rob vanharten

Hello all, I have just recently purchased a 99 28TD Daytona. The boat has twin 572 natural aspirated motors making 625hp, imco drives with standard lowers and 31p 5 blade props turning in. X dimension is even with the bottom of the boat, and no boxes. Only had the boat out 3 or 4 times, and have only made a handful of top end runs but I have only seen 97 on GPS so far.... this was at about 5200-5300 rpm's and trimmed out decently well. Looking for some driving tips on this hull, or set up changes. I have had cats in the past, and everything I have read and discussed regarding this setup I thought I would be up in the 110-115 mph range. Thinking I need to go with the imco -2 lowers to get some speed back? What does everyone think? Thanks in advance for any input. (as I understand it 496HO boats are breaking 100)

antisocial

My 28 with twin 496 did 102 all stock

Tinkerer

If drive height is proper you should be able to trim back down to neutral trim at top speed. The RPM will stay the same but the speed will go up because the prop and bullet are more efficient and more in line with the direction of travel. Mine does this now. Last year I was 1 inch above the low point of the hull. I added boxes and went up another inch. With a twin engine boat You should be able to go up higher than a single engine boat. Have you tried 4 blade bravo props?

JRider

I take it you bought the one in Indiana?

With 502 mags my friends 28 TD never broke 90, the guy who bought it put 600 phaffs in it and hit about 112 if I remember correctly. With 572s I would think you shoud be at the same. You should be winding over 6500 and you could go up to 6750 with that motor, I think it is over propped.

28 pitch props at 6750 on a 1.5 drive @ 8% slip = 110
31 pitch props at 6750 on a 1.5 drive @ 8% slip = 122.32 (unrealistic)

Going with a low slip number due to 5 blades...btw I would stick with 5 blades for the stern lift.

rob vanharten

Yes this is the boat from Indiana, I need to confirm drive ratio. Do you really think I should be spinning these gm crate motor 572's over 6500.....?

JRider

Peak Hp is @ around 6500 and doesn't really fall off drastically until after 6750...which is also recommended limit from GM on the 572.

rob vanharten

This is the 620hp version, not the 720hp version. The specs I looked up for the 620 said peak rpm 6000, and this has a hydraulic roller cam. Was GM really conservative with the numbers?

JRider

Oh $hit, my bad, was looking at it on my smart phone...you are correct!

The numbers work out to be 108.73 with 31 pitch at 6k so your props might be about right just not sure why you are not turning more RPM. I wonder what your timing is at?

rob vanharten

I had the same thought with timing, I need to check my total timing to make sure I am seeing enough advance.

SB

Super Chevy Mag dyno'd it: