Compression Ratio vs Octane Rating

[yellowsnow4free]

Hey everyone,

I'm not 100% sure where to post this, so I apologize in advance if this is in the wrong place. Mods, please feel free to move this post if necessary.

I usually post my questions on Club4AG, and for the most part I get good responses, but if I mention "4AC" anywhere in my post I immediately start getting replies along the lines of, "y u no 4ag?"

Anyway, onto my question.

In case anyone cares, I'm seriously considering a 7AC build for my AE86 SR5. No 4AG because the parts to swap are expensive (especially here in CA) and I'd have to go to a smog ref. The smog ref isn't that terrible if you swap everything properly, but I don't think all that hassle is worth going from a 4AC -> 4AG. It would be cooler to put in something like a 2RZ or 3RZ, but you can't legally swap those into a car here in CA, as they're truck motors.

I've read about the handful of 7AC builds I could find, and it seems like a better option than a stock 4AC rebuild. I'm currently trying to piece together all the info I can find on it.

One of the biggest things I'm wrestling is the compression ratio. I know how to calculate it, and what all the measurements are, but one of the biggest hurdles is here in CA we only have 91 octane max. Plus, since it'll have to be smogged I can't advance / retard the timing too much.

Keeping all timings stock (cam, crank at TDC, ignition at 5 degrees BTDC), what's the max compression ratio I can achieve running 91 octane? I found this chart online, but I'm not 100% sure if it's accurate:



Looks like 91 octane would put me at 9.5:1 CR?

I certainly don't want to be below the 4AC's 9:1 CR, and since the pistons will have to be swapped anyway (4AC pistons don't have a 20mm wrist pin to match the 7AFE rods), why not look into a little more CR?

Finally, is it even worth bumping the CR so little? I've read advancing the timing has a similar effect. If that's so, keeping the CR around 9:1 would allow me to adjust the timing, and run lower octane fuel if necessary.

Just spitballing here, but would it be possible to machine the 4AC pistons to accept a 20mm wrist pin?

Thanks!

[dr.occa]

@yellowsnow4free
You can't really determine the ideal compression for any given octane unless all engines were identical. Static CR isn't what you're looking for but dynamic due to the variations from engine to engine. Your current engine's CR with Cam duration/lift (which affects camshaft pressure bleed-off - that also factors in to determining your dynamic CR) are a few bits of info you'll need to discover. That should be a good start.

[yellowsnow4free]

Thanks for the reply dr.occa!

Yeah, I've been digging and digging and have determined dynamic CR is the real issue here.

Having said that, I posted on Tercel4WD and got a pretty informative response.

I haven't decided whether or not to run a cam (basically not sure if it'll affect my emissions), so it's hard to determine dynamic CR.

[dr.occa]

Good deal man. To reiterate what one of your Tercel4wd members stated: The higher the octane the more resistant the fuel is to combustion hence advancing your timing in order to get the burn started early.

The smog restrictions in CA are such a Debbie Downer on hot rod dreams. The Delta regrinds are a common mod for the 4AC motors along with dual valve springs. Some have gone with larger valves and a weber 32/36. Even still, w/ the 7A block, even if you were to just mate 4AC head and 7A block, you'll need a carb that would allow you to re-jet for the stroke. The factory 4AC carb would most likely run lean.

[yellowsnow4free]

Yeah, you've got no idea how much I want to run a 32/36, even if it's just so I can get rid of some of the vac lines

@raccoonracing posted in these forums that his 7AC passed smog in CA, so I assume he used the factory carb. Unfortunately he hasn't been an active user in a while, so it's hard to know for sure.

What's tough about 7ACs, unlike 7AGEs, is there isn't much info on them. I've had to dig and dig to get what little info I could find, then piece it all together.

I should really compile my links here...

But here's a promising note to end on! :D

60 kW (80 HP) atw 7AC

[dr.occa]

Is there already an appropriate timing belt identified for the frankenstein?

[yellowsnow4free]

Is there already an appropriate timing belt identified for the frankenstein?

Yes, the part number is floating around somewhere, but I know it's from a Dodge Colt.

Unlike a 7AGE, apparently there isn't a need for an adjustable cam gear or a re-cut (broached) crank timing gear. However, 8Ton at 4AC Powered said he would get a broached crank timing gear if he builds another 7AC...

Like I said, not a ton of info