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Cam timing effects on compression 
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In my thread on my C Sedan 510 I discussed the build of my new race L16, it has 12.5 :1 comp pistons, 580/278 Electromotive can, V912 head built by Gerolamy....and I just found out 160 compression across all cylinders and pulled 110 hp on the dyno. I have read about how different lobe angles on different cams can effect compression, but not how changing the cam timing in an existing motor can do so. I haven't done a leak down test yet, or put oil in the pistons and re check the compression, but can compression be off 50psi just from timing the cam incorrectly?
The dyno showed good air fuel up at red line (13.5:1) but midway thru the rev range (around 4000rpm) A.F. went hi 16.5:1. Cant these two issues be related?


Last edited by defdes on 06 Feb 2009, 18:44, edited 1 time in total.



06 Feb 2009, 00:10
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It would make sense that if the cam timing were off, you could see a loss in compression. If the cam timing was retarded enough - and the intake were to remain open during the compression stroke..... In practice, I don't know how possible that is and still have it run....

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06 Feb 2009, 00:45
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James wrote:
It would make sense that if the cam timing were off, you could see a loss in compression. If the cam timing was retarded enough - and the intake were to remain open during the compression stroke..... In practice, I don't know how possible that is and still have it run....

Thanks James, yeah, that's the kind of info I am looking for. The car definitely runs, like I said in the above it doesnt have the greatest throttle response (especially at low rpm's under load) and mid range power which could be attributed to cam timing. It's the compression figures I am most concerned with.
1 and 2 cylinders were also popping allot out of the air horns. I took it apart on while there, blew compressed air thru all passages with no change.


06 Feb 2009, 00:57
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I had my l-16 rebuilt (a long time ago) and they installed the cam one tooth off. The car would run, but was blowing out my su's, backfiring etc. I never checked the compression, but the engine was not what it should have been. I drove it that way for a bit, and then decided it wasn't right, and went through everything (dist, valve lash). Once I checked the cam timing on with the notch on the back of the cam sprocket - man what a difference. I would start there since you are having some of the same symptoms....Might not be a tooth off, but advancing it may help.

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06 Feb 2009, 01:07
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Depends on intake closing. The stock L20B intake closes 52 degrees after BDC!!! The piston has already started well on it's way to TDC. Theoretically, during this time the piston is pumping air back into the intake. In reality the time is short and inertia helps trap the air before the valve snaps closed. At high revs the time shortens and the inertia of the high speed inrush of air helps even more.

A cam with a delayed closing time will give more time for air to back into the intake as the piston travels upward. This is even worse at low speed where there is increased time as well. At low speeds it's like taking a bight out of an apple then spitting 1/3 of it out. At cranking speed the cylinder that is fully filled on the down stroke, can expel a sizable chunk of that due to the late intake closing and will show a somewhat low compression. This is fine at cranking as it is easier on the starter. If you could check the compression at high RPMs you would find the compression much higher. Volumetric efficiencies can exceed 100%.

If your cam is timed incorrectly, say retarded by one tooth on the sprocket. It would delay intake closing by 9 degrees. If it already has a long duration late closing valve, this could seriously affect compression at cranking speeds.

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06 Feb 2009, 01:09
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Right now cam lobes are "straight up" at TDC, and the - and the "V" are right on top of one another, dash might be a bit to the right of the "V". I will try advancing the cam, so the dash is clear right of the "V". My friend did the assembly with a degree wheel and degreed it, he said, to the cam card specs. I wasnt there, so I dunno.


06 Feb 2009, 01:20
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While I am not the whizz as to how many degrees effect things, I can attest to my real world use of changing the cam timing for different events, either more or less bottom end, with comensurate increase/decrease in top end.
With even my cam now, I can vary the static compression reading from 190 to 220 cranking pressure, and that is with a so-called static compression ratio around 10:1. The last stupidly BIG cam I had in there, had quite low static cranking pressure, and had stupid REVERSION untill I got up past 4,500 RPM. I even had to make a special baffle for the top of my carb, as.....the air goes down through the carb, picks up it's fuel load, then, would pulse back up the venturi, carrying the fuel load, then, go back down, of course picking up more fuel...can you say.....BOOOOOOOGGGGGGG!!!!!!!! It was horrible. It did make it though, that I actually could run regular pump gas, and not get any detonation, till the boost got up. The overlap was horrible, and of course I sufferdd the truly "over cammed" engine scenario.

160 on that just seems too low. Just for comparision, not that it relates, but Andy's killer 1600 made about 190 HP on the consevative dyno of his.

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06 Feb 2009, 02:27
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bertvorgon wrote:

160 on that just seems too low. Just for comparision, not that it relates, but Andy's killer 1600 made about 190 HP on the consevative dyno of his.


i wish I had made 160 HP!! that was my target. Unfortunately it was the COMPRESSION that was 160, the HP a paltry 110.
I hear what you are saying about the drivability, these motors don't really come on until 4K or so, anything below that it will run like poo. I am expecting that. It's the compression/power numbers that have got me vexed.


06 Feb 2009, 03:03
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Yah, I did mean the compression. It just seems like your cylinder is not filling fully, to get the maximim squeeze....HP. I think my lawn mower makes more than that :wink: . I would sure recheck that cam timing.

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06 Feb 2009, 06:16
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http://www.rbracing-rsr.com/comprAdvHD.htm

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06 Feb 2009, 07:52
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Defdes
590/390 ?????
or did you mean 490/290 cam?????

since thee might be more overlap is thats why the numbers would be down?


110hp?????????

Damn I must have 89hp then on my car!!!!!!!!


06 Feb 2009, 15:32
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banzai510(hainz) wrote:
Defdes
590/390 ?????
or did you mean 490/290 cam?????

since thee might be more overlap is thats why the numbers would be down

Sorry....typo. 580/278 cam based on the NISMO "L4"
Here's the cam card:
Attachment:
cam.jpg
cam.jpg [ 111.66 KiB | Viewed 626 times ]


06 Feb 2009, 16:47
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510rob wrote:
http://www.rbracing-rsr.com/comprAdvHD.htm

Based on that site's calculator:
Camshaft, Rod Length, Boost and Altitude Correction to Compression

Your engine summary is as follows: Bore 3.327 inches, stroke 2.904 inches, rod c-c length 5.236 inches, with a static compression ratio of 12.5 :1. Your camshaft specifications call for an inlet valve closing of 61 degrees ABDC (after bottom dead center).

Your chamber volume is 35.97 cc's. With this camshaft your dynamic, or effective stroke is 2.31 inches. Your dynamic compression ratio is 10.13 :1 corrected for cam timing, altitude, and rod length. Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 215.17 PSI. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 1 PSI is 10.82 :1.




06 Feb 2009, 20:43
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Add 9 degrees so it closes at 70 degrees and re-calculate. 9 degrees is one tooth out (advance)

Stock closure is 52 degrees on an L20B.

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07 Feb 2009, 02:11
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This is the punch out for that, drops CR significantly...but not as low as actual.
Your engine summary is as follows: Bore 3.307 inches, stroke 2.904 inches, rod c-c length 5.236 inches, with a static compression ratio of 12.5 :1. Your camshaft specifications call for an inlet valve closing of 70 degrees ABDC (after bottom dead center).

Your chamber volume is 35.54 cc's. With this camshaft your dynamic, or effective stroke is 2.13 inches. Your dynamic compression ratio is 9.41 :1 corrected for cam timing, altitude, and rod length. Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 195.90 PSI. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 1 PSI is 10.05 :1.


Why did you ask me to do that? Assuming that the cam timing is off where it is now, that would put me at 61 degrees minus the 9 for one tooth ? or is adding the 9 degrees putting me where I am now? Sorry...not too swift with this stuff.
Either way, the above figures are way higher than my actual. If I had gotten 195 compression, I probably wouldn't have been that concerned.
One thing that does concern me, is that this calculator puts our closed chamber head volume at 35.9 when I thought they were 41cc's + -, what's up wit dat?


07 Feb 2009, 02:58
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