skinny z

Last years engine had a cranking pressure of 200-205 psi across the board. That would come from a 10:1 SCR and a relatively short 218 @ .050" cam. 110 LSA and a measured ICL of 100 degrees. ( 10 degrees advanced ). DCR measured about 8.14:1.
This years engine has been upgraded with a XR276HR installed straight up with 110 LSA and a measured 106 ICL. (4 degrees advance ground into the cam).
Compression pressure is considerably lower at about 175 PSI (+/-).
SCR remains the same at 10:1 while the DCR is a little lower at 8.04:1.
While I'm a little suspicious of the previous flat tappet cam ( adv duration of 284 which I find hard to believe ) I'm curious as to the relationship between DCR and cranking pressure.
A decent street engine should have cranking pressures in the 200 psi range while 180 is considered the low end of what's acceptable. That's with a DCR around 7.5:1.
Any experiences with cranking pressure and calculated dynamic compression ratios?
Have cam upgrades or even mods to the static compression ratios affected your compression test results?
I'd like to get the 200 psi cranking pressure back however my DCR is already creeping up there.

Atilla the Fun

I live at 4500 feet, and up here, 140 psi is considered acceptable, so long as every cylinder is above 137-138 or so. 200 almost never happens.

skinny z

600' above sea level here.

Rolling Thunder

Well if i understand what your asking correctly i may have a basic answer for you. First off as you know the SCR is simply based on the geometry of your combination. Its very simple if you compress 10 ci of fuel and air down into 1 ci you have a 10:1 comp ratio. Now this is all great and fine in an ideal world this would be you compression at all RPMs. However this world is far from ideal. Now lets talk about the cam specs for a moment specifically duration and overlap. Now without getting into a long scientific explination of whats going on ideally you want your cylinders to be completely filled with fuel air mixture. In the low RPMs you can run a short duration cam shaft with no or very little overlap and everything works fine. However when your motor gets into the high RPMs theres not really alot of time for this to happen so your cylinders to take some fuel air mixture but not its maximum potential. This is where duration comes into play. With longer duration and some overlap in the high RPMs the valves will be open longer allowing for more time for the cylinders to fill. It also takes advantage of the fact that once airs moveing like anything it builds up some momentum and wants to continue to fill the cylinder even when the pistons hit the bottom of its stroke. The escaping exhaust gasses also help suck in the fuel air mix. However on the flip side lets look at this bigger cam in the low RPMs like when its cranking. Because the valves have a larger duration the valves are open longer past the point where the piston has reached the bottom of its stroke. Now as stated earlier in the high RPMs the air is moveing so fast it wants to keep filling the cylinders even though the piston is starting to come back up. However in the low RPMs the air is moveing slower and this cylinder cramming momentum is not built up so the intake gasses are actually being pushed back out into the intake manifold befor the intake valve closes. This effectivly reduces your cranking compression and is also part of the reason why low end torque suffers with a big cam. Now that being said in the higher RPMs your DCR will increase as result of more momentum being built up in the intake charge. So as crazy as it sounds your DCR changes constantly with RPM so you cannot likely increase your cranking compression back up to 200 without putting your high RPM dynamic compression past the point of detonation with that cam.

skinny z

You bring up several interesting points, one of which I've overlooked entirely and that's the issue of overlap and how it affects low RPM cylinder filling ( like during a compression test)
I'm going to back into my notes and check/calculate the overlap values for each cam.
I will have to comment on the changing DCR though. The definition as I understand says the DCR does not change at any time. Effective cylinder filling would change with respect to RPM as you state however the factors that make up the DCR values are fixed.

Rolling Thunder

Yea your right about the dynamic compression ratio i used the term improperly i should have said cylinder pressure but all and all not bad for a saturday morning after a long friday night lol.

SpitotRs305

if you are looking to increase your DCR without changing your SCR you have 2 options
1 shorter rods
2 decreased IVC deg

skinny z

I've had many "morning after the night before" episodes. Some are even during the night before.
I appreciate any and all dialouge. That's what makes the hobby enjoyable.
I still have to check into the overlap point you made. Makes perfect sense. I want to investigate the numbers.

Damon

You added a fair bit of duration to the cam and you moved the ICL back from 100 to 106. Both serve to close the intake valve later than your old combo. That's your cranking compression difference right there. DCR and cranking compression go hand in hand.

175 cranking is about where you want to be for pump premium gas.

skinny z

That part is understood. What I'd like to know is if there's a formula for calculating how one changes with respect to the other.
Example: If you lower the combustion chamber volume by 1 cc, you gain about a tenth in compression ratio. 64cc yields 9.7:1. 63cc would be 9.8:1. We've all worked with that one.
If I add 10 degrees in intake duration or move the ICL by 5 degrees how does that pan out with regards to the DCR? Or if I want to restore my 200 psi cranking pressure, I can add 'x' amount of advance to the cam timing. I suppose you could just substitute values into the DCR formula and work backwards. Still that doesn't give you any cranking compression values.

skinny z

I'm thinking of advancing the cam timing ( although that increases the IVC angle BTDC). At least I was clever enough to install a hex adjust timing set.

I'll correct that BTDC. My calculator uses ABDC for intake angle which would give DECREASED IVC angle when advancing the cam. (Thanks SpitotRs305)

SpitotRs305

here KB pistons DCR calc
http://kb-silvolite.com/calc.php?action=comp

Damon

Leave it alone unless you can compare the performance changes on a dyno. Maximum cranking compression is not the name of the game. Maxumim performance is. Advancing everything can help build low end torque but usually at the expense of higher RPM power. If you went too big with the cam and lost too much low end torque a smaller cam is what you want, not dialing in large amounts of cam advance on the too big cam.

Here's a DCR calculator that also estimates your cranking compression for you:

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

Rolling Thunder

Alright now that my minds a little clearer lets take another stab at it lol. Well heres the problem i know what your getting at the with calculations but the probelm i see is there are so many variables efffecting the cranking pressure you could spend days weeks even months grinding through the math to get only a decent approximation. I mean theres flow rates to concider displacemets cranking speed altitude temperature ect. Now a mathamatical modle could be made for calculating the DCR based on changing the cam timing or useing a different cam in general as thats based off fixed known values although again it would be hard to equate that to specific cranking pressures. Back to the concept of cylinder filling you can adjust the cam timing to achieve better cylinder filling down in the low RPMs but theres also the high RPM cylinder pressures to tend to as well. Further more by correcting the cranking pressures to 200 you will effect the high RPM performance adversly effecting the cylinder filling in the high rpms when the valves need to be open alittle longer. This would hurt the high RPM performance which was the whole point of the larger cam in the first place. Now you could tinker with the cam timing a bit but for why? If your goal is alittle more low end torque go for it but keep in mind everything comes at a price.

skinny z

http://kb-silvolite.com/calc.php?action=comp
http://www.rbracing-rsr.com/comprAdvHD.htm
Both of these DCR calculators use the intake closing angle at .050". Comp cams only specs theirs at .006" (adv). Is there a general rule of thumb for adv vs .050" specs.

skinny z

Bench racing can be almost as much fun as the real thing. I like to build engines on information or as David Vizard would say "based on first principles".
I just figure if I try to maximize all of the components in my build, I may not have the best or the most up to date small block however I will have one that makes the most of what I have.
I've built some adjustability into this engine . Maybe I can use that to get a little more performance.

Rolling Thunder

Yea i dont mean so much why tinker with the valve timing as in theres no point but more what are you trying to accomplish by adjusting the timing? I mean if your trying to build more top end sure you can adjust the cam timeing to get better cranking compression but that would go against your goal of more high end. I love bench racing and discussing the theoretical aspect of motors its all alot cheaper and more interesting then actually physically doing it. Thats all i meant by why do you want to adjust the cam timing as more what you hope to gain in terms of performance. On another not comp cams does measure there cams at .050. To look it up just to to there web site type in the part number or the grind number and look up the spec card for your cam. The spec card has all the info listed on it.

SpitotRs305

comp gives the .006 timing which is as good as closed i think KB says to add 15 deg to find the true angle of closure i just wouldnt add it in

skinny z

I attached my cam card to this posting. It shows valve timing at .006" however, as I mentioned, both the previous DCR calculators ask for valve timing at .050".

KittDrifter

theres a 92 camaro z28 350 tpi i was interested in but he says the compression is shot to hell. he says the min compression for a 350 is 90psi, but the 6 pistons are at 87-89.3 psi. other than that everythings great. is the $1500 he wants for it a good deal and it will be ok or should i keep looking?

skinny z

You kind of jacking this thread however.......
With that kind of compression, despite the altitude, I'd say that engine is done. If the rest of vehicle is in good condition you'll have to decide what the balance is worth.
For the record, I picked up a (near) mint condition 87 Coupe with a blown engine for 400 bucks.