I did think about using the XE250H (idle-4800rpm), but thought the XE256H might give a bit more in the 1500-4500rpm range without loosing too much below 1500rpm. What do you think?

The 525 Speed Demon may be better with this cam (XE250H).

 

You really do need to open up the exhaust ports. Otherwise, the money you're spending on everything else is mostly wasted. You don't have to go deep into the port, only 3/4" or so. 2+2GT has a picture stashed somewhere showing the difference between a stock port casting and one that's been gasket-matched.

 

DynoSim Results for my engine with a 525cfm Demon and the XE250H cam vs a 575cfm Demon and the XE256H cam. Thanks to urban_cowboy.

Dotted lines are torque in ft.Ibs and solid lines are HP.



Which would you choose?

 

that tells you something right there........how much time do you spend under 2k? and at your cruise rpm it wont make much difference. Look at power and tq peak at cruise and it will give you best mpg's or Kpl's where its highest, the best bet would be add an OD and some better gears.

 

It spends virtually no time under 1.5K, but a fair amount of time between 1.5-2.6K, and very little over 3.5K. I was thinking the smaller cam, but was interested in what others thought. The smaller cam is ok so long as it does hang out that well until 3.5K, if it had of matched the bigger cam until 4-4.2K it would have been simple.

See, I wouldn't mind loosing a bit on day-today performance if it was going to be significantly better under "hard" acceleration (up to just over 4K).

As for an OD and better gears, it isn't going to happen. I feel bad enough modifying the engine in this and I do not want to alter anything else. This means I need an engine that works well with what I have; irrespective of how much better it could be with different gearing.

 

While your best bet if money is not the issue is an AOD and steeper gears, I hear your desire to keep it as stock as possible. The combination you are suggesting is fine with the exhaust gasket match it will serve you well. There is little diference in flow between dual 2 1/4" (31 sq. in. of area) vs. a single 3" (30 sq.in. of area). In fact, the slight back pressure of the single may provide better performance at the low rpms you run. I say, build it as you have suggested and have fun!

 

Do you mean the XE250H or the XE256H?

By the way I think you calculated those areas using 3 and 2.25 as the radius, not the diameter (need to divide your results by 4). But in any case I agree that the exhaust is plenty, it is designed using empirical data from David Vizard.

 

Oops, you are correct on my poor math skills!

I mean the 256H, 575. In this range of cam specs. and in a non-truck application, go with the larger cam. Your VE losses will make up the difference and the performance will be a bit better than shown on the desktop dyno.

 

Thanks mate. I did think because the results are so close in the lower end that perhaps the 256/575 would be better in the top end with no appreciable loss down low.
Comp recommends up to the XE262H, but I think it is too big for by rev range and gears. I was always planning on the XE256H, until I saw the simulated dyno. results.