Dynamic compression ratios DCR

Good day gents. A few questions:

1) Do any of you know the DCR of the stock 2000 motor?

2)Specifically to my cammed NA gen 2 viper owners, do any of you know the DCR you are running? If so, could you elaborate on your setup such as if your bottom end is stock, worked or aftermarket heads, and if you are willing, your cam specs. Im not trying to copy anyones setup here but am more interested in what DCR's are safe. I have purchased my cam and heads already and am trying to optimize the compression ratio and need this info to calculate a safe optimal DCR and translate it into the SCR to shave the heads.

Thanks for any feedback.

I have a long thread on this intake valve closure and cranking cylinder pressure issue. http://forums.viperclub.org/threads...reg-Good-Heads-Dyno-708-Cam-Stock-Improvement.

The answer is 190 psig cranking cyclinder pressure is the wall right before you cant run on 93 pump gas. My car is 189 with no problems but i am on the edge because I produce 1.29 ft pounds per cubic inch. Be careful as my target was 182 cranking cyclinder pressure. One bank came out 181-183 the other 187-189 and when I asked the machinist why he said we did not want to take the sleaves out to deck the block and both sides were in tollerence but one was high and the other low.

If you target 189 and get 195 on one bank you will more than likely have predetination problems at WOT and peak torque.

Eric

I think the advertised gen2 comp ratio is 9.5:1. My combo for my old gen2 was 10.25:1 and was achieved by JM CNC Heads with a 80cc chamber, 4.020 bore, .040 gasket and the the piston .007 out of the block. Camshaft was a 228/234 @.050 with .587 int lift and .575 ex lift. Also had a set of SVS stepped headers. Make sure you get someone that knows how to tune a Viper!!!

What was your intake valve closing event?

ericj96 said:

I have a long thread on this intake valve closure and cranking cylinder pressure issue. http://forums.viperclub.org/threads...reg-Good-Heads-Dyno-708-Cam-Stock-Improvement.

The answer is 190 psig cranking cyclinder pressure is the wall right before you cant run on 93 pump gas. My car is 189 with no problems but i am on the edge because I produce 1.29 ft pounds per cubic inch. Be careful as my target was 182 cranking cyclinder pressure. One bank came out 181-183 the other 187-189 and when I asked the machinist why he said we did not want to take the sleaves out to deck the block and both sides were in tollerence but one was high and the other low.

If you target 189 and get 195 on one bank you will more than likely have predetination problems at WOT and peak torque.

Eric

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Sorry, I have a hard time getting time to post. I read your thread before I posted this and it does have good info in it. It doesn't have quite what I'm hunting for though. I'm looking for the Dynamic Compression Ratio. I'm sure you know, but for anyone who doesn't, this is the compression ratio measured from when the intake valve closes to Top Dead Center (TDC). The cranking pressure is no doubt important, and I will delve into that eventually but right now I'm more interested in the DCR as it is a fixed number that doesn't change due to outside variables like atmospheric conditions. I was really interested in the stock DCR of our engines and am currently trying to calculate it but, as of now, can't seem to align the numbers with the Static Compression Ratio (SCR) of 9.6:1. I wanted the stock DCR to evaluate what Dodge was considering safe out of the box to give me some sort of base of comparison. I wanted the DCR of other Gen2 cammed cars to further see what others are "getting away with". Much like the 190 psig being the wall as you posted, a DCR of 8.5:1 is the seemingly arbitrary limit for pump gas NA engines and other reports I've read being 8.0:1. I wanted to test these numbers by comparing them to cars like yours and other cammed Vipers.

As for my intake valve closing event...I don't know it yet. I am still waiting for my cam and cam card so I can't even theorize as of now. Sounds strange, I know, but to get the heads I wanted I had to purchase this cam that came with it. The heads were originally purchased by another member with a cam custom made for him but he backed out of the sale and the seller insists the cam come with the heads. My arm was twisted if you will. The heads are, in my opinion, more than worth the gamble on an unknown cam. This is what has prompted the interest in DCR as I want to optimize this cam with the appropriate compression ratio. It is supposed to be a big fat lumpy evil cam (which is fine by me) but big cams are, for the most part, worthless without a compression to compliment them. If I don't like the numbers the cam is giving me I will sell it and have a custom one ground to my specs. This cam maybe just what I'm looking for though.

As far as the stock DCR I think I may have some numbers incorrect. I can't find some of the info on the 2000-2002 cam. What is the intake center line and angle between lobes for these years? Your post revealed for gen one and the 708 to be 117 deg and 114 deg respectively. I'm thinking the 2000-02 was different no? Also not sure the stock piston to valve clearance.

I am going to take measurements when I pull my stock heads of course but right now I wanted to do some bench numbers while I wait for my heads and cam to arrive. I was interested in getting Dynomation 5 or Engine Analyzer Pro. Any experience with these? I plan on many further mods on my car and my other cars and was thinking this would be very informative and fun.

Again, good read on your thread. Need more posts with raw numbers like that. Tech data is tasty.

vp97 said:

I think the advertised gen2 comp ratio is 9.5:1. My combo for my old gen2 was 10.25:1 and was achieved by JM CNC Heads with a 80cc chamber, 4.020 bore, .040 gasket and the the piston .007 out of the block. Camshaft was a 228/234 @.050 with .587 int lift and .575 ex lift. Also had a set of SVS stepped headers. Make sure you get someone that knows how to tune a Viper!!!

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Good info. Thanks. Do you know when your intake valve closes After Bottom Dead Center (ABDC)? If you have that info I can calculate the DCR. Hey, I went to Marcus HS. Lived in Lewisville most my childhood. I live in Greenville TX. now.

A couple of things to note, 2000+ vipers have high bleed down lifters, in addition to different exhaust valve timing. I have the info. your looking for but can't seem to locate it now. I do have my current cam specs. but unless your running Striker heads they really won't mean much to you. I can also tell you that I was warned about compression and did not listen to those who told me I would have issues. I will say this, with Striker heads you can run a bunch more compression than I was told would work on 91. I honestly can't comment on the results with factory heads.
It's really nice to see other guys on this site who are actually gear heads, where were you guys 10 yrs. ago!

" I'm looking for the Dynamic Compression Ratio"
Answer: My dynamic cylinder pressure is 7.6 at 0 Ft elevation.

"I was interested in getting Dynomation 5"
Answer: I used an older version of this and it was very helpful.

"a DCR of 8.5:1 is the seemingly arbitrary limit for pump gas NA engines and other reports I've read being 8.0:1."
Answer: A DCR of 8.5 will ping like hell in most NA Viper pump gas applications.

By the way I'm at 8.85:1 DCR with 91 octane and zero issues. Where are you pulling these hypothetical assumptions from?

I am assuming full timing advance with a factory 195 F thermostat and 12.8-13.0 to one air fuel. I have the aggressive alternate factory tune for the gen one that had full timing. Its very hard NA to pull much more than 1.3 Foot pounds per cubic inch without problems on older head designs I am already getting there at 1.29. Richer colder motors with great heads can get away with more.

With a 8.85 DCR, a 3.88 stroke and 6.123 rod your cranking cylinder pressure should be about 224 psi on a healthy motor. Great that 8.8 worked out for you. I could not run street gas with a 205 CCP which equates to 8.25 DCR. Maybe your heads are superior to mine & manage heat better, run less total time, running richer AF and 180F thermostat. I dont know.

It's driving me crazy, that I can't find my stats, but off the top of my head my ccp was no where near 224. I want to say it was some where in the 190 range. Cylinder heads are the key ingredient here. The stock head design is really archaic compared to the newer designs. The fact that I added a camshaft that was huge compared to the stock cam with those cylinder heads and not only lost no torque anywhere is unheard of, at least in my experience. In fact the engine picked up an additional 120 lb/ft of RWtorque from bottom to top. I've never, ever seen that before on a naturally aspirated engine.

Right 190 is reasonable but thats not 8.8 DCR. I have no doubt that if you have a 190 CCP you are fine. I am at 189 and have no problem at all so we are in agreement.

We shoot for 190-210 cranking compression with a 228/232 cam, 114 intake centerline. It requires a recalibration. If the engine has less than 190 cranking compression it does not make good peak power numbers. The cam you run and where it is centerlined make a big difference. I tuned a car with GG heads and big cam, it made 220 cranking compression and 630rwhp (Gen 3) on 91 octane fuel. It did require less than normal timing.

Just my experience, yours could differ.

Dan have you found that gen 3 intake maifolds with short runners can carry higher cranking cylinder pressure than gen 1/gen 2 intakes can? I would expect a short runner intake to carry higher cranking pressure than the long runner design on the older motor. Actual cyclinder pressure is directly related to peak torque and some manifolds pull higher peak torque numbers per cubic inch due to intake velocity "supercharging" in a more narrow operational band when its in resonance. Tune port injection has a history of producing great peak torque but its not so great on the top end. I would expect a gen 3 intake to carry more CCP than my older intake design all else equal.

I am not surprised that a gen 3 intake with reduced timing runs fine around 200+ ccp. Were you running a little rich too like 12 AF? Did you pull timing at peak torque and then throw it back in on the top end? Did you throw in fuel heavy in the mid range then lean back out after 5 grand?

So in the end you were able to push the design out to about 8.25 DCR and get good results but you did it with reduced timing and a short runner and a little rich in the mid range? Maybe a 180 thermostat too?

I think my gen one would be unhappy at full time and 13 to one AF at those pressures.

Dan thanks for posting I follow your comments frequently but dont say anything.

sanguissaevus - Sorry, my cam was a 225/234, same lift as in my other post. My int valve closed at 44.5 degrees @.050 with the cam installed straight up. I ******** it 2, 3 and then 4 degrees. Ended up with sticking with 46.5 degrees. I couldnt tell any change. My cranking pressure held at 195psi on a hot engine.
Never ran into any engine ping with 93oct gas. I will give credit to the JM heads, tight quench and the Dan Cragin tune for that.

Camfab said:

A couple of things to note, 2000+ vipers have high bleed down lifters, in addition to different exhaust valve timing. I have the info. your looking for but can't seem to locate it now. I do have my current cam specs. but unless your running Striker heads they really won't mean much to you. I can also tell you that I was warned about compression and did not listen to those who told me I would have issues. I will say this, with Striker heads you can run a bunch more compression than I was told would work on 91. I honestly can't comment on the results with factory heads.
It's really nice to see other guys on this site who are actually gear heads, where were you guys 10 yrs. ago!

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Well, actually yes, the heads I am waiting to ship are Striker streets. Guess I should have mentioned that considering different design from stock like spark plug location and such. I think I bought the last ones on the market. So, your setup would be of high interest.

10 years ago I was a Lance Corporal with barely enough money to buy a toy model of this car I was driving her every chance I could though on Gran Turismo This has been my dream car since I was a tween.

Camfab said:

By the way I'm at 8.85:1 DCR with 91 octane and zero issues. Where are you pulling these hypothetical assumptions from?

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A few different online write-ups and posts. None of them substantiated these limits though and that is why I wanted to dig a bit more to see where our engine stands in regards to these limits. Here is one write up many seem to refer to: http://cochise.uia.net/pkelley2/DynamicCR.html Most of what I found involved chevy engines.

Your DCR is a prime example of this not being a set number across the board. It is high by all accounts I have read. I'm reading through Internal Combustion Engine Fundamentals by John B. Heywood. Maybe it will touch on this limit or "sweet spot" or lack of. I haven't made it that far in the book though. I'm hoping the Engine Analyzer software will also give some insight on this. Great info. Thanks for the input.

Dan Cragin said:

We shoot for 190-210 cranking compression with a 228/232 cam, 114 intake centerline. It requires a recalibration. If the engine has less than 190 cranking compression it does not make good peak power numbers. The cam you run and where it is centerlined make a big difference. I tuned a car with GG heads and big cam, it made 220 cranking compression and 630rwhp (Gen 3) on 91 octane fuel. It did require less than normal timing.

Just my experience, yours could differ.

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Cool. Thanks for the info. Seems like the 190 cranking compression works for many here. I will be running exclusively 93 oct here in Texas. Would you by chance know what the ICL and lobe separation is on the 2000 stock cam?

vp97 said:

sanguissaevus - Sorry, my cam was a 225/234, same lift as in my other post. My int valve closed at 44.5 degrees @.050 with the cam installed straight up. I ******** it 2, 3 and then 4 degrees. Ended up with sticking with 46.5 degrees. I couldnt tell any change. My cranking pressure held at 195psi on a hot engine.
Never ran into any engine ping with 93oct gas. I will give credit to the JM heads, tight quench and the Dan Cragin tune for that.

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Great insight. To clarify, these are stock heads ported by JM not street Strikers yes?

Eric what is your exact formula for calculating DCR, as it may be possible that I am using a different formula. I also remember having a long conversation with Jeff Morys, the developer of the Striker heads. I was under the impression that the Gen III manifold would be an upgrade for my application. Jeff revealed that the Gen III manifold gave up huge torque numbers throughout the entire midrange as compared to the Gen II.

Sanguissaevus, let me know which cam you get when you receive your heads. I have no issue with revealing any and all information regarding my combination.

Eric what is your exact formula for calculating DCR, as it may be possible that I am using a different formula.
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Answer: There are many online calculators for this and they all read the same assuming you select the same atmospheric pressure. Not2fast has a good one. I am lazy and do not run the calc by hand. I would if I had to.

I also remember having a long conversation with Jeff Morys, the developer of the Striker heads. I was under the impression that the Gen III manifold would be an upgrade for my application. Jeff revealed that the Gen III manifold gave up huge torque numbers throughout the entire midrange as compared to the Gen II.
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Answer:As I mentioned above the actual working cylinder pressure at peak torque normally is higher for longer runners that are in resonance than short runner intakes. That’s ONE of the reasons I think that guys like Dan can get away with higher CCP/DCP on the gen three intake designs with big cams. I have never seen anyone do a back to back dyno of the gen 1/2 manifold vs the gen 3 intake so I can not say with any certainty. It is my view that our intakes (gen 1/2) that go out of resonance by about 5700 PRM are poor mates for big cams. They were designed to make tons of mid range torque at the expense of high end flow. Gen three intakes with their larger plenum volume and short runners were design to make good power at higher RPM.

Gen 3 intakes are the ticket with big cams because they both want to peak at relatively high RPMS.
Gen 1&2 intakes work great with fast acting cams with longer than stock durrations at .200 but moderate durrations at .050 because both the cam and manifold want to peak in the same RPM range.

Cam, that’s the best answer I know how to give you.

Running the stock head casting ,stock sized valves (11.5 to 1 compression )Edelbrock Headers ,Crower State 2 Cam with o tuning my old Gts trapped close to 132 mph in the late 90s .Iwas never one for chasis dyno numbers as Dynojets are extremely easy to manipulate and or give different power numbers daily (even corected ) while a trap speed says it all The car was 488cu on pump gas and ran 11-2 to 11.3 (stock Pilots )at 130 plus all day long. Hello Dan Craigen.

1BADGTS said:

Running the stock head casting ,stock sized valves (11.5 to 1 compression )Edelbrock Headers ,Crower State 2 Cam with o tuning my old Gts trapped close to 132 mph in the late 90s .Iwas never one for chasis dyno numbers as Dynojets are extremely easy to manipulate and or give different power numbers daily (even corected ) while a trap speed says it all The car was 488cu on pump gas and ran 11-2 to 11.3 (stock Pilots )at 130 plus all day long. Hello Dan Craigen.

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Ps for you guys tuning these cars now trying these mega heads cam packages beware the manifold will never support it .Ispent thouands on different heads ,cams and maniold work for nothing as the car was never faster.The old Caldwell Dev Car had used the race manifold for a reason

1BADGTS said:

Ps for you guys tuning these cars now trying these mega heads cam packages beware the manifold will never support it .I spent thouands on different heads ,cams and maniold work for nothing as the car was never faster.The old Caldwell Dev Car had used the race manifold for a reason

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Right if you are going to keep the gen 1/2 intake then the rest of the parts have got to peak in the same RPM band and focus on lifting the entire opperational RPM band which does not extend beyond about 5700 RPM or so. These intakes are good for ABOUT 550 RWT and 500 RWPH without much trouble. After that you are fighting the intake, spending stupid money and sacrificing idle characteristics for tiny gains.

ericj96 said:

Right if you are going to keep the gen 1/2 intake then the rest of the parts have got to peak in the same RPM band and focus on lifting the entire opperational RPM band which does not extend beyond about 5700 RPM or so. These intakes are good for ABOUT 550 RWT and 500 RWPH without much trouble. After that you are fighting the intake, spending stupid money and sacrificing idle characteristics for tiny gains.

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Intakes are good for a bit more power

Tuned with stock exhaust manifolds

Added SVS 1 1/2 stepped to 1 5/8 Headers, same tune (in blue). Tried another tune as it had more timing and more injector %.(in red) As you can see it ran out of fuel again and sooner. Had to add a voltage booster to the fuel pump and it solved the problem. When it was all tuned, it made power all the way to 5800rpm. Final numbers were 585hp and 610tq.

Camfab said:

Sanguissaevus, let me know which cam you get when you receive your heads. I have no issue with revealing any and all information regarding my combination.

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Roger that.

For the immediate future I am going to stick with the stock manifold due to funds and the extensive time needed to research and test a properly sorted intake setup. I fully plan to go with a custom manifold in the future though. Again, much research is needed for this and I am hoping Engine Analyzer or Dynamation will help in this, but I'm thinking maybe ITB's or something in sheetmetal from Hogan. Just dreams as of now. The heads and cam are more than enough on my plate with the little time I have. I also would like to play with header design with this software. It would be really interesting to play with primary sizes and lengths which both programs are setup to do. Good times.

Well obviously you can get more power out of the gen 2 intake but at what cost in terms of 600-1500 RPM drivability and idle quality. Your fighting an up hill battle with the long runner intake. More power would be found with a gen 3 retrofit.

If original engine look and behavior is not important why not just put a blower on the engine and be done with it? Seriously if it isn't blown is ***** anyway.