[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.

[steve25]

My point exactly!
It’s still additional heat the gets into block water jacket and then goes up into the heads!

[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

[tom s]

For street diven engines I tend to do 5 in the hole to allow for a later cut if necessary.JMHO,Tom

[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.

[67Fbird]

just some info on PISTONS!!! BEFORE you deck anything make sure you have your PISTONS in hand as they all vary in their compression height number.

IIRC Icon is 1.490
DSS is 1.480

that's .010" difference just in the same XXXX dish piston. Also Head gaskets matter. Seems the Felpro's are about .038-.039 while the Butler's come in at .045-.046 so...again another .007". When putting everything together you have .017" of deck difference JUST N WHAT YOU BUY..

[Formulajones]

If you're serious about an overdrive, Shiftworks sells all the conversions for different factory automatic shifters to work with the new transmissions.

https://shiftworks.com

With the addition of overdrives you can expect about a 30% or more increase in gas mileage.

My wife's camaro went from 17 mpg highway with 3.55's and a muncie to 21 mpg highway with a TKO600 5 speed.

My Chevelle went from a little over 12 mpg highway with 4.10's and a 400 turbo to 16.88 mpg with a 4L80E and this car makes a little over 600hp. It's now a nice cruiser.

It'll completely transform a car. Like Formulabruce has shown it cuts the rpm cruising down to an area you just can't get with a 1:1 ratio at those speeds.
Not just better mileage and less wear on the engine, but less noise in the cabin too.

The biggest drawback is cost.

[Ramairnacho]

Originally Posted by 67Fbird

just some info on PISTONS!!! BEFORE you deck anything make sure you have your PISTONS in hand as they all vary in their compression height number.

IIRC Icon is 1.490
DSS is 1.480

that's .010" difference just in the same XXXX dish piston. Also Head gaskets matter. Seems the Felpro's are about .038-.039 while the Butler's come in at .045-.046 so...again another .007". When putting everything together you have .017" of deck difference JUST N WHAT YOU BUY..

The machine shop owner indicated buying thick forged pistons and he custom dishes them saving money vs. Buying the butler dished pistons. And what's the best gasket set. I'm guessing Butler or will I be ok with a standard Fedorov engine kit. Inalso plan on reusing my freeze plugs or do they go bad too?

[Dragncar]

Originally Posted by Ramairnacho

The machine shop owner indicated buying thick forged pistons and he custom dishes them saving money vs. Buying the butler dished pistons. And what's the best gasket set. I'm guessing Butler or will I be ok with a standard Fedorov engine kit. Inalso plan on reusing my freeze plugs or do they go bad too?

Find a new machine shop. Buy a real piston dished by the people who made them. It will not save you a thing and leaves open variables, making mistakes.
If a machine shop told me that I would be out the front door with my stuff and never come back.

[1969400HO]

your cyl heads will need the pushrod slots cut- milled to allow the 1.65 ratio for clearance, so weight that into the mix.
The bigger ratio is a way to gain lift the cam doesnt have in its profile so just get a bigger lift cam!
Making sure your vlv spring and seat accommodate the lift amount.
Target your numbers and RPM range you want to drive at and seek parts that render most HP from 2000-4500RPM where 95% of driving occurs.
Adjust the cam advance will more Torque range down aiding your zone driving RPM. oF course use clay -verify the piston clearance to vlv as always it's a few hours to prevent epic issues.
You can get some compression adjustment with certain head gaskets. It slips memory there is a company that specializes in cyl head gasket are any thickness you want them or composition ratio based on their gauge.
From 020 to 060 and anything in between. Butler has a few various kinds.
#13 heads like most all Pontiac heads work rather well with a certain cam and will make you 400-450HP and 500TQ..That's plenty to move your rig.
That is a low 12 second 1/4 mile performance type 3500# car. Once you have the HP, you will find weak mechanical links as typical occurs so Beware.

[unruhjonny]

there is some very good advice here already.

Please take the following points (covered earlier) to heart:
1) actual compression is well below advertised;
I have always run 91 octane pump gas in my engines - all were 1970 400's with #12 heads;
Little did I know that as I bought my car it had a puny "A" cam in it, so even though it was a smaller duration cam, it would have actually increased static compression.

2) when these guys say that you should have pistons first before machining the block, take this to heart - these details on a build, weather you're looking to eek out max performance, or just be able to run ping-frre on pump gas with a "10:1" engine, it's all in the details.
- piston installed height
- head gasket (compressed) thickness
- how far 'down the hole' our pistons should sit
are all of paramount importance.

3) Skip the 1.65 ratio rockers;
If you are already contemplating an aftermarket cam, get your additional lift from the cam.

4) if at the end of the day you are worried about compression after ingesting point 1 & 2, consider running the incorrect #13 heads;
I have run the 1969 model year version of those heads on a 400 and drove to California and such with zero issue.

With my YZ engine;
- I did not zero deck
- I ran trw forged slugs (0.030" & 0.040" over)
- I used Fel-Pro gaskets
- I ran the "A" cam (0.030") & "H" cam (0.040")

I tend to be of the opinion that the closer to factory, the more worry free you might be.

[jerry455]

I got a set of Diamond forged pistons for my 455. You can have them make the compression height whatever you want them. I had the deck squared. I did not take it all the way down. Pistons are within .002-..003 in the hole, which is pretty close to what I wanted. My buddy, who is building my engine, has had a lot of success with Diamond over the years. It is convenient that they are only about 10 miles from me so I could pick them up.

[Formulabruce]

KB pistons you can BUY with 6-15CC dish.They are CAST, WITH Silicon. They put WAY less wear on your block.
and they LAST. They typically weigh less than Forged as well, which is good.
>>Although not as strong as forged pistons, hypereutectic pistons offer increased strength that raises piston reliability at a lower cost. Hypereutectic pistons also don't experience as much thermal expansion as forged units, which means they can be mated to their respective cylinder bores with tighter tolerances.They have more Silicon than Stock cast piston
which decreases cylinder wear.
DONT , DO NOT, have the engine built ( Again) and have it sit for years ( body shops are booked way up right now)
A replacement piston with 4 valve reliefs (6-7 cc) , or a small dish)and thicker than stock head gasket will be fine on pump gas

[Ramairnacho]

Originally Posted by Formulabruce

KB pistons you can BUY with 6-15CC dish.They are CAST, WITH Silicon. They put WAY less wear on your block.
and they LAST. They typically weigh less than Forged as well, which is good.
>>Although not as strong as forged pistons, hypereutectic pistons offer increased strength that raises piston reliability at a lower cost. Hypereutectic pistons also don't experience as much thermal expansion as forged units, which means they can be mated to their respective cylinder bores with tighter tolerances.They have more Silicon than Stock cast piston
which decreases cylinder wear.
DONT , DO NOT, have the engine built ( Again) and have it sit for years ( body shops are booked way up right now)
A replacement piston with 4 valve reliefs (6-7 cc) , or a small dish)and thicker than stock head gasket will be fine on pump gas

I thank all the input on this. Im.a few months out at least. I want to have all parts ready to go when I pull engine and inspect in my garage. I will be taking pics and asking more advice and do realize it may be a can of wor.ms, not sure how or if any monster damaged the inside of block. I'll probably remove intake and heads while it's on the stand. I just let my project sit too long.