11:1 too high for pump gas?

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Jbarker

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Hello all
Let me start by saying thanks for all the advice on my previous posts. Much appreciated!
The question today is when I reassemble my .030" over 383 based 489 stroker, should I put it together with the dished pistons that I already have for a 9.8 to 1 compression ratio using .039" gasket or spring for flattop pistons which would make it 11 to 1. To make it 11 to 1 (and not higher) I would need to use a thicker head gasket (.051" or thereabouts). Motor is completely tore down at this point, and things such as head gaskets and camshaft have not yet been purchased. There was minor scuffing on the piston skirts and some loss of the crosshatching on the thrust faces of the cylinder walls so I had the pistons polished and the bores honed. Machine shop said the fit was still good. The block is zero decked. I have 440 Source CNC'd aluminum heads. This is a street car and must absolutely run on pump gas. It's not difficult for me to find 93 octane.
The obvious question is if 11:1 is too high for a pump gas aluminum head engine. The secondary question is what are the chances that the pistons I buy will fit the bores that have already been honed? The pistons in question are all 440 source.
Lastly, I can't use a .039" head gasket if I go with the flattop pistons because that would push my compression ratio to 11.3 to 1 which I think is asking for trouble. But going with the thicker head gasket and flattop pistons to get to 11 to 1- is that just a dumb idea since I would simultaneously be increasing the quench height? I know that if I want to get away with higher compression I need a tight quench.
It seems to me that 9.8 to 1 is it touch low for aluminum heads and 11 to 1 is pushing the envelope a bit for pump gas. Your thoughts?
 
I ran pump gas with my 11:1 stealth heads on the street.
I put 100 in when I went to the track.
 
You will have to have a VERY good tune to run 11:1, no chance of a lean condition, and timing must be precise.
 
I have been in that boat many times before. I will NEVER go over 10 to 1 on a street engine again. You'll never feel the extra compression in a street car other then always adjusting the timing and carb. Sure you have aluminum heads but when it is 85 of 90 outside and the sun it shinning down (when you will be driving it the most) it will be pinging away. Then the move the timing around game begins. I was at 11.5 to 1 with my 440 with aluminum heads in my Cuda. I just rebuilt the engine in the fall and it is now at @ 9.5 to 1 and it is so much better. Same camshaft everything, just refreshed it and changed the pistons. From the sounds of it you have plenty of motor. I would stay below 11 to 1 for sure, and trust me pump gas it a hell of a lot cheaper and you're not building something for fuel mileage :)
 
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IMM Engines insisted I not go over 9.4 on the 408 stroker engine he is building for me using pump gas in my daily driver dodge ram. Pump gas is crap and it isn't getting any better. I'm running a 5.9 engine in it now ...completely stock engine with add ons with a SCT tune on it using 93 octane gas and hear detonation ever so often! Stock compression!
From what I've read on line doing research on how far I can go with static compression on pump gas it all comes down to dynamic compression ratio. Using DCR if factors in elevation, cam intake valve closing(bleed off), and other factors. The DCR should fall between 8.00-8.20 DCR on pump gas using iron heads ... you will be OK. I can't remember what is allowable in DCR using aluminum heads but don't remember seeing 10.00 in DCR. Another words you can go over 8.20 with aluminum.

Here is the calculator: Wallace Racing: Dynamic Compression Ratio Calculator

I went thorough the "I can change out the head gaskets and get 9.8 static compression and fall in the safe DCR range" but was told "is that little extra compression really worth it on the street when you can be facing detonation issues?"

Heads iron or aluminum, cam(bleed off), timing, altitude, engine cooling efficiency, outside temperature, all play a huge roll in what you can get away with on pump gas.
 
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Ran 11.9 comp. in my aluminum headed 416 using 93 octane no issues what so ever..
 
I think we calculated in about 11.3 to 1? I've been running a 100LL since the beginning and mostly just drag racing in the car or cruising around town. This year I plan to put it on the street more and would like to start running some pump gas through it. I know my cranking compression is down around 190 195. But my cam should have enough bleed off I think to get away with high octane pump gas. We'll see...
 
And I quote..............."This is a street car and must absolutely run on pump gas."

Haven't you already answered your own question?

Oh heck, go ahead,

"Take it,"

"to the limit,"

"one more time!"

It's so much fun tearing them down to fix something you knew could be wrong in the first place.

"Aluminum heads need more compression." Is BS.
 
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Vehicle weight is around 3200
all depends on vehicle weight, cam and rear gear
Vehicle weight is around 3200 pounds. Camshaft will be chosen after compression ratio is decided upon. I will use a solid lifter cam. 355 rear gears.

I posted this same question 10 years ago when I first built the motor and was trying to decide between flat top and dished pistons. I ended up taking the safe route- went with dished pistons and 9.8 to 1 compression rate and was very happy with the results. However, I have not built many motors and I always wondered if I left a lot on the table by going with the lower compression ratio.
And by the way, I agree with IQ52 that an aluminum head motor does not NEED a higher compression ratio. Hot Rod did an article where they swapped absolutely identical heads on the motor, except one set of heads was iron and the other was aluminum. Dyno test showed minimal difference.
 
I don't have a bit of trouble with my aluminum headed 10.8 small block 360 in my cuda.
This is exactly where I’m at. Runs well on 93. The cam is sizeable for the street. Bleeding off low rpm and drive speed compression. Paying attention to the engine timing is a key factor.
Ran 11.9 comp. in my aluminum headed 416 using 93 octane no issues what so ever..

My 440 is 11.8-1 edlebrock heads and run 94 in it. Never have I herd a ping. Street and strip car.

What are the size cams you guys are running? And, if you don’t mind, the gears, tire size and converter stall speeds.
 
Rumble...533/.552 cam.. 275 60/15 drag radials..3.91 gears..Ran a best of 11.02@121:D:D
 
And I quote..............."This is a street car and must absolutely run on pump gas."

Haven't you already answered your own question?


"Aluminum heads need more compression." Is BS.

x2.
 
I don't like anything over 9.5 to 1 on pump gas. That is plenty run what you need for performance on the street. Flow is more important. My opinion would be to back off a tad on the compression
 
I don't like anything over 9.5 to 1 on pump gas. That is plenty run what you need for performance on the street.
I have gotten away with a half point more in iron heads and 11-1 with aluminum. A point drop would be a bit friendlier as you could ping easy with something being not up to par (fuel) being off or broken. Like a vacuum line leaking or distributor malfunctioning.

I agree, flow over squeeze on the street. The cams I have used in the slightly has their ratios were big for the street bleeding off compression.
 
Crazy the Audi 8 I get to drive at work is 12.7:1 and no issues. That’s stock. 93 pump all day.
 
Oh yea! I agree. Not busting your chops!
The modern car has every racer trick and modern electronic trick it can stuff into the car.
 
Have any of you fine mopar maniacs ever have the idea of switching to E85 fuel?

(For the sake of reference, an idea stoichiometric air/fuel ratio for gasoline is 14.7:1, but only 9.765:1 for E85.) Fuel economy suffers about the same percent. The fuel-heavy mix has a cooling effect, allowing for more timing and compression. The addition of alcohol to gasoline in such a high percent gives us a significantly higher octane rating, which is posted as 105 at the pump. At about $2 a gallon on average, which is about the cheapest octane in the country, even after you account for the lower fuel economy. This octane level can easily support compression ratios up to about 14:1. Experimentation is bringing that number even higher. With this increase of compression tolerance, many engine improvements can be made.

Gas Versus E85 - Converting To Corn - Popular Hot Rodding Magazine
 
Went from 110 octane to E85. Dealing with the corrosion I experienced in my fuel system turned me off. Then I found out I could make more horsepower with 91 octane than most people could make with race fuel. I will never build another high compression engine. It's simply not worth it, to me.
 
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