Quote:
Originally Posted by philminotti
Ed is spot on with his comments. My cam is 248/252 .633 on a 108 center...warm street cam. I'm running FAST XFI sequential port injection. Self learning was for the birds. It needed a dyno tune by a real pro before it would even think about behaving.
Phil
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Having experienced the phenomena first hand with FAST's XFI, Phil you can speak with considerable authority.
Quote:
Originally Posted by DWRAT
Your specs look pretty hot to me, it's the biggest hyd roller Comp offers for your motor. 
This cam is very close to your specs and it states "NOT FOR E.F.I." 35-427-8 - Xtreme Energy
Worst case you may need a different cam but I know Bob Reems will take care of you. |
Hot is a relative term DWRAT. As Phil indicated, he thinks of his 248˚/252˚ .633 lift cam as a warm street cam. Terms like mild, aggressive and hot are the modern day incarnations of the old 3/4 race and full race camshaft terminology. God bless his soul, Isky was the most creative of all the cam manufacturers when it came to naming conventions and terminology.
Because of what is available and what people demonstrate an appetite for many cam companies provide warnings like that of CompCams for certain profiles. The reason has less to do with the cam profile and more to do with the most common types of entry level EFI systems being sold/bought.
Today most systems use a speed density fueling model. The less sophisticated, earlier Alpha-N model is sometimes also included in the EFI computer but not always. Alpha-N systems basically relied on throttle position and engine speed to determine engine fuel and spark requirements. Change a carb, intake, or cam and you had to reprogram the EFI system because all its tables relating to throttle position, spark and fuel demand had to be redeveloped on a dyno.
Speed density systems were a step up from the Alpha-N engine management model because they used engine volumetric efficiency tables to map engine load. Still, when you made changes that materially changed the engine's volumetric efficiency you still had to redevelop the engine's Ve maps on the dyno. The FAST XFI, system for example, is an excellent speed density system and I believe they may also offer a MAF based option in their EFI today.
The Mass Air Flow (MAF) system model was a major step forward in engine management controls. With the introduction of the MAF based system you no longer had to guess about Ve because the MAF flat out told you exactly how much Air Mass just went into the engine making it now a self contained system. Programmed (tuned) correctly with some head room for "good air" the MAF base system could actually accommodate changes in engine design such as cams, ported heads and stroker cranks.
If you can lock down engine specs to a manageable range you can build a simpler, not all that programmable EFI system that can, within limits, properly manage fuel and spark for the target engine. Increase the engine Ve with a hotter cam and that simpler EFI can not accommodate the additional air mass and still properly fuel the engine — hence the warning from Comp.
Now contrast that with a fully programmable speed density system and suddenly the camshaft upgrade is possible to accommodate — although a dyno session will be necessary to properly map out the spark and fuel requirements for the new Ve. Now consider a MAF based system and you have a much wider latitude before the trip to the dyno becomes necessary.
The down side of the MAF based system is the need to upgrade the MAF to a larger MAF if the engine's Ve is
substantially increased — think of a bigger blower as
substantial, that's the bad news. The good news is after the new MAF is installed and the MAF xfer table is loaded into the EFI computer you are, with a few tweaks, back in business.
When Mercedes, BMW and other high end manufacturers go racing they use MAF based systems for a reason — certainty, predictability and performance. As luck would have it so does Ford. Even better Ford systems can be bought directly from FRPP and also through the aftermarket. The aftermarket source for a system that looks so close to a Ford corporate racing ECU is offered by these guys, Pro-M Racing.
If you are interested in learning more about the EFI they offer this is an excellent link to begin at, click here =>
Pro EFI
If you want to get a better look at the kit click here =>
Universal Kit
The system is predictably a MAF based system that is extraordinarily flexible.
Ed
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