I'm having a hard time understanding how a 1970 350 Olds cutlass engine with a 2 BBL carb has less of a CR than a 4BBL engine. Does this mean I would be able to switch from a 2 BBL to a 4 BBL and increase to my CR? I understand the 4 BBL has more HP also.

 

I believe the pistons are different, it's my understanding that they use the same heads.

 

Why would GM do that? I thought the heads and blocks were the same. Why not the pistons.

 

I have the opposite of yours but I'm not complaining: D but yea it does make you wonder

 

My car has a 4 bbl but I was just wondering how they made that call. I have been on other sites and seen factory spec sheets and they all say the same .

 

It was all about money and upgrades on pre ordering the cars

That's what makes tracking the documents so fun and worth$$$$ while

 

Because then the CR would also be the same. The whole point of a lo-po 2bbl motor is economy (ie, lower operating costs). It wouldn't be very economical if it required premium fuel due to the 10:1 CR. THAT'S why Olds lowered the CR on lo-po motors.

 

Except that in the early- and mid-sixties, the 10.25:1 CR was standard for both 2- and 4-bbl carbs, and you had to request the 9:1 ratio if you wanted to use cheap gas with your 2bbl.

In '68 (if I recall), presumably because of low interest in HC 2bbl motors, they simplified the line to offer only LC 2bbls or HC 4bbls.

Of course, with the federal requirement that all engines run on low octane unleaded, all CRs dropped between 1971 and '72, and didn't come back up until 4-valve aluminum heads and computer management became standard in the '90s.

- Eric

 

I wonder how the math pans out on this one; a higher cr requires more expensive gas, but will be more efficient, that is more power produced from a given amount of fuel.
This translates into better mpg for the same driving conditions.
Cheaper at the pump to fill the tank but you will need to refill more often. Could be long term a high cr using high grade fuel works out a better deal. Anyone got some proper test reports on this?.
Don't forget use the lowest grade gas your engine is happy with, using higher grades than your engine needs is wasting money.


Roger.

 

Ya right 28 cents a gallon for regular versus 31 cents for ethyl who cared about fuel economy back in the sixties.It was about performance.

 

The irony being that 2 bbl carbs had way bigger throttle bores than the 4bbl primary bores.

So, putting around town with a 2bbl dumping lots of gas into a gutless engine was kind of a waste of......pretty much everything.

 

Except that in 1967, that was the equivalent of $1.96 and $2.17, which don't sound bad right now, but are still significant.



To answer Roger's question practically, to reliably get today's 93 octane up to yesterday's 100 octane (equivalent to about 95-96 in today's numbers), you need about 1oz of tetraethyl lead additive ("Octane Supreme") per gallon.
At about $150 (including shipping) for 192 ounces, that adds about 78¢ per gallon to to roughly $3.50 a gallon you'd pay today for 93 octane, for a total of about $4.28 a gallon.
Even though you probably pick up 2-3mpg on the highway (I've never tested high vs low compression in the same car), you'd still pay $23.77 to go 100 miles with the octane boost at 18mpg, versus $22.33 to go 100 miles with regular gas at $3.35 a gallon and 15mpg.

- Eric

 

I'm having a hard time understanding how a 1970 350 Olds cutlass engine with a 2 BBL carb has less of a CR than a 4BBL engine. Does this mean I would be able to switch from a 2 BBL to a 4 BBL and increase to my CR? I understand the 4 BBL has more HP also.
=================
I suggest a youtube search on the definition of Compression Ratio. There are only so many things you can vary to obtain different CR's for a given engine. Gasket thickness, head cavity volume, and piston dish or dome volume. Olds chose to use pistons of varying dish volume, leaving the other variables unchanged.



I wonder how the math pans out on this one; a higher cr requires more expensive gas, but will be more efficient, that is more power produced from a given amount of fuel.
This translates into better mpg for the same driving conditions.
Cheaper at the pump to fill the tank but you will need to refill more often. Could be long term a high cr using high grade fuel works out a better deal. Anyone got some proper test reports on this?.
===============
This was NOT my experience on the newer car. I altered the computer to run on 93 octane instead of the 89 it usually requires. MPG plummeted. Perhaps the canned tune varied many other things in the wrong direction. Currently running the factory tune to see how that does, keeping track with actual numbers, not guesswork.



Don't forget use the lowest grade gas your engine is happy with, using higher grades than your engine needs is wasting money.
=================
Correct.
Keeping 93 in the tank instead of the 89 was 10-15c a gallon more, or about $3-4 a week, so I was willing to spend that to do some research.



The irony being that 2 bbl carbs had way bigger throttle bores than the 4bbl primary bores.
==============
The beauty of that is lower pumping losses because the 2-bbl having a larger opening available at typical highway speeds. Less loss to drawing in the air. The Olds engineers were really quite a clever group. Hard to improve on whatever package they devised. E.g., the Turnpike Cruiser. If you have never driven a 2-bbl Olds engine, consider trying it for a while. Great throttle response. Just get an L69 intake and block off the front an rear carb holes.

 

An engines compression ratio is determined by the difference in cylinder volume from the piston at bottom dead centre to the piston at top dead centre. If the difference is 100cc to 10 cc then you have a compression ratio of 10:1, if it is 100cc to 20cc it is 5:1.
None of this is affected by carbs, manifolds, camshaft etc.
Stroke length, rod length, piston height and bowl shape, and head design are factors involved in determining compression ratio.


Roger.

 

Thanks Eric, well thought out and presented as usual.


Roger.

 

I think I got the idea! Thanks everybody,

 

You get back here, Mr. Crank Shaft
We're not done cramming stuff into your head yet!
:-)

 

[QUOTE=Octania;622877]You get back here, Mr. Crank Shaft
We're not done cramming stuff into your head yet!
:-)[/QUOTElol

 

Really thanks for all the info, like I said in the past.I love this site, just hold on to your shorts though, I have a good thread coming up on rocker arms!
Mike

 

If you ever drove a properly tuned Olds 350 2 bbl, you wouldn't be calling it gutless. My first car had such an engine, a lower-compression 1972 model even, and it was a lot of fun.

 

It's all relative. My "lo-po" 350 two barrel ran so good I had a very hard time with the decision to make improvements. - Steven

 

I know I had several, I used to roast the tires with them!

 

Chet's tires roasting on the open road
Jack handle dropping on your toes...

 

Lol

 

Man I had a 1974 olds cutlass with the 350 4bbl and a friend of mine gave me his 4:56 gears out of his old camaro and I used to shift gears when the motor started to pop I think the points couldn't keep up.. And my friends would tell there friends..seems like I was always racing that car..everyone thought I had a big block in it..it was just an over 100,000 mile car motor looked like it sat under water for years..I raced a guy who just built a 350 Pontiac motor and we would stay side by side until I would winde out that second gear and slowly pull away before I put it in third..He tried several times but could not out run that old stock motor..good motors.

 

Ok, so what would the compression ratio, bore and stroke of a 350 that's bored 30 over?

 

I would guess stock dished pistons, Its been awhile since I tore it down

 

This is incorrect. Bowl shape and head design do not factor into compression ratio, the volumes of both do. Rod length also has no bearing on the compression ratio, you will not find it as a factor in any static compression ratio calculators.

 

You are right about rod length, happy to be corrected, however bowl shape and head design are factors which vary combustion chamber volume.


Roger.

 

Two chambers of different shape with the same volume will have the same CR. Shape and head design alone does not affect CR. Chamber volume is what maters.

 

Determining CR is, like said above, directly the ratio of cylinder volume at BDC divided by its vol at TDC. That is (volume displaced by that cylinder + combustion chamber) / combustion chamber.

Rod length, while not directly involved, influences stroke and thus displacement.

My grandfather tried detuning a Thunderbird and running on regular. He said that the car's performance was horrible, and gasoline bills were worse than a lesser amount of premium with a proper tune, provided one kept one's foot out of it.

 

All other things being equal, about .1 different than one that is not bored.

If the engine will be bored .030" over, the main thing that will impact CR is the new .030 over pistons that will be used. Many cast replacement pistons are .020" to .030" shorter than the original pistons, which had a fairly large impact on the CR unless the block and/or heads are milled to compensate for this. It is much simpler to get correct pistons instead.

 

It was either the points bouncing or the valves floating - I used to have the same popping issue for both reasons. Either way, it was because the springs (points or valves) didn't have enough tension. Way back when I ran points, I used Echlin/Accel street points with 23 ounce springs from NAPA and would double up the spring, using one from the old points set on top of the new one. This was good for 5800 RPM easily. Now valve float is another thing and required a new set of springs to cure.

 

This has been an interesting read, I had no idea that the pistons were different between carbs. I have a 72 S with a 350 2bbl (7A heads) and am planning on swaping in a qjet. But if I swap out that carb I still won't be equivilant to a factory 350 4bbl is that correct? The 2 bbl pistons were dished more?

Thanks
Tim

 

"All other things being equal ...."
And the pistons are not the only thing usually not equal. A stock Olds engine came from the factory with head gaskets .017" thick. The most common gasket used in a rebuild is the FelPro at .040" thick. Most other aftermarket gaskets are similar. This has a tremendous effect on the compression ratio (making it lower).

People who ask what their compression ratio is AFTER they build their engine drive me nuts. You have to DESIGN and BUILD the ratio you want into the engine.

 

Then you should read the Chassis Service Manual.
Many mysteries are revealed therein.


Actually, no.

In 1972, due to the federal requirement that all cars run on low octane unleaded, there was only one compression ratio and only one dish size, so both 2- and 4-barrel carbs used the same compression.

The compression difference applies to 1970 and earlier V8 engines, which, in the US market, were either 9.0:1, 10.25:1, or 10.5:1, while all '71s and '72s were 8.5:1.

- Eric

 

Ok, that makes sense. I have really only scoured the 72 service manual and never ran across that information that's why I was surprised to hear it. I might have to read up on the earlier manuals for some comparison info.

Thanks
Tim

 

To add to Erics post, and to the confusion, Cr was in the low 8's in 73-76, but they did it with 14cc dishes and the large chambered (80cc +/-) #8 head. One easy way to put together a cheap and easy 350 is to mill some older 350 heads a tad and pop them on a 73-76 350, which for some reason many guys think are junk. Cr will be close to 9 to 1.

 

I disagree with this, too. While the rod length does affect dynamic Cr due to different piston speeds, the rod length has no bearing on the stroke or displacement. You could take an engine, use a 1" shorter rod, chop 1" off the deck (I know, just theoretically) and all other factors being the same, Cr and displacement stay the same. The throw on the crank determines the stroke and that along with the bore equal displacement. The rod length is a crucial component along with p/h and deck height in the design of an engine, but it does not affect Cr or displacement.

 

Jim is EXACTLY correct. Do the math for yourself if you don't believe it. The chamber volume is the same at both TDC and BDC. The only difference at BDC is the swept volume of the cylinder, which is governed ONLY by stroke and bore. Rod length can be anywhere from zero to infinite and the CR will be the same.

Now, if you change the rod length and DON'T change the piston compression height to match, then you have changed the chamber volume, so obviously that changes CR, but the difference is due to the change in chamber volume, not the rod length. ANY change in chamber volume, due to piston shape, head gasket thickness, etc, will cause a CR change. More to the point, you DON'T change the rod length without also changing the piston compression height.