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Dual down shafts performance

Old 07-20-2021, 10:59 AM
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Smile Dual down shafts performance

I switched from Imco SC lowers (because I was eating XR gears) to same length dual down shafts. Have about 15 hours on them and all is going well but I lost solid 5mph over the SC lowers. I am not complaining because I bought them to be strong but was surprised in speed lose compared to SC. I have run with same props same loads and boat is 5mph slower every day. Just thought I would post my experience for guys looking to switch because no one could answer this question when I asked few months ago. Imco was straight up and just said your going to loose a little. If your swapping from stock XRs to full SCXs and dual down shafts figure 6 to 7mph total lose but you will be boating not breaking XR's.....
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Old 07-20-2021, 10:28 PM
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How deep are your drives? I've talked to a few guys that went from Sportmasters to DDL and losing 2 mph was the average. I was thinking about switching my standard SCX lowers to DDL, but not if I'm only going to gain back a few mph.
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Old 07-21-2021, 08:20 AM
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The way I see it, yes the bullet is smaller on the DDL, but you’re still spinning more gears and mass through the heavy torco RTF. Props to you Jesse for posting your real life results for others to read. Most are too proud to give their real numbers after dropping a lot of coin on new complete drives or lowers.

What are your slip numbers like sc lower to DDL lower?

Last edited by IGetWet; 07-21-2021 at 08:39 AM.
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Old 07-21-2021, 08:34 AM
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Rv, what kind of power are you running to damage the SC lower
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Old 07-21-2021, 10:02 AM
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I am running prop shaft height 3/4 inch below bottom of the boat no boxes. Motors started out as 525s and Todd Goodwin designed new 525 intake manifold with cnc heads and 540 ci running 100 percent mercury injection and ecms dynoed 712hp and 747tq with wet exhaust and all accessory's in good air. More like 650 to 675 in summer air. I pitted both sets of XR gear sets in under 30 hours. I do run harder then most guys. The boat is a heavy 42 and I ran it last year at loto Shootout 94mph in 3/4 mile with SCX uppers and SC lowers. The boat would run 96 mph in longer stretch. This year with DD lowers I have all I can do to run 90 MPH. It was a big change.
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Old 07-21-2021, 04:51 PM
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Thanks for the detailed info. Just for comparisons sake, I'll post my initial results from my single down shaft SCX lowers.

I have a 39 TG with 600's that last year had a 700 Stage 2 tune, yes not the optimal set up. Last year max boost with the 600 cams was around 8 psi. With XR uppers and -2 Sportmasters and 34 factory labbed Maximus props my best speed was 96 mph, normal around 94. I was on the limiters at 5400 rpms.

This year, I have the correct 700 cams, but so far only 6 psi boost. With the full SCX drives and 16"x 35 5 blade cleavers my best was 92 mph at 5000 rpms. Immediately after that run, my oil pump pick up fell out of the engine, and that was that. So now I wait until the engine is rebuilt. It's hard for me to compare, as I did change the cams. My drives are -2 below the hull. Nothing was dialed in yet, so time will tell where I end up.

Last edited by endeavour32; 07-22-2021 at 07:13 AM.
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Old 07-21-2021, 05:23 PM
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Thanks for data. I would think with 16 inch diameter props you could raise drives to get the bullet out of the water and free the boat up a lot. I have a good friend that runs a 39 TG with 700s stage 2s or 3s with NXTs and 17x35 herrings and boat runs 105ish every day and best ever was 112mph. I could measure his drive heights if you wanted to know? The boat really runs good. He is planning on racing it this year at Shoot out. I think it will do good in 700 class.

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Old 07-21-2021, 06:20 PM
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Yes, that would be great if you could measure his drive height. Ha, I've had a post in the Cigarette section for a year trying to get that info.

When I bought the SCX I talked with Tres Martin on whether to go SCX or SCX-4. He told me to go SCX with sportmaster lower. Not wanting to deal with broken drives anymore I just went full SCX. He said the issue with the SCX-4 is due to the large bullet, it will want to surf on the wake and in turn lift the stern. His opinion was to keep the bullet buried or get it high. He said it would take a lot of work to get the -4 set up right on a 39 TG. With all that said, the NXT bullet has to be every bit as large. It's not a slim drive. So I am curious where the NXT prop shaft is located.

Thanks in advance!

Last edited by endeavour32; 07-21-2021 at 10:16 PM.
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Old 07-21-2021, 07:02 PM
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the scx-4 is a true surface drive, the sc is not and probably shouldn't be ran like it
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Old 07-22-2021, 05:58 AM
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02 T/S w 785 HP over 800 TQ. I went w full SCXs 6-7 yrs ago now and will never look back.
Best the boat saw w XRs was 96. That was w old factory blowers, no chiller and not optimum tune.
Yes I lost 3-5 probably now. Still playing w props. And water pressure issues
BUT w XRs I was eating drives 1or 2 Per Year. One Shootout weekend Both. I got to where I had a spare set of XRs that I always carried on trips out of town.
So how fast does your chit run when you are floating w one drive puking oil out the top cap that is no longer there ?? And the Wife is biching in your ear?? Not fast enough.
Sorry but top speed is no longer a priority in my life. Trouble Free time on the water is what matters. Maybe one day I will be there


Did I mention its for sale??
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