How does HIM time relate to IM time?

I have seen a formula for "scaling" races to different distances, but Tom is probably right that the difference between a 1/2 and a full plus all the exogenous factors probably make this of marginal predictive value at best. 

That siad:

t2=t1*(d2/d1)^x

t2 is the time you are trying to guess and t1 is your observed time for an event (so say your 1.2mi swim from you r 1/2). d1 & d2 are the distances, so for a 1/2 to full this ratio is 2.  x is the important factor.  I have seen suggesttions for 1.05-1.06 for thw swim & bike and 1.07-1.08 for the run.  Add them up and that might give you a VERY ROUGH idea.

This question came up on TNO quite awhile ago, and it received several responses from folks who'd completed both distances (see link below).  Of course, there is very wide variability, but it can be helpful to see how others have faired over the two distances.

http://www.trinewbies.com/phorum2/forums/thread-view.asp?tid=32720&posts=48

Factors such as the difficulty of the course and the weather can be very significant considerations, and can have a proportionally greater effect in a full IM.  Also, you can make some pretty significant pacing and nutritional blunders in a 1/2IM and still do okay.  Those same mistakes in an IM can bring your day to a premature end, or cause you to fall way short of your potential.

My very brief advice.... proper pacing on the bike is critically important in an IM.  Head over to Rich Strauss' site (cruciblefitness.com), or Gordo's site (gordoworld.com), and check out their tips on IM pacing (and nutrition).  It's excellent advice, IMO.  Aim to get off the bike knowing you could have pretty easily gone 10-15 min faster.  Your race is about not slowing down on the run.  Being able to actually run, and keep running, is all about how you paced your bike leg.  Get that right, and get your nutrition right, and you'll have yourself a good experience in your IM.

Just my $.02.

-Steve

OK, since no one else will tell you...  I did a Half (with a 57.4 Mile Bike leg) in 5:27.  I did IMAZ in 11:46.

HOWEVER, as has been said above, it's not a direct line from one to the other.  There is so much more 1) training; 2) pacing and 3) nutrition/hydration that has to be done to successfully execute an Ironman.  But, if you know you can do a 5.5 hour half, if you train accordingly and have a good race day, you should be able to do a 12 hour full.  Much more easier said than done.

IMO, you'd be more accurate to try to predict a Half time later in the season from a Full time earlier in the same season.  But that don't help you now... 

FWIW, here are my two specific examples:

July 2002:  Muncie Endurathon 1/2IM = 4:59

Sept 2002:  IMWI = 11:37

--

July 2003:  Dairyland (Spirit of Racine) 1/2IM = 5:13

Sept 2003:  IMWI = 12:00

-Steve

Your training paces are likely a more accurate predictor. My recommended benchmarks:

1. 3800m race pace TT, in a long course pool. Mine are usually within about a minute of my race time.
2. 112/4 race rehearsal brick. See my article on this site for the protocol. However, I highly recommend you begin to train with a heart rate monitor. PE can be very deceptive in an IM, especially in the first 30-40 miles of the bike. This is the most important part of your day and having at least two measures of intensity (HR and PE) is very important. Adding power to that list is even better. That said, realize that there is a difference between the bike split you _can_ do and the split you _should_ do. They are not neccessary equal. The optimal bike split yields a run that is close to your long run pace, per some of the other comments above.
3. Long run pace: the spread between long run pace and IM run split time is a indicator of bike pacing, nutrition, running strength, and mental focus. Cracks in any of these four can have you falling off the pace quite a bit. The course is full of 3:10 marathoners walking 4:30's, usually because they didn't train the run appropriately and/or pace the bike well.

The quote I'm using these days, something from Gordo's board: bike strength is like a nuclear arsenal: most useful when not used. The lesson: build ludicrous strength on the bike...and then use it to ride the split you should, not the split you can. I just came off a 350-400 mile training trip through Utah, now prepping for a holiday weekend of 10k/320/25 with 14k of gain. Next weekend is a 112/4 race rehearsal and I know I "can" ride a 4:52-55. However, I will ride a 5:05-10, as I wrap my head around the split I "should" ride at CDA. For me it's all about running 3:30-35 off the bike, nothing else really matters.

So in my opinion the critical training event as an indicator of potential IM time is doing a 112/4 race rehearsal brick and pacing it so that you absolutely learn the value of the lesson that it pays to hold back on the bike so you can simply not slow down on the run. If you enter the race with absolute faith in that lesson then you will be better able to ride your race, ignore those around you, and set up the run.

Cheers,