how much compression is too much?

[AJ/FormS]

The 557 lift cam P4120665 has specs of 296/296/110 and [email protected]
The ICA is about 74*, in at 106
This cam wants a lot of compression.
With an Scr of 11.0, the Dcr comes in at 8.2/164psi
With an Scr of 11.5, the Dcr comes in at 8.5/173psi
With an Scr of 12.0, the Dcr comes in at 8.87/183psi
With an Scr of 12.5, the Dcr comes in at 9.22/192psi
I run my 367 at 10.9/8.9/180 with aluminum heads , and 34* of power-timing and it burns 87E10, has for over 100,000 miles. My timing is delayed to all in at 3400rpm.It idles on 14 to 18. This on .034ish Quench.Street
I have also run 11.3/8.6/176 with a 292/108 cam; also on 87E10.This on .028ish quench.Street
That said, there are some fellows on FABO that have stated they run 400plus CID-engines with aluminum heads, at Dcrs of up to 9.3/200psi, on pump gas. Street.
Dcr is a guideline for determining freedom from detonation,generally below 3500 or so and up to perhaps the torque peak.
I know nothing about running WOT for 10 seconds atta time well above those rpms. I can say with confidence tho, that my engine has never detonated, even running up to 7200rpm, and/or up to 120 mph, and always on 87E10.
.
Jus FYI. Pick your poison.lol

 

[hemichuck]

Thanks for that explanation, very informative. I knew someone had the answer.

 

[AJ/FormS]

With iron heads, you probably want to stay around 165psi; I think that was mentioned.I've heard this bandied about, but it usually refers to open chamber heads and as good as no quench.
With closed chambers and proper quench, It's my opinion that you could run another half a point, still on pump gas, so that would be pushing 175 psi. I haven't tried it so it's just an opinion. If you or anyone else does or is, please let us know how it worked out.
They say aluminum can run a half a point more than iron; they say.
But I think actually, aluminum HAS to run a half point more,just to break even.I also think that when running aluminum and tight-Q, the detonation wall hits much softer.
Like with iron and loose-Q it's easy to hit that wall with just the tiniest bad tune. Either a hair to much timing, or a hair too much compression.It's like a switch; good,good,bad; back off the throtle and good again.
But with aluminum, it seems like; where's the wall?I'm still running 180psi on 87E10 so, I still have lots of room on the Dcr. I would like try 200psi some day. I'm sure I could get it sorted at below 3600. I think I could get it sorted to 4600 or the torque peak. I hope I could get a tune in it from there to shift-rpm without pulling more than a few degrees of timing.
And see,that's another thing; on the street, my combo seems to have the same power, running power-timing from about 32 to 36. Even at 30 It was hard to tell if I lost anything. And the reason this is so,I think, is cause I am geared to hit 65 wound out.
Wound out in First-over( 8.56 final-drive ratio) at 6900. So the engine is not really working hard on account of she's spinning the 295s all the way.With an automatic you would need about 5.56s to do that; 5.56 x 1.45 x 1.05=8.47 final drive. So the engine is barely working.
But if you are gonna race it qtr mile, with slicks,and traction, and once the wind gets up, your engine will be working as hard as it can, and that's when you would have to be careful.I would try it cuz there are still other options. Like anti-detonants, and water-injection, and timing retards. Streeters got it easy.........ok my engine has got it easy,lol.

AJ's ramblings
I ran 4.30s for a while with my GVOD. This was 3 ratios from a stop to 65mph,topping out at 6700 in 2nd. And I even tried 5.13s to get 4 ratios to hit 65 in 2nd-over at 6200; that was crazy! But I had the gears and I was curious,lol.The road-ratios were 15.85-12.36-9.80-7.64, crazy I tell ya. Waste of time too, 15.85 was useless even running 325/50-15Drs. I ended up with 3.55s, a 3.09 low and hitting 65 in second-over, at 6900, I think I said that already,lol. And this is all a 360 needs.

What I learned from all that was this;
1) a street-360 with a manual trans,needs no more than 11/1 starter gear. And 4 ratios from zero to 65 is crazy; the first gear gets to be waaaay too low; 3ratios is about the max. 2 can do it with the right combo using a very close ratio. Do not try it with the A833-od. The only way to get the required 8.4/8.5 road-ratio( to trap at 6800/stick car,is with a 5.03 rear gear.Ever hear of one? and the starter would be 15.51 crazy I tell ya,lol. With an A904, you would need a 5.80 rear; so say 5.38s to hit 65=6600 at the top of second, also crazy.Next best is 3.23s to hit 65@6700 in first. To hit those trap rpms will take a bit of a cam, say a power peak around 6200 . Yes she will be very quick;if you can make it stick. But that's really too much rpm for a streeter. But yours will be up around that, I don't think it will be any lower than 5800.
2) in the 1/8th; 4 ratios will do it; just chose a rear gear that will get you thru the traps at the top of the power curve plus maybe 300 to 400 more rpm,depending on how fast the curve noses over.
3) no more than 5 ratios for the qtr. for a 12 second car
4) the faster your car traps at, the more important it is to keep the engine on the power curve. Trying to get a NA teener-Abody to go 120 is kindof crazy. Even 112 at 3450# will take around 388hp. You might be able to suck it out of her, but the power band is gonna be really really narrow. And 5 gears may not keep her on the pipe to ET all that well. And once you get to 4th gear, all you got is motor and its a long way from say 80mph@4800 to 112@6800. yeah that's a long hard slog for a 318 with a 4 speed. But a 5 spd with an 80% 4-5 split, well now that would be from 90@5440 to 112@6800. The powerband is now 1360 vs 2000 on the previous. Now yer talking.

 

[moper]

If it's a race only car, build with the specific fuel in mind. There's power to be had by running high compression. But if you're like me, pump swill fits the budget. Its imperative whatever your choice you build for it. Otherwise you'll leave power on the table.