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I'm new to this sight and already know I'll be a "regular". I just bought a 37 special model 41, haven't even got'n it home yet. (the cars in Phoenix, I'm in Iowa). Quite sure its going to end up modified but am taking my time to think out how I'm going to build it. Has the original 248 ci inline. I understand there were 4 different displacements for this engine. Are they all the same length? Mount the same? Considering using an old long 8 with a modern automatic. The original 248 sounds a little small for modern speeds, what you think. Maybe a 320? How much can these blocks be bored? Would really like to find a two carb manifold, are they hard to find? I'm sure some of you guys really know your Buick stuff, what's your opinion?

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theres only 2 versions of the straight 8 engines: 248 cid 107 hp, (dont know torque) and 320 cid, 141 hp, 269 lb ft of torque (well those are for the '38, i know its specs, but im not sure about '37. id guess the numbers are real close though.) i would assume the engine is overall bigger and longer with a bigger CID. im sure there are people here who could tell you what mods could be done as far as boring it out, new carb like u said, etc BUT they might not be to keen...this forum is full of people who are rather against the people who customize and trick out and mod the old classics. but youll still get good advice, just dont totally ruin the car. myself i think it would be a good idea to get some more power out of these engines. keep the engine just hop it up a bit to get the car moving well along with normal traffic. though the 320 cid probably have gobs of torque, i dunno about the 248. ill find out soon once i get my '38 model 41 n the road (read here: http://www.aaca.org/ubbthreads/showflat.php?Cat=&Number=285904&page=0&view=collapsed&sb=5&o=&fpart=1 )

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There are actually 3 straight 8 engines. The 248CI and the 263CI from 1950 up will interchange. The 320 CI is longer and would take major surgery to fit in the 40 series. The 263 has inserted bearings and hyraulic lifters and makes a GREAT engine in the early cars. I have one in my 1940, and drive it everywhere at 70+ MPH .The biggest drawback to these cars is the rear end gearing. The tallest gear you can get for a 37 is a 3.6, and they are VERY hard to find. There are places that will install an overdrive in your torque tube drive, and that is probably the simplest way out. The 1941-42 Buicks had dual carbs, and you can still find them on E- bay occasionally.

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For some reason, carnut.com doesn't give torque figures for the Buick engines prior to 1950. In 1937, it lists the 248 at 100hp and the 320 at 130. Compression ratios are 5.7:1 for the 248 and 5.75:1 for the 320.

For 1950 it lists the following specs:

248ci 6.6:1cr 115hp @ 3600 210ft/lb @ 2000

248ci 7.2:1cr 124hp @ 3600 220ft/lb @ 2000

263ci 6.9:1cr 124hp @ 3600 220ft/lb @ 2000

263ci 7.2:1cr 128hp @ 3600 225ft/lb @ 2000

320ci 7.2:1cr 152hp @ 3800 280ft/lb @ 2400

320ci 7.5:1cr (same hp & torque)

Evidently, you could get some real improvement by increasing compression ratio, IF everything else in the engine will take it and IF the head could be planed that much on yours.

This kind of modification is nice because it's basically "invisible."

You can look up more details at the site. Just click on "Car Specs" to find the link to Buicks.

They're not always 100% accurate, but it should give you a starting place for comparisons.

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55packardguy----just curious, what do you mean by getting the head "planed" do you mean like boring out some of the underside of the head, creating more space in the combustion chamber? how would that increase compression ratio? i thought to do that you have to use a bigger crankshaft so the piston comes up further? if im wrong on some of those assumptions (or all!) could you please set me straight on that?

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"Planed" or "shaved" heads involves machining the "underside" of the head, as you say. This decreases the effective size of the combustion chambers, so that when the piston is at Top Dead Center, the air fuel charge in the cylinder has to be "squeezed" into a smaller space. A definition of "mechanical" compression ratio could be: The ratio of the volume in the cylinder at Bottom Dead Center to the volume at Top Dead Center. "Planing" the head does NOT affect cubic inch displacement, because the volume of this "swept area" in the cylinder does not change.

Increasing the bore increases the "swept area," so it also increases cubic inch displacement (volume of the air/fuel charge in the cylinder that is "displaced" by the piston). Increasing the stroke requires shortening the rods, as the piston is generally already flush with the top of the cylinder at Top Dead Center. Strokng also increases the displacement. Neither "boring" nor "stroking" NECESSARILY increases compression ration, depending on any changes in combustion chamber size (for instance, going to a different head with larger combustion chambers, or "dished" pistons, can offset the change in displacement and keep the final ratio the same or even lower it).

Higher compression ratio generally puts more stress on things like pistons, rods and bearings. You can't just assume that the other parts will take the extra load, but must be very careful to build up the rest of the engine as needed. I believe someone said that the earlier straight-eights like yours did not have some of the modern improvements in the bottom end like bearing inserts. If this is the case, you'd definitely need to upgrade parts if you were to try to modify the compression ratio of your present engine. There is also the issue of piston/valve interference, although I believe these older engines have plenty of clearance.

There are also limits to how much material can be safely machined off the head. I know of cases where up to .125" (1/8") can be removed, but this can vary between engines. Sometimes, just going with a thinner head gasket (if available) accomplishes as much as you can do. Yes, that's the poor man's way to increase compression.

I'm no hot-shot mechanic, but I believe the "theory" I presented here is accurate. I invite comments or corrections...

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  • 1 month later...

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">There are actually 3 straight 8 engines. The 248CI and the 263CI from 1950 up will interchange. The 320 CI is longer and would take major surgery to fit in the 40 series. The 263 has inserted bearings and hyraulic lifters and makes a GREAT engine in the early cars. I have one in my 1940, and drive it everywhere at 70+ MPH .The biggest drawback to these cars is the rear end gearing. The tallest gear you can get for a 37 is a 3.6, and they are VERY hard to find. There are places that will install an overdrive in your torque tube drive, and that is probably the simplest way out. The 1941-42 Buicks had dual carbs, and you can still find them on E- bay occasionally. </div></div>

Hi do you have any links to these places? Another question, is it possible to change the manual gearbox for a dynaflow in the ser 40 specials (1946-49's)?

Johnny

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<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">"Planed" or "shaved" heads involves machining the "underside" of the head, as you say. This decreases the effective size of the combustion chambers, so that when the piston is at Top Dead Center, the air fuel charge in the cylinder has to be "squeezed" into a smaller space. A definition of "mechanical" compression ratio could be: The ratio of the volume in the cylinder at Bottom Dead Center to the volume at Top Dead Center. "Planing" the head does NOT affect cubic inch displacement, because the volume of this "swept area" in the cylinder does not change.

Increasing the bore increases the "swept area," so it also increases cubic inch displacement (volume of the air/fuel charge in the cylinder that is "displaced" by the piston). Increasing the stroke requires shortening the rods, as the piston is generally already flush with the top of the cylinder at Top Dead Center. Strokng also increases the displacement. Neither "boring" nor "stroking" NECESSARILY increases compression ration, depending on any changes in combustion chamber size (for instance, going to a different head with larger combustion chambers, or "dished" pistons, can offset the change in displacement and keep the final ratio the same or even lower it).

Higher compression ratio generally puts more stress on things like pistons, rods and bearings. You can't just assume that the other parts will take the extra load, but must be very careful to build up the rest of the engine as needed. I believe someone said that the earlier straight-eights like yours did not have some of the modern improvements in the bottom end like bearing inserts. If this is the case, you'd definitely need to upgrade parts if you were to try to modify the compression ratio of your present engine. There is also the issue of piston/valve interference, although I believe these older engines have plenty of clearance.

There are also limits to how much material can be safely machined off the head. I know of cases where up to .125" (1/8") can be removed, but this can vary between engines. Sometimes, just going with a thinner head gasket (if available) accomplishes as much as you can do. Yes, that's the poor man's way to increase compression.

I'm no hot-shot mechanic, but I believe the "theory" I presented here is accurate. I invite comments or corrections... </div></div>

i dont question your expertise, but it doesnt make sense to me that machining off some of the underside is decreasing the amount of space. wouldnt that increase the space thus deacreasing the compression ratio?

although i think i understand from what you said this does not increase the displacement because this new area is not "swept" by the piston aka it doesnt enter this area, it still travels the same distance?

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<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">I've heard that the 320cid straight eight from the thirties weighed 1200 lbs. Can that be true? Wouldn't be an engine I would want to swap. [color:\\"black\\"] </div></div>

The 320 out of my '41 Century weighs 862 pounds dry without accessories. shocked.gif That's a lot of iron!

There are specs on at least the '41 Buick motors here. The blocks and specs are pretty much the same, but increases in compression, redesigned pistons and the dual carbs explain the horsepower increases between 1937 and 1941.

As for planing the head, yes, that increases compression ratio. Remember that the head isn't just a flat piece of metal, but has a combustion chamber cast or machined into it, kind of like a dish in the head (see photo below). By planing the head, you're decreasing the size of that dished area. Shaving, planing or "decking" a head is simply using a flat grinder to remove material from the flat part of the head (the deck surface), not hogging out the combusion chamber itself (which is often known as "porting").

Think of the combustion chamber as an inverted teacup. It holds 6 ounces or so. Then you cut off the top inch of the cup (at the rim--the open end). Is the volume of liquid that the teacup will now hold greater or smaller? It's smaller, of course.

Cylinder%20head.jpg

In an engine, the piston traveling up and down determines displacement (in fact, that's pretty much the definition of <span style="font-style: italic">displacement</span>: the difference in volume between top dead center and bottom dead center). The piston traveling down will pull in a known quantity of air, just like when you inhale using your lungs. (Making your mouth smaller won't affect how much air you can fit in your lungs, right?) But when the piston comes up, that volume of air gets squeezed, and by shrinking the size of the chamber where it gets squeezed, you increase the pressure--called the compression ratio. Continuing my crude example, if you made your mouth smaller, forcing the same amount of air from your lungs into it would increase the pressure on your cheeks, right?

The thing with increasing displacement is that it usually <span style="font-style: italic">decreases</span> the compression ratio. This seems counter-intuitive because you're theoretically sucking in more air due to the greater displacement. But the greater displacement also <span style="font-style: italic">increases the size of the combustion chamber</span> (increasing the bore makes it wider, for example). So you have to take that into account when boring or stroking a motor.

With a stroker motor, you're increasing how far down the piston travels, increasing the size of the displacement. But it can only travel so far up until it hits the head, so you have to change the crank's throws as well as the piston and rod combination to keep the piston inside the cylinder. It's tricky and too much to explain here. But hopefully you can see how shaving the head will increase compression without affecting displacement.

Compression ratio and displacement are related, but not 100% dependant on each other. You can increase compression ratio without affecting displacement, but you can't increase displacement without affecting the compression ratio.

Hope this helps.

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<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">The thing with increasing displacement is that it usually <span style="font-style: italic">decreases</span> the compression ratio. This seems counter-intuitive because you're theoretically sucking in more air due to the greater displacement. But the greater displacement also <span style="font-style: italic">increases the size of the combustion chamber</span> (increasing the bore makes it wider, for example).</div></div>

Nice explanation and illustration. I'd just add that (as I see it) boring won't increase the volume of the combustion chamber, it will just increase the "quench" area where the piston reaches "flat" area of the head surrounding the combustion chamber. So, all things remaining equal with the head, I believe boring and/or stroking will increase the compression ratio, because the swept area will increase, and compression ratio is the ratio between the swept area and the combustion chamber size. If the combustion chamber volume is enlarged, it would offset some or all of the compression increase, or even as you say, decrease the compression ratio.

Other than that, I think your explanation and mine are in complete agreement. Let me know if this makes sense to you or not.

Question: Do dished or crowned pistons actually change the compression ratio, or just the combustion chamber "pressure?" I think the ratio actually remains constant, since the volume at bdc is increased or decreased as much as the volume at tdc.

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Hi <span style="font-weight: bold">Guy</span>,

I don't know if there's a hard-and-fast rule about how displacement affects compression ratio--I think it varies from engine to engine. On the Corvette LS-1 strokers I used to build, the compression ratio increased because the original pistons were flat-tops and our replacement pistons were the same, since you can't bore an LS-1. In this case, you're right--the added displacement adds compression. We decked the heads on the hot motors to increase the compression when more displacement didn't add enough.

However, on my 5.0L Mustang, I had to deck the heads to bring the compression ratio up after boring it .030" oversize when I built my stroker motor. Even with the flat-tops that I used in that application (the stock pistons are dished), it wasn't enough. The wider bore apparently <span style="font-style: italic">added</span> combustion chamber volume, dropping the compression ratio.

I think it really depends on the size/shape of the combustion chamber and its relation to the piston. You're right that the quench area may not change much, but even .030" can add up when you figure the volume of a cylinder vs. the circumference. However, when you do the math, you are 100% correct that in most cases increasing displacement <span style="font-style: italic">should</span> increase compression ratio.

Domed pistons increase the compression ratio because they reduce the volume of the combustion chamber (likewise the dished pistons decrease compression). I think this is because the dome will consume a much greater proportion of the combustion chamber than it does of the overall cylinder volume. Even though it's the same at both ends, the <span style="font-style: italic">ratio</span> implies a proportional relationship. The dome may take up 40% of the combustion chamber volume, but only reduce total swept volume by 5%. I don't think they cancel out when doing the figuring (looking over the formulas, it seems to be a common numerator in the calculations, so there is definitely a proportion involved and not a simple difference).

We often <span style="font-style: italic">cc'd</span> the heads (measuring the volume of the combustion chamber alone) to determine the compression ratio in a new combination. The numbers we generated were always called "compression ratio" by the engineers at GM we worked with, too.

Here?s the math I always used (remember to convert cc?s to cubic inches!):

<span style="font-weight: bold">Compression ratio= (Swept Volume + TDC Volume) / TDC Volume

Swept Volume= (pi x Bore x Bore x Stroke) / 4

Top Dead Center (TDC) Volume= Head volume + gasket volume + deck volume + dish/dome volume

Gasket Volume= (pi x Gasket Bore x Gasket Bore x Compressed Gasket Thickness) / 4

Deck Volume= (pi x Bore x Bore x Deck Clearance) / 4.</span>

NOTE: Any volume below the piston compression height (dished piston) is a positive value and any volume above the piston compression height (domed piston) is a negative value. With this perspective, you can see how adding or removing volume here affects the overall volume of the combustion chamber)

Here?s how I determined the compression ratio for my Mustang?s stroker motor (man, I had to dig through some <span style="font-style: italic">old</span> files for this stuff?I built the engine way back in 1993!):

Head Vol.= 70cc = 70 x .061 = 4.27 cubic inches. (I milled a 74cc head)

Dish/Dome Vol. = 0cc = 0 x .061 = 0 cubic inches. (flat-top pistons, no dish/dome)

Gasket Thickness = .035".

Gasket Bore Dia = 4.090".

Deck Clearance = .010".

Deck Vol. = (3.1416 x 4.030" x 4.030" x .010") / 4 =.128 cubic inches.

Gasket Vol. = (3.1416 x 4.090" x 4.090" x .035") / 4 =.460 cubic inches.

Swept Vol. = (3.1416 x 4.030" x 4.030" x 3.40") / 4 =43.369 cubic inches.

TDC Vol. = 4.27 + 0 + .064 + .477 = 4.857 cubic inches.

<span style="font-weight: bold">COMPRESSION RATIO</span> = (43.369 + 4.857) / 4.857 = 9.93

Here?s a diagram:

ratio.jpg

So I guess we?re talking about the same thing and just muddling our definitions?the quench area you mention is part of the deck clearance and head gasket thickness (top dead center thickness). From the math, it looks like swept area doesn't include the combustion chamber, which might be where I got confused. To get my compression back up to the 10:1 that I wanted (stock was like 9.5:1), I had to mill the head even with flat-top pistons and more displacement.

Of course there?s a whole bunch of stuff about static and dynamic compression ratios and cylinder pressures, but that's a much bigger discussion, and frankly one I don't have much experience with. I usually just used the static compression ratio above as a rule of thumb, knowing that it wasn?t the total picture, but a good guide. The car runs great on 89 octane is all I know!

I plan on milling the head of my Century a bit to increase compression, too and will use this same math to do the figuring.

Does this help answer the question? I think we?re thinking too much!

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There's a lot to this, isn't there? Like I said, I'm no engineer, but kind of know the basics of how to "figger" compression ratio and generally what increases and decreases it. One thing you pointed out that's interesting is that the "quench" area, even though the clearance is very small, DOES add some volume to the combustion chamber. So overboring can increase this volume.

Those formulas can be very helpful to someone who is seriously working on modifying an engine. That kind of work requires a lot of thought--so I guess we aren't thinking too much when including that information here. grin.gif

Thanks for the reply!

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matt, i completely understand what you mean now. i thought what you meant was what in that post with the 3d CAD picture, you referred to as porting. now it makes perfect sense because of the word "planing" too, i dont know why i didnt consider that, as i know that a wood planer is what you put a plank of wood through to shave off its thickness.

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  • 5 months later...

One other thing to consider is the thickness of the head gasket. The typical 350 Chevy engine with flat top pistons and a 64cc combustion chamber yields 10.28:1 compression with a .064" thick head gasket, swap the head gasket to a .015" and you get 11.68:1 compression with no other changes. The average sbc head gasket is .040" thick. Of course I doubt you could even find different thickness head gaskets for a 1940's straight 8. Check out the cool compression calculator at:

http://www.rosspistons.com/calculator.php

Regards,

Duane

1946 Buick Super Conv.

1971 Chevy Truck - 420 sbc stroker

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with my 37 i first found a set of nos pistons and bored it to that size.they can go.0090 safely.i only went .0020 to clean mine up.the heads can be milled up to .0080,and ive heard of going as much as .0110.these engines thrive on high compression.you can use 38 or later high dome pistons and get about the same result.a 41 or 42 dual manifold will also work.find a late 49 or 50 special with insert type rods and these will drop right in.these engines respond very well to this treatment.your axle ratio is probably 4.4 which is very low.acording to buick engineering with standard tires it goes 1910 mph for every 1000 rpm in high.they will turn 4300 to 4500 in stock form in high gear

or a true 83 to 86 mph.not big but remember this was 1937,and it was good enough to edge out a ford or chev.3.9 and 3.6 gearing is available,and with some

machining will go in,and of course give a better top end.if you plane the head shim the rocker stands equal 1/2 the amount taken off as it will change the angle of the rocker arms.good luck wink.gif

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  • 3 weeks later...

ZondaC12, maybe some information about the influence of the compressionratio on the thermal efficiency=horsepower will help you. If you raise the ratio from 7:1 to 10:1 the efficiency will gain about 12%. The mechanical losses will raise too due to the increase in internal forces, so you could expect about 10-11% more horsepower. For 7:1 to 8:1 these figures are 4-5% and 7:1 to 9:1 about 7-8%. The gain is interesting but be sure the engine is healthy to cope with the higher forces. Before you do check the cranckshaft and rods for cracks. Check the bearings too. You will need a very good oilpressure to enjoy it for a longer time. Also advance the ignition to prevent detonation, because detonation gives enormous forces and will ruin the engine in short time.

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