moneyman380

What are the cons of running a low compression n/a 350, like say 8.1-8.7 range ?

rx7speed

engine isn't going to be as efficient. so low power, lower gas mileage, going to be sluggish. if you have a good sized cam it's going to be even worse.

try driving a late 70's domestic with a 350.
most of them seemed to have 7.5-8.5 compression and had no *****

84z28350

In that case try driving my car, even with all the bolt ons and goodies i swear my sisters v6 corsica has more ***** . My motor is a early 70s which means big 76cc smogger heads and -13cc dished pistons so its way down in the 8:1 or even lower.

With that engine a th-350c, 4.11s i get horrible gas mileage and for power wise its pretty bad for a 350 with that many bolt ons.

moneyman380

What if i bolt up some 58cc heads on the -21.1cc dished pistons to bump the c/r up to 9.72. Will using the smaller chamber heads hurt anything ?

rx7speed

smaller chamber head is more preferable to running a larger chamber head and then running dome pistons.
dome pistons impeade the flame travel causing inefficient running and higher chaces of detonation vs doing a smaller chamber and slight dish when running the same compression.

ever notice that on the old heads the combustion chamber was almost one bit area taking up almost the whole area of the bore where now the combustion chambers instead have a nice little flat spot on there? even when comparing heads of the same volume. at least it seems that way to me.

Vader

Been there:



See those dished pistons? They're in an L81 ('81 Corvette). It came with a whopping 8.2:1 or something like that, with 76cc smogger heads (462624 castings). It now has 14014416 castings with 1.94" intakes, unshrouding, porting from 161/56cc to 177/64cc ports, screwed studs, and a decent cam. It now has closer to 9.4:1 static compression. It runs notably better. You can probably expect similar results with similar treatment.

Get the heads ahead of time, fit the valves, do the porting, accumulate the cam, lifters, timing set, gaskets, and supplies. You should be able to perform a clean swap in a weekend.

rx7speed

since mr vader knows a lot and is a little older then me (no offence) he might know. but when did they start getting away from the heads that where more round in shape for the combustion chamber and instead switching to a more tringlish shape like the one you posted?

AlkyIROC

Racers like high compression because it makes torque. 2 identical engines except one is high and the other has low compression, the high compression will make a lot more torque. Higher torque will also make more HP.

Higher compression also has other factors. You need a fuel with enough octane to resist detonation. Higher altitude driving can run higher compressions with less octane.

On the other end of the scale, demo derby cars are built with very low compression. Normally in the 5-6:1 range. They do this because although the engine will still run, it doesn't produce a lot of heat with very low compression. If they lose their cooling system, the engine can still operate longer without coolant. After a race and the car's destroyed, they yank the engine out for the next car to use.

Any power adder engine is also built with lower compression. The boost increase from a blower or turbo raises the compression ratio. Top fuel engines are only 6:1 compression but the blower produces enough boost that they're actually closer to 16:1 and there's enough fuel dumped into the cylinders that they're almost to the stage of hydrolocking.

The engine in my 91 454SS truck came from the factory with 7.9:1 pistons. They did this to pass emission laws. It runs happily on 87 octane fuel and still produces about 300 hp and 400 pounds of torque.

Vader

The chamber shape varied across several years. Different engines got different heads. 1984-85 had a wide variety of heads designs. Carbed engines in trucks had 76cc chambers that had nearly 80% of the bore covered by an open chamber. The later design heads had reduced that slightly, probably closer to 70%, but the big differences were in the depth of the chamber.

The more significant chamber design changes didn't really occur until the LT1 heads were produced (not counting the LT5, or course).

Vader

Those were some photos I accumulated. I think all the casting numbers are right. Sorry for the grainy images, but I wasn't sure if I could compress the jpg enough to make the 100K attachment limit. Just made it!

Damon

You can use the 993 photo for an earlier 882 head, too. Chambers are identical. The model of inefficiency.

moneyman380

So i will be ok running small 49cc heads to bump the compression up on the dished -21.1cc pistons ?

Red Devil

Low compression? Perfect excuse for a power adder.

moneyman380

Thats what i originaly bought the pistons for, but changed back to n/a.

HalfInchWrench

good stuff!!!

CraZ-28

You can still run a power adder. That's what I'm working on with my new motor. I'm just trying to keep the cr in the 9.5 and lower range if possilbe. If you can do that, you should be fine as long as the motor can handle it.

MaNiAk86

iTrader: (2)

i am planning a build up of a 350 that has slightly dished pistons and i wanted some smaller cc chamber heads to raise compression. What's a safe compression ratio for a small 125 shot of nitrous?


Do you know what the cc sizes on all those heads are and what is the equation for determining the final compression ratio?

MaNiAk86

iTrader: (2)

anyone?

84z28350

check out this link for a CR calculator

http://www3.sympatico.ca/peter.matte...lc/calcupg.htm