Unshroud AA 914 Heads?

[jayxian]

Hello All,

This is my first post, but I've been stalking this 4rum for awhile. I am in the development stages of a conversion (1960 bug) engine build. I have the cooling (DTM), case (1.7 914), exhaust (A-1 Sidewinder conversion), and a recent set of AA knockoff 2.0 914 bare heads (black Friday AA deal). At this point I am planning a 2258 78x96 with a Webcam 163/86b and all the valve train goodies (maybe even the Type4store cam kit variant).

When using the UEM engine calculator https://uempistons.com/p-27-compression ... lator.html I'd like to keep my compression around 8.8:1 or lower. This would require me to have 65cc chambers with .040 deck. Per Ham https://newsite.hamheads.com/2017/07/01 ... knockoffs/ the chambers in these heads will be around 58cc when built. The jump from 58cc to 65cc seems drastic. I would have European Motor Works or Headflow Masters do the work, but 7cc in each chamber would be a lot in an already large chamber.

Is unshrouding 7cc even possible/advisable in these AA knockoff heads?

Or should I run a smaller crank(76mm), increase the deck (to .045) which would require only 62cc chambers? Unshrouding 4cc seems more doable.

I could also increase CR to 9:1 with 78x76, .045 deck, and 62cc chambers (4cc unshrouded)?

Obviously all of this is in the planning stages as my heads are still bare. I wanted some opinions and options to mull over. 2258 seems to be a common combo. What CR are people running? What CC are your head chambers? What direction should I be looking?

Thank you
- J

[wreck]

I would just leave the heads alone and run the higher compression . Have you factored in the valve pockets in the pistons ? they are around 2cc from memory .

With that cam your dynamic compression ratio is 7.3 to 1 which is safe .

I'm running a larger cam ,Scat C55 which is 264@ .050 . with 53cc heads on a 103x78 . with .043 deck . dynamic is 8.151 . when new I didn't have any pinging issues , but 20,000km on ,with carbon build up etc and not perfect tune , a lean transition from idle to main jet circuit it will ping . I fitted water/meth injection that starts at 1/4 throttle and 2000 rpm . No more pinging and I'm running 34 degrees at WOT and the engine runs cool , average head temps are 120C on a 30C ambient day .

[patto]

I seem to recall the chambers in the stock 914 heads being 65cc ?
Im guessing Ham modifies them to achieve the 58cc chambers

[jayxian]

Per the Ham article above 58cc chambers are "about average for uncut O.E. heads", but you are correct that I won't know for certain until they are built and I accurately measure.

I was planning to use flat top 96mm KB pistons so I won't get the extra ~2cc from the dish.

Using the calculator and assumed 58cc chambers my DCR will be 7.7:1 with a static CR of 9.6:1. This is a lot higher then I would like even with a DTM in a light vehicle (1960 bug).

How many CC's would be advisable to take out of these heads? Max 7cc?

Wreck, thanks for sharing the combo a DCR of 8.151 sounds fun. I'm not sure I could get away with that due to the 91 octane pump gas in California.

Thanks,
- J

[patto]

I just measured my AA 914 Heads at 62cc so i would suggest wait until you build them up!
One difference I noticed between mine and Hams is that both of my valves have a small concave in the valves probably not a huge difference though.
Also I was curious how much our heads could be fly cut so I sacrificed a head and was surprised to see we can comfortably fly cut 2mm I would even go 3mm hopefully a pic is attached? the black is where i drilled straight through the head and the green is where the actual thread finishes.

[Clatter]

The non-square bolt pattern of type 4 head studs can contribute to the heads 'lifting' or not sealing.
For this reason few will flycut for compression.
The heads need support to keep sealed.
Especially in a performance build..

With a Web 163/86b it wouldn't bother me to run 9:1 or more on pump gas.