Mechanicall VS dynamic compressio ratio and fuel ?

Posted By: Cab_Burge

Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 05:06 AM

I'm putting together a 440 block 505 C.I. bracket motor(4.350 bore x 4.300 stroke) that has a mecahical compression ratio of 12.60 with a dynamic compression ratio fo 9.08 to 1 according to United Machines on line calculator. The customer wants to mix 110 octane race gas with non ethanol pump gas that has 92 octane here in OR, my question is what percentage would be better for this motor, 50/50 (2.5 gallons of each) or 3 gallons race gas to 2 gallons pump gas for a 66.6 percent of race gas to pump gas

I've never heard or paid any attention to dynamic compression ratio before coming onto this site many years ago

This is a bracket car only, not street
raced

The solid roller cam is 108 LSA in at 106 on the intake and it has 269 @ .050 on the intake and 276@ .050 on the exhaust side, ported 440-1 with no flow numbers, single 1050 dominator on a 440-2 intake modded to fit the dominator carb. pattern.
I am planning on dyno testing it next week on the local Studka dyno that has been calibrated recently, I'll ask to have it recalibarted for this motor, it has had a bunch of SB Chevys on it and may have been tweaked for those tests

Just kidding on the tweaking

Posted By: ademon

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 08:04 AM

i run 93 on my 11.25 Mechanical CR, SB. It does have about 200 cranking psi.

Posted By: gregsdart

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 01:34 PM

The fuel octane will be in direct proportion to the mix.(Edit; I stand corrected, non leaded only) No expert hear, but would think 50/50 should work. It would be nice to have a knock sensor on that dyno, just in case there was a hint of trouble. It would be good to have the extra octane in the tank though. Like for when my electric pump quit in the staging lanes just before the burnout! I didn't see it till the return road.

Posted By: Challenger 1

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 02:45 PM

Personally I think it needs straight 110 leaded to be bracket raced on. Maybe can get away with mixing it on the street, but racing it's gonna need all that 110 can provide at near 13-1. I've been selling race fuel for 25 years.

Posted By: MR_P_BODY

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 02:59 PM

I've never played with the dynamic pressure to octane
to see what will work and be safe... plus different
fuels will knock at different rates so start on the
safe side... but I always try to reduce octane a little
at a time... I see the engine running better and quicker
with a lesser octane thats ends up being correct..
I wish there was a quick chart to use for a relation
of dynamic to octane(and be close)... when I was
talking with Dan on my SB he gave me a number for
the dynamic that I should stay under for pump gas
but I would have to find it

Posted By: DaveRS23

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 03:20 PM

Will the dyno graph show detonation? A local racer told me a while back that he could identify detonation on the dyno by comparing the graphs. He said detonation makes the line much choppier. Any truth to that?

And have you thought about water/meth injection? It would be a lot less trouble than mixing and may save some money in the long run with the cost of race gas today.

Posted By: dogdays

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 07:43 PM

The 110 race gas is rated by the Research method.
The 92 pump gas is rated by the (R+M) / 2 method. Rated by the Research method it'd be around 97 RON.

If the 110 has tetraethyl lead in it, adding the unleaded to it will not change the octane rating in direct proportion to the mix, because TEL follows the law of diminishing returns. So, the first gram changes the octane more than the second gram, which is more than the third gram, etc. This means that the last gram in the race gas turns into the first gram in the unleaded and the result is higher octane than you would expect, although lower than 110.

If the 110 is unleaded, then the octane changes in direct proportion to the mix. So 50/50 would get 97/2 + 110/2 = 103.5 RON.

R.

Posted By: Challenger 1

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 07:57 PM

Quote:

The 110 race gas is rated by the Research method.
The 92 pump gas is rated by the (R+M) / 2 method. Rated by the Research method it'd be around 97 RON.

If the 110 has tetraethyl lead in it, adding the unleaded to it will not change the octane rating in direct proportion to the mix, because TEL follows the law of diminishing returns. So, the first gram changes the octane more than the second gram, which is more than the third gram, etc. This means that the last gram in the race gas turns into the first gram in the unleaded and the result is higher octane than you would expect, although lower than 110.

If the 110 is unleaded, then the octane changes in direct proportion to the mix. So 50/50 would get 97/2 + 110/2 = 103.5 RON.

R.

I believe 104 is as high as you can go without lead. And 104 is full of ethanol, way more than 10% to get it to 104. So I'm sure 110 is leaded.

Posted By: Blusmbl

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 08:33 PM

You can buy 110 unleaded all over town here in Detroit.

Posted By: Challenger 1

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 08:43 PM

I said I believe.
I think you should look closer, Here's Sunoco's list of unleaded fuels which I have been selling for years. Does not mean someone else is not selling 110 unleaded but I have never seen it. Show me some proof.
Here's sunoco list.
web page

Posted By: Pacnorthcuda

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 09:07 PM

Quote:

I've never played with the dynamic pressure to octane
to see what will work and be safe... plus different
fuels will knock at different rates so start on the
safe side... but I always try to reduce octane a little
at a time... I see the engine running better and quicker
with a lesser octane thats ends up being correct..
I wish there was a quick chart to use for a relation
of dynamic to octane(and be close)... when I was
talking with Dan on my SB he gave me a number for
the dynamic that I should stay under for pump gas
but I would have to find it

I hear to stay under 8.5 dynamic for pump gas (92 here). On my motor thats 10.2 static--I was going to shave the AL heads to 10.5 but decided I better not.

My Voodoo cam intake closes at 60, thats pretty early, not too forgiving to high compression ratios.

Posted By: justinp61

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 10:37 PM

Quote:

You can buy 110 unleaded all over town here in Detroit.

Same here, says right on the pump "unleaded".

Posted By: Challenger 1

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 10:47 PM

Quote:

Quote:

You can buy 110 unleaded all over town here in Detroit.

Same here, says right on the pump "unleaded".

What does a gallon of 110 unleaded cost?

Posted By: Crizila

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 10:49 PM

Quote:

Quote:

You can buy 110 unleaded all over town here in Detroit.

Same here, says right on the pump "unleaded".

Trust me, Sunoco 110 is leaded fuel. Don't are what the pump sez. Look on their web sight.

Posted By: justinp61

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 10:54 PM

Quote:

Quote:

Quote:

You can buy 110 unleaded all over town here in Detroit.

Same here, says right on the pump "unleaded".

Trust me, Sunoco 110 is leaded fuel. Don't are what the pump sez. Look on their web sight.

I don't see Sunoco mentioned in my post or the one I quoted. There are no Sunoco stations here.

Posted By: justinp61

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 10:55 PM

Quote:

I've never played with the dynamic pressure to octane
to see what will work and be safe... plus different
fuels will knock at different rates so start on the
safe side... but I always try to reduce octane a little
at a time... I see the engine running better and quicker
with a lesser octane thats ends up being correct..
I wish there was a quick chart to use for a relation
of dynamic to octane(and be close)... when I was
talking with Dan on my SB he gave me a number for
the dynamic that I should stay under for pump gas
but I would have to find it

Mike if you find the number Dan gave you can you post it?

Posted By: justinp61

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 10:57 PM

Quote:

Quote:

Quote:

You can buy 110 unleaded all over town here in Detroit.

Same here, says right on the pump "unleaded".

What does a gallon of 110 unleaded cost?

The last time I bought any was six or so years ago and it was $6 a gallon then.

Posted By: Challenger 1

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 10:58 PM

Quote:

Quote:

Quote:

Quote:

You can buy 110 unleaded all over town here in Detroit.

Same here, says right on the pump "unleaded".

What does a gallon of 110 unleaded cost?

The last time I bought any was six or so years ago and it was $6 a gallon then.

Thanks.

Posted By: AndyF

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/01/14 11:06 PM

For bracket racing the guy will want to run straight VP-110. If it was my motor I'd probably run C-12 but VP-110 should work.

Dynamic compression doesn't mean anything on a race engine. Once the engine is up on the cam you'll have 100% compression if not more due to the ram tuning.

You can get away with pump swill if you're just idling around town, once you get that engine under full load you'll need good gas.

Posted By: Crizila

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/02/14 05:34 AM

Quote:

Quote:

Quote:

Quote:

You can buy 110 unleaded all over town here in Detroit.

Same here, says right on the pump "unleaded".

Trust me, Sunoco 110 is leaded fuel. Don't are what the pump sez. Look on their web sight.

I don't see Sunoco mentioned in my post or the one I quoted. There are no Sunoco stations here.

Ok. So, what 110 unleaded fuel is available all over Detroit?

Posted By: justinp61

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/02/14 02:34 PM

Quote:

Quote:

Quote:

Quote:

Quote:

You can buy 110 unleaded all over town here in Detroit.

Same here, says right on the pump "unleaded".

Trust me, Sunoco 110 is leaded fuel. Don't are what the pump sez. Look on their web sight.

I don't see Sunoco mentioned in my post or the one I quoted. There are no Sunoco stations here.

Ok. So, what 110 unleaded fuel is available all over Detroit?

You'll have to ask Blusmubl, I live in KY.

Posted By: BradH

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/02/14 07:29 PM

Quote:

Dynamic compression doesn't mean anything on a race engine. Once the engine is up on the cam you'll have 100% compression if not more due to the ram tuning.

This also means that the crutch of using a "big" cam to bleed off cranking pressure to run lower octane w/ a higher CR only works up through a certain RPM range before the engine's static CR is driving the WOT octane requirement.

That's not my opinion, that's info that came from both a successful cam grinder and an engineer for a race fuel company.

Posted By: maximum entropy

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/02/14 10:18 PM

cab, i've found those dynamic c.r. calculators to be inconsistent, depending a lot on what they consider the closing of the intake. the best way to determine actual dynamic c.r. (in my humble opinion) is to measure cranking compression. if you've got zero deck, closed chambers, aluminum heads, you can get a way with quite a bit. the specific numbers escape me, but you know that engine better than we do. if you nailed zero deck, it will be a lot more detonation resistant. personally, i would run 110 in that thing.
karl

Posted By: Porter67

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/02/14 10:57 PM

Im probably wrong here but.

Doesn't the longer duration camshaft events lessen the effective stroke (where both valves are closed)? And this is where the motor hits it powerband?

So with less effective stroke don't you have less air to compress and is why some cars run so well with a lesser duration camshaft because they do have a much longer effective stroke which increases cylinder pressures.

So wouldn't a deciding factor be how much cranking or running cylinder pressure a setup could handle without popping a head gasket.

Small blocks I know are different.

Im a bit confused on the "If" you nailed zero deck? With the proper tools or a good moch up checking each end of the deck then deck the block to achieve a zero deck "with consideration to piston rock" its easy to achieve zero deck.

I see many just taking a block in to have it square decked and cut to the oem deck height which to me is hit or miss at times.

.032 on a true zero deck small block has worked well for me to 7200 area.

Id think a BB has a bit more piston rock and would not go that tight on quench.

Its possible at least on a small block zero deck with a modern domed piston to still hit a good quench # on the pad.

Factor in the al. head aspect and the heat lost couldn't you run 110 or better on track and at times if needed non eth. premium?

And does that seem reasonable as when running on spray, some have a separate cell with the C116 to handle the increased cylinder pressures that nitrous produces

Posted By: AndyF

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/02/14 11:27 PM

Quote:

Quote:

Dynamic compression doesn't mean anything on a race engine. Once the engine is up on the cam you'll have 100% compression if not more due to the ram tuning.

This also means that the crutch of using a "big" cam to bleed off cranking pressure to run lower octane w/ a higher CR only works up through a certain RPM range before the engine's static CR is driving the WOT octane requirement.

That's not my opinion, that's info that came from both a successful cam grinder and an engineer for a race fuel company.

If you run the numbers you'll see that most race engines have dynamic compression ratios down around 9:1 or so. Even Pro Stock engines. They certainly don't run those on pump gas.

The dynamic compression ratio doesn't mean anything when picking a fuel for racing. Once the engine speed gets into a range where the cylinder is getting a full charge of air then you need good gas.

Call any race fuel engineer and ask them or look on their websites. Nobody picks fuel by dynamic compression ratio when racing, all of the charts are static compression ratio.

Posted By: @#$%&*!

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/04/14 05:23 PM

Quote:

The dynamic compression ratio doesn't mean anything when picking a fuel for racing. Once the engine speed gets into a range where the cylinder is getting a full charge of air then you need good gas.

Call any race fuel engineer and ask them or look on their websites. Nobody picks fuel by dynamic compression ratio when racing, all of the charts are static compression ratio.

There's nothing 'dynamic' about the so-called Dynamic Compression ratio. It's just Static compression based on intake valve closing point instead of BDC. A true Dynamic Compression Ratio would include Volumetric Efficiency and the Thermodynamic conditions inside a running engine. Unfortunately, you can't calculate that kind of stuff on a calculator or spread sheet, it's just too complex. However, if you just substitute 100% Volumetric Efficiency and calculate you're right back at the good old (volume@BDC)/(volume@TDC) compression ratio of old. (That's me agreeing with Andy)

It sounds like a good E85 build to me but it seems that the owner has an aversion to Ethanol, at least as a fuel.

Posted By: Pacnorthcuda

Re: Mechanicall VS dynamic compressio ratio and fuel ? - 10/04/14 05:36 PM

Quote:

Quote:

The dynamic compression ratio doesn't mean anything when picking a fuel for racing. Once the engine speed gets into a range where the cylinder is getting a full charge of air then you need good gas.

Call any race fuel engineer and ask them or look on their websites. Nobody picks fuel by dynamic compression ratio when racing, all of the charts are static compression ratio.

There's nothing 'dynamic' about the so-called Dynamic Compression ratio. It's just Static compression based on intake valve closing point instead of BDC. A true Dynamic Compression Ratio would include Volumetric Efficiency and the Thermodynamic conditions inside a running engine. Unfortunately, you can't calculate that kind of stuff on a calculator or spread sheet, it's just too complex. However, if you just substitute 100% Volumetric Efficiency and calculate you're right back at the good old (volume@BDC)/(volume@TDC) compression ratio of old. (That's me agreeing with Andy)

It sounds like a good E85 build to me but it seems that the owner has an aversion to Ethanol, at least as a fuel.

Agreed. Volumetric efficiency plays a BIG part, and is incredibly complex.

Regarding cranking compression, I am in the middle of a head-replacement project on a 440. Before tear down the cranking compression was 188-200! Much higher than expected.9.25 iron heads. Now going to 10.2 aluminum, with quench, but I am concerned what the cranking psi will be. I was going to go to 10.5 before I did the compression test...