydromotive 4 blade props for some stern lift. They really helped my 311. Get some base lines with the 21 and 23 props and give them a call

the boat came with a pair of 21 mirage props that I ran for a very short while. I had a 23 mirage that I ran on my Checkmate and found a matching prop so i switched to the 23's. I'd like to tryna pair of 23 mirage pluses but not having counter rotating drives makes finding props a bit harder (aka more expensive..)

I did end up switching to carbs but I still pull the same RPM's as before. Only reason i changed is because I was having trouble with keeping the motors from stalling at low RPM's and nothing I did seemed to fix the issue. The EFI was very cool but the fact of not being able to accurately diagnose the issue and then even if I did figure it out most of the parts are no longer available made it a pretty easy decision.

Oh Okay great thanks for the response. So is it fair to say when you switched to Carbs you didnt lose any performance? What combination of carb intake etc you used?
Also when you had the 21s on there what was the MAX rpm you got? thanks

I haven't noticed any performance loss at all switching to the carbs. I used Edelbrock RPM Air Gap intakes and QuickFuel Marine 650cfm carbs. Throttle response is very crisp and low RPM manners are fantastic now.

I only ran the 21 props the boat came with a handful of times and never ran them out to see how high they would go. I backed off after it was over 5500 rpm and that was around 63-65mph depending on conditions. With the 23 mirages I am right around 5300 RPM but that is with full fuel and 4-5 people on board and a couple coolers. I would like to prop the boat for a max of 5200-5300 with low fuel and just myself on board. I would really like to try a set of 23 mirage pluses to see how it performs.

Oops