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Old 27th March 2017, 08:51 PM   #1 (permalink)
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tomei b poncams low compression test results

I have poncams with the intake cam well advanced (can't remember exactly but lots). Runs ok, no smoke, doesn't use oil. I does however run hotter than it used to on track. And I have suspected the cause is the amount of overlap from running the intake cam so advanced. Of course it could just be HG or other head issues.

Anyway, I finally did a compression test. I get 115-120psi across all six. That's hot, WOT, two batteries connected. Don't forget it runs fine.

Could low compression potentially result from a big cam overlap? Could it also explain the hot running?
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Old 28th March 2017, 04:36 AM   #2 (permalink)
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Sorry to hear this Alex. When did you change the cams?

Keeps my fingers crossed for a simple explanation.

Hope you're well
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Old 28th March 2017, 09:51 AM   #3 (permalink)
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Did compression tests on both type A and Type B across two engines never not less than 150psi on either cams, one was running a 1.4mm head gasket. Suspect overlap if it's just as a result of changing cams.
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Old 28th March 2017, 10:13 AM   #4 (permalink)
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Did compression tests on both type A and Type B across two engines never not less than 150psi on either cams, one was running a 1.4mm head gasket. Suspect overlap if it's just as a result of changing cams.
Thanks, I read similar results from people on SAU.

The cams went in a while back, I noticed running hotter on the dyno shortly after, then started to see it on track this time last year, I put in every cooling mod I can think of and its not really helped. It used to maintain 80deg lap after lap, now just climbs slowly during a session until I come in when it hits 100deg.
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Old 28th March 2017, 10:57 AM   #5 (permalink)
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Maybe a full drain and refil. Was it properly bled fully to remove any air? What radiator are you using? Do you still have the shroud?
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Old 28th March 2017, 11:03 AM   #6 (permalink)
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yeah the cooling system has been flushed and bled, first thing i tried. The same cooling setup ran the same power before at more boost, and it ran cooler, so the shroud/rad isn't the issue i think.
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Old 28th March 2017, 06:49 PM   #7 (permalink)
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Very strange....
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Old 28th March 2017, 08:16 PM   #8 (permalink)
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I think in most instances you wouldn't even notice it. I track the car and watch the temps and have logs to show the change.

I've run it by Abbey and they didn't seem concerned with the compression test results given the cam setup. Which is reassuring.

Still there is this question of overlap induced excess heat in the system. The water, oil and EGT are all up on where they used to be.

One possibility is a head gasket failure but it's a 1.2mm tomei gasket that is only 3-4 years old, a more expensive alternative would be an issue in the head itself. Can't see how these would raise EGTs, whereas the cam timing/overlap might do that.

In case of the worst scenario I've priced up a rebuild to a built 2.6 or even a 2.8 which are both in budget and are something I want to do at some stage but I don't like throwing money at un-diagnosed issues.
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Old 28th March 2017, 08:28 PM   #9 (permalink)
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What compression do you get with the engine running?
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Old 28th March 2017, 08:50 PM   #10 (permalink)
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What compression do you get with the engine running?
I've not done a running test, what would that tell you?
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Old 31st March 2017, 03:26 PM   #11 (permalink)
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Spoke to Mark again re the hot running and he suggested retarded ignition timing would be consistent with the high temps (ECT,EGT esp).

The ignition timing is based on the cam gear position, the CAS synch with the ECU and the proper mechanical timing between the crank and cams.

I put a timing light on the car and found it was running 10degs measured on the crank when the ECU was instructing 20degs. I pulled the CAS off and the exhaust cam didn't look like it had moved. So with Mark's guidance I've reset the CAS calibration offset in PC Link (reduced by 10deg) and set the timing at the crank to match the timing at the ECU.

10degs sounds like a lot and would put a lot of heat into the exhaust ports and head, and I guess it would have been well down on power too.

Just need to run it on the dyno next week to check it isn't going to knock and then take it out for a track day to see if the issue is fixed.
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Old 31st March 2017, 08:41 PM   #12 (permalink)
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Interesting reading and fingers crossed that's it
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Old 31st March 2017, 08:42 PM   #13 (permalink)
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Also the temps could be due to you driving it harder
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Old 31st March 2017, 09:23 PM   #14 (permalink)
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Also the temps could be due to you driving it harder
I take it pretty easy

One upside was that it made mega flames
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Old 13th April 2017, 11:10 AM   #15 (permalink)
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Same problem my side Alex. Jun 272 deg on the RB34, flat tops. Cranking pressure 7 bar ! 115 lbs ! Im really searching for a cr increase. Done the math and she has a static cr of 8.8:1.
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Old 13th April 2017, 12:14 PM   #16 (permalink)
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I think I'm reassured that the slightly low compression test results is a result of cam overlap, which may also be your issue BigKriss, especially with a longer duration cam.

Regarding this ignition timing issue, Mark very kindly slotted me in at short notice last week for a quick check over on the dyno. His timing light matched mine, which was reassuring, so the timing is now definitely 20deg at idle. And the car doesn't knock at all, so the original ignition map is still fine.

However the power was the same as the last check over in December (440 at the rear hubs running in 2wd at 1.2bar). (The run in December was to check the fueling after I fixed the fuel pump wiring issue that cropped up last year)

This means that it wasn't running 10 deg retarded for long (as a test Mark ran the car with 10 degs take out and it made 40hp less), and the hot running on track must be unrelated.

It is more than a little worrying that the CAS should go out of calibration like that. The trigger scope in the Link looked fine but something is clearly not right there. I've ordered a new sensor for the CAS from Australia, which may take a little while to arrive. At some point crank trigger is the way to go but not something I'm urgently going to drop a grand on for parts and fitting.
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