Hello Chris Craft SBC Think Tank!
I have been faced w/ some interesting questions regarding my 327Q.
Fisrt has to do w/ compression ratio. Cylinders bored 30/1000ths over. I have new pistons, however the face is flat w/ 2 slight reliefs for the valves. The old pistons were "dished". I am told in order to lower compression ratio to maybe 8 to 1?
Question. Can I use my new flat top pistons w/o causing a problem. I know that compression ratio will be increased, but how much?
Has anyone replaced "dished pistons" w/ flat faced pistons?
Also, Intake valves have been changed to reflect 2.02.
Cam is Extreme Marine Comp Cam which is mild but somewhat hotter than original.
Any advice on the pistons w/ that setup?
Some quick thoughts late at night. First you are smart to be concerned with compression ratio. I would avoid increasing the ratio very much in these days of high fuel prices,especially marina gas. YOu don't want to get in a situation where you are requiring high test/octone gas to keep your engine in one piece. The ultimate goal for me anyway, is to set the engine up to run on mid grade gas which is usually a 8.5 to 1 compression ratio with a carbureted engine. You can get away with a little more depending on the cam grind but not much more. Maybe 8.8 to 1. Running 9 to 1 comp ratio is treading on thin ice with no electronic timing controls to retard timimg.
More valve opening duration and overlap in the cam will effectively kill some of the cylinder pressure at lower RPM's but that can be a complicated subject in itself. Be very careful with the cam selection, you don't want to get to radical with a marine cam. Increasing the cylinder bore .030 over raises compression ratio a little in itself,surfacing the block and/or the head mating surface a little more, and the thickness of the head gaskets are a another factor. I don't think the 2.02 intake valves are going to be much of a benefit at the below 4500 rpm range this engine will be running in. If the stock heads are running 2.02 intakes those are Corvette or SS heads, that found there way onto some marine apps., not really necessary on a smaller engine/heavy boat combination.
Bottom line: The piston manufacturer usually can tell you what the compression ratio will be for a certain piston and head combination. Let them advise you. I am guessing the flat tops will raise CR too much though, because the 327's are 1960's vintage and the head chamber volumes are small compared to the later so called smog heads with the larger chamber volumes.
Your 23'is a fairly heavy load for the little 327 to push around so don't kill it by running to much cylinder pressure.
It is already going to be working hard enough. Prop choice is critical to letting the engine breathe.
Many years ago my boss at the time decided he was going to get into computer drafting. He purchased things one at a time, got a deal, shopped around, let his girlfriend at the time even give him some direction. Eventually he got everything hooked up, the software didn't work well, the guy he hired to run the thing didn't have a clue, and I saw a client come in one day and read him the full Monte riot act because he didn't meet a deadline. The stuff didn't work together at all.
We have to beware buying pistons and cams, with increased bore, etc., because those all need to work together.
The increase in bore will lower the compression if you use the stock type piston, because there is a bigger volume to compress with the same crankshaft stroke. Therefore, it may be good to use a piston that will compensate for the bigger volume, because that flat top may give you back the compression you lost when the cylinder bores were made bigger.
Just using a flat top piston isn't very telling, because we need to know WHERE (in relation to the centerline of the crankshaft connecting rod) that flat top is located. If it is foreshortened, the compression will be lower, if it is a little closer to the valves, that flat top will provide higher compression.
I looked up the Xtreme Marine Cams at Comp Cams, and I didn't find any listed for below 1000 RPM. Most of them are in the .440 to .475" range, which is okay. There are quite a few listings.
I agree the larger valve won't do much in the rpm range you'll be seeing. Some systems actually use smaller valves to promote torque. With the bigger valves, a little hotter cam, perhaps a compression ratio in excess of 8.5:1, you should have a nice running machine. There are charts about the compression ratio for each of the various SBC cylinder heads. I posted one here not too long ago. I am sure the piston manufacturer will KNOW exactly what the compression is, if you can find someone who will take the time to look it up.
I think you need to find out what cc your heads are in order to get the actual compression. There are a few web-sites you can go to and put your engine stats in and get a idea of what your cubic inches, compression, torque and HP are. I would think you would want more torque then hp? (always been a good topic)
From a previous post, here are the common GM cc listings.
The marine motor most likely has the larger volume and lower compression 76 cc head.
The 11.2 = 57cc cylinder head
The 10.3 = 64cc cylinder head
The 9.9 = 69cc cylinder head
The 9.0 = 76cc cylinder head
You can probably get the number off the cylinder head and quickly look up the spec here on the Mortec web site. http://www.mortec.com/
The cylinder head, however, is not the only thing that regulates compression in the motor. It is a combination of the volume of the head, and the piston too. If Bill had a dished piston, did a bore job, now has a flat top which might give more compression, that would appear to be self regulating to a point. There are so many variables with the SBC, there are probably thousands of potential combinations depending on the manufacturer used, etc.
If you check the head and do the math, that should settle this. Again, the piston manufacturer is the best source, because they will know what their product will do with each of the heads noted above.
That is quite a nice reference site for the small block Chevy guys. I agree with the last statement you made, Paul, to contact the piston manufacturer, let them know what the numbers on the cylinder head are and the new bore, and they will validate the resulting compression.
One thing I would like to add about the small Chevy motors, especially older ones. There is a chance they have been rebuilt in the past, and due to the fact that small block Chevy cylinder heads are very plentiful, there is no assurance the one that came on the motor was the one intended for that application. Cylinder heads are often done on an exchange basis, and if it will fit, it was sold sort of thing. Some people made out better than others with that system.
I assumed and that is always dangerous, that piston pin location is to remain standard. There are so many different pistons available these days mainly for racing apps that nothing should be assumed. If using a stock piston the top of the piston at the perimeter edge will be the same
distance up from the pin whether it is a flat top or dished. Paul is right about that. One item I would disagree with is the effect of an overbore on compression ratio. It will raise the CR a little because you are squeezing a larger volume of air into the same size combustion chamber space, so the ratio goes up. The amount is almost negligible sp? but when added with other small changes it can start becoming a factor. Using a .040 head gasket from FELPRO is a very smart choice and will give excellent sealing but since it is thicker it would raise the head off the block deck that much. That would increase the head chamber volume a little and would lower CR. Bottom line: The .030 overbore will raise the CR a little and an .040 gasket would lower the CR a little therefore the final CR would be relatively unchanged. DO WHAT?
Best advice so far.
1. Consult piston manufacturer, use the best piston you can afford
2. Stay modest with cam selection, not too much duration over stock or you will lower cylinder pressure at lower RPM's
More lift is OK
3. Find out what your head casting numbers are to determine chamber volume
4. Brass freeze/core plugs
5. Concentrate on making TORQUE, not horsepower. Torque is king on a boat engine. A boat engine is always pushing uphill
6. HAVE FUN
Thanks Greg,
I will get the specs from the piston company "Sealed Power Pistons".
Also, my heads are original equiptment, however they have been ground incorrectly to the point that the valve was seating too far in. My machine shop is of the opinion that the heads are a good candidate for 2.02 intake valves.
I am going through all this work and would like to get something more out of this motor. I don't want to do it in a way that will shorten it's life.
I have a cam from my 327F that I do think is the same as the 327Q motor. They both are "automotive,Standard,left-hand" rotation motors.
I was thinking of several minor changes for the power. .30 over pistons, mild cam, 2.02 intake.
It's not too late to stay stock, however .30 over is mandatory due to cylinder specs.
I hope you don't mind sharing what you learn from sealed power. I just installed Sealed power flat top pistons (want to say PN# 310P) in my 327Q block, which has also been bored out by .030". I'm also curious if I will be increasing the CR too much.
I have an extra head gasket (came with my gasket kit, but I'll be using a marine head gasket). So if Sealed Power can't/won't give an answer I could install one head and just measure the CR; should be ballpark except for the crushed thickness of the head gasket.
Over the weekend I've tried to educate myself a bit better about compression ratio and how to calculate it. Based on the information that I have been able to gather on these Sealed Power 310P pistons that I put in my 327Q I've come up with the following:
The head volume is estimated (I knocked .5 cc off of the stock nominal value to account for the re-machining of the heads), the "eyebrow" volume is estimated based on figures available from Summit for similar pistons, and the deck height was measured.
The stock compression ratio was 8.8:1, and with the modifications that have been a slight increase is expected. Do you guys think that by retarding the ignition I can run regular or mid grade gasoline in this engine? If I switched to 13 cc dished pistons I could get down to 8.29:1 or so, it would be inconvenient to do, but it would be much worse if I decided it was necessary later on into the build. Just not sure if I am so far off from stock that it is worth the extra time/money.
The stock 427 marine motor is 8.9:1 compression. I run marine 93 octane, which is the only thing they sell at our marina. Have encountered no compression or octane related issues.
Your calculations are interesting, but in practice those numbers can be off a bit, and even vary between cylinders, and you'll never know it.
If you end up with the designated compression ratio, you should have a pretty healthy motor. You should be able to tune it to run very well too.
Tom mentioned the 93 octane marine fuel. I don't know of any difference in the gas we're pumping at our marina or not, when compared to local pump gas, I don't know if there are more detergents in it, or what. I would shy away from any ethanol, because it's a solvent for the kind of varnish that forms in fuel tanks, and I understand fuel efficiency is reduced with ethanol too. I had to pump some 10% ethanol into the car this morning and I didn't like it. Hopefully I'll find some "real gas" nearby. BP and Shell are both pumping 10% ethanol in Nashville now, but believe it or not, Kroger is pumping the real stuff.
In any case, with your compression ratio still below 9.0:1 you should be okay if you can get good gas at your boating locations. Even if you have to use ethanol, get yourself a BIG fuel filter and tune accordingly.
That's my take on it. Hope everything works out for you.
Sounds good, just remember if you start pulling timing out it will start negating the performance enhancements you are trying to do. It may be fine, try it out with mid grade, keep your hatches open on some test runs to listen for detonation and look at your spark plugs after a few runs. They will show effects of detonation on the ceramic, little spatters of gray. Make sure your props are letting the engine run up to high side of RPM range, too much pitch will lug the engine, raising the cylinder pressure and increasing the risk of detonation. It is good to see someone really do their homework when rebuilding an engine instead of just throwing it together "shadetree" style. GOOD JOB!!
Early Chevy heads need a close "quench" area to create turbulence in the chamber. The distance from the piston top to the flat area of the head should be 0.040 or a little less. This turbulence helps prevent detonation, and requires less ignition lead. Make sure there are no sharp edges in the combustion chamber. With a slightly fat mixture and not too aggressive timing (30 degrees @ 3000 rpms) you should be ok with 89 octane. You can't hear detonation.
Bill, when you hear spark knock that is detonation. An engine can stand small amounts of spark knock and still live a long time but severe spark knock is trying to tell you something. It is a resonance or vibration of the block, and computer controlled engines use a knock sensor screwed into the side of the block to detect this. It sends a small voltage to the computer which in turn pulls timing out until the knock goes away. Detonation is uncontrolled combustion after ignition. Pre-ignition is spontaneous combustion before ignition. Similiar results though. UGLY!
OK - little tuning and higher grade gasoline required, know what to watch for, guess I can keep going on the build then....phew! Thanks again for all your help,
Yes, you hear spark knock, which is light load pre-ignition. Detonation occurs at high rpms and high load conditions, and is inaudible. While similar, spark knock doesn't create the immense temperature/pressure spike that melts stuff and beats bearings to pieces. You can have "pinging" without detonation. My truck has been "pinging" for 150,000 miles. The "ear" on a digitally controlled engine hears the metallic clatter of spark knock and incrementally bumps the timing back. It is frequency sensitive. Usedta be able to tap the valve cover with a wrench and shut the engine off.
Detonation gives no such noise, and has MUCH higher temperatures and pressures. I've melted forged pistons in a matter of seconds from detonation, which I never heard.
Exhaust temperatures are around 1000 degrees fahrenheit..aluminum (pistons) melts at 1300-1500 degrees.
Don't be lulled into a sense of false security. Pinging is an indication of too advanced ignition, causing a pressure spike before TDC in a partially filled cylinder. Detonation is compression combustion like a diesel. Since a gas engine runs with a throttle plate, part throttle operation results in much lower pressures in the cylinders from incomplete filling of the cylinder, while wide open, high rpm operation creates maximum filling, with correspondingly high pressures and temperatures.
This spontaneous combustion is instantaneous, while a spark plug ignites a mixture that burns at a finite rate. This burn time is the reason timing has to advance with rpm. Pre-ignition means you've lit the spark a little too early, but the mixture still burns rather than explodes as in detonation.
Sorry my thumbs are too big for the buttons on this cellphone! Try doing a search for that article, I could not link it for some reason, pretty good info for motorheads!
