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Dyno Day! Question for engine builders - A/F correction factor for dry dyno headers?

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Old 05-30-2014 | 04:30 PM
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Just for grins, here is the startup cart I slapped together. I guess I need to paint it when I have time.

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Old 05-30-2014 | 04:32 PM
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So, based on those numbers, what RPM do I shoot for when selecting a prop? 5500?
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Old 05-30-2014 | 05:28 PM
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Originally Posted by Budman II
BB, I was thinking about trying one of those. Friend of mine has the identical hull to mine, 235 Force, and was running low 80's with a blown 502 a few years ago. He tried a 4-blade on his and found that it gave him too much stern lift and not enough bow lift, which is what these boats need. They have the helm and driver's seat pushed further forward than a lot of other go-fasts for more cockpit room. Also has fridge, head, etc in cabin, which makes them nose-heavy. He found that a 3-blade with a lot of cup helped to lift the bow and cut down on drag. My boat tends to be very nose-heavy. Always had a hard time lifting the bow with the 454 that was in there before. I have a 23 Mirage Plus that belongs to a friend that I am going to use as a starting point. I know it is probably not enough prop, but I want to see how it carries the bow. I will also be looking for a 25 three-blade and probably a Bravo 1 like you described to try.

At this point it will just be nice to have it moving without a paddle!
Nice run stand, I dig it! I'd probably start with the 23p mirage plus and see what you get. I'd probably shoot for 5500rpm at wot. If you rev too high, then try the 25p mirage plus. I wouldn't bother with the bravo 4 blade. Too much diameter for a deep drive setup. I'd bet ya a six pack a bravo 4 blade will kill the speed on your boat. If you want to try a 4 blade, the Revolution 4 blade works well, in applications where a mirage plus works well. The Rev is 14 5/8 diameter, but high rake and provides a lot of lift.
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Old 05-30-2014 | 06:57 PM
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Someone had asked about jetting and timing - forgot to list that. They have a pretty good carb guy at the shop who used to work for Quick Fuel. He took a look at my carb right after we fired it up for the first time, and found that the power valve was blown. He also noted that the A/F reading at idle was around 12 - which explains why the boat made made everyone's eyes water at the dock. He slightly enlarged the low speed air bleeds to take care of this, and put in a new PV. Problem solved, at least at idle.

We started with the stock jetting that Holley puts in the list# for that carb - 72 primary and 87 secondary, with a 6.5 PV. First run was with this setup and 32* total timing. Went easy on it and only took it to 4700 RPM, and we got 484 HP and 540 ft/lbs. Next run we richened it up a little, and got 530 HP at 5400 RPM, similar torque numbers. We eventually got to 76 primary and 84 secondary on the jetting, same PV, and 34* total timing. At this point we were at about 565 HP and 580 ft/lbs. We tried an HVM Super Sucker 2"spacer on it, and it netted us a gain of about 10 HP.

At this point we were pretty satisfied with the A/F numbers we were seeing, and power numbers were about what I had expected. Probably could have wound it up to 6200 or so, but I didn't see the point because the power appeared to be peaking around 5500. So the next test we did was kind of a steady state loaded cruise test. We put a pretty heavy load on it with the water brake and ran it at about 3200, 3800 and 4400 RPM for about 3 minutes. I was watching for signs of oil starvation from drainback, elevated oil temps, and the all-important A/F numbers. Oil temp started to creep up (no water going thru cooler), but not as much as I expected. I don't think it ever went over 200*, which surprised me. The thing that caught my attention was that we started going lean - high 14's to a little over 15 at times. Headers started to glow so it was time to shut her down and make some adjustments. The carb guy thought that having the secondaries kicking in a little sooner would help even out out the fuel distribution, so he changed the secondary linkage to 1:1 to make them come in much sooner. I'm still a little concerned that I may have to break out my Valdez Platinum Card at the pumps with this tweak, but he assured me that I will still have the same amount of throttle area open overall, just not all in the primary circuit. I had to trust his judgement on this, and it certainly took care of our part throttle high speed cruise issues. A/F numbers fell right in line at around high 12's to low 13's. I'm really glad we did this test, because I could have melted it down just doing full throttle runs on the dyno.

We finished up with a couple more runs for good measure, and this last time out we hit our best numbers. That is the dyno sheet that you see listed. Oddly enough, after the mod to the linkage, we saw very little difference with the Super Sucker, so at this point I am probably going to keep my hatch clearance and my $150. Might have made a bigger difference if I had a more appropriately sized carb for the engine, but honestly, I am quite happy with the power level I'm at now, and I'm pretty confident that I won't melt it down on the water or wash my cylinders down with fuel.

This was probably the best $400 I could have spent on this build. I have to say that the people at Dale Meers Performance were very good to work with. I never knew where Buffalo Kentucky was, but he has a helluva nice shop out there!

Last edited by Budman II; 05-30-2014 at 07:15 PM.
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Old 05-30-2014 | 07:23 PM
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Great to see everything working out for you! Did you ever have the bores notched to unshroud the valves, or just leave them as is?

I will be assembling my motor next week, and am still on the fence regarding the bore notch. Time wise, if it works out I may do it. We will see!
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Old 05-30-2014 | 07:29 PM
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Hey Bud, why don't you give a spec sheet on the build, for those reading who haven't been following it for the past several years. From what I remember

489CI?
AFR 265 oval port heads
Compression?
Custom Madera Cam?
Intake?
Carb?

Im curious, as far as the carb having a lean spot during cruise, did you guys try a power valve that would open a bit sooner?

Last edited by MILD THUNDER; 05-30-2014 at 07:31 PM.
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Old 05-30-2014 | 07:54 PM
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Originally Posted by Borgie
Great to see everything working out for you! Did you ever have the bores notched to unshroud the valves, or just leave them as is?

I will be assembling my motor next week, and am still on the fence regarding the bore notch. Time wise, if it works out I may do it. We will see!
Thanks Borg. Yes, I did bore notch for the intakes. I left the exhaust alone. t was pretty easy - just mount the heads on the block and mark the area where the valves land. Just don't get too crazy with it - make sure you stay well above the top compression ring. From what I hear it is much more important on the 4.25 bore blocks than the 4.5's.

How do you like that Nordic? Friend of mine bought a 28 Heat MCOB. Our daughters all want to ride on his "cool" boat now. That or an Advantage MCOB might be my next ride.
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Old 05-30-2014 | 08:02 PM
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Originally Posted by MILD THUNDER
Hey Bud, why don't you give a spec sheet on the build, for those reading who haven't been following it for the past several years...


Im curious, as far as the carb having a lean spot during cruise, did you guys try a power valve that would open a bit sooner?
Hey MT, I posted some basics about it at the beginning of this thread, but I'll go back and edit that post to flesh it out a little more. Good idea.

I asked him about jetting it richer or going with a different PV, but he thought getting the secondaries into play a little sooner was key to evening out the fuel distribution. The back two tubes were getting the hottest. I guess he knew what he was doing because it seemed to take care of it. Just to make sure I understand, if we went with the PV, we would go up in number to get them to open sooner, right? Like a 7.5 or 8.5?
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Old 05-30-2014 | 08:04 PM
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Originally Posted by Budman II
Hey MT, I posted some basics about it at the beginning of this thread, but I'll go back and edit that post to flesh it out a little more. Good idea.

I asked him about jetting it richer or going with a different PV, but he thought getting the secondaries into play a little sooner was key to evening out the fuel distribution. The back two tubes were getting the hottest. I guess he knew what he was doing because it seemed to take care of it. Just to make sure I understand, if we went with the PV, we would go up in number to get them to open sooner, right? Like a 7.5 or 8.5?
Yes, an 8.5 would open sooner than say a 6.5.
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Old 05-30-2014 | 08:33 PM
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Originally Posted by Budman II
Thanks Borg. Yes, I did bore notch for the intakes. I left the exhaust alone. t was pretty easy - just mount the heads on the block and mark the area where the valves land. Just don't get too crazy with it - make sure you stay well above the top compression ring. From what I hear it is much more important on the 4.25 bore blocks than the 4.5's.

How do you like that Nordic? Friend of mine bought a 28 Heat MCOB. Our daughters all want to ride on his "cool" boat now. That or an Advantage MCOB might be my next ride.
I will probably just notch it. After hearing most of the big engine builders endorse it, sounds like a no brainer. As far as the Rage goes, I LOVE IT!!! Thing is mint and has a stepped hull with pad v,very lightweight, handles really nice, has tons of room and Nordics styling and build quality are like no other for a Performance boat in this size/class. I only drove it for an hour up in Havasu before removing the 502 mag lol. Should hit 90 fairly easily with this new motor. The MCOB is awesome, highly recommend!

Last edited by Borgie; 05-30-2014 at 11:36 PM.
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