1) What is the recommended dynamic compression ratio for pump gas?

2) I think I am over the limit, will someone check and see if they are getting the same numbers I am?

The motor:
-383 --350 bored 0.030 over (4.030 bore)
-64 cc heads
-LPE 213 camshaft (also known as a Blue Racer 290-2h)
---cam card here: http://www.cranecams.com/?show=brows...73&lvl=2&prt=5
-5.7" Rods
-3.75" 400 crank
-Piston-deck clearance is 0.012"
-Dished hyper pistons with a 12 cc Dish
-Head gasket size is in flex to try to get the best dynamic compression ratio

 

i don't want to start a war, and this DCR stuff is way populer. Still when i called isky about my cam numbers and started talking to Ron Iskendarien about what DCR i was looking for he was like "what the hell are you talking about". he seemed to think it was a pretty small issue when all the other variables where considered. in fact he asked me to fax him the info that was leading me to think this was important stuff. just thought i'd throw that out there cause i found it interesting considering all the importance i have seen placed on it in more then one forum. I have to think Ron Iskendarian has some small amount of knowledge when it comes to engine. Of course I could have been unclear in my understanding of his thoughts, but i don't think so.

 

Ok, I will admit, having the plan change on you midway...is a very bad thing.
So, here is my current satus:

Recommend DCR is 8.0-8.5:1
I now have a SCR at 10.2:1 and a DCR at 9.38:1

So I am way over the limit on DCR, I also have only 91 pump gas and I will not be mixing fuel on this car.

I am basically screwed, my cam's intake closing point is at 41deg.
It appears I need something in the 60-67 range to get the DCR down to the 8.0-8.5 range.

So, what are my options here?



Thanks guys, I think I made a huge mess of this one...

 

See thats the thing panchop, its been like that with everything with this (my first build). It seems like if you are looking for an answer, all you have to do is go out there and ask enough people because someone will claim it is the truth!

Now, yes I would agree, I would think Ron Isky would lay more faith on this. At least thats how it seems to be according to all that I read, so that is very interesting indeed.

The other thing I don't know is what effect EFI has on this. I have a motor with some great flowing heads and a SuperRam and I don't know how much this is all applicable in EFI ether.

Dahh!!! I just want to finish this motor and drive my car...

 

Actually your intake closing point is NOT 41 deg. Please read the cam spec card again. The given degrees are at .050 tappet lift. The actual intake closing point is closer to 60 degrees. The only way to know exactly the closing point, is to measure it.

Good luck!

 

This same situation came up recently, but I can't get the search function to prove it.

There is no way your DCR is 9.38 with a 10.2 SCR. As suggested you cannot calculate the DCR from the .050" lift point. THINK for a moment, what DCR IS. Then think what the valve position is, when degrees are given at .050" LIFT. It doesn't take Einstein to figure out that it is going to be tough to start building cylinder pressure with the valve .050" away from the valve seat.

RACE ON!!!

PS. I found the previous thread on this exact subject. Click HERE.

 

Thanks CFI, I was freaking out a little bit. I thought I had verified everything would be ok and then I found this, a set back like this probably would've pushed the motor back another year.
I really appreciate it guys. Sorry for all the dumb questions, still a first timer.

 

The DCR is definitely NOT computed from the .050" closing point. An SAE standard defines the valve opening/closing points at .006" valve lift, and these timing points are usually used by aftermarket cam grinders to specify "advertised duration". From this and the specified centerline, you should be able to compute the .006" lift inlet valve closing point to use in the DCR calculation If you google on the topic you will find several online DCR calculators, and the Engine Analyzer simlulation program calculates it. Of course, the .006" closing point is a function of rocker ratio, too, which varies, and it's not that close to the "claimed" rocker ratio. For OE SB rockers the lash point ratio is about 1.37:1 and 1.44:1 at max lift. Since EA also allows user specified rocker ratios, it yields fairly accurate DCRs.

The key to reasonable accuracy is understanding both the closing point context that each calculator requires and the ACTUAL static CR of your engine, which must be measured for each build. Also, the rocker ratio behavior should be known so you can "adjust" the .006" valve lift closing point to what is actually achieved in the engine since "specs" are based on a fixed specified value, which are not very accurate.

IMO you want to keep DCR below about 8:1 (based on the .006" valve lift closing point and actual rocker ratio as computed by EA) to be able to use pump premium and have no detonation without running excessively retarded timing in the low to mid range.

This is usually satisfied by a TRUE static CR of no more than 10.5:1 and a high performance cam with a relatively late closing point - what you would get with a cam that has about a 110 ATC inlet centerline and about 220-230 degrees .050" duration. A "smaller" cam with an earlier closing inlet valve will have a higher DCR at the same static CR, so you might need to cut static CR a bit to keep DCR below 8.

Duke

 

The reason ( more than likely) Isky and the rest of us who happen to choose a particular cam for a particular application don't really care about the Dynamic C.R. is due to the fact you have to choose the cam to work best with the entire ***'y. If you pick a cam specifically to lower the Dynamic C.R. you'll merely have the wrong cam installed. You can't run (properly) an 8:1 C.R. with a 270 @ .050 cam, the same is true opposite, you can't run a 13:1 motor with a 218 @ .050. (These are just examples) We're on the dyno (almost) "24/7" testing all types of combinations. Thanks, Gary in N.Y.
PS If you like, Performance Trends has the best software out there for figuring you're C.R. both static and dynamic. Just contact them, it's not expensive. We use the "Compression Ratio" calculator all day long.
Not for dynamic C.R. reasons, but for more for piston "dish & dome" volumes and stroke/rod length combos.

 

I'm still trying to get my head around this concept. If you don't mind please explain why this is true and how it works. When GM first lowered CR's in '71 to run lower octane unleaded fuels, they didn't change the cams in their high performance engines. On the solid lifter LT-1, the HP and TQ dropped but the engine remained a potent cointender with the same cam and a lower CR.

Thanks for any insight you can provide.

 

When the decision was made by GM management that all '71 engines were to operate on 91 RON unleaded fuel there was barely a year to implement it on ALL GM engines, so there wasn't a whole lot of engineering that went into optimization - just modify the head and piston designs as required to achieve what was advertised as 8.5:1 for most engines (but actually closer to 8:1 as built). It was a crash program. Because of the LT-1's later closing inlet valve the advertised ratio was only dropped to 9:1, but like the other engines, was about half a point lower as built.

The gross HP reduction was nearly ten percent from 370 to 330, but what most probably don't realize is that torque/power at EVERY point in the rev range was reduced by a similar percentage.

Also, the combination of lower compression and retarded timing map to meet emission control caused a dramatic increase in fuel consumption.

If you compare a 11:1 LT-1 with a pre-emission ignition map with a production spec 9:1 LT-1 there will be a signficant SOTP difference.

Because high performance cams typically have more overlap the reduction in low end torque is even more dramatic with lowered compression than on a typical high torque engine because of the greater exhaust gas dilution due to the larger clearance volume.

This and the lower dynamic compression ratio with a later closing inlet valve on a high performance cam is why big cams need high compression.

Duke

 

Seems like one defeats the other. More consumption equals more emmisions. How did the Fed measure tailpipe emmissions? pr gallon of fuel consumed with MPG no part of the equation?

What is SOTP?

Thanks for explaination. Okay, lets talk solid lifter SHP cams of yesteryear in short stroke small blocks. Since those cams made HP/TQ higher on the curve, where the overlap could be utilized, these engines made up for a lack of low end torque with a higher compression ratio, in effect building TQ at a lower rpm? Is that correct? Another way to put more effective TQ to the rear wheels is by increasing gearing, plus with the SHP cams it would get you into the power band more quickly.

Does a higher compression ratio equate to higher temperatures in the combustion chamber by itself?...and if so, why slower burning, high octane fuels are needed for those combinations. Is that primarily why a lower compression is typically recommended for towing applications?

TQ is obviously a good thing when towing to get mass moving. So a typical high torque engine is going to build most of the TQ with a lengthened stroke, thus giving up some "high-end" HP where it is not needed.

 

I can't address all your questions in the time I have available. Suggest you get a good textbook on IC engines - like Taylor's two volume set or Heywood. If you have a college in your vicinity that offers an IC engine course, you should consider taking it.

Duke

 

SOTP = Seat 'o the pants. That one got right by me.

 

High octane fuels are not slow burning fuels but ones which are less likely to spontaneously combust at high temperatures caused by higher compression.
For towing you need lower compression as the motor is under high contineous load and more likely to detonate plus lower compression allows a shorter duration cam to develop higher torque output at lower revs on pump grade fuels.

 

Excellent post! Nice to know we've got some knowledgeable engineers in Oz.

Duke

 

I had a 10.7 static C/R 355 ci iron headed motor. It was always right on the edge of detonation and I always carried octane booster. I was using the Crane Power max 272 cam. When I changed to the 278 cam my DCR went down and I was able to advance the ignition and change my advance curve back to something more normal.

I really liked this cam because of a broad TQ courve, enough RPM, decent mileage. I used 1.6 rockers

Part Number: 113801 Grind Number: H-278-2 (REPLACES HMV-278-2)
Engine Identification:
Start Yr. End Yr. Make Cyl Description
1957 1987 CHEVROLET 8 GOOD IDLE, DAILY PERFORMANCE USAGE, MILD BRACKET RACING, 3000-3400 CRUISE RPM, 9.5 TO 10.75 COMPRESSION RATIO ADVISED. BASIC RPM 2500-5500
Engine Size Configuration
262-400 C.I. V

Valve Setting: Intake .000 Exhaust .000 HOT

Lift: Intake @Cam 3114 @Valve 467 All Lifts are based
on zero lash and theoretical rocker arm ratios.
Exhaust @ Cam 3294 @Valve 494
Rocker Arm Ratio 1.5

Cam Timing: TAPPET @.004
Lift: Opens Closes ADV Duration
Intake 26 BTDC 72 ABDC 278 °
Exhaust 80 BBDC 30 ATDC 290 °

Spring Requirements: Triple Dual Outer Inner
Part Number 99848
Loads Closed 114 LBS @ 1.700 or 1 45/64
Open 324 LBS @ 1.230
Recommended RPM range with matching components
Minimum RPM 2200
Maximum RPM 6200
Valve Float 6800

Cam Timing: TAPPET @.050
Lift: Opens Closes Max Lift Duration
Intake 2 BTDC 40 ABDC 109 222 °
Exhaust 56 BBDC (2) BTDC 119 234 °

 

I have to echo SWCDuke's recommendation of some independent study.

The true issue, here, is of cylinder pressures. At higher compression ratios, we expect higher cylinder pressures. Normally when we speak of "compression ratio" we are referring to the "static" compression ratio. That is the relationship between the volume with the piston at TDC vs the volume with the piston at BDC. All else being equal the cylinder pressure will be higher in a cylinder with the higher compression ratio. But "all else" is seldom equal. As the closing point of the intake valve becomes delayed, by the use of a "wilder" camshaft, the point in the crankshaft rotation that cylinder pressure starts to build, becomes delayed. That shortens the effective length of the compression stroke. The dynamic compression ratio measures the ability of the engine assembly to produce cylinder pressure.

As camshaft durations increase, a higher STATIC compression ratio is needed to maintain power producing cylinder pressures. At lower levels of duration the (static) compression ratio must be kept in check, to prevent excessive cylinder pressures from causing detonation or knocking.

Detonation is the air/fuel mixture "auto igniting" from the heat of compression. As a gas (as the air/fuel mixture) is compressed, the temp of that gas increases. When the temp of that gas becomes high enough the mixture ignites, like a diesel. Higher octane gasolines simply have a higher temperature of "auto ignition".

The torque an engine will produce is mostly a function of the cubic inch displacement. The stroke, alone, except where it increases the displacement has little to do with the specific torque output of an engine.

The power (really torque) range of the engine is the highest when the cylinder pressures are the greatest. If the camshaft has a lot of duration that bleeds off cylinder pressure at low rpms and "packs" the mixture in at higher rpms, then maximum torque will come at a higher engine speed than if the valves opened later and closed earlier to build cylinder pressure at lower speeds.

The camshaft is the brain of the engine, but for the best results, the rest of the engine must be "optimized" to fully take advantage of the characteristics the cam was chosen for.

All the above is expressed in it's most simplistic terms for the sake of being understandable. I hope that it helps some.

RACE ON!!!

PS. What are "solid lifter SHP cams "?

 

SHP = Special High Performance

Back in the sixties and seventies if you needed an engine part, you didn't order it by power rating or RPO. SHP were the high horse engines, most of which had mechanical lifters, but L-79 is also SHP. In '65 the camshafts were referenced as "Spec. Hi. Perf. w/ mech. lifters" for L-76 and "Spec Hi. Perf. w/ hyd. lifters" for L-79

FI engines were referenced "FI"

The L-71/72 were referenced as "427 w/Spec. Hi. Perf."

L-88 parts were referenced as "427 w/ HD" - "heavy duty"

Duke

 

Thanks, yes it does help. I know I could use some "textbook" study but I'm not looking to become an engineer. I do like having a better grip on how things work, more from a curiosity standpoint than needing it fundamentally to pursue a living in the field.

It's funny how some fairly good mechanics don't seem understand engine dynamics even though they could overhaul a short block. Obviously, there are many disiplines involved and a mechanic doesn't necessarily need to know, or understand, thermodynamics to install a camshaft, or tune an engine. However, if the mechanic does have the extra knowledge then they are better equipped to properly match components for the best performance desired. That's how I see it. I think in many cases, do-it-yourselfers choose cams by duration, without much thought given to matching other components and then end up over cammed. At least that was true in yesteryear.

Camshaft voodoo is probably the least well understood, or misunderstood part of the equation by most of us amatuers. In my mind, I try to get a mental picture of what is going on internally with something like overlap, the effect it has on the combustion process and how the CR and cylinder pressure relates. I guess if I want to know the "why's" I'm going to need to crack the books and maybe invest in some good desktop simulation software.

I have alot of respect for those early Chevy small block engineers and what they were able to accomplish, in relatively short order, with a slide rule and long hand.