I have a 390 Caddy motor that I'm having rebuilt. Just a stock rebuild with a mild cam. These motors had 10.5 to 1 compression from the factory,but in 1960 gas was a lot better than it is now a days. Just wondering if I should use thick head gaskets to lower the compression a little. If so who makes thick ones?
Advertised compression wasn't always actual compression. You need to figure exact compression and go from there, if anything I would have a dish cut in the top of the replacement pistons if forged and go that route instead of a thicker head gasket.
Ignition timing has a lot to do with this, too. Sometimes on a modified engine, you have to modify the advance to get the low speed performance you like and still not rattle at higher speeds and load. If the cam is not too crazy vac advance is necessary, too. frank
I'd suggest using the thicker head gaskets. That will lower your compression some. It's less trouble and expense than getting the pistons cut. Should be alright with premium pump gas.
Are you replacing th pistons or useing the original pistons? If you are replacing the pistons just tell your rebuilder that you want to run in the 9-9.5:1 compression range. If you have a competent builder he can hit that mark for you.
As a general guide, your compression ratio should look like your octane rating. If you want to use 87 octane regular, 8.7:1 compression is about right. If you want to use 92 octane super, you can go to 9.2:1 compression. This is not a hard and fast rule, just a guide. There are a lot of other factors involved. Lots of engines have higher compression ratios, by .5 or so and work fine. On the other hand some other engines are not so forgiving.
You can probably get diaper head gaskets from FelPro or Victor. That will lower you compression a bit. 10.5 :1 is right on the ragged edge of drivability with pump gas. if you can get premium you can tune it to run OK. Put a little more lag and less lead in your timming, keep the engine as cool as possible and maybe run it a little fat. Where you run into a real problem is that you cannot always get premium. MOROSO make a real good octain booster. It comes in a purple can (I think, looks purple to me). It like brill cream a little dab 'll do ya. if you out too much in it will turn your plugs orange. Carry a can with you. You are comprehending what I am endevoring to convey, right?
Have you actually measure everything and computed the 10.5:1 or is this simply what the factory claimed? Are these NOS pistons or recent manufacture? Many times the manufactureres took some 'liberties' with their advertising, just like the aftermarket piston folks who have taken some 'liberties' with the compression for older engines. Until you have an actual measurement it is hard to know what to do. It may be that you can cut the piston tops a little or perhaps your particular package would benefit from enlarging the combustion chamber a few cc's to put the actual compression in the 9-9.25 range. Best to talk with some Caddy guys to see what practical maximum is advised. .
I have about 9. to one in a Y block and I need 89. I could back the timing down, but then its not fun to drive.
You could also deburr and polish the combustion chambers. While this will have just a small effect on the compression ratio, doing this will deter later carbon build-up and remove any sharp edges which could cause hot spots, both of which can lead to pre-ignition.
Falcon George, you beat me to it on the fact that thicker head gaskets can actually INCREASE octane requirement. Squish area is important in a wedge type combustion chamber. If you decide to try a cut on piston top, first make sure you have the "meat" to do so without weakening the piston. And then make sure you cut only on the part of top that is underneath the open part of chamber @ TDC. Dave
I'll be using a cam from SLR--the same one they used in the HR Deluxe article Caddy-O. 0.478/0.486 lift 0.268/0.274 duration,and he's going to cut the lobe center so it will have a lumpy idle
I have a 71 Lincoln Mark 3 that has a Tag on the Engine compartment that says 100 Octain Fuel Only 460/365 hp if you don't use it it will Ping Big Time! Just my 3.5 cents
I can't hear pinging or anything else---run lakes headers wide open-----what did you say---I can't hear you haha
forget---didn't write it down---will know later today The guy is a cam genius---don't think he will steer me wrong,and I want most of my vacuum retained. Had a real hot cam in my T and it had hardly any vacuum---damn pcv valve wouldn't even work right
If your car pings on the hi test because of too much compression water injection will tame it. They have been around a long time, in 1951 you could buy a Crosley Hot Shot sports car with 10:1 compression when the best hi test was about 80 octane. They came stock with a Thomson Vitameter water injection. The fluid they used was actually made of water, alcohol and a shot of tetraethyl lead.
Cam selection is a science definitely. Falcongeorge is right. I like a wider LSA for any street use. We just built a 496 BBC for a 3500lb car that will see some street use and went from a 108 to a custom ground 111LSA and stil made 718 HP @ 6300RPM and 637lb/ft of torque @ 5400RPM. Everyone thought we were crazy 'til they saw the results. This is a pump gas engine with modern design chambered aluminum heads.
Gary, if I may make a suggestion here. Learn how to read your spark plugs. Yes you should buy a spark plug magnifier with a light. You dont need the $100 one, just your basic light and magnifier. Learn the difference between the tell-tale signs of " pre-ignition " and " detonation ", and understand the differences too. TR
Plugs always looked good---no pre-ignition,but engine had lotza blowby and low compression-- it will have more compression when rebuilt
this is the shurest way to deal with the problem looked in several old manuals and couldn't find the 390 cad. specs. starting in 1957 many cars had 10to1 as high as 10.5 to one. blows my mind to think some of you said i dought its that high
Have any of you seen the new Mazda adverts? They are hyping a small displacement, high reving, high compression engine (with all the modern stuff like variable cam timing, direct injection, etc.) in one of their new small cars that gets BETTER mileage than the small motors they now have. The compression ratio is like 13:1!! Perhaps there is some hope in the immediate future for modern versions of Z-28-ish 302s ICW this high tech stuff? Gary
An example of this is my '63 327. Factory rated at 10.5:1, figuring it out in the real world, it's in between 9.6 and 9.7:1.
Don't have anything to add other than I am re-learning a lot. You guys are great to share the info. A lot of this will be food for thought when I tackle my 283. Thanks!
What with the small cubes, you'll have to work at it to get the compression high enough to cause serious concerns.
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