87 OCTANE can we guess C.R. ?

[1974dodgecharger]

My engine builder who built my engine when I went to get my car put in octane 87.  I said, 'whoaaaa' what you doing?  He said, 'you said pump gas'!!!  He claimed he built the car to run on 87 and not 91.  This was awhile back......

We drove it for 500 plus miles after break and no pinging/knocking sound with 87 octane.  Does this mean the compression ratio is less than 10 to 1? 

I just put in 91 octane since that day....I don't know why maybe thinking, '91 is better safe than sorry attitude', but he put in 87 and it was fine.

[Ghoste]

Probably less than 10 to 1 but other things affect it as well such as cam choice and head material.

[c00nhunterjoe]

If you know the exact cam used, and do a compression test, you can get a decent estimate. Does it have iron or aluminum heads?

[1974dodgecharger]

Probably less than 10 to 1 but other things affect it as well such as cam choice and head material.

at the time of build up edelbrock only had 2 choices for heads in the RPMs 88cc and 84cc

509 Cam and the head gasket was the edelbrock head gaskets provided with the heads.

4.505" bore
0.038" compressed
9.69cc volume

[1974dodgecharger]

what readings am I expecting coonhunterjoe?

Just saw an old thread and cooter responded, lol....

[Ghoste]

You could have 10 to 1, aluminum heads will allow for a little higher compression with a lower octane.  I think that cam will let you get away with it too but someone else will know better on that.

[1974dodgecharger]

I been reading alot.....good info....in the end I could pull heads but then I would want to upgrade things.....

[firefighter3931]

Dynamic compression determines octane requirements to a large degree. Dynamic compression is a result of the engine's static compression coupled with the cam timing events. Closed chamber aluminum heads are more efficient than open chamber heads....so that also comes into play. 

The 509 is a large cam so it's bleeding off a lot of cylinder pressure. That coupled with the aluminum heads significantly reduces the octane requirement. 


Ron

[BSB67]

My engine builder who built my engine when I went to get my car put in octane 87.  I said, 'whoaaaa' what you doing?  He said, 'you said pump gas'!!!  He claimed he built the car to run on 87 and not 91.  This was awhile back......

We drove it for 500 plus miles after break and no pinging/knocking sound with 87 octane.  Does this mean the compression ratio is less than 10 to 1? 

I just put in 91 octane since that day....I don't know why maybe thinking, '91 is better safe than sorry attitude', but he put in 87 and it was fine.

If you tell us what piston and how far in the hole they are we could probably estimate pretty close the actual CR.  Based on the info that you gave, you could probably build a 10.5:1 motor and run on premium fuel at sea level.

[1974dodgecharger]

My engine builder who built my engine when I went to get my car put in octane 87.  I said, 'whoaaaa' what you doing?  He said, 'you said pump gas'!!!  He claimed he built the car to run on 87 and not 91.  This was awhile back......

We drove it for 500 plus miles after break and no pinging/knocking sound with 87 octane.  Does this mean the compression ratio is less than 10 to 1? 

I just put in 91 octane since that day....I don't know why maybe thinking, '91 is better safe than sorry attitude', but he put in 87 and it was fine.

If you tell us what piston and how far in the hole they are we could probably estimate pretty close the actual CR.  Based on the info that you gave, you could probably build a 10.5:1 motor and run on premium fuel at sea level.

that I wont know how far into the hole he used Keith black pistons the hyperentic pistons. 

[1974dodgecharger]

Did driver side pressure test in the hot az and headers don't help after driving.

I was getting 140 to 130 range psi readings.  Each cylinder kept getting lower my batter juice was low....cranked it many times.


This is with a 509 cam 292 so big stick.....

[Kern Dog]

You need to keep a battery charger on while cranking the engine. This helps with maintaining consistant readings.
4.505 bore? Are you sure? A stock 440 is 4.32. This would have to be an aftermarket block or you'd be into water at that bore size!

[chryco_psycho]

i was thinking the same thing kern dog. there is an "ok" calculator on summit's website and as an example this is what my 440 popped out on there
Bore: (diameter)   4.350 in
Stroke:   3.750 in.
Cylinder Head Volume:   80 cc in.
Effective Dome Volume:
Use (-) for Dome and (+) for Dish.   +5 cc in.
Deck Clearance:   -.016 in.
Compressed Gasket Thickness:   .04 in.
Number of Cylinders:   8
Compression Ratio :   11.05 : 1
Total Displacement (in.3) :   445.85
Total Displacement cc's :   7309.02


while i highly doubt i built an 11 to 1 motor for $3500 i wanted a torque'y one so i got the alum. heads from 440source and a hughes cam

[BSB67]

i was thinking the same thing kern dog. there is an "ok" calculator on summit's website and as an example this is what my 440 popped out on there
Bore: (diameter)   4.350 in
Stroke:   3.750 in.
Cylinder Head Volume:   80 cc in.
Effective Dome Volume:
Use (-) for Dome and (+) for Dish.   +5 cc in.
Deck Clearance:   -.016 in.
Compressed Gasket Thickness:   .04 in.
Number of Cylinders:   8
Compression Ratio :   11.05 : 1
Total Displacement (in.3) :   445.85
Total Displacement cc's :   7309.02


while i highly doubt i built an 11 to 1 motor for $3500 i wanted a torque'y one so i got the alum. heads from 440source and a hughes cam

I get 10.2:1 CR.  I think the deck clearance is suppose to be a positive number if the piston is below the deck.

[flyinlow]

One problem with a larger cam is they are usually in cars with headers and louder exhaust. You don't always hear the detonation at 4000 rpm where the cam is working best and making the highest cylinder pressure.

If you had a small block chevy for a starter and did a compression test at 3000 rpm the compression numbers might be higher.

So.... crank the timing up until it knocks on 91 octane ,then back off a couple degrees    Mopower.

An engine will not detonate from compression unless it is a diesel. It is the rise it temperature and pressure after the sparkplug ignites the mixture that causes lower octane fuel to flash across the combustion camber rapidly increasing the pressure, banging on the piston/rod /crank.

[Challenger340]

i was thinking the same thing kern dog. there is an "ok" calculator on summit's website and as an example this is what my 440 popped out on there
Bore: (diameter)   4.350 in
Stroke:   3.750 in.
Cylinder Head Volume:   80 cc in.
Effective Dome Volume:
Use (-) for Dome and (+) for Dish.   +5 cc in.
Deck Clearance:   -.016 in.
Compressed Gasket Thickness:   .04 in.
Number of Cylinders:   8
Compression Ratio :   11.05 : 1
Total Displacement (in.3) :   445.85
Total Displacement cc's :   7309.02


while i highly doubt i built an 11 to 1 motor for $3500 i wanted a torque'y one so i got the alum. heads from 440source and a hughes cam

Calculators are fine, but only as good as the "imput" information which can sometimes be inaccurate, without actual "CC" measurements being taken.

For example:
* the 440 source Stealth Heads "out of the box", without milling, are generally 82 to 83 CC's....  NOT "80" CC's.  I've checked dozens, and I have never found one below 82 CC.
* All "composition" style BB Mopar Head gaskets are .039" to .040" compressed thickness. But what the calculators cannot do is compensate for the fact they are NOT a "round" hole at that thickness ? (due to their compatibility for the older max-wedge bores with cylinder flycuts).
Not a huge difference, but 9.9 CC's instead of the calculators typical 9.6 CC's.
* just me... but I have never found a BB Mopar aftermarket Piston with Flycuts below 6 CC's. Yep, I know some publish "5" CC's, but those are the ones I checked were 6.4 CC's ??

just sayín...
without actual measurements, Calculators can be off quite notably ?
Even with an extra 2.5 CC's on the Stealth Heads and 1 CC on the Piston Flycut.... your Comp ratio could drop from 11.05:1 down to 10.67:1 ?? even WITH the NEGATIVE .016" which the calculator is interpreting as ABOVE deck.

Try it again with + .016" deck, 82 CC Heads and still the even 5 CC Flycut.

[c00nhunterjoe]

130-140psi on a 440 with a 509 cam does not sound like 10:1 to me. More like 8.5- 9:1. I crank at 195-200 on my engine with a 509 equivalent.

[1974dodgecharger]

GREAT INFO flyinlow....thanks.

One problem with a larger cam is they are usually in cars with headers and louder exhaust. You don't always hear the detonation at 4000 rpm where the cam is working best and making the highest cylinder pressure.

If you had a small block chevy for a starter and did a compression test at 3000 rpm the compression numbers might be higher.

So.... crank the timing up until it knocks on 91 octane ,then back off a couple degrees    Mopower.

An engine will not detonate from compression unless it is a diesel. It is the rise it temperature and pressure after the sparkplug ignites the mixture that causes lower octane fuel to flash across the combustion camber rapidly increasing the pressure, banging on the piston/rod /crank.

[1974dodgecharger]

130-140psi on a 440 with a 509 cam does not sound like 10:1 to me. More like 8.5- 9:1. I crank at 195-200 on my engine with a 509 equivalent.

I was thinking that also...Coonnhunterjoe I would ask my builder, but he is long gone with no phone anymore.....so no more contact

[cdr]

i thought you had a 383 ?

[c00nhunterjoe]

I do have a 383, the basic numbers will be very similar. 130 psi static compression is low unless you have a ridiculous cam, and even thats pushing it.

[1974dodgecharger]

i thought you had a 383 ?

what coonhunterjoe said CDR....I do have a 383, but 440 should be similar as he said.....

I upped the timing the other day and man it made my car idle really high like 1300 or so....I didn't adjust the carb just the timing.  Then back my timing back a tad to original spot....The car is loud with the headers and 3inch exhaust so its hard to hear knocking when I rev it to 3k or so....I will have to try it again next time....