Early Hemi Compression for street motor?

Just trying to get some feedback from Hambers running Hemi's in there street driven rides. What kind of compression can I get away with on 91 octane. Also, hard seats? Needed, or not? Info, opinions, experience, all appreciated.

 

Don't know if the old Hemi is more or less efficient than my combo, but I am running 10.7:1 on 91. (517 BBF) I have never expirienced any detonation or pinging, but then again I do have aluminum heads. With iron heads I wouldn't go much past 10:1, but probably even lower. Just my two pennies.

Joe

 

The "word" is the Hemi can run 1 point higher compression on the same octane than a wedge engine can.

 

HIJACK! How about some pics of your truck? Did you find a door?

 

I could be wrong, but weren't the seats already hardened? Also, I think you should be fine with 10:1 and 91 octane on stock heads, as mentioned above.

YES! Pics! What Hemi are we talkin' about here!!?? Let's see what you're up to!

 

seats/valves...supposedly the heads either have hardend seats, or Chrysler just machined the head & put valves in w/o actual seats, others say there are non hardened seats. I've heard 1. Only 51-53 have them. 2. Only 55 had them, & then only some. 3. One of the "guests" in the Tex Smith book said only 55 didn't have hardend seats. I can say my 55 331 has OEM hardend seats/valves.

 

Wow! That keeps the rumor partrol's heads spinning . I can officially say I don't have clue now...

 

The '56 331/354 used no seats and were a favorite candidate for oversize valves , just machine the seat area larger and run the valve against the cast iron . This was strictly for compe***ion only , not street use . ---You should be able to run at least 10 to 1 static CR on 91 as the hemi design keeps the flame travel consistant . More,if your cam has enough overlap to bleed the cyl. pressure off at lower RPM's. .

 

My 58 392 did not have hardened seats and I had them installed and I run 10.7.1 compression with steel heads and have no problem with 92 octane.

 

Hopefully not too O/T, but a tangent for sure... how about Industrial heads? What's the scoop with those seats?

 

I'm gonna guess that the Industrials were hardened. they were built for continuous low maintanance use after all...

 

I would guess that too... but then you saw my last guess which was right from the hip!

 

My '57 354 industrial heads have hardened seats (even though they have the automotive-style heat crossover). My '56 331 marine heads (also used for truck engines) have hard seats too as well as sodium-filled exhaust valve stems.

Hard seats are more necessary with propane fuel as well as heavy continuous engine loads (i.e. marine and truck applications). Keep in mind the gasoline in those days was leaded anyways.

 

Hey thanks for the info so far guys. The engine is a 56' 354 Chrysler. On the cheap side, I could bore the motor 20 over and stick with a 9:1 piston, which would cost about half of a set of 10:1 pistons. But, I plan to run a pretty healthy cam, so compression is a must. I could always put a lead additave in the gas every few weeks i suppose. Out here in California, 91 is all the hippies let us have. Although, dont know how helpful one more octane would be, for those of you that still have 92 at the pump. The weight of the pickup will be fairly light which will be better. Im expecting about 2200 2300 pounds. So the engine aint gonna lug down. Thanks again for the responses, keep them coming. I want to learn all I can.

 

Cool! Thanks man! I have the '57 354 heads w/out the automotive style heat crossover. I'm sure they're would be a difference though. Thanks! Very helpful!