496 CMI header myth?
 

Once and for all, are the CMI E-tops or the Sport tubes netting any real speed gains? :cuqui:

496HO in a Donzi 22 Classic switch from stock to CMI 496...zero gain. I am sure they would help with other changes. Blower, heads, cam, etc.

that would seem to follow what happens with most computer controlled car engines, - in that you have to change the program to really take advantage of any changes to the engine package.

Yeah. thats what I was thinking...
It might be a good combo with the Whipple ECM tune.

Did anyone get dyno numbers on the Etop yet?

Anyone have performance gains?

BobL dynoed the Lightening, Dana Torque Flow and (recently)CMI Sport Tubes on a 496. The HP gains for all three with out tuneing mods were in the 20's. The CMI and Dana published HP gain numbers for a 496 were in the 60's.

Dana Marine results are up to 48 HP on a 496 HO. As 496 engines will see up to a 5 percent difference in stock HP output, it will greatly depend from one application to the next. The engine we dyno'd ran 433 HP stock and 478 HP after the exhaust install.

Bob L's dyno is a wet dyno, where most dyno test are not. The dyno test that we performed was on a DTS4000 SAE certified dyno which is not a wet dyno. Bob L has this information documented in his testing. A wet dyno will have different HP results as it is mixing water in the exhaust stream slowing up the exhaust output.

This subject has been gnawed over here a few times. The key thing to keep in mind is the power to weight ratio. 40-48 HP will mean alot more to someone in a smaller, light weight boat compared to a larger heavier boat.

Speed increases can vary tremendously based on hull design, X-dimension, weight, propeller, etc. etc.

There is no one across the board answer to speed increases. We've seen a wide range of numbers. Some as little as 2mph and some as big as 7 mph.

Alot of boats can be on the verge of breaking free, not that the exhaust upgrade can account for a full 7 mph gain, but the exhaust system may have gained 4 mph allowing the boat to free up and ride on a different spot on the bottom giving up a few more MPH.

Some boats have huge differences between a 525 and a 496, I know it's roughly 100 HP more, but some boats will run 15 MPH faster with a 525. That 525 may have given that particular boat 10 more MPH, but the boat is airing out so much better it will yield another 5 MPH.

Dana,
Excellent explanation for the variances the speed as related to changes in HP. Exhaust seems similar to props, each boat reacts differently to the change.

Correction. CMI posted dyno results show 56.1hp increase and Dana 48hp increase as noted in preceeding. Attached is a copy of the raw dyno data that is posted for each. I should learn to double check my memory.

Attached.

I've been told 8-12hp of the hp increases with performance exhaust is the result of the turbulators being removed.

Yep. See the "496 Mag Dyno Testing & Myth Busting" section in:http://www.fullthrottlemarine.com/

Hi, I have a donzi 35zr with 2 496 with Corsa Q & Q, do you have an exhaust kit for me that replace the original exhaust?
I need 2 kit so, could you make a special price?
Thanks

Here are before and after prop shaft dyno sheets for CMI sport tubes on a stock 496 HO.

Anybody have the dyno results for the E-top headers?

My 2 cents: 27 Fever, 66mph on Fountain performance report with stock engine and 24" prop.
When I bought her, the previous owner had her fitted with sport tubes and a labbed 26" prop.
Now good for 70mph, but I can't say how much is courtesy of the prop or the exhaust.
My guess is that neither - alone - could have given a 4mph improvement. So half and half, maybe?

anyone have before and after sound clips of CMI E-tops?

I would like to see a pic of the E-tops on the 496's if anyone knows where one is.