can see they at least surfaced the deck.....it sounds like it is all apart ? I still wouldn't be afraid of that cam, but i would degree it and get your cylinder pressure up as high as you can. After that you can time and fuel adjust with various methods to make it a happy camper. Mark
Degreeing the cam may indeed help and is well worth the effort. http://www.buickperformance.com/camdegree.htm That cam may be in completely wrong.
I just read that and it sounds pretty complicated. I also read the instructions for my timing chain and it says that as long as line up my key with the right shape on the gear my degree will be correct on the cam. Is this true? http://compcams.com/Technical/Instructions/Files/174.pdf There is what ive been reading.
What is your name ?? First off ....do you have any idea what your cranking cylinder pressure is ?? Might be a good indication if you are still assembled. If not ....it really is not very hard to degree a camshaft....you would be best off to read some articles and sleep on them for a night to think about what is really happening when you degree a cam and what happens as a motor runs with camshaft timing. Just a single degree off can cause a huge difference in cylinder pressure.....time it and then you will sleep a whole lot better and will be able to work some of the other things out much much easier. Mark
Unfortunately that will depend on how much advance was ground into the cam, as well as whether or not the timing gears were machined at nominal. That's why we always recommend checking with a degree wheel. Devon
My name is Kody. The motor is still assembled, but im not sure what the cracking cylinder pressure is. Is that just normal cylinder pressure that i can check with a normal compression tester? My cam says it needs to be at 4*, and the bottem gear has 3 ways to line it up. O, which is basically nuetral, square which is retarted, and triangle which is 4* advanced.
Yes, cranking compression is measured with the pressure gage screwed into the spark plug hole. Disable the ignition and keep the carb at WOT while you crank the starter. Best to do it at operating temp, so you might have to warm the engine up once or twice before you make it through all eight cylinders. Devon
Also, most q-jets don't like more than 5 psi total fuel pressure. Does your car seem to run rich? -Cort
What exactly does the cylinder pressure tell me? Im not sure what my fuel pressure is, but it doesnt seem rich. I read a lot about cam degreeing last night. From what i can tell degreeing shows you if you cam is identical to the cam card, but it also gives you an accurate tdc rating. The biggest thing it seems to do is tell you if your tolerences are off and yur cam doesnt matxh to the cam card, and if so you can correct it. You set the actual advance degree by the timing sprovcket. Does this seem about right?
Hey Kody.. Your pretty much correct about the degreeing thing, yes it will tell you the acurracy of the camshaft machining. But you also have to think of how the accuracy of the timing of the camshaft in relation to your cylinder timing affects the motor as a whole. In one sense as the camshaft is advanced you are closing the valves a bit earlier to increase cylinder pressure at slower rpms. ( you are faking out the piston as to the whole duration thing ) .....on the other hand as the camshaft is timed with the valves open longer and earlier your pistons cylinder pressure is actually building as your engine speed increases ( kinda like your piston is chasing your camshaft ). Thats why when you have alot of duration ( valves open during a longer period )...as well as seperation ( yours 110 degrees ) ....the difference at which both vales are fully open you need alot of cylinder pressure to over come the absense of closed valves as the piston approaches the top of it's stroke.....or in other words you can compensate for this by advancing your camshaft to close the valves a bit earlier..... Help ? Mark
Ok that all makes perfect sense. Now my question is do i have to degree the cam shaft or can i just advance the degree on the timing sprocket? Thanks for all the help, you guys are great.
Kody, I posted this link before. http://www.empirenet.com/pkelley2/DynamicCR.html I urge you to read it. It will tell you the difference between Static compression ratio, and dynamic compression ratio. Different cams close the intake valve at different times in the compression stroke. This increases or decreases cylinder pressure because actual compression cannot start until the intake valve closes. A cam that closes the intake valve later will have less cylinder pressure because the pressure is bled off as the piston moves up the cylinder and the intake valve is still open. The article explains this and gives you a downloadable calculator that lets you calculate everything we have been talking about in this thread.
larry is 100 percent correct with the figuring of your compression ratios.....you do as a " hobbiest " have the luxury of being within the ballpark as far as perfect timing and compression matching as you are a daily driver and not a racer....but he is right, to have it matched proper you need to do your calculations prior to changing any parts or timing events. It does sound though that by advancing your camshaft you will remedy alot of your problems. Mark
Sorry if it doesnt seem like im listening to you guys, but im doing my best. Im cramming in a lot of new info and ive having a hard time retaining it. I never crammed this hard for school. Larry, ive read that link a couple of times but its so long and a bit complicated so its takinge me a while to comprehend it all. Ill read it a few more times and hopefully i will understand it better. I got a full gasket kit today and im gonna start tearing the car apart on wednesday...hopefully. Thanks for all the help.
Start with the Cam Timing and Overlap Calculator(Center Tab). Input the Advertised Duration #'s. For your cam they are 280/284, Lobe Seperation is 110. Intake Lobe centerline would be 106 (4* advanced). Then hit calculate Click the Dynamic Stroke Length Calculator (Left Tab) Input the crank stroke (3.85 for SBB). Input the rod length (6.387 for SBB). Hit Calculate Then click Compression Ratio Calculator Input what you know. Bore diameter would be 3.80 +.030 = 3.830. Stroke would be 3.85. Combustion chamber volume for the SBB would normally be 59. Head gasket would be .040 Head gasket bore can be 3.9 Piston to deck clearance will vary. (This is one of the things you need to know, try using .040 ) Click dished for piston type, and input dish cc (again you need to know from measurements try using 20) Then click calculate. At the bottom you will see the static compression ratio. If you click the box opposite stroke, the crank stroke will be replaced with the dynamic stroke (stroke remaining after the intake valve closes) Then hit calculate again. You will see the Dynamic compression ratio(DCR). It should be between 7.5 and 8.4 for a strong running street engine on pump gas. Probably better to stay towards 8.0 though. You can try different cams and see how it affects your DCR. The Static Compression will not change with different cams, but the DCR will Hope that helps.
That means it won't run up to it's potential. You can try advancing it 8* or increase you SCR by using a thinner head gasket or milling the heads. That 6.91 is only an estimation. We really don't know what your true SCR is because we don't have your true numbers. Try messing around with the head cc's and deck height, and head gasket to up the SCR. Then see what effect it has on your DCR. Play with the numbers.
Well i know the block has been decked and the heads have been resurfaced. So hopefully that helped. I can see about shimming my heads to get higher compression. How would i advance my cam to 8*? To my understanding you advance it by the key groovs cut into the timing sprocket. My sprocket only goes to 4*. Whats the downside to advancing it to 8*? Im assuming the up would be more compression?
