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Discussion Starter #1
I recently purchased a dart big m aluminum 4.5" block still in factory packaging and want to make a 509 out of it. I have a GM 502 forged bottom end that I want to pair up with a set of AFR 315 aluminum heads (121cc chamber) comp solid roller .256/.260 @ .050 with .660 lift on a .110 LSA cam. Eddy Vic JR R intake with an HP 950 Holley. All I need are pistons...what is max 93 octane compression you can run safely with this combo? I've heard 10.5:1 with alloy heads & iron block but is it safe to go any higher with the alloy block? This will be in a 68 camaro weekend cruiser pro touring car with 4.10 rear and 6spd trans.
 

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in my opinion its more about cranking compression. camshaft and static compression ratio work together to create dynamic compression. then there are a few things that may affect resistance to knock, like quench distance, and piston shape and bore size. weather conditions, engine temperature and load also affect it. with all these things in mind im sure somebody on this site will be able to tell you the absolute maximum, i would probly try to be at 10.0:1
 

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I recently purchased a dart big m aluminum 4.5" block still in factory packaging and want to make a 509 out of it. I have a GM 502 forged bottom end that I want to pair up with a set of AFR 315 aluminum heads (121cc chamber) comp solid roller .256/.260 @ .050 with .660 lift on a .110 LSA cam. Eddy Vic JR R intake with an HP 950 Holley. All I need are pistons...what is max 93 octane compression you can run safely with this combo? I've heard 10.5:1 with alloy heads & iron block but is it safe to go any higher with the alloy block? This will be in a 68 camaro weekend cruiser pro touring car with 4.10 rear and 6spd trans.
I know a guy running 12.5-1 compression with 93 octane. He has a big roller cam and aluminum heads.

Personally, I would stick with 11-1 compression. You can ask your cam company what your dynamic compression will be with their cam. That should tell you if 93 octane is ok.
 

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Like Cory said above dynamic compression is more than just your cam specs. Piston dome, quench (what little there is with a big block) chamber shape, any special thermal coatings or mirror bright polish, your stroke and oil control can all be contributing factors to staying out of detonation.

Personally I like build high compression engines and using alcohol and water spray to increase octane rating for the street and pure race gas for racing.

Larger Dave
 

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Brandon, If you want to be on the safe side stay with 9.5:1 with iron heads and 10.5:1 with aluminum heads. You can stretch it to 11.0:1 if you had to but might have to lower the timing if you hear any knocking. My 496 rotating assembly has 10.3:1 c.r. with Edelbrock Performer RPM 454R heads with 119cc chambers. My cam is a Crower mechanical roller .641/.636 lift and 256/266 duration @.050. I have the Victor Jr. intake with a Holley Race 850 carb. It has been upgraded with a Proform Billet base plate, Holley fuel bowls with quick change jets, and a Proform main body with air bleeds and no air horn.
 

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Yep Dave and Cory are on to it
Bascally compression means squat...what is important is the cylinder pressure at max piont of explosion at a given piont between 12 and 18 ATDC at a given rpm and load.
This is called the dynamic compression ratio
A cam has the inlet and outlet valve open at the same time which means as the piston comes up one looses potentual compression because of the TIME they are open together.
As the rpms increase this time reduces.. reducing the amount leaking out the out let
This is what basically determines the rpm range of the cam.
So if have a large duration and excessive high lift , at low rpms the engine becomes very inefficient... add a big carb on and it also is working outside the rpm range, dramatic drop in bottom end torque.
Which is why a track cam n the street will have 20/ 22 degree idle advance... no compression, lower the compression slower the flame across the chamber.

upping the compression to get around this bottom end issue , increases the top end dynamic compression ratio dramatically... to the piont where there is far too much cyclinder pressure to get the best potential out of a lower octane fuel.
Bit of a catch 22 situation.

So it comes down to this....
Design the engine.. carb cam, headers, compression for the best efficiency in the rpm range it will be used for over 95% of the time...

Not doing so is the biggest and most common mistake guys make....
So lets say you want top end , big cam big carb bragging rights....fine..
Or the alternative is to deesign towards lower street end.....u end up with smaller carb cam, and a car that on the street has huge bottom end torque with big time show off charactoristics...and when ppl ask, expecting big brag, they get something like
Oh mild cam, bit higher lift (gives the lop) x small carb and does x sec at x mph under 5000K... and economical
Or the big cam, yes sounds good , yes spectacular, but doesnt perform anywhere as well bottom end as the above, and chews thru a gallon a mile.. very expensive on a road trip or if driven much.

And should also mention, that high cylinder pressure up in the rpms, where u will never hear it, on low octane fuel, will have a real bad tenancy to detonate.
yes u can go up around the 10 or 10:1 but once u start getting over 9.5 9.8:1 depending on your cam and rpm range u are targeting, u will be loosing full potential of the fuel
 

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i learned the way on the 383 in my nova. i didnt think you could have too much compression for a given cam. i ended up with a motor that cranked 285 psi!!! the opposite is true as well, the guy that picks the cam at the bottom of the page with stock 8.5:1 compression...
 

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Discussion Starter #10
Thanks for all the great info guys! Does the block being aluminum allow you run more compression too?
 

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with aluminum heads the thermal conductivity is better thean iron. this means it draws heat out more evenly and you do not get "hot spots" in the combustion chamber. hot spots make the fuel ignite before you would like. im sure over all surface temperature is less on the surface of the chamber. while i cant prove it, my opinion is while the aluminum head allows more compression than the iron head i dont think your actually running more. i think its to stay even performance wise you need to have more.
 

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I run 105 ron with over 10.5:1 and a cranking pressure of 218 to 223lbs.. as a my daily driver for over 30yrs now.. cars don coming up 300K miles
Its slightly cheaper to run than the 96 ISUZU bighorn/ trooper diesel and most 2.5 L cars.
 

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Cam sounds like around a 290 advertised duration. That should put intake closing point (conservatively) at around 70 degrees. That comes out to dynamic compression of around 8.4:1 with 11:1 static compression. Upper limit on dynamic is typically quoted as around 8.5:1 with aluminum heads with some safety built in. I'm at around 8.3 with mine and zero issues on 93. On a side note, not sure why you wouldn't just go with a nice hydraulic roller with that setup and usage unless you just already have the valvetrain.
 
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