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Old 04-03-2007, 06:59 PM   #1 (permalink)
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Question Head Shaving, Ignition Timing Changes

Hey, I recently re-did my ZR 580's engine with Wiseco pistons, port polishing, Psi-twin pipes and .020" taken off of the heads, but I was told several times that the ignition timing must be changed in conjuction with the head modifications. Is this true?, and has ne1 here ever tuned a sled's ignition after head mods just to give me a ball-park, (prefferably a 580 ZR engine)????
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Old 04-03-2007, 08:47 PM   #2 (permalink)
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Changing the squish will require you to change the timing.
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Old 04-03-2007, 09:02 PM   #3 (permalink)
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The squish is the domed part of the head, right? because we just machined off of the endge, but not the dome.
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Old 04-03-2007, 11:17 PM   #4 (permalink)
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squish is the measurement from the top edge of the piston to the head. if you shaved the head and did not cut the squish back in you may be in for a surprise. (not a good one)
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Old 04-04-2007, 06:22 AM   #5 (permalink)
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wut do u mean, cut the squish back in? I shaved the head to get more compression??#@ and does ne1 kno about ignition timing changes??
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Old 04-04-2007, 06:44 AM   #6 (permalink)
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Higher copression = quicker ignition = preignition/knocking. You're going to have to retard the timing.
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Old 04-04-2007, 08:49 AM   #7 (permalink)
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I have done alot of perf work! Take a piece of soldeir and bend it to reach fro the plug to the cyl wall. With it parralel to the crank rotate the engine over then measure the soldeir, this is your squish, and if you look there should be an angle to the tip. The mesurement should be .057-.078 don't go below the .057 and you wont need to change anything! What all did you change on the porting?
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Old 04-04-2007, 05:19 PM   #8 (permalink)
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I smoothened out the casting flaws in the ports with a die-grinder to make them really smooth.I don't think I made them all that much bigger though.
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Old 04-04-2007, 05:26 PM   #9 (permalink)
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Thanx for ur help guys but I'm just curious as to how higher compression makes you have quicker ignition, because in the end the motor is still turning at the same RPM, right? TDC, shudnt change, shud it? I'm just really curious and was wondering if you guys cud tell me Y? and also Kccats, how much do u usually retard the timing??
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Old 04-04-2007, 06:22 PM   #10 (permalink)
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i understand cutting the head to gain compression, i also understand when you do so and do not have enough clearance between the edge of the piston and the head, when the thing heats up, the piston grows, rod stretches, BOOM

SOOOOOO whenever you cut a head, you must maintain a minimum squish to allow for this.
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Old 04-04-2007, 07:31 PM   #11 (permalink)
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Compression causes the mixture to ignite, more compression = quicker ignition or pre-ignition in a bad case. (look at diesels).
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Old 04-05-2007, 06:36 AM   #12 (permalink)
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ok, so basically wut ur saying that pre-ignition will occur due to the increase in compression. Using high octane fuel should help this though, shouldn't it?
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Old 04-05-2007, 09:04 AM   #13 (permalink)
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thats what SEG is saying, which BTW is correct, you must be totally ignoring everything else
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Old 04-05-2007, 05:25 PM   #14 (permalink)
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actually upersleder, i asked SEG a question about using high octane fuel to help eliminate pre-ignition. I asked for help, not commentary. Also, I have had the sled running at operating temperature so I don't have to worry about the squish being to close. I am not disagreeing with or ignoring nebody Im just looking for info, and im grateful to neone who can give me useful pointers, so try and leave the ignorant comments to a minimum.
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Old 04-05-2007, 08:39 PM   #15 (permalink)
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Now, now, Getting back to the squish, Mesure it! Anything tighter than the .057 and you will need race fuel? Your piston is not going to hit with only .020 off! but it could be just enough to melt the exhaust side of the piston off!(hence the retarded ignition!) If you are in that window your good! But do only run Prem if your close!
As for the porting, I always knife the transfer ports too!
More compresion wont actually give you more HP but will accelerate quicker!
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Old 04-06-2007, 02:10 AM   #16 (permalink)
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Thanks for the tips, kccats. I'll measure my squish and I'll get back to you. Let's say I'm in the position where I have to retard my ignition, how many degrees should I retard it??
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Old 04-06-2007, 09:01 AM   #17 (permalink)
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sorry, was just trying to help you not blow your motor, ignorance is lack of knowledge, so you figure out who has left them. and NO i wont be bothering you anymore
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Old 04-06-2007, 09:14 AM   #18 (permalink)
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Chill uper...Higher octane will help, but like kccats said, too much compression you might have to go way up on the octane. They sell flywheel keys that are offset to adjust your timing.
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Old 04-07-2007, 07:13 PM   #19 (permalink)
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Right like SEG said the keys are made to advance the timing but can be used either way! Don't get too excited about moving your timing, I don't see .020 as alot, I have made my own head inserts and I had one set that was real tight and I was over 200 psi and it cracked the allum insert! and it didn't melt anything, My original CDI box had too much advance in it and I melted a stock set of pistons!
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