tomulrich

iTrader: (2)

I built a 427 ci motor. Currently being tuned at Norris Motorsports in Florida. A recent compression test showed 125psi on all cylinders. I have been reading and learning about static vs dynamic compression ratios. My static compression is 11.1 and my dynamic compression is 9.6(I did not add 15 degrees to the Intake valve close ABDC like some web sites tell you to do.). My intake valve close ABDC is 55.2.

I was told that because my cam is so large that the cylinder is bleeding off pressure..causing my cylinder pressure to be low. Cam is a futral custom hydraulic roller 250/257 duration, 113 lobe seperation, lift in the .665/686. Valve overlap is 28, intake centerline is 110, Exhaust centerline is 116.

I know 125psi is low. I have been searching and trying to understand this better....I am finding that 175-200 psi is what I should be shooting for.

Is this something that other people are dealing with??? Can someone that is running a similar cam check their compression PSI with a compression tester????

Anyone with large cams.....what is your cylinder PSI. I did degree the cam when it was installed ...it was not installed with any retard or advance.

All help would be appreciated......Thanks Tom

fast98

iTrader: (16)

sounds low but not bad nesacarily. as long as they are all even or close and above 100, i dont see a problem. it coulbe be due to the cam bleeding off pressure as u said.

LSONE

iTrader: (1)

what type of pushrods are you using with a cam that big? low compression can be caused by pushrods that are too long causing the valves not to seat properly (if that makes sense) letting air escape.

ssheets

iTrader: (1)

Or long duration cam timing that doesn't close the valves until the piston is already traveling up.

2001 Pewter WS6

iTrader: (43)

Did it make good power or not? Did it seem like something was wrong with the setup from the way it was running?

tomulrich

iTrader: (2)

pushrods are 7.3 I have adjustable jesel mohawk lifters. I set lifter preload at .025. cold. The compression was tested cold.

Havent done a dyno run yet...still working on idle issues/surging.

I did drive it...didnt get on it....but felt very sluggish. Norris didnt seem concerned that there was a mechanical issue. He felt strongly that it was the I was using.

does this pressure increase at higher RPM...Is this why big cams dont make power at low rpm.....but having the intake valve close later as the pistin is going up helps with higher rpm cylinder filling??

anyone with big cams(245 plus duration}...what are your psi numbers?

Thanks

tomulrich

iTrader: (2)

no one with big cams........anyone with more advice???


Thanks..Tom

71CamaroLS1

A hydraulic roller with 250 degrees duration at .050 will have about 300 degrees or so duration off the seat. It's seat-to-seat duration that determines a cam's impact on cylinder pressure when cranking and at low speeds. Remember, the .050 duration specs are just a convenience for comparing cams between manufacturers - they don't mean squat from the perspective of a running engine.

Let's assume that your seat duration on the intake is 300 degrees. Your intake centerline is 110 - add 150 degrees, subtract 180, and you get intake close at 80 degrees ABDC. In other words, between one third and one half of your compression stroke will revert right back past the intake valve at low engine speeds. That's why you have low cranking pressure, and why your engine feels sluggish. It is sluggish, because cranking pressure relates directly to cylinder pressure at lower engine speeds, and cylinder pressure is what makes torque.

Your engine will simply not create the torque it should unless you use a smaller cam or raise the compression or both.

tomulrich

iTrader: (2)

will it make the torque at higher RPM's? How do people run big cams if you cant get cylinder pressure. Raising my compression to 12:1 isn't going to raise cranking pressure that much. correct?

going to a smaller cam will give me low rpm torque .....bu thigh RPM will suffer....Correct?

What type of power HP/torque and at what RPM do you think my motor is capable of?

Thanks Tom

71CamaroLS1

Although, 125 PSI still seems too low - but I don't have any engine sim tools handy to model what it should be ...

71CamaroLS1

Consider the new LS7 - it makes 500HP at the crank, through short tube manifolds, low noise intake, etc with an intake duration of 211 (and 11-1 compression). With heads that flow big, you should still make very good power with less duration.

Did you run a leakdown test? And is the pressure that low when the engine is warm?

When it comes to cylinder pressure, there are big cams and then there are BIG cams. 10 degrees later intake close makes a big difference in cylinder pressure when the piston is moving up the bore quickly later in the compression stroke. And I'd bet my guess of 300 degrees true intake duration is conservative ...

TrendSetter

yes that is why bigger cams make less low rpm power is because of all the cylinder pressure that is bled off. that is necessary for higher rpm power, and always why you have to raise your static compression to make use of a larger cam. its all part of the horsepower game.

tomulrich

iTrader: (2)

I am at 11:1 now static. Dynamic is 9.5-9.6. Do you think I should be higher than that? How much is there to gain. Also is there a way to figure out what the cylinder pressure should be at a dynamic compression of 9.6. Is there a formula?

Thanks for all the help;;;;its appreciated. I will see how the car runs and what kind of dyno numbers it makes....if I am not happy I will tear the motor apart and re think my setup!!!

Feel free to chime in...

Tom

Patrick G

iTrader: (14)

At 9.6, your dynamic compression sounds way too high (for the cam you're running). What formula did you use to calculate it?

71CamaroLS1

Here is a link to an online DCR calculator:

http://www.rbracing-rsr.com/comprAdvHD.htm

When I plug in 80 degrees ABDC for your combo, it comes up with 150 PSI cranking pressure for sea level. Keep in mind that these calculators usually assume zero leakdown, which is far from true especially in a cold engine.

Note also that this calculator makes the same mistake of telling you to use .050 duration specs - this will appoximate your DCR at some higher engine speed, but not at cranking or lower engine speeds.

tomulrich

iTrader: (2)

Well just went to the above site. bore 4.125, stroke 4.00, 11:1 compression, rod 6.2, cam intake ABDC 54 degrees (this is what the cam card says...I assume it is at.050,

Results yield dynamic compression to be 9.48. It also says PSI should be 197psi. Why do I only have 125?

Right now the car is at Norris motorsports.....I wish I was able to do the compression test myself.......maybe thier gauge is bad?/...

Tom

strokedls1

iTrader: (1)

I know a local 383ci with 11-1 compression and a 244 250 .625 .625 on a 114LSA. The compression test showed between 195 lowest to 220 highest with average of 210 with the motor cold. I have never seen a hydrallic cam for a ls1 that big. Sounds like you have an issue with the cam, lifters, or piston rings. What are the rings gapped at? What kind of power is it putting down?

JakeFusion

iTrader: (7)

Well I don't know about the 125... but 11.1:1 static CR is to low for extracting the maximum potential from the setup. 11.7:1-12.5:1 is where you want to be.

Patrick G

iTrader: (14)

When calculating dynamic compression, you need to use ADVERTISED duration. Most calculators will ask for duration at .004" or .006" lift. If you plug in your duration at .050", the dynamic compression will be lots higher (and incorrect).

DavidNJ

Three words:leak down test