Max cranking psig, DCR for pump gas?

[juuhanaa]

Fiat/Lancia TC?

[Stan Weiss]

Stan, thanks for that interesting graph.

That 66 ABDC IVC seems just about right for the stock cam.

He could retard the intake cam one tooth, which is 17 crank degrees for 83 IVC, which is what some hotter street cams run about.

Any chance I could ge the same chart for my engine?

Same bore, 84mm. Stroke: 79.2mm, Roc C/C: 136mm, cranking compression 185psig.

Yes here it is. Also If your friend is 10.5:1 CR how his PSI will change as he retards the cam.

Stan

Nut124_CGP_IVC_2.gif

Nut124_IVC_CGP.gif

[Nut124]

Stan, thanks for those curves. Very interesting.

My IVC is 75ABDC. I degreed the cams very carefully. During the build, I buretted the head and piston volumes and calculated static CR to be 10.5:1.

If I read your graph correctly, it shows 10.6:1 at 75ABDC, which is very close to my 10.5:1 calculation.

[jeff swisher]

I have always checked cranking pressures on engines and found it is not an indicator of what octane is needed.

1982 Ford 200" Inline 6 carbureted cranked 210 psi his was with 300,000+ miles on the engine and it ran best on 87 octane.
165 psi Olds 350 ran best with 91.
Brand new 1989 Dodge truck 318" 195 psi.
11.9 compression 350" with 270H comp cam 224@ .050 would run on 87 but liked 92 better.

I have a buddy with a 383 stroker with 11:1 compression and it really needs 92 or better but he pulled timing back to 22 total and runs 87 octane and it runs in the low 11's in the 1/4 in a chevy Luv truck with a tunnelram.

Just depends on how you tune it.

[gunt]

so on say every stock bike or vvt engine , a rebuild is stated at 180 compression , 225 is std , there's so many variables ,

[rebelrouser]

I have always checked cranking pressures on engines and found it is not an indicator of what octane is needed.

1982 Ford 200" Inline 6 carbureted cranked 210 psi his was with 300,000+ miles on the engine and it ran best on 87 octane.
165 psi Olds 350 ran best with 91.
Brand new 1989 Dodge truck 318" 195 psi.
11.9 compression 350" with 270H comp cam 224@ .050 would run on 87 but liked 92 better.

I have a buddy with a 383 stroker with 11:1 compression and it really needs 92 or better but he pulled timing back to 22 total and runs 87 octane and it runs in the low 11's in the 1/4 in a chevy Luv truck with a tunnelram.

Just depends on how you tune it.

It is not the only indicator, but it is one of them. And when you say tune, I would say the timing curve is the main thing to look at. A lean air fuel ratio would also contribute to spark knock. If you look at the data on a newer engine that has a knock sensor and see what the computer is actually doing with the timing it will prove out the timing curve. If you just retard the total timing, you can get by with low octane fuel, but you are leaving HP on the table.

[juuhanaa]

Op could open up a little what engine he is working with. If its a TC engine what i think it is... I could give a advise lol

Screenshot_20221122-193455.jpg

-juhana

[Nut124]

Op could open up a little what engine he is working with. If its a TC engine what i think it is... I could give a advise lol

Screenshot_20221122-193455.jpg

-juhana

Juhana, yes, it is a Fiat twincam. I have posted about mine here quite a bit when I rebuilt in 2-3 years ago.

[RDY4WAR]

I went rounds with someone on this topic this morning. He's having a 383ci SBC built, and his engine builder has been trying to talk him into a set of 64cc heads which, when combined with flat top pistons and .041 quench, gives 10.82 static compression. The builder stated he could use a bigger cam to bleed off cylinder pressure to run on pump gas.

The guy was asking if that's a logical move as he's never had an engine on pump gas with that much static compression. He also stated the engine is going in a casual cruiser that'll rarely, if ever, see north of 4500 rpm and paired with a TH350 and 2.73 gear. He wants to keep all the low end torque he can with good driveability. I ran some simulations for him in Engine Analyzer Pro.

383ci SBC
4.03" bore x 3.75" stroke
Promaxx Freedom series 185cc aluminum heads (comes in 64cc or 72cc chambers)
Performer RPM dual plane intake
750 VS carb
Zero deck w/ -7cc flat top pistons
4.100" x .041" head gaskets

The first simulation I did is in red in the graph below. It's using the 72cc heads for a static compression ratio of 9.92 and a more appropriate cam for the use of the engine.

72cc chambers
SCR = 9.92
DCR = 8.14
HR cam
- 270 / 276 @ .006"
- 220 / 224 @ .050"
- .536" / .530" lift w/ 1.5 rockers
- 111 +3 LSA

The second simulation is in black. That's using the smaller 64cc chambers for 10.82 static compression and a larger cam than ideal to bleed off the dynamic pressure.

64cc chambers
SCR = 10.82
DCR = 8.18
HR cam
- 286 / 292 @ .006"
- 236 / 240 @ .050"
- .543" / .539" lift w/ 1.5 rockers
- 113 +3 LSA

The higher compression engine is compromised quite greatly to give better manifold vacuum for power brakes while also bleeding enough pressure off to get a similar DCR for pump gas. The result was a -8 lb-ft average torque loss from 1500-4500 RPM and -17 lb-ft average torque loss from 1500-3000 RPM where that engine will spend 95% of its life. The higher SCR / bigger cam combo would likely make +20-30 HP at 5500-6000 RPM, but that's irrelevant if the engine is never spun that high. If it was a street/strip engine with a 3000-3200 stall converter and 3.73-4.10 gear, then sure. That wasn't the case here though.

I think the effort to push DCR to the very limit is more harmful than helpful in some applications. Take a long look at how the engine will be used before going down that road.

383ci low rpm build low vs high comp.jpeg

[Tom68]

I went rounds with someone on this topic this morning. He's having a 383ci SBC built, and his engine builder has been trying to talk him into a set of 64cc heads which, when combined with flat top pistons and .041 quench, gives 10.82 static compression. The builder stated he could use a bigger cam to bleed off cylinder pressure to run on pump gas.

Higher comp will probably get better mileage.

Higher torque is higher cylinder pressure, low comp motor may actually be more detonation sensitive in the rev range that he spends most of his time. Might need a bit more cam in that one, the extra top end will be taken care of by the enthalpy of vaporization of the greater fuel flow.

[F-BIRD'88]

All the sbc engines that have high compression also employ knock detection that dials up/ down the spark advznce as required depending on operating conditions at the moment.
A carb and a diz with mechanical and vacuum advance is a very different thing.

All the LS engines same same.. You'll note the cast iron block versions All have lower cr. Even with active knock detection and spark control.
With a carb on premimum pump gas a SBC get beyond 10.50:1 and your chance of detonation damage $$$
increases exponentiually.
Most end up dialing back the spark advance to let it live while making wide claims.
Your money your car. Depends on how far you want to go fast. $$$.

[RDY4WAR]

I went rounds with someone on this topic this morning. He's having a 383ci SBC built, and his engine builder has been trying to talk him into a set of 64cc heads which, when combined with flat top pistons and .041 quench, gives 10.82 static compression. The builder stated he could use a bigger cam to bleed off cylinder pressure to run on pump gas.

Higher comp will probably get better mileage.

Higher torque is higher cylinder pressure, low comp motor may actually be more detonation sensitive in the rev range that he spends most of his time. Might need a bit more cam in that one, the extra top end will be taken care of by the enthalpy of vaporization of the greater fuel flow.

EAP seems to think otherwise. It's showing higher peak cylinder pressure, and a slightly higher knock index, with the 10.82 SCR and bigger cam despite being down on torque. It's showing higher volumetric efficiency, thermal efficiency, and mechanical efficiency with the lower SCR and smaller cam.

383ci low rpm build low vs high comp pressure and knock index.jpeg

383ci low rpm build low vs high comp pressure graph.jpeg