Increasing the Compression Ratio for the 1929 6

[Guest frshcatch]

I am having a cylinder head cast for my 1929 Commander 6. In the process we are considering increasing the compression ratio. My understanding is that the original ratio was about 5:1. We are going to increase it to 7.5:1. My question is whether they produced engines in that period with low compression ratios because the engines could not handle the higher compression ratios, or whether they were kept low because of the low octane of the gasoline at that time?

Has anyone increased compression for these cars? Is it a wise thing to do? How much can it safely be increased?

I would like to get a little more "performance" but don't want to tear the engine apart in the process.

[nvonada]

Both of those reasons were a considerdation. There was a third reason too. Cooling jackets and manifolds were not as well designed (many cars did not even have cooling for the entire cylinder). This created hot spots that would trigger pre-ignition at higher compression ratios. It is your car and your money but you are off into an area that has not been all that well explored (at least in the last 1/2 century). I would go stock if it were me. The engine might not last long or it might still be running another 85 years from now. I am glad you are not dropping a SBC into it though. Good luck and post lots of pictures!

Nathan

[studeq]

If your car is a 1929-30 model GJ Commander the standard compression ratio was 4.8 to 1 (see chart above). You will also notice there was not an optional head available for this model. The bottom end of that engine was not designed to handle a compression ratio that high (i.e. 7.5), or anything close. You will have trouble if you try to drive it at sustained speeds above 50-55 mph. In any event increasing the compression ratio will not provide a significant amount of increased h.p. to your Commander (you will also be limited by the relatively low rear end gears 4.36 to 1 standard). These cars were designed for the roads and speed limits of the day. That is 35-40 mph. I believe in the long run you would be better off keeping the standard cyl. head ratio and slowing down to enjoy the scenery. Let me know in 3-4 years. And by the way I hope you are not having that new cylinder head recast by the guy down in Texas? If so plan to spend 3-4 times the quoted price and be prepared to wait 3-4 years. If that is the case I recommend you get out NOW!

[rbk]

Good that you warned this guy. I know that a group contracted with him to cast heads and lost their $10.000.

I have several cylinder heads , give me the casting number and I will have it checked out.

I do not have a head for 8 cylinder cars.

Robert Kapteyn

[Guest frshcatch]

Thank you all for the excellent and timely feedback. I will back off of the increased compression. Although it appears somebody should try it, I guess I won't volunteer :-).

If I don't get this problem solved I may end up with a SBC. That would be a shame.

[dictator27]

Problem? What problem? Slow down and smell the roses. You'll still get there.

What kind of "performance" are you looking for? Cars of this period were designed to do almost all their work in high gear. Second gear was good for about 15 mph without straining. The rest was high gear.

Terry

Edited December 14, 2012 by dictator27 (see edit history)

[Gary_Ash]

My engine simulator software (Dyno 2000) won't accept compression ratios less than 6:1, so I can't model the Commander 6 with 4.8:1 compression. The factory specs for 1930 were 75 hp at 3000 rpm. The model says at 6.5:1, that you might be up to 96 hp at 3500 rpm. Torque might be 170-175 lb-ft over the 2000-3000 rpm range. This assumes the carb can deliver about 100-150 cfm at 1.5" Hg. I used stock bore/stroke and valve sizes. At 7.5:1, you might get 113 hp at 3500 rpm and 190 lb-ft of torque at 2000-3000 rpm. Adding more carburetion would help.

So, you might cruise right along until it throws a rod or something else blows up. Can I watch?

Of course, George Rohrbach put a later Commander 6 in his 1922 car, including dual carbs. The car looks stock with the hood closed. You could go with a 245 cu in version of the engine, like a 1960 Studebaker truck engine with dual carbs, and have a real goer. Here's George's engine:

Edited December 15, 2012 by Gary_Ash (see edit history)

[Guest frshcatch]

That is a beautiful and impressive engine.

I assume he also changed the transmission and differential. And, probably the brakes? It would take an upgrade of the rest of the mechanicals to handle that engine. I don't think I dare go nearly that far with my engine.

[Guest jpatino]

It's amazing what some people have done with some of these old engines. Take a look at what this guy did to a Packard straight 8.

Edited December 16, 2012 by jpatino (see edit history)