Pump gas or race fuel?

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Jonnylightening

Jonnylightening

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I'm building a 408 for my 4 door Aspen. The other night I was playing around with a compression calculator. My static compression came to 11.82:1 but with my cam my dynamic compression came to 7.42:1. Soooo would I technically need to run race fuel or would I be safe to run 93 pump gas?
 
Jonnylightening said:
I'm building a 408 for my 4 door Aspen. The other night I was playing around with a compression calculator. My static compression came to 11.82:1 but with my cam my dynamic compression came to 7.42:1. Soooo would I technically need to run race fuel or would I be safe to run 93 pump gas?
Click to expand...

If that dynamic compression figure is correct, it should run on 87.
 
RustyRatRod said:
If that dynamic compression figure is correct, it should run on 87.
Click to expand...
Thats what the calculator said. My cam is a howards cams roller. Here's the specs

20210408_230319.jpg
 
ok. If it's accurate, you're in good shape.
 
Aluminum heads are much more forgiving than iron heads also a quench design will help considerably over a open chamber. So aluminum quench design yeah you should be good open, chamber iron or even Magnum style maybe not so much.
 
I ran a 12 1/2 to 1 piston in my iron head closed chamber max wedge. Since i didnt want to deck a collector block, the measured compression ratio was only 11 1/2, and i used a considerably bigger solid roller than yours, but yes, i ran it on pump premium.
Now that i have access to 100LL, I would have run a 50/50 mix..... or maybe just the avgas.
 
yellow rose said:
There is more to running pump gas with compression ratios like that then just cam timing. You can do it, but you have to have everything in order to do it. And, you have to be able to tune for it.
Click to expand...

His dynamic is 7.4, it'll run on 87.
 
Jonnylightening said:
I'm building a 408 for my 4 door Aspen. The other night I was playing around with a compression calculator. My static compression came to 11.82:1 but with my cam my dynamic compression came to 7.42:1. Soooo would I technically need to run race fuel or would I be safe to run 93 pump gas?
Click to expand...

You did not specify your elevation, and I do not see what heads you are running, but,
with an Ica of 72*, and at 1000 ft; I got this off the Wallace
Static compression ratio of 11.82:1.
Effective stroke is 2.92 inches.
Your dynamic compression ratio is 8.90:1 .
Your dynamic cranking pressure is 182.42 PSI.
V/P (Volume to Pressure Index) is 162

So IDK how you got your numbers.

To get to 7.42 dynamic I have to fudge the numbers to
Ica of 76* and Scr to 10.22
Static compression ratio of 10.22:1.
Effective stroke is 2.80 inches.
Your dynamic compression ratio is 7.45:1 .
Your dynamic cranking pressure is 143.69 PSI.
V/P (Volume to Pressure Index) is 123
123 is pretty weak

For a 408 to have 11.42Scr, the total chamber size needs to be 80.2cc.
For 10.22, it is 90.64
 
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AJ/FormS said:
You did not specify your elevation, and I do not see what heads you are running, but, with an Ica of 72*, and at 1000 ft; I got this off the Wallace
Static compression ratio of 11.82:1.
Effective stroke is 2.92 inches.
Your dynamic compression ratio is 8.90:1 .
Your dynamic cranking pressure is 182.42 PSI.
V/P (Volume to Pressure Index) is 162

So IDK how you got your numbers.

To get to 7.42 dynamic I have to fudge the numbers to
Ica of 76* and Scr to 10.22
Static compression ratio of 10.22:1.
Effective stroke is 2.80 inches.
Your dynamic compression ratio is 7.45:1 .
Your dynamic cranking pressure is 143.69 PSI.
V/P (Volume to Pressure Index) is 123
123 is pretty weak

For a 408 to have 11.42Scr, the total chamber size needs to be 80.2cc.
For 10.22, it is 90.64
Click to expand...
I'll give the Wallace calculator a try. The one I used may not be accurate???
 
It kindof depends on what parameters that calculator is assuming and/or what numbers you actually input-ed. You know what they say; garbage in=garbage out.
I calculated the Ica from the 288 advertised, installed at 108.
I used 4.02 x4.00 for engine size, and 6.123 for the rod length, and
used 1000 ft elevation. But I noticed that the elevation-input is not working correctly today, giving the same Dcrs for any elevation from zero to 3000.
That is a nice cam-size for a street combo, so I hope you get it sorted. Some guys here on FABO have reported running 200psi, still on pumpgas, with alloy heads. I myself have run my Eddie-headed 367 at up to 185psi, still on 87E10.
Yellowrose is, IIRC, at 170 on iron heads, an excellent number to be sure.
The 20psi or so difference, between the two heads , is not actually a serious benefit to horsepower production, on the street. The alloy heads NEED some extra pressure, just to keep up to the iron, because of their propensity to steal heat out of the chambers. Heat is power. But, the alloy heads are better at NOT producing detonation as easily as are iron. And of course, are easier to port and such. Plus, a good chamber size and shape, makes power on less ignition timing.

Post up your volumetric specs, and we can help you with the math;
 
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I used wallace too, and with the numbers i put in, i got 9.02 dynamic, 189 compression pressure.
I think you will need something better than pump 93.
(My solid roller was 30° bigger at .050 than yours.)
 
yellow rose said:
I saw his numbers...not sure I’m buying them. If that’s the correct numbers it will run on 87. That seems like a low DCR for his CR and cam timing.
Click to expand...

Did you see my quote? "If the dynamic is correct"
 
It might not detonate at low rpm, detonation can happen at any rpm and you won't hear it.
 
33IMP said:
I did.
I think there was justifyable scepticism at that number. I just dont see an almost 12 to 1 engine with an under 300 advertised cam, and iron heads, running on pump gas.
E 85, oh yeah!
Click to expand...

Right. I've just been through all the calculations for TWO engines very recently and I thought his numbers were too big a spread. Mine was 9.98 static and 7.92 dynamic. That was the milled head slant six I am installing now and the closed chamber head engine on the stand came out to almost the same. I was thinkin over 11:1 satic and 7.4 dynamic was a stretch. Possible I guess, but questionable.
 
I put in the wrong head gasket thickness in the first go round but now here's my numbers. Also I'm at around 2000' elevation but If I go to the track its right around 1000'

20210409_220052.jpg


20210409_220109.jpg


20210409_220122.jpg
 
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@Jonnylightening, first question, dome or dished piston? If a dish, how sure are you of the volume of the dish.
When i ran your cam through wallace, it said your intake close was at 70° atdc. Four degrees probably makes a difference. Have you degreed your cam? Is that where the (almost) 74° comes from?
Whoa, wait a minute, back up the truck! What the hell is intake closing point AT FIFTY THOUSANDTHS ? Is it closed, or open at .050?
I think they mean at what degree is your valve .050 away from closing, tho that is NOT what they said. There is your error, you input your ACTUAL closing point. Probably thirty or so degrees later than the number they ask for.
 
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33IMP said:
@Jonnylightening, first question, dome or dished piston? If a dish, how sure are you of the volume of the dish.
When i ran your cam through wallace, it said your intake close was at 70° atdc. Four degrees probably makes a difference. Have you degreed your cam? Is that where the (almost) 74° comes from?
Whoa, wait a minute, back up the truck! What the hell is intake closing point AT FIFTY THOUSANDTHS ? Is it closed, or open at .050?
I think they mean at what degree is your valve .050 away from closing, tho that is NOT what they said. There is your error, you input your ACTUAL closing point.
Click to expand...
Engines still down to the block. Piston manufacture said the reliefs(flatops) are 5cc. Ill post up the formula they calculate to find the IVC@ 50
 
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