Straight-8 Performance Upgrades?

[Matt Harwood]

At the risk of becoming a blasphemer here, I'm embarking on a rebuild of my Century's 320. While my car will be a Concours restoration, I plan to drive it often after I'm done showing it. I'm the kind of guy who says, "Well, as long as I'm at it..." and over-engineers my stuff. I don't want to make any visible changes to the engine, but I was thinking that the following list might be possible:

<ul style="list-style-type: disc">[*]ported & polished head

[*]0.100" shaved off the head (should raise compression to about 7.8:1 by my calculations)

[*]Hardened valve seats (mandatory, I think, as long as the head's apart)

[*]Big-block Chevy exhaust valves, which are a direct-replacement and are undercut and slightly larger (1.188")

[*]Insert bearings instead of babbits

[*]Reground camshaft

[*]Slightly firmer valve springs (again, BB Chevy parts are virtually identical)

[*]Extrude-Honed intake and exhaust manifolds (see Extrude Hone for more information on the process)

[*]Jet-Hot coatings inside and outside the exhaust manifolds and inside the intake manifold for heat control--click here for Jet-Hot information)

Again, all these modifications will be invisible and will not detract from the originality of the car, but can potentially increase driveability and performance considerably. There's no reason a 320 inch motor can't make 200+ reliable horsepower without undue stress, and very few of these modifications will negatively affect driveability (in fact, they will probably improve fuel mileage).

Does anybody have any experience hot-rodding a 320" straight-8? Any additional tips and clues would be very much appreciated. Also, since I will be building the motor myself (sending it out for machining and balancing, but I'll do the assembly), are there any tricks to it? I'm an experienced engine builder (I have built many, many small-block Ford engines and worked for a major Corvette tuner building LS1 hot-rods), but I care more about making this one right more than any other I've built. For instance, I'm guessing the rear main seal is a rope seal--what's the deal with that and is there a better replacement available?

Any advice is greatly appreciated. Please don't flame me, because all I want is a better driving Buick that looks and acts 100% original.

Thanks in advance!

[Guest 53and61]

Could you gain enough displacement by boring to 0.040 or 0.060 oversize to increase HP a bit? Was the dual-2bbl setup legit for the '41 Century? If so, would going from 1 to 2 carbs help? I've seen dual-carb manifolds for the 320 for sale, but not for cheap.

[Guest]

all I've seen done is the dual carb intake, headers, and maybe the hydraulic lifters out of a 1950 road master to get ride of the tapped noise.

[Matt Harwood]

Thanks for the replies, guys. I'm probably going to bore it .020" oversize, which seems to be a common sized replacement piston. I could probably go as far as .125" oversize if I got radical (for 344 cubic inches), but I don't know if I want to take that much out of the block.

It already has the dual carbs as stock equipment from the factory. These are probably the single biggest improvement over the single carb motors that I could make, so I'm glad I don't have to track down a manifold!

Anyway, I appreciate the feedback. Thanks!

[NTX5467]

Matt, that sounds like what I'd classify as a "good rebuild" orientation with great machine work and mods that would hopefully increase durability, reliability, and maybe even a little bit of power too, without being obvious.

As with any rebuild, boring the cylinder walls just enough to get a smooth surface for quality honing operations is all that's really necessary, so I concur with your .020" overbore completely (dependent on piston availability too). The few extra horsepower from the bigger overbores .040" or .060" would not be felt except in a tight drag race when another 2-5 horsepower at the flywheel might make a difference. Does anyone build any torque plates for that engine (had to ask)?? I suspect you'll also check the line hone on the main caps too? Plus resize the rods? Plus balance it?

As for the porting and polishing of the head and manifolds, I understand the Extrude Hone works best for that. A polished exhaust port in the head might help with decreasing heat absorption from the exhaust gases into the head itself just as your coating the inside of the exhaust manifold would--basically keeping all fo the exhaust heat in the exhaust system on the way out. I suspect it would not help exhaust flow that much to get excited about as that's usually more dependent on shapes and such than surface texture.

While what you mentioned might be different than what many would do, I would say that as long as it's invisible when assembled and increases durability and driveability, then let it happen. A valve is a valve regardless of what it's application is, same with the valve springs too. There might even be a more modern lip seal that could be used for the rear main seal too.

Using the high quality machine work which is usually available "everywhere" in modern times, rebuilds of vintage engines can be done that can turn out much better than they ever could have in earlier times. Add the much better lubricants and there's no real reason that I can see NOT to drive those cars. Of course, putting rear axle gears that will keep the cruising rpms down on the highway is another incognito swap (as others have mentioned too).

Good luck on your project!

NTX5467

[Matt Harwood]

NTX5467:

Thanks for the detailed post! I should have added that all this work will be in addition to standard rebuilding prodcedures (bore, line hone, balance, resizing, decking, etc.). I'd really like to build a straight-8 that can run with modern motors, while keeping its legendary smoothness intact. We'll see how it works out.

I'll probably also add a 3- or 5-angle valve job, since the original valve seats were just blended and radiused. This should improve flow and sealing, especially with the hardened seats and the bigger valves. The biggest problem is the siamesed intake ports, which seem to be kind of thin, too, so I can't take too much material out. I may just clean them up a little. I don't want to Extrude-Hone the heads because with a carbureted motor, the fuel tends to "wet-out" on smooth surfaces. A slightly rough surface helps keep the fuel atomized and in suspension. There are also those pesky copper ring gaskets to deal with...

Right now, I have 3.90 gears which are probably OK, but I have found an NOS set of 3.60 gears that I might install. It isn't much of a difference, but it might help highway cruising and with more power in the motor, I'll never notice the reduced acceleration around town. How difficult is a ring-and-pinion swap in one of these, anyway?

Thanks for the input!

[Mrbuick714]

There was an offset rocker-arm which would let you get more valve action with a standard cam.The outfit was "Barker" Your car will surprise lots of people absolutely stock.I was with a dealership when these cars wre new. Ralph

[Jyrki]

Ralph, would you recall what the stock and/or "barker" rocker arm ratios were?

[The Old Guy]

You may find that the 3.6 gears will make the car cruise the interstate better, but you wil probably lose some MPH. I went from a 3.9 to a 3.42 and lost 5-8 MPH on the top end. My 40 with a 3.9 would run 105-108 MPH ,and now it just makes 100 on a good day. I have a 263 with the 41 exhaust manifolds to give me duals, and a carb off the 320 engine,and it makes a nice cruiser, but it is no race car. I have a 37 with a 455 for that!

[Grant Z]

Hi Matt,

 

I'm starting to plan the rebuild of my 320 from my '41 Roadmaster and came across this post by you. How did that go? What mods did you end up doing?

 

I want to do it right and do it once.

[Matt Harwood]

Man, this is an old thread. I did rebuild my 320 about 15 years ago and it's still sitting on a stand. I believe we bored it .060" oversize (it was already .030" over when I got it). We also removed a bit from the deck surface and a bit more from the head to end up with like 8.2:1 compression or somewhere in that neighborhood. I found an NOS Howard 3/4 race camshaft on eBay that we used as well. A few other tricks including insert bearings on the rods and a later front cover with a neoprene seal instead of a rope seal. Nothing radical. The manifolds are still the biggest obstacle to horsepower, particularly the intake, so it's not really worth going crazy with internal modifications. I just wanted reliability and maybe 180-190 horsepower. I haven't driven it, but given the modifications I think that's reasonable.

 

Aim for reliability more than all-out performance. The insert rods are a good idea. Balance the rotating assembly. Buy good pistons with a modern ring pack. Those are all smart upgrades that will pay off in durability.

 

I also just finished the engine for my 1935 Lincoln and we had custom pistons made that were like 30% lighter than the originals and added a little compression on top. We also reprofiled the camshaft to open the valves a little faster (not higher lift, just a slightly steeper ramp profile) to help flow more air. Opening the valve faster has the same effect of opening it farther--more area under the curve. We also used custom valves that were likewise lighter than stock and had a deeper undercut that should flow better. This was a Lincoln flathead V12, so again, we weren't after huge horsepower but these were easy, invisible things that wouldn't affect reliability or tuning, and as long as we were spending the money, we may as well do it. Of note, the Lincoln uses a roller camshaft, so I'm not sure what the right profile would be on a Buick flat-tappet cam. You can do things with a roller cam that aren't a good idea with a flat-tappet cam.

 

Some porting on the Buick might help, but again, you're limited by the manifolds. Don't go crazy with the porting since it probably won't do much. It's never going to be a screamer, but a good running engine will be a joy to drive.

 

Hope this helps!

 

 

Edited March 24, 2023 by Matt Harwood (see edit history)

[Shootey]

in the early 1990’s there was a guy in California who had a 1938 Century coupe that had weber carbs on it. I think it may have had a chopped top. At the time I was restoring the same model to original and so wasn’t overly interested. In fact my coupe as bought was kind of a low grade drag car with dual exhausts and wider rear wheels. 
 

what’s kind of funny is my relatively recently acquired 1941 Estate wagon has dual exhausts. The restorer, Craig Johnson (done in the early 2000’s) did everything stock except for the duals. Maybe his people didn’t want to wrestle with the y-pipe. He also was basically a Ford man and so duals were perhaps not viewed as blasphemous. 

[Matt Harwood]

I'm not convinced that dual exhaust adds any real performance on a straight-8 Buick. Plus it sounds weird. I went with slightly larger tubing (2.5-inch) when I had a custom exhaust system made and that probably added more flow than the engine could use, even with the headers I built. As I mentioned, I think the limited flow of the factory manifolds is your bottleneck, and even if you eliminate those, you probably won't get a whole lot more out of it without some rather substantial internal modifications (cam, compression, porting). Even the guys drag racing these engines back in the '50s, I'd be shocked if they were getting more than 300 horsepower out of them, naturally aspirated. If you want a stock-looking motor, there's not a gold mine of horsepower hidden inside.

[JBP]

Somewhere among my files I have a copy of an article for a custom/hot rod magazine that was written decades ago. The author had spent some time looking at and working on their Eight, and had made some interesting mods. 
 

One change I remember most clearly was the rerouting of water flow through the engine for cooling, which he believed vastly improved it. I want to say it was for the smaller series engine (248 or 263), but can't recall with certainty. 
 

Will see if I can dig it out. Other forum members may already know it.