Hyper Piston owners, what is your compression ratio for street fuel

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dodge freak

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Got talking with a builder saying how 9.5 to 1 should be the max compression with iron heads for hyper pistons--well all pistons but hypers do not like knocking at all.

I have two sets of cylinder heads. One has 65cc chambers the other is uncut J heads which I have not yet cc...guess around 72. Those J heads need to be re-done and I hate to put $300-$400 in them when my 65cc MP 308 castings with 2.02 intakes are ready to bolt on. Then again I don't want to crack a piston on a hot summer day--if its ready by then, lol.

Oh street fuel in Michigan is 93 octane--the pump says.

Its a 340 .040 over with KB 243 hypers
 
You won't crack a piston unless you detonate hard & often.jmo

I'm running 9.9 in the 410, no pinging/detonation, pistons look sweet.

how much compression?
it's about the cam profile too, 180'ish psi 'dynamic compression is on the edge for pump gas but works and makes power too, but that could be a static of 10.8-11.2+ depending on the cam amongst other things like curve timing/programable curve distributors etc.....

is the 'builder' by chance also the machinist?
 
No, I get to visit two engine shops since my 340 block had an oversize lifter and needs a bronze bushing which nobody around me can do expect for him. His shop is super clean, you could eat off the floor--no kidding but he has a CNC machine and his prices are too high for me--$570 to assemble a short block and degree the cam. $850 to bore-line hone, deck, etc (which he claims all motors need cause the CNC is perfect afterward). Anyways he is just doing one lifter bushing and its getting out of there and back to the stone age shop, lol.

My builder thought 10 to 1 is perfect for street fuel and I be ok with 10.5 if its that high..might be with the cut heads. The builder with the high buck CNC machine thinks no more that 9.5 to 1 with iron heads..even with 30-32 total timing, no vacuum advance and a bit rich

The cam is a Crane with 226-230 @ .050 on 110 lbc, only .455 lift..should last a long time I hope.
 
A general rule of thumb that works very well is 10-1 iron, 11-1 aluminum. Carbon build up can push the ratio up and how much depends on the amount collected. This is also due to start of tune, amount of use, driving conditions and fuel grade and quality.

In the past, I have cleaned up OE pistons and scraped an 1/8 off the piston top. This adds up very quickly and I would not be surprised at all if it added another 1/2 point to the ratio. So less/safe is better in the long run.

While it is true that a larger cam will bleed off compresion to allow more comprsion ratio, the carbo issue never goes away. You will meet guys running 11-, 12-1 iron head on 93, but you won't see them use the car everyday, every year, year after year.

A 1/2 point error on the side of caution is worth it. The HP difference is insignificant.
 
I am having a 360 built right now. Zero deck with hyper KB107 pistons. My engine builder is shooting for 10-1 using ported J heads. He also said 10.5-1 would be pushing it.
 
I've run that combo before, zero deck 107+030, iron J's= 93 octane and no problems.
 
We have a station selling leaded turbo blue 110 octane for $6.50 a gallon. It be a pain but I might be adding 1 gallon to each 15 gallons of 93. Doesn't sound like much but it be 6% ratio and should spike the 93 up a bit.

I drive my car everyday 6 months of the year and with 3.90 gears and traffic going 75 mph on the freeways--which I stay with traffic, the motor buzzes. Once fall comes and cool weather I try the 93 straight.

I'm breaking the cam in on 110 fuel but too expensive to run straight. I likely spike the fuel more to start with.
 
( Gallons_of_gas * Octane_of_gas ) + ( Gallons_of_toluene * 114 )
___________________________________________________
Total Gallons
Here is a formula from a site for adding toluene to gas as an octane booster. According to that it raises it to 94.
Here is the link.
15x93=1395
1x110=110
1395+110=1505
1505/16=94
I know it's toluene, but substitute the octane (110 instead of 114)


http://www.elektro.com/audi/toluene/
 
I'm running KB 107's in my 360. I didn't deck the block so their a little in the hole but I figured the compression ratio out to 9.85 to 1... The heads are old iron open chambered X heads and it has no problem at all running on 93 octane. Running 35 degrees total timing also. Cam's not all the big either at 234* at .050 on the intake.
 
I agree about 'in general' a 1/2 point won't be critical, but in the case that.. say you wanna run a cam thats a lil too big for your equipment/usage etc...thats when the 1/2 point might keep/help you from fouling plugs and ...'carbon build up'

carbon is mostly in the tune, and it can, like mentioned, raise compression some but what I always worry about with an engine in this shape 'besides the poor tune' is the carbon heating till it turns into a glow plug...at that point even an 8.1 motor will kack out, I see it on old slants daily.

higher compression= more efficiency, power etc

JMO shoot for a 180psi dynamic and you'll have room still and not be selling it short on the power/response, you'd probably be able to run 87-89 octane with 32*
 
Well Manici Racing is posting the KB 107 as 9.8 to 1 with 72cc heads and the 340 KB 243 at 9.5 to 1 with 72cc heads. The motor has not been deck and I will check to see if the pistons are in fact .018 out of the hole. The rods have been re-done and I was told long ago that lowers the piston height a bit--removing metal from the big end when its hone.

This CNC builder seems very how can I say it, like a worrier. I told him while he was trying to talk me in to CNC the whole block that I recall in the 80's we used to rebuild these engines all the time with only a hone and new bearings and rings and the motors ran and last. No comment on his part.

This shop sells racing motors starting at $42,000....so I'm like waaaaaay over my head over there but like it needs a bronze lifter bushing and he can do it. Lucky I found a shop close by.
 
If you are using open chamber factory heads and a cam smaller than 228°@.050 I would not go beyond 9.5:1.
 
I agree with moper. I was going to mention a duration @ 050 of 230. The 2* is nothing.
I'd just hate to build a engine and then have to runless advance that I thought I could or could get away with. Something lower than normal set up for performance. You'll end up loosing power due to timing not being all that it can be. A few degrees mean a lot sometimes.

I don't think you'll need 100+ octane.
 
dodge freak said:
694301
cch 288-2
Click to expand...

thats small, run 9.5 'or even a lil less'

FWI as long as you can still have 30* total, you're good.

you guys at 35* 'under load' are just a lil short on cylinder psi for the cam, jmo

the guys who designed the !!!angled plug!!! sb chamber state 30*
I believe as long as you have at least 30* with the high cy you are losing no power! it's called balancing between the 2.

sure you pick up mph with more timing... when you had picked a big cam and then crippled it with low cylinder psi cause you were scared to cack out/detonate, or you wanted both race/street and we know how that usually turns out.
c it all the time.

don't confuse this now, I'm telling people that if they can run 35 not to, just that the cylinder psi is on the nilly. stock 318's run around with 35* under load

my 9.9 170psi 410 could even squeeze 36* under load in the right weather, BUT it's slightly on the nilly for the [email protected], heck I bet I could get 87 to run fine @ 30*
thinking out loud here...
big blocks like the kind of timing I'm hearing some sb guys are running [35-38*]
Take a good look at the plug angle....no look at a small block chevy plug angle...
they are both straight and both typically need 35+* of timing, now taking that into account you can see where lowering the timing could hurt power...
do what ever you are comfortable with.
 
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