snowstorm
Forum Newbie
Posts: 13
Reg: 09-05-09
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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 |
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 grumpyvette
Senior Chevytalk Moderator
Posts: 13827
Age: 62
Loc: FLORIDA USA
Reg: 03-16-01
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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 FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK !" |
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pheyden
Forum Newbie
Posts: 4
Reg: 10-16-09
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 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.
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pheyden
Forum Newbie
Posts: 4
Reg: 10-16-09
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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.
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snowstorm
Forum Newbie
Posts: 13
Reg: 09-05-09
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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 |
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octanejunkie
Contributor
Posts: 211
Loc: Los Angeles area
Reg: 11-12-09
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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
0.040-over 350, mild (214/224 @ .050) 112 LC cam,
vortec heads, air-gap, 600cfm carter, 2-1/2" rams horns,
built 700R4 w 2200rpm stall, 3.42 posi rear on top of 31.5" BFGs |
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a-1canvas
Forum Newbie
Posts: 1
Reg: 05-16-10
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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
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snowstorm
Forum Newbie
Posts: 13
Reg: 09-05-09
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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 |
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kitabel
Contributor
Posts: 105
Reg: 11-28-09
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08-09-10 06:04 AM - Post#1957917
In response to snowstorm
VE for cranking compression is never, ever, more than 100.
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tri5ss (4)
