I know it's kind of an old argument as to whether you get more power with a smaller combustion chamber and deeper dish but I was curious of the MPG perspective. I hear you can make decent power with this kind of set up but you get better fuel mileage compared to a similar compression ratio with flat pistons and big chambers. Has anyone been able to test this IRL? (sorry if there's a related thread but I didn't find it.)

 

I run a 32cc chamber fastburn style head with 25cc dished pistons, .010 popup for 9.8:1 SCR. On the highway i run over 40 degrees of advance and run around a 16:1 AFR. 89 octane gas with no knock or overheating issues.

All this equates to between 30 and 35 MPG highway. City driving still sucks, might be better with the auto trans this year though.


as far as power, 214 CI making 320 HP is pretty decent IMO...

 

no offense but anybody with a v8 want to chime in?

 

an engine is an engine, small combustion chambers + a dish makes for a better burn.


My cammed 327 with flat tops and '882 heads is lucky to get 15 on the highway with about 32 degrees advance. The carb does need some fine tuning but i doubt it would do much better than 20.

 

Well the dished pistons and small cc head promote a better flame travel which means a more complete quicker burn. Does it mean two engines of equal cam and compression ratio with a different configuration of heads and pistons both 9:1 ratio the dished piston engine will gain 3 more mpg? No but it might get you an extra mile or 2 out of a tank full of gas.

 

The factory has researched this continuously for years in search of better power and better fuel mileage. A quick look at what they have done is a good study. Turbulence promotes complete combustion (witness: swirl-port heads).
But the best configuration for mileage sucks at performance so it's a compromise. The best configuration for both can be found in the latest design engines like LSx's. They experimented with 55cc chambers in the LT heads for mileage. Too small of a combustion chamber limits breathing ability (power) because you can't put 2.02 valves in a 57 cc chamber without shrouding it. The LS heads use a 65cc chamber and add dish to make the compression ratio right with different displacements (4.8, 5.3, 6.0L). They also manage to get decent fuel mileage because the turbulence is created by the shape of the intake runners.
This is why the difference in the designs of the gen1, II, and gen III engines.
So for our gen1 engines the best mileage is the small chamber swirl-port heads and a dish (but eeeuuuw!).
"Decent power" is a relative term...

 

k so when building an engine get 65cc heads and dished pistons like in the lsx series. purple82TA: sorry if i sounded like a jerk i was just hoping for some info like yours (cc and dish size, compression ratio, and mpg) in a v8. but i have to admit you're right. 320 hp + 30ish mpg is definitely a good thing. that's about what the new direct injection v6s in 5th gens are reporting. By decent power i was thinking 300-400 hp and similar torque.

 

lol.. no prob.


Remember to watch your quench as well, perfect quench is .040". If you can get that with a dished piston and a good head chamber you'll be golden. Don't forget about port flow as well though. It's gotta breathe to make power.

 

For my build I used:
350 stroked 388
"D" 16cc dished forged alum 4.06" x 1.125" pistons
Compcams XE262
GMPP fasburn alum heads 62cc 2.00"/1.55" int/exh
.040" head gasket
I run 87 octane.

 

any ideas on mpgs and hp you're getting from that?

 

Stock Vortec castings 64cc.
Flat top pistons .025" below deck.
.026" head gasket.
XR276HR cam w/1.6 rr.
Chassis dynoed at 350 chp with single exhaust.
Best highway mileage of 25mpg (cdn) 22mpg (usg)
12.7 @ 105 weighing 3700lb

 

That is not really true. Depends on the chamber design. I run 60cc chambers with 2.05 valves, and the valves are not shrouded. If the chamber is shaped properly the flow curtain around the valve will work in your favor. It's only the really old castings with huge chamber walls, and the chinese procomp heads that need unshrouding work.


-- Joe

 

Not at the moment, As soon as I got it running right, I tore into the interior. I'm installing sound/heat mat, new black carpet, 95' seat set, and A/C. I'll posted it when I get around to it, but it will be a while.

 

chp? is that canadian hp?

 

I have some 305 heads (081) that I am porting and am going to put 1.94 intake valves in, as well, for a 355 build. I have not cc'd the chambers yet, but let's assume they are right about 58cc's (59cc after some unshrouding). I am looking at dished pistons around 13 ~ 16cc to keep compression in the mid 9's.

This build is not for racing, just a nice daily driver for cruising (occasional red light to red light romp) and hopefully decent mileage.
Any advice on this?

Deck clearance - 0.025" (stock)
Compressed gasket thickness - 0.015" (stock)


CG

 

Jeez.. I don't know that you are going to be happy with that. The chamber and compression is one thing, but the size of the intake and exhaust runner is going to play a huge role in how the car responds on the street.

I always go for the smallest runner with the highest flow. Problem with those heads is your going to have a small runner that barely flows enough.

So while, your building a conservative street machine with moderate compression for pump gas, my feeling is it's going to be very RPM limited, might be snappy off idle if you cam it right, but I think it's going to have horrible mid range and top end.

-- Joe