Dave_B Posted October 4, 2007 Share Posted October 4, 2007 I have heard that increasing the compression ratio in these old low compression engines can cause them to run cooler. I think, for example, my '28 standard has a compression ratio of about 4/1. When rebuilding the engine would raising the ratio to about 6/1 cause it to run cooler. It would still run on regular and would most likly have more power. Could be a win win situation. Or it could be a load of bull.Anyone have any thoughts?Dave Link to comment Share on other sites More sharing options...
Guest ZondaC12 Posted October 4, 2007 Share Posted October 4, 2007 Well I do know that a higher compression ratio means more energy from the burning fuel goes to moving stuff around, hence more force on the piston...look at the immense torque diesels make...its all from the like 22:1 compression! So I would think it would run cooler.But I've also heard of some engines having better cooling through the heads so higher compression was possible. But I was told THAT situation was pre-igntion due to the extra hot spots caused by the compression stroke...so extra cooling helped that. Dont know enough to know if that would cause a net increase in running temperature. Sheesh it could make one go crazy thinkin about this!!! I wanna see the experts' opinion on this...very curious myself. Link to comment Share on other sites More sharing options...
Guest rlbleeker Posted October 4, 2007 Share Posted October 4, 2007 I have no specific knowledge regarding '28 engines, but I expect it would run well with a ratio between 6:1 and 8:1. The increased efficiency of the exhaust stroke may very well lead to cooler running. Link to comment Share on other sites More sharing options...
Grandpa Posted October 9, 2007 Share Posted October 9, 2007 Hi Dave, To move your car down the road takes a specific amount of horsepower for that speed. Thus, increasing the compression ratio a little should not cause the engine to run hotter or cooler, as the required hp for that speed remains the same. A little more heat is generated because the engine is compressing the mixture more, however, increasing the compression ration to 6:1 should not cause noticable additional heat. Your engine should give slightly better mileage and slightly improved performance. Grandpa Link to comment Share on other sites More sharing options...
Guest rlbleeker Posted October 9, 2007 Share Posted October 9, 2007 I'd expect there is a fairly fixed amount of energy in a gallon of gas, assuming proper combustion. When burned a portion is converted to mechanical energy (via pistons, crankshaft) and the rest is converted to heat (exhaust, radiator). Assuming mileage increases slightly, which seems likely, then total energy consumed per mile is reduced. With the mechanical requirement being fixed, that should mean less energy converted to heat. Another way to look at it is, in a 4:1 compression motor, on exhaust stroke approximately 1/4 of the exhaust is left in the cylinder (ignoring the finer points of exhaust systems). That remaining exhaust is mixed with the incoming charge, raising it's temperature. If you increase the ratio to 6:1 only approximately 1/6 is left, that's a 66% decrease in residual exhaust gases. That should result in cooler cylinder temperatures throughout the cycle and less demand on the cooling system. All speculation on my part of course. Link to comment Share on other sites More sharing options...
Dave_B Posted October 16, 2007 Author Share Posted October 16, 2007 OK if I am understanding all this right... higher compression means more energy is going into motivating the pistons therefor less energy goes into heating the parts up. Also there is increased exchange of fresh fuel and exhaust which also increases efficiency. With todays fuels what is not to like here? Next question is what is the best way of raising the compression?Dave Link to comment Share on other sites More sharing options...
Guest brh Posted October 16, 2007 Share Posted October 16, 2007 No expert and I don't mean to offend but the phrase that keeps popping into my head on this thread is "if it ain't broke don't fix it". You will need to reengineer some of your engine to do this. Will you create or put strain on old parts to accomplish this? I am sure a dome piston, or putting the head on with a thinner gasket etc will bring up the compression some. Just a thought. Link to comment Share on other sites More sharing options...
Guest rlbleeker Posted October 16, 2007 Share Posted October 16, 2007 I you are rebuilding the engine anyway, I don't see a downside to raising the compression a bit assuming you can get parts to do so. I don't think strain is an issue if you don't drive it hard. That is, the strain on the crank/rods/pistons during moderate accelleration is pretty much the same regardless of compression. If you routinely took advantage of the expected power increase, that of course might lead to bottom end problems. I'm not familiar with these engines, so don't know how durable the bottom end is.The simplest way to raise compression, like brh said, is a thinner head gasket. Next is decking the block and/or shaving the head, often part of a complete rebuild anyway. Boring increases compression slightly and of course pistons with a dome or greater pin height. Anytime you change any of these things you need to make sure you have adequate valve - piston clearance. Shouldn't be a problem on an engine such as this though. Link to comment Share on other sites More sharing options...
Guest Posted October 18, 2007 Share Posted October 18, 2007 I took .050" off my head and block and bought pistons with a .125 dome on them when I overhauled my Pontiac engine in 1978, 340,000 miles ago. The car is my daily driver and in the mid 60,s I changed to a 195 degree thermostat. I still use the same temp stat and found absolutely no difference in operating temperature. The changes boosted my compression from 4.5 to 6.0 to 1. The extra power is noticable and I think was worth the bother. If your cooling sysstem is working properly the engine should operate at the temperature of the thermostat regardless of compression. Link to comment Share on other sites More sharing options...
Guest rlbleeker Posted October 18, 2007 Share Posted October 18, 2007 True, heat is only an issue if your cooling system isn't up to the job. I was reading the manual for my Marquette last night and they had an optional high compression head that raised the ratio to 5.7:1. I wouldn't mind a little more power. Link to comment Share on other sites More sharing options...
Dave_B Posted October 22, 2007 Author Share Posted October 22, 2007 I hear ya brh. The problem I am trying to address is warm weather over heating. Anytime the air temp gets above the mid 80s the radiator can't handle it. Funny thing though I can run a low speed with less trouble. At about 40 mph or a long hill climb it will get hot in a minute. This all started when I had it recored with a honeycomb type repro core, but the new core does not have the surface area of the original equipment core and so it don't work so great. I was hoping for greater difference in running temp by raising the comp ratio. It sounds like it's not going to happen that way. Btw, I saw in the parts book that Buick sold a high compression head in '28. I'd love to get my hands on one of those. Based on Tinindian's experence when I rebuild this engine I will raise the compression.Dave Link to comment Share on other sites More sharing options...
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