[steve25]

In a nut shell that effect / action is due to the expansion ratio.
We are all too use to looking at the compression ratio and the pistons travel up the bore.
The thermo dynamics involved in the expansion ratio looks at residual heat left over once the crank has rotated passed the point where it can still use that heat to power it.

A high compression motor burns its inhaled air and fuel much faster then a low compression one and Is why less total advance is needed as compared to a low compression motor.

The post last week of the W72 400 motor build and the 42 degrees of timing that was employed is a clear cut example of this!

[Ramairnacho]

Originally Posted by steve25

In a nut shell that effect / action is due to the expansion ratio.
We are all too use to looking at the compression ratio and the pistons travel up the bore.
The thermo dynamics involved in the expansion ratio looks at residual heat left over once the crank has rotated passed the point where it can still use that heat to power it.

A high compression motor burns its inhaled air and fuel much faster then a low compression one and Is why less total advance is needed as compared to a low compression motor.

The post last week of the W72 400 motor build and the 42 degrees of timing that was employed is a clear cut example of this!

Thanks for the good info. I need cooling , timing , dish,forged rods,heads build right, stainless values,new guides, roller cam, brass plugs,distributor recurved, rear end should be 308 or 331 ,and need to figure out Trans and tires size too. I'm slowly getting there. I n not sure why everyone knocks the 273 252 or 242 gears they where found in Trans am and formulas in the late 70 s and with those gears I think.gas milage is better or am I wrong? But at end of day I might slap some 331 posi and scream down the freeway at normal speed limit getting 10 miles to the gal. Not!

[Formulabruce]

Originally Posted by Ramairnacho

I'm not sure why everyone knocks the 273 252 or 242 gears they where found in Trans am and formulas in the late 70 s and with those gears I think.gas mileage is better or am I wrong? But at end of day I might slap some 331 posi and scream down the freeway at normal speed limit getting 10 miles to the gal. Not!

I do Not knock these ratios, and I own a very early 1971 2.73 rear, and a late 76 2.41 .
What OPH said was pretty much true though. A 2 series ( 8.75 ring gear) "Safe-T-Track" or "Posi"carrier for a 68-70 12 Bolt Chevy /Pontiac is a rare piece. Not reproduced. The open ones are around though.
In a 10 bolt ( '71 -81 for this conversation) you can use a 3 series carrier for a 2.73
gear ( have built a few myself), but NOT for a 2.56 or 2.41 . Also New carriers for these 8.5 rears are all over and some are inexpensive and have carbon fiber clutches vs steel.
If you want to use your TH400 and cruising and economy is paramount,
Call Franks Pontiac and grab a 2.41 Posi from a Firebird. With 15 inch 25 inch tall tires youll be at around 2500 at 72 mph ( a mere guess without any wind resistance). There are calculators to try online.
I happen to like Overdrive, and this is a pic of a car of mine with 3.42 gears,
26 inch tall tires ( 225/70/15 -- same as in 1979) and .67 OD .. taken on I-95 --- Th2004R transmission

[Formulajones]

3.31's aren't all that terrible. I daily mine with that gear and get 17 mpg with it highway. I buzz along at 70-ish, sometimes 75 mph. I keep a 28" tire on it which helps. I think that's where people get in trouble as most cars I see guys want to run bigger 17" wheels and can't get more than a 25-26" tire on it.

If I had to run that short of a tire I wouldn't like the 3.31's so much.

Like lust4speed said, overdrives are really the best scenario anymore. You get your affective gear ratios down low for easy starts AND your low rpm cruising on the highway. It's the best of both and why they are so popular.

[Ramairnacho]

Originally Posted by Formulabruce

I do Not knock these ratios, and I own a very early 1971 2.73 rear, and a late 76 2.41 .
What OPH said was pretty much true though. A 2 series ( 8.75 ring gear) "Safe-T-Track" or "Posi"carrier for a 68-70 12 Bolt Chevy /Pontiac is a rare piece. Not reproduced. The open ones are around though.
In a 10 bolt ( '71 -81 for this conversation) you can use a 3 series carrier for a 2.73
gear ( have built a few myself), but NOT for a 2.56 or 2.41 . Also New carriers for these 8.5 rears are all over and some are inexpensive and have carbon fiber clutches vs steel.
If you want to use your TH400 and cruising and economy is paramount,
Call Franks Pontiac and grab a 2.41 Posi from a Firebird. With 15 inch 25 inch tall tires youll be at around 2500 at 72 mph ( a mere guess without any wind resistance). There are calculators to try online.
I happen to like Overdrive, and this is a pic of a car of mine with 3.42 gears,
26 inch tall tires ( 225/70/15 -- same as in 1979) and .67 OD .. taken on I-95 --- Th2004R transmission

I like those numbers . You guys are good. It is simply difficult to get that with a turbo 400 but with the new Trans I can do that. Hmm what about the shifter is it the same? Or did you put the 82 through 92 shifter in it. And is a 700 r4 stronger than the 200 r4. Happy new year. Doing this is going to side track my stock build but those cruise numbers are very good.

[Ramairnacho]

Originally Posted by Formulabruce

I do Not knock these ratios, and I own a very early 1971 2.73 rear, and a late 76 2.41 .
What OPH said was pretty much true though. A 2 series ( 8.75 ring gear) "Safe-T-Track" or "Posi"carrier for a 68-70 12 Bolt Chevy /Pontiac is a rare piece. Not reproduced. The open ones are around though.
In a 10 bolt ( '71 -81 for this conversation) you can use a 3 series carrier for a 2.73
gear ( have built a few myself), but NOT for a 2.56 or 2.41 . Also New carriers for these 8.5 rears are all over and some are inexpensive and have carbon fiber clutches vs steel.
If you want to use your TH400 and cruising and economy is paramount,
Call Franks Pontiac and grab a 2.41 Posi from a Firebird. With 15 inch 25 inch tall tires youll be at around 2500 at 72 mph ( a mere guess without any wind resistance). There are calculators to try online.
I happen to like Overdrive, and this is a pic of a car of mine with 3.42 gears,
26 inch tall tires ( 225/70/15 -- same as in 1979) and .67 OD .. taken on I-95 --- Th2004R transmission

I like those numbers . You guys are good. It is simply difficult to get that with a turbo 400 but with the new Trans I can do that. Hmm what about the shifter is it the same? Or did you put the 82 through 92 shifter in it. And is a 700 r4 stronger than the 200 r4. Happy new year. Doing this is going to side track my stock build but those cruise numbers are very good. I bet it's good gas milage and less wear on engine. Do you need a computer box with that tranny. I'm very curious.

[lust4speed]

Originally Posted by steve25

...The post last week of the W72 400 motor build and the 42 degrees of timing that was employed is a clear cut example of this!

Agree with almost all of your analysis. The slower burn is why we start the combustion process earlier so it will be done by the time piston reaches BDC. If we didn't bump timing then the combustion process would still be going on at the bottom of the stroke. With proper timing both high and lower compression should have the optimum burn ratios and be done at the same position of the crank.

Most "hot" engines are those where the compression ratio is too high and timing is retarded in a band-aid approach to subdue detonation, and this process does have the process starting late and finishing late with a lot of heat still being released at BDC.

RAMAIRNACHO, our builds are always compromises. 2.41 gears are excellent on the highway, but starting off is like taking off in 2nd gear compared to a 3.31. With a 1:1 final drive ratio decisions have to be made, and why many of us end up with an overdrive trans.

[Formulabruce]

Originally Posted by steve25

In a nut shell that effect / action is due to the expansion ratio.
We are all too use to looking at the compression ratio and the pistons travel up the bore.
The thermo dynamics involved in the expansion ratio looks at residual heat left over once the crank has rotated passed the point where it can still use that heat to power it.

A high compression motor burns its inhaled air and fuel much faster then a low compression one and Is why less total advance is needed as compared to a low compression motor.

The post last week of the W72 400 motor build and the 42 degrees of timing that was employed is a clear cut example of this!

While your point is fact, and well taken, that left over heat is just Block heat and very easily dissipated into the cooling system. Lower CR engines just dont build the Head heat. Higher compression means more HEAD heat in nearly Every situation. More calories, more energy, power
produced.