What is the highest compression ratio i could run on pump gas with afr 195 eliminator aluminum heads and flat top hypereutectic pistons at sea level? Would 10.7:1 be possible?

 

Maybe. Depends a lot on the rest of the combination. You probably won't be able to run low octane but 91-92 should work.

 

I didn't see what engine this is going to be on... 305? 335? 350? 383? 400cid?

If you've got some cam, say over 230@.050", and some decent overlap, I think with super premium fuel and a cold air intake, you could survive.

 

People say a static ratio of 10 for iron, 11 for aluminum. To me, that doesnt mean much, dynamic is what really matters. I'm not sure what a safe dynamic ratio is, but from what I've read, if you don't want to do the math and consider all your variables, then 10.5 seems to be the safe point. 12 can be done if you really know whats going on.

I knew a guy that was running 12:1 static in his truck with iron heads on 94 octane. I told him it was a bad idea, and the motor broke a piston after a year of daily driving. Timing advance also has an effect on the detonation.

Like Sonix said, if you've got a cam that bleeds off a little pressure, I think you'll be fine at 11.7. AFRs have nice chambers.

 

12.4:1 with 10 lbs of boost was my best on pump gas

 

Uh huh.... I'll bet the rest of the real world would love to hear details on how you pulled that on off....

 

i dont thnk i should have ever worked.
steel heads to boot.
i ran with 34 deg of timing and a btm to pullout timing when i was in boost.
it was a 6-71
pic

this was taken when i was tubbing it.
i made one pass on street tires and 94 pump gas
mt sportsman 31x18.5x15 and 4.56 gears
it hat a 3.3 60 ft and crossed at 130 mph. lol
i felt like john force couldent get boost because the tires would lite up
i ran it on the street for about 1 year before i sold the blower to go turbo.
the same motor was in the s10 after that on spray running 9.93 at 134.98.
so the blower and pump gas didnt hurt it.

 

Tony Mamo says that here in California with 91 octane one should not exceed 11:1 cr with the AFR Eliminator heads. In my new build I will be going for 11:1 cr and a 8.5DCR.

These heads have a dual quench pad with a modern chamber that allows them to run a little higher compression. I'm currently running 10.7:1 with my older design Dart Pro One heads with no problems.

 

The engine is going to be a 383. Does that hurt or help my chances at making 10.7:1 cr work?

 

Well with the same parts a 383 will have higher compression. You can do a lot with gasket thickness. Also if you use 6" rods, it makes finding the right piston harder.

 

Actually, it helps because you can run a bigger cam than a 350 which will lower your DCR.
The people that get into trouble are the ones that do a head swap and up the C/R on a stock engine.
With the stock cam the DCR is too high.

 

its all about dynamic compression.. i'm at 11.1:1 with only about 7.8ish dynamic.. not entirely sure though but i'm perfect on 92 octane.. i even take her to 7k when i race..

 

There are some calculators online. They even get the cranking pressure right. They recommended 110 octane though with 8 lbs of boost. I'm running 93.

 

Any suggestions on a cam? How big am i guna need it?

 

We're going to need the static compression numbers first.
Deck height, combustion chamber size, gasket thickness and diameter, piston eyebrow volume.
You can plug them in HERE to get the C/R.

 

I'm running iron Sportsman heads on my car with 64cc chambers. According to SpeedPro I should be at 10.8 and I have to back the timing off to 22 total advance to run WOT with 93 octane. Add some octane booster and I can run 36 degree advance. I will use a thicker head gasket next time the heads are off.