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 Page 2 of 2 <12
Username Post: understanding DCR (dynamic compression )        (Topic#92966)
snowstorm 
Forum Newbie
Posts: 43

Loc: Norway
Reg: 09-05-09
09-07-09 08:13 AM - Post#1767594    
    In response to snowstorm

Hi again. I tested the compression tester, and it seems to be fairly correct, at least up to 120psi.

First, sorry for my typo, the combustion cambers in my AFR heads are 68cc, not 66cc, witch support my previous estimations of CR to10.5:1

Following calc. by Wallaceracing.com:

Static compression ratio of 10.50:1.
Effective stroke is 2.89 inches.
Your dynamic compression ratio is 8.32:1 .
Your dynamic cranking pressure is 167.25 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 8.32 :1.
V/P (Volume to Pressure Index) is 149

Grumpy, your calc’s shows some less DCR than mine (Wallaceracing), but even mine is far from 220psi. Have any of you guys experienced this much difference between calculated and measured compression? What do you think the reason fore the difference can be (I didn’t use any oil to do the test)?

Maximum cranking pressure:
-Which cranking pressure numbers do you have on your pump gas engine?
-I read an article of Ray T. Bohacz where he says “If the compression ratio of the engine is up near 11.0:1, it will love 100-octane unleaded”. Again, remember that here in Norway we have 98 on the pump.
-I don’t remember the author, but I remember reading someone say they feel 220psi is the max fore 100octan gas. What’s your opinion on that?

Please, I’m only trying to find why my readings are so much higher than calculated, and I hope I don’t have to rebuilt my engine, again

Cheers


-86 Z28, -73 sbc 4bolt, -385cid 3.75"x4.04",-CompCam HR 280 224@0.050" .565" w/1.6 rocker arms, -14in/Hg@idle,
-AFR 180cc/68cc, -9.6:1 SCR, -HSR induction, -950 ComPro,-MSD electronic ignition, -TH 700 Raptor stage III, -Strange 12bolt 3.42:1


 




grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17142
grumpyvette
Age: 70
Loc: FLORIDA USA
Reg: 03-16-01
09-07-09 08:34 AM - Post#1767606    
    In response to snowstorm

well, the first reasonable answer would be that the calculations are based on erroneous data, and the most likely is that the combustion chamber, piston dome , deck clearance or quench figures are wrong.
if your getting 220 psi that shows about a 14:1 cpr

http://victorylibrary.com/mopar/cam-tech-c.htm

now I know your sure your figures are correct but you can,t get to 220 psi unless your compressing the volume above the piston, more than the 10.7:1 ratio your figures predict

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


 
pheyden 
Forum Newbie
Posts: 4

Reg: 10-16-09
10-16-09 06:37 AM - Post#1789137    
    In response to grumpyvette

I see one problem straight away.

In the USA we rate fuel octane using the (RON+MON)/2 method. However in Europe all fuel pumps ONLY list the RON Octane number.

As there is a difference of 10 points between RON and MON (MON being lower), the REAL octane rating of a fuel like Shell V-Power 98 octane at a european filling station is 93 (using the US fuel rating system (RON+MON)/2.





 
pheyden 
Forum Newbie
Posts: 4

Reg: 10-16-09
10-16-09 07:14 AM - Post#1789167    
    In response to pheyden

There is one other consideration that I would offer. If a particular engine design (intake and exhaust) has good scavenging, then at certain RPMs (usually around peak torque) the Volumetric Efficiency (VE) might be 105-144%.

This increased VE will also affect the dynamic compression. For example, if we have 114% VE then instead of having 1 atmosphere (14.7 PSI) of pressure on the intake system, the pressure would be 16.75 PSI. Likewise, if the dynamic compression were 8:1 @ one atmosphere, under the improved VE it would rise to 9.12:1.

Certainly if you are running race fuel with an octane rating of 108-114 (using the USA rating method), then all of this means little. IF, however you are trying to make do with pump fuel, it may impact how much ignition lead you can run.



 
snowstorm 
Forum Newbie
Posts: 43

Loc: Norway
Reg: 09-05-09
10-18-09 10:13 AM - Post#1790360    
    In response to pheyden

Thanks Pheyden. Very interesting information you gave about the octane and the VE. For a long time I have wanted too check to see if the ratings here in Norway were the same as over there in the US.

I pulled off the heads to check different aspect reg. the CR. The piston to top deck is 0.027”. The volum above the piston at TDC is 12cc. The combustion camber is 68cc. I decided to use a Fel-Pro 1003 instead off Fel_Pro 1094, this change will increase the volume off the head gasket to 9.1cc instead off 3.2cc. I also checked the camshaft installation, ICL 105.75 degree and intake valve close at 0.005/67 degree.

In the meantime I called CompCam teck support. He told me to not pay any attention to the cranking compression!! He also told me that 220psi was not way off, he was running 210psi on his private car, using 93 pump gas.

The next week I’m going to travel, so I have no time to play around with my car. I do appreciate any replay.

Cheers



-86 Z28, -73 sbc 4bolt, -385cid 3.75"x4.04",-CompCam HR 280 224@0.050" .565" w/1.6 rocker arms, -14in/Hg@idle,
-AFR 180cc/68cc, -9.6:1 SCR, -HSR induction, -950 ComPro,-MSD electronic ignition, -TH 700 Raptor stage III, -Strange 12bolt 3.42:1


 
octanejunkie 
Contributor
Posts: 595
octanejunkie
Loc: Los Angeles area
Reg: 11-12-09
11-22-09 05:16 PM - Post#1810528    
    In response to snowstorm

Found this article online, very easy to follow in my opinion, hope it helps!

http://www.empirenet.com/pkelley2/DynamicCR.html

'59 Chevy 3100 Stepside
0.030-over 350, mild (214/224 @ .050) 112 LC cam,
vortec heads, air-gap, 600cfm carter, Holley Avenger EFI, 2-1/2" rams horns,
Bowtie Overdrives 700R4 w 2200rpm stall, 3.42 posi rear on top of 31.5" BFGs


 
a-1canvas 
Forum Newbie
Posts: 1

Reg: 05-16-10
05-16-10 03:48 PM - Post#1915839    
    In response to grumpyvette

Do you know what the compression test of a 1971 454 (in a boat) should be?? comp ratio is not in question just triing to understand some problems. It wont idle under 1200 rpm. carb's clean/rebuilt edelbrock1407 800 performer. plugs are tan/good.
Any help would be great



 
snowstorm 
Forum Newbie
Posts: 43

Loc: Norway
Reg: 09-05-09
07-31-10 09:27 AM - Post#1953427    
    In response to a-1canvas

Hi, guys. Well I dropped 220 USD at the local store to get a Lisle compression tester just to find out that my first tester was almost correct. My new readings are rating between 198-210 psi (6 cylinders 198-203psi and 2 cylinders 209-210psi). Anyway it’s too high for the Norwegian 98octane gas, which equal to a US 93 octane.

I’m realizing that I’ve messed something up during the planning/assembling of the engine. I didn’t measure the bore and stroke, but the possibility that Scat and the machine shop made the same mistake must be like zero. I measured the cylinder head with the liquid method. The head gasket spec by FelPro must be trusted. The volume above piston was measured with the liquid method (involves some math, which can be is a potential source of error) to 12cc(including the valve reliefs). The piston to deck clearance was measured to 0.027. The next, and most likely source of error, is the camshaft installation. I measured it several times, and back then I was sure I had it all right. But I’m not a professional engine builder, so maybe I did it wrong.

I’ve played around with different calc’s. Let say that the bore/stroke/combustion chambers/ head gasket/volume above the piston is correct. That will give a SCR of 9.86:1. With the above numbers in mind it requires a DCR of 9.86 to achieve a cranking compression of 208psi. And in the same example, to get a DCR of 9.86:1 I need a dynamic stroke of 3.75:1. This mean the intake valve have to close at, or before BDC. Would the engine run at all if the valve closed at BDC?

If the camshaft was installed correct. And I’m focusing on the VE. If I just plug in my measured numbers, Wallaceracing estimate the DCR to be 7.83:1. The difference between 7.83:1 and 9.86:1 is 126%. Could my combo really reach a VE of 125%?

I do feel I understand the empirenet article, maybe that’s why my problems are so frustrating.

Cheers


-86 Z28, -73 sbc 4bolt, -385cid 3.75"x4.04",-CompCam HR 280 224@0.050" .565" w/1.6 rocker arms, -14in/Hg@idle,
-AFR 180cc/68cc, -9.6:1 SCR, -HSR induction, -950 ComPro,-MSD electronic ignition, -TH 700 Raptor stage III, -Strange 12bolt 3.42:1


 
kitabel 
Contributor
Posts: 291

Reg: 11-28-09
08-09-10 06:04 AM - Post#1957917    
    In response to snowstorm

VE for cranking compression is never, ever, more than 100.



 
VetteRed1965 
Forum Newbie
Posts: 6

Loc: Woodstock,Georgia
Reg: 05-06-12
05-06-12 06:48 AM - Post#2223031    
    In response to grumpyvette

Hi Grumpy, I'm running linux and can't run these calc.I'm building my 327 and before I put it together I want to make sure my machine shop is using the right cam. I'm not saying that they do not know what they are saying, I only want to do this once. Here are my specs.
My heads are overhauled with new 2.02 and 1.60 valves with screw in studs. The combustion area is calculated to be 64.4 cc's. Hydrolic lifters.
The machine shop says to use a head gasket with a compression of .35 to achieve a compression of 10.1 to 10.5. On another forum they say that I'm leaving a lot of HP on the table with the cam selected.
Any help apreciated ! I will be assembly of the heads and components starting next week !
Here are the specs for the block and cam below. Thx VetteRed

Block
Deck -.003
bore 4.030
stroke 3.250
CID 331.6
Pistons TRW 4 valve reliefs piston to cyl wall .0032

Cam Elgin Prostock E1011P
Cam Lift
Int .325 Exh .325
Valve lift
Int .488. Exh .488
Lobe centers
Int 106 Exh 110
Adv Dur
Int.292 Exh 300
.050 Dur
232int 234 Exh
Adv.timing
BTC-39 ABC-73 BBC-79 ATC-41
.050 Timing
BTC-10 ABC-42 BBC-47 ATC-7

Any help is appreciated, VetteRed



 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17142
grumpyvette
Age: 70
Loc: FLORIDA USA
Reg: 03-16-01
05-06-12 07:03 AM - Post#2223037    
    In response to VetteRed1965

  • VetteRed1965 Said:
Hi Grumpy, I'm running linux and can't run these calc.I'm building my 327 and before I put it together I want to make sure my machine shop is using the right cam. I'm not saying that they do not know what they are saying, I only want to do this once. Here are my specs.
My heads are overhauled with new 2.02 and 1.60 valves with screw in studs. The combustion area is calculated to be 64.4 cc's. Hydrolic lifters.
The machine shop says to use a head gasket with a compression of .35 (I assume you mean .035 here?)to achieve a compression of 10.1 to 10.5.

(my calculations put the true static compression closer to 9.8:1 )On another forum they say that I'm leaving a lot of HP on the table with the cam selected.
Any help appreciated ! I will be assembly of the heads and components starting next week !
Here are the specs for the block and cam below. Thx VetteRed

Block
Deck -.003
bore 4.030
stroke 3.250
CID 331.6
Pistons TRW 4 valve reliefs piston to cyl wall .0032

Cam Elgin Prostock E1011P
Cam Lift
Int .325 Exh .325
Valve lift
Int .488. Exh .488
Lobe centers
Int 106 Exh 110
Adv Dur
Int.292 Exh 300
.050 Dur
232int 234 Exh
Adv.timing
BTC-39 ABC-73 BBC-79 ATC-41
.050 Timing
BTC-10 ABC-42 BBC-47 ATC-7

Any help is appreciated, VetteRed





whats the car weight? ,
whats the transmission being used?
and
rear gearing ratio?
and
tire diam?
why would you seriously handicap a 327 with hydraulic lifters if your goal is impressive power?



IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


Edited by grumpyvette on 05-06-12 07:12 AM. Reason for edit: No reason given.

 
VetteRed1965 
Forum Newbie
Posts: 6

Loc: Woodstock,Georgia
Reg: 05-06-12
05-06-12 07:29 AM - Post#2223047    
    In response to grumpyvette

weight 3230
tires 205 75R15
axle ratio 335
4 speed

Thx



 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17142
grumpyvette
Age: 70
Loc: FLORIDA USA
Reg: 03-16-01
05-06-12 08:17 AM - Post#2223063    
    In response to VetteRed1965

if IT was my car, ID stick a crane 110931 solid lifter cam and 1.6:1 ratio roller rockers on the car(keep in mind the rpm band in a 327 will be higher than shown which assumes a 350-383 displacement as those are currently more common displacements)


while its true a larger cam would produce more peak hp, your rear gears not ideal for racing,and better matches the milder cam I linked too, this is a street car that will spend a good deal of its time at lower than peak rpms
if it was strictly used for racing a cam like a crower 00355 and a 4.56:1 rear gear ratio might be used, don,t make the mistake of caming the car for max power when youll spend 5% of your time at peak rpms, youll be happier caming the car for where you spend 95% of your time




IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


Edited by grumpyvette on 05-06-12 08:32 AM. Reason for edit: No reason given.

 
VetteRed1965 
Forum Newbie
Posts: 6

Loc: Woodstock,Georgia
Reg: 05-06-12
05-06-12 10:40 AM - Post#2223108    
    In response to grumpyvette

(my calculations put the true static compression closer to 9.8:1 ).

Is my DCR with the cam I listed good.

I'm now running a 1970 350 4 bolt main 290HP at the rear wheels and 310Lbs Tq.

Will my 327 have at least the same HP, making a guess ? We were shooting for 350 hp




Edited by VetteRed1965 on 05-06-12 10:42 AM. Reason for edit: No reason given.

 
black thunder 
Forum Newbie
Posts: 60

Reg: 01-20-15
04-21-15 04:30 PM - Post#2541426    
    In response to snowstorm

here is another thought for all of you.
I heard grumpy say he lives in Florida.
Thats just above sea leval ,
where I live in reno 5400 ft above.
Ive done hundreds of compression test on stock
chevys.
8 and a half to 1 dish pistons , 76 cc combustion
chambers.
I get 120 psi brand new motor,
up to when it usualy starts getting carbon bad head gaskets cracked valve seats.
how come these guys build high performance engines and getting 120 psi.
are we designing cam grinds?
Doesent air density have a big part .
I know when it gets cold I have to up octane
or youd think somebody is hitting my pistons with hammer.
I generaly degree cam to cam grinder spec.
Are WE kissen ? LOL
COOL way to up OCTANE.
Cox and Testors air plane fuel 12 % nitro cost
15$$ a gallon I dump half in 32 gal. tank.
I think 1/4 gal take it past 100 proof.
never messed up any thing in 10 years .




Edited by black thunder on 04-21-15 04:38 PM. Reason for edit: No reason given.

 
scottm 
Member
Posts: 105

Loc: Scottsdale, AZ
Reg: 03-13-05
10-27-15 10:00 PM - Post#2585298    
    In response to black thunder

Grumpy, I didn't follow all the links you posted, but something here seems to be overlooked. Air is only pushed back out the intake on the piston upstroke at cranking rpm. That is when a compression gauge shows much lower psi than you would expect based on static cr.

Within the cam operating range, air continues to flow in long past bdc on the compression stroke. That is why the valve is held open past bdc, and why high rpm cams hold it open even longer. Phayden (sp?) hit on it with his posts on VE. Performance engines can exceed 100% VE easily. Max torque occurs at max VE, and that happens at the rpm when everything comes together - Optimum exhaust scavenging combines with cam overlap and intake characteristics to ram in well over 100% of cylinder volume. My own race engine makes 460 ft-lb from 372 cid at 4500 rpm, which I guess is maybe 125% VE. So my 11:1 static cr is more like 14:1 at 4500, which is why it requires 110 octane Sunoco to run 36° spark timing. I think a lot of people will find they either get detonation or have to reduce total timing to 30° or less with 11:1 and pump gas. So cranking pressure does not necessarily indicate what the engine's octane requirements will be.





I spend most of my money on beer, women and racing. The rest I just waste.

77 C10 Class 8 desert race 520hp sb372/th400, 82 Titan RV 454/th400, 94 Blazer 6.5 diesel/4l80, 94 C2500 6.5 diesel/5sp, 97 K3500 454/4l80 dually, 01 S-10 ext cab 4.3


 




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