Liberator21,
What’s interesting is the techs recommendation of the XE over the XM being based upon reversion considerations. While the XM is 2º longer on intake and 6º longer exhaust, due to centerline placements and resulting LSA, the two cams have identical rotational overlap of 54º. Being that the lobes on both cams are the same family the overlap area will be nearly identical also. Seems to be a non-issue. The increased duration (XM) may actually be beneficial due to the additional displacement you have @ 383 CID.

With equal mechanical parameters and as a comparison of only these two cams, the significant difference is in the valve timing of the two profiles shown in the timing chart below. The XE 268 H is the top cam and the XM 270 H is the second cam listed.

The XE (intake valve open) IVO is 3º advanced over the XM, which may factor in depending upon the intake tract.

The XE (intake valve close) IVC is 5º sooner than the XM. Give the XM the edge with the 9.5 CR and additional CID.

The XM (exhaust valve opening) EVO is 3º advanced to the XE. Advantage to the XM given exhaust restriction and the apparent pumping losses with your current cam.

The XM (exhaust valve closing) EVC is 3º later, keeping the overlap in line.

The XM cam should also carry the power curves past peak more efficiently than the XE in a marine application, where the XE would seem more suited to automotive application with efficient exhaust.

Bob

since you all are on the subject of small blocks and cams. I have a 27' formula with twin small blocks. I have so far decided on trick flow heads and the victor jr intake I have the edelbrock 1410's (750) cfm's. For cam I was considering the xm270h and the xm278. the 270 is the rpm range I want but the fun in a cam is when it chops hard at idle. do you all know if the 270 has a good noticable idle? The guy doing the work on the motors wants to go huge with the cam, he wants everyone to know that I have some heat when I idle by. I still have the center rise manifolds is the only problem. If add a 3" riser spacer and make the exhausts exit angle steep will that help with the risk of reversion? Is anyone running the revolution marine headers? I saw them on ebay for 899.00 they are on there web site for 949.00. that has to be half the price of anything else out there. unless you all know of another maker.

SB, I don't think anyone is mixing up between the XE and the XM (I know I'm not), It's just the guy at comp recommended the XE over the XM.
RM, your points are well taken, and I completely agree. Although I don't have the level of knowlege that you do when it comes to this, I have built an engine or two , and have researched cams for other applications. Your explaination of the valve timing events is excellent, and I had actually come to the same conclusion regarding the exhaust valve timing. I felt the XM cam would have the advantage given my more restrictive exhaust system. Your final assessment makes total sense given the fact that a marine application would require a broader power curve than an automotive application. I appreciate all the help. It's also apparent that I'm not the only one looking for small block info. I'm glad I started this thread
My decision is getting easier. Flatout, sure, the choppy idle adds to the "Cool Factor", but there are other things to consider. Not the least of which is reversion, and the associated expense of purchasing the proper accessories to avoid it. I'm sure RMbuilder (and others), can better inform you of the advantages and/or pitfalls of building your engine a certain way.
Remember, you're the one who will have to live with it, not your builder.
I want to make informed decisions, that's why I'm here.

I am taking all this in,this is the most small block info I have found yet!
I am being told my cam is too small (see my post) but now I am trying to see if bringing the boat home and doing a quick cam change will be worth it,I don't mind doing the work if I gain something.

The xm270 has a noticable idle,I personaly like it,I spend alot of time idling because of it.it is not real radical, but friendly.I do believe however I am at the edge of reversion ,where the xm 278 with the stock center exhaust will put you over the edge,not worth it for me.

I like the noticable idle as well (I'm a guy after all!), but I'm also concerned with dock manners, and overall cruising performance. According to what you've been telling me (blkhorse), it sounds like the 270 is what I want.
Correct me if I'm wrong, but it also sounds like rmbuilder is giving this cam the thumbs up for my situation.
245limited: I don't know what year engine you have, but I'm putting in a 2 piece timing chain cover to make cam changes easier (just in case). I have a later engine with the one piece oil pan gasket, and trying to remove and replace just the timing cover can be a real pain (and a big leak!).

who makes a 2 pc timing cover? I have the one pc pan gasket,all I did to avoid leaks is I had to take out all the side bolts in the oil pan (fun!!) and drop the pan just enough to get the cover on,the 270 cam is not a split duration right? I looked at a 270 and it is 224/[email protected] is that the one you guys are talking about?

A bunch of people make a 2 pc cover. The ones I'm looking at are the Mr. Gasket Quick-Change cover kit #1099, and the Proform cover #66666.
There's even cheapie covers on Ebay. I don't know how they are, so I'm staying away from them (I hate oil leaks!).
The cam I've been talking about (and I hope everyone else has been), is the Comp Cams XM 270H-12. Part # 12-240-4. It shows an advertised duration of 270 intake, and 286 exhaust. Duration at .050" is 226 intake, and 236 exhaust. Lift is .480 in. / .489 ex. LSA is 112 deg. The best price I've found so far is $120 for the cam, or $320 for the K12 kit. That's at flatlander racing. If anyone else has a better price, let me know.

I wonder how that cam would do in a 350 CI?

Just look at the posts above from blkhorse.
A 355 is nothing more than a 350 bored .030" over.

Paul