max compression on a 494 b uild

[fusername]

so I may have gotten overexcited and ended out with a tighter motor than I had planned! already have the parts sitting on the shelf, but the CR is way higher than I wanted:

494 cam, flat top pistons, new amc heads. Every motor I've ever built had larger deck then i wanted os I planned on .045 min out of hte box, hence the flat tops. this is going into a fiberglass buggy street toy/weekend warrior, but my CR?!?

ended out with:
49cc head
flat top
.025 deck!
494 cam,

so all that spells out 10.2:1 CR, 9.0:1 dynamic CR. is that too much for a motor in this light of a car? I'd hate to spring for a set of 7mm dish pistons this late in the game, but the head vol was smaller than expected, and the deck was nice and tight for once, when i didn't need it lol.

[H2OSB]

Cylinder base shims. I bet if you got yourself to 1mm deck height, your CR would be ok.

What's the diameter of your pistons? 94x71?

H2OSB

Edit: Assuming you have a 94x71, even at 1mm deck, you're still at 9.8:1 CR. Might need to spring for the 7cc dish.

Edit2: Adding 7cc and a 1mm deck on a 1971 gets you 8.8:1 CR. Much more livable.

[Piledriver]

Are the heads flat deck or step cut?
If step cut, remember to add the step into your calculations.

Also check valve to piston clearance especially carefully if is a tight LC cam

[cal 67]

You have specs for the 494?

[H2OSB]

494 Grind: .465" Valve Lift with stock 1.3:1 Rockers, 280 degrees of advertised duration, and 244 degrees of duration at .050". This info from the AC.net website

H2OSB

[fusername]

Yes I'm building a 2.0 using stock amc head'swhich have the step. Measured 49cc, had expected 57cc. I bought the flat tops before I had the heads so I kinda screwed myself there.
Ica on the 494 is 50deg.
So if my math is right,as built will be 10.2:1 compression, 9;1 dynamic
Would that be a bomb in a buggy? Hate to give up the squish this motor has but shimming the deck to .065 would bring me down to 9.2:1 CR and 8.0:1 DCR.

[fusername]

ok I'm thinking I'll put the pistons on the mill and put some 7cc pockets into them. they are AA pistons, think they can survive being pocketed? Have to hit a deadline and get this motor on the road asap, so buying new pistons is out. somewhere in a box i have used 7cc pistons, but no clue what kind of shape they are in, and rods are already made to match the wristpins, so swapping them in is not an option. this motor is way too far along for a stupid surprise like this.

I had a friend re-cc the heads and he came up w/ 49cc as well, so i guess that is real.

[fusername]

found some .060 shims, tempted to put it together with those, not machine the pistons and just resign to opening it back up in the winter when its off the road. that nets me a decent 8.7:1 but i lose my squish with that .085 deck. could probably live with that vs the risk of melting things/taking too much off in the mill

[Clatter]

.065 at 9.2 seems fine to me in a light buggy.

Run premium if you're worried about it..?

Can you un-shroud the valves a bit?

[fusername]

as is my luck, looking closer at the .060s, its not a full set, they are different sizes. the only full set i got is the .009s. since this MUST close the upper end tomorrow (I know you never have time to do it right, but there is a health emergency on the horizon so the deadline is real) I am going to work with what i have. .034 deck thanks to the .009 shims. I found some good used 7cc pistons, but they have even lower pin height (.025 lower to be exact) so I can only run those if i trim the tops of them down, but i don't have rings on hand, i could probably find them in mine or my buddies stash in an emergency tomorrow morning.

I could open up the chambers but i'd need to get the second head from my friend, which is already assembled, and I don't have a CC rig anymore, it wasw borrowed, so that also complicates things. I'll stick with using the 914 pistons for reference on how thick the crown needs to remain, and hopefully there is enough meat.

[fusername]

9.2:1 sounds fun tho. every other motor i've built ive been a wimp on CR. I want to go to the opposite extreme (and get 20k miles out of this motor) so maybe I'll keep the dish small and go for 9.2:1 instead.

[fusername]

oh and I do plan on running premium at all times. this thing is supposed to be fun, not cheap

[fusername]

good news everyone! I am an idiot!

re-measured the heads while getting ready to open up the chambers, they are actually ~55 cc. like I expected! Now all i have to do is polish the chambers and this 9.1:1 motor is ready to go together. 8:1 DCR is still a little higher than i meant to, but maybe if I'm over-zelaous with the polish we will come down to 7.9:1.

[Piledriver]

with a 108 lc 494, and a 9.1:1 static cr, I get 7.2:1.
http://www.webcamshafts.com/pages_vehic ... 01084.html
Make sure to follow the instructions...
https://uempistons.com/p-28-effective-c ... lator.html

[fusername]

using that calculator and the following inputs:
9.1
2.8
5.157
50
I get a CR of 7.8

not sure why you get 7.2. I am a little confused there. I am using a calculator i wrote myself, so it could be wrong. using the same values as above in my homemade sheet, i get the 9.08:1 SCR, 7.96:1 DCR

Can't really explain the difference.

I am going to go with me .034 deck and 9.1:1, because that is really .064 deck due to the step in the AMC heads, which is the biggest i'd like to run. I understand a tight deck helps keep detonation at bay. I will have a CHT guage, and if it runs too hot or pings/detonates I'll pop it open and put some modified 914 pistons. since they are taller I get to machine the lip down and get exactly the deck i want (.040 including the head step) which will also change the piston volume to closer to 6cc, giving me 8.9:1 and 7.8:1.

fingers crossed here! I regret not opening hte heads up a bit more, but the season ends in 3 months so i get to do it all again if i want...