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| Compression ratio https://slantsix.org/forum/viewtopic.php?t=65856 |
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| Author: | CoyoteDan [ Fri Mar 19, 2021 2:57 pm ] |
| Post subject: | Compression ratio |
Hello all, So I have an OCG #2106 cam shaft and I am trying to get optimum compression ratio for it. From the searching I have done I'm looking to have 8 to 1 dynamic compression. Does anyone have advice other than that? Is static compression really important or should my focus be achieving that dynamic value? The car is a 1974 D100, bore is .060 over (not sure if that matters for this). I'm aiming for a nice truck build and I wanna keep it 87 octane. Thanks |
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| Author: | volaredon [ Fri Mar 19, 2021 4:25 pm ] |
| Post subject: | Re: Compression ratio |
I'm watching this too. I have a slightly different Oregon cam but also not stock. What carb and ignition are you gonna run? What exhaust? What trans and gear ratio? Looking to bracket race or tow or daily drive? Some of these factors play in as well, when determining best compression ratio. I'm building up a slant for a truck, too. |
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| Author: | Joshie225 [ Fri Mar 19, 2021 4:44 pm ] |
| Post subject: | Re: Compression ratio |
If you want to run 87 octane then 8:1 dynamic is too high. Post your cam specifications, a picture of the cam card is best, and your static compression ratio so we can calculate the dynamic CR. |
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| Author: | CoyoteDan [ Fri Mar 19, 2021 7:39 pm ] |
| Post subject: | Re: Compression ratio |
Right now everything is stock except the head has been ported (I did it myself so I could have helped it or hurt it ha.) Pistons bored .060 decked .020. Machine shop is doing the valves but they are staying stock size. I have a super six intake and Carter BBD but I don't know the condition of the carb yet so I figure it's probably not a good Idea to attempt the cam break in with a carb that may need a rebuild. Exaust is 2" the whole way. I've read that 2.25" is the way to go so I will be looking into that when I get to the exaust. I plan to cut an old manifold and fab my own Dutra Duals see if that works out. I have an A390 standard three speed I belive the ratios on that is 1st 2.99, 2nd 1.77, and 3rd 1, 3.55 rear gears. 15 inch wheels. I would like to daily drive this car and tow when I need to (that will be rare and nothing too big). Cam specs: 210/214 @ .050”, 254/254 adv, .434”/.444” lift, 108 sep I need to get all of my stuff back from the shop and calculate my static compression ratio. I realize it was probably foolish to bring the engine originally without knowledge of these things. I should have had a plan going in so I'm learning the hard way Thanks |
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| Author: | volaredon [ Sat Mar 20, 2021 6:27 am ] |
| Post subject: | Re: Compression ratio |
Sounds real close to what I have/have done so far except for I'm going o/s valves, I decked the block 0.030, (it'll take 0.020 to make up for the thicker head gasket as any commercially available one is double the thickness of the original) and I'm running an auto trans (at least for now) |
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| Author: | Dart270 [ Sat Mar 20, 2021 8:03 am ] |
| Post subject: | Re: Compression ratio |
That is close to a stock cam, so if you want to run 87 octane, you should not run above about 8.8:1 static CR. If stock 74 head and block, then you would be somewhere between 7:1 and 8:1 with a replacement composite head gasket (most are around 0.04" compressed). You could assemble the short block, measure the TDC piston-deck distance, then get them to mill the head to get the CR you want. There are other ways to do this, but that is a decent one. They/you could also install the crank and 1 rod+piston, measure deck ht, then deck block further. For that application, I would likely go for 8.5:1 static CR, which will be substantially above what it actually had stock (even though the spec would say 8.4). Lou |
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| Author: | CoyoteDan [ Sat Mar 20, 2021 9:47 am ] |
| Post subject: | Re: Compression ratio |
Well thank you for the advice. My original plan once it came back was to go with the method Lou suggested and instal the crank and a piston, measure deck height and take the block back to the machine shop. Is it better to make these compression adjustments to the block or the head though? |
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| Author: | Joshie225 [ Sat Mar 20, 2021 10:15 am ] |
| Post subject: | Re: Compression ratio |
Quote: Well thank you for the advice. My original plan once it came back was to go with the method Lou suggested and install the crank and a piston, measure deck height and take the block back to the machine shop. Is it better to make these compression adjustments to the block or the head though?
With the open combustion chamber it doesn't matter if you take material off the block or head to raise the compression ratio, it all works out the same. If the block was not given a clean-up surfacing I would mill the block. Also, check the deck height on #1 and #6 (with the same rod and piston) to see if the block is even end-to-end.
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| Author: | CoyoteDan [ Sat Mar 20, 2021 12:31 pm ] |
| Post subject: | Re: Compression ratio |
The machine shop cleaned it up they took .02 off the top. Is it safe to assume it's correct then? Or should it be rechecked? Also I am wonerding if 200 Hp at the crank is attainable with this harware and 87 octane goal? |
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| Author: | Dart270 [ Sat Mar 20, 2021 1:33 pm ] |
| Post subject: | Re: Compression ratio |
If block is decked well, then can just assemble and get head milled to remove the extra material and not have to take the block apart and take it back. You could make around 200 HP if you had a well ported head and hipo int and exhaust, otherwise, with that CR and cam you are looking at 150 HP or so. HP numbers do not mean that much in daily life, unless you like to rev to 5000+ RPM all the time. Most people inflate theirs or have no idea, so I would not worry about it. Yours will make plenty of torque assuming you assemble it all well and degree the cam. Lou |
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| Author: | CoyoteDan [ Sat Mar 20, 2021 5:34 pm ] |
| Post subject: | Re: Compression ratio |
Quote: HP numbers do not mean that much in daily life, unless you like to rev to 5000+ RPM all the time. Most people inflate theirs or have no idea, so I would not worry about it. Yours will make plenty of torque assuming you assemble it all well and degree the cam.
That is a very good point.Lou Well I guess the question is to maximize torque. What would you say my torque numbers are around? A little under 200? Also is the HEI all that it's cracked up to be I've seen strong opinions for and against it. Thanks |
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| Author: | Dart270 [ Sun Mar 21, 2021 6:39 am ] |
| Post subject: | Re: Compression ratio |
HEI works fine. I recommend NOT getting a cheap HEI module and make sure it has a proper heatsink. Pertronix works well too. Stock coil is fine assuming ballast works with whateve ign you decide to run. Torque should be over 200 ft-lbs at normal RPMs. Perhaps over 225. Lou |
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| Author: | DusterIdiot [ Sun Mar 21, 2021 7:35 pm ] |
| Post subject: | D |
D |
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| Author: | CoyoteDan [ Sun Mar 21, 2021 10:55 pm ] |
| Post subject: | Re: Compression ratio |
Quote:
To maximize torque you will install the cam and degree it to be advanced (how much will depend on the cam and static compression...too much will hurt things).
Ah man, I got to figure out how far I need to advance it?I thought I was gonna measure and make sure the engine spec matched the card spec. Well with say an 8.5 to 1 compression ratio and the rv10 what am I looking to advanced it to? Intake centerline is 104° to advance it say 3 degrees intake centerline is now at 101° did I do that right? |
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| Author: | Dart270 [ Mon Mar 22, 2021 5:05 am ] |
| Post subject: | Re: Compression ratio |
Yes, you did that right. Anything between 100 and 104 will work very well. I would shoot for 100-102 myself with that cam and comp, but 104 would be just fine too. Lou |
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