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Old 01-12-2011, 03:55 PM
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compres ratio n timing???

i bought a 88trans am recently and was wondering if it had the right amount of timing. im unsure about the compres ratio, and im using 93 pump gas right now. I know that sometimes the motor will spit, cough or whatever when i turn it off, but that might be because the throttle spring is stiff??(if im in the motor bay and close the throttle lever, then it idles from 1400 back down to 950, n shut it off no hickups). all i have are machine shop recepts to work with, and it looks like he took the motor there a coulpe of times. I dont have any concrete evidence that this is a stroker motor but the persons word, the machine shop receipts aren't specific enough for me to tell. it was said to be a 388 stroker, but the first machine shop receipt shows a 30bore and theres newer receipt that shows a 40bore. there was engine balancing, decking, boring/cut&hone, turning the crank,cut/set seats&spring tension, dip the block, new cambrg/F.plgs, "epa chg?", and assembly of the LB.All the information provided is from the motor or machine shop receipts.

"3970010" 350 chevy bored 40
"SPD L2256F40" speed pro pwrfgd flat 4valv relief-
comp ratio: 9.72:1 w/64cc head or 8.62 w/76cc head
ex energy hyd roller - "CS XR276HR-10" the receipt shows they bought the-
kit Comp Cams "K12-423-8"
comp cams roller rock "1001-16" Stud 716 in. Ratio 1.6 in
sportman II heads "1-037"
distributer "1111483" w/petronix ingnitor II, w/ accel super stack coil
GM 7.5-10 bolt 3.42.
700r4 with 2500 stall

im a little confused about the timing deal. At idle i have 14, then when i rev it up till it stops is at about 50. so 50-14=36 is that my total timing??? At what RPM is the timing supposed to stop advancing? I noticed the cam rpm range is from 1900 to 5600. If anyone could help me clear this up would be great. Another thing is that the timing tape is on the harmonic balancer and i have a lil gauge above that that reads 15 before TDC and only 8 after TDC. Im assuming to match the timing tape with the timing mark of the gauge at TDC or 0. is the gauge the left overs of a stock motor?
This car seems to top out at about 125, but the spedo is about 10% slow, so a 137 would that be average? lol starts getting hot too.

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Old 01-12-2011, 04:10 PM
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read this.

http://www.corvette-restoration.com/.../Timing101.pdf

are you using a carb or efi??
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Old 01-12-2011, 04:10 PM
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It looks like you have somewhere around a 9.7:1 cr. My engine has a 9.5 cr and i run the lowest grade pump gas, 87 octane i think, and i have no problems. Not sure what my timing is set at as i timed it by how it runs, not with a light. I don't get any pings or detonation. I'll try to put a light on it tomorrow or the next day and see what it's at so you have an idea.
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Old 01-12-2011, 08:05 PM
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Its a 350. The pistons are 350 pistons for a stock 3.48" 350 stroke.
Its not a stroker motor.
The timing should be checked with the vacuum hose disconnected from the distributor. It should not exceed 36deg BTDC (full mechanial advance) when you rev it up.

To check the centrifical + vacuum advance plug the vac hose back on the distributor and rev it up. It should not exceed 51deg combined.

If the mechanical advance springs are very light tension the advance curve will be very quick and tends to be unstable at idle.
This can cause the advance to not return to base timing at idle.
Creates a in consistent idle.
In order to set the ignition timing accurately you first have to check and verify that the TDC mark on the balancer and timing tab are accurate.
They are always off a bit.
The timing curve probabily needs work.

I suggest 16 to 20 deg base idle timing 34 to 36deg max mechanical maxing out around 3000 rpm. Then + 10 to 12 deg of additional vacuum advance at high vacuum part throttle cruise (hiway cruise).
The distributor will need to be modified to get this curve. They are never right, out of the box. The length of the curve needs to be shortened.

Make sure you have the right type and heat range spark plugs for the cylinder heads. If your heads use the .460" reach plugs, use a Champion RV8C (AC R42T or R42TS) gapped at .035".

Last edited by F-BIRD'88; 01-12-2011 at 08:17 PM.
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Old 01-12-2011, 08:20 PM
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The run on can be a number of things but the best case senario is that by increasing the base timing and adding vacuum advance at idle you will be able to close off the base idle adjustment screw on the carb some more, this may help with the run on.

According to the info you are running a old points type dist that has been retro fitted with the petronix pickup. You need to ensure that the air gap between the rotor and the coil is correct, there is a special feeler gage that comes with the kit, its about 0.030 thick. Also there is a ground wire inside the dist to make contact between the module base and the dist base, check that these connections are tight.

I am not savvy to these kits/modules but they play around with dwell, this is taken from the vendor:

Quote:
This Pertronix Ignitor II Electronic Ignition Conversion Distributor kit fits most 57-74 GM all models with 8 cylinder engine with Delco distributor that has the mechanical advance at the top of the shaft. The Ignitor II senses current levels in the coil and adjusts the dwell to maintain to maintain peak energy throughout the entire RPM range. Dwell angle is increased or decreased with changing engine RPM and operating conditions. This provides more energy at high RPM , reducing misfires while improving engine performance. Compared to point-type systems, develops on average 4 times more available energy between 3,000 and 5,000 RPM, and 2 times more available plug voltage. Compared to the original Ignitor, the Ignitor II develops an average 3 times more available energy between 3,000 and 5,000 RPM, and nearly doubles available plug voltage. Higher RPM performance is improved when used with the new Flame-Thrower II super low resistance (0.6 ohms) 45,000 volt coil. Also compatible with other coils. Peak current level is reached just prior to spark, so ignition energy is sustained with less heat build-up, increasing coil and module life. Adjusts spark timing over the entire RPM range to compensate for the inherent electronic delay, resulting in more stable timing. Senses engine start up and increases dwell time, providing more energy for starting. Senses incorrectly wired Ignitor II or a “key left on” condition and shuts down the system, protecting the module, coil and other components from damage. RPM range is from 0 to 15,000 RPM. Compatible with 12-volt negative ground systems. Legal in all 50 states and Canada. Guaranteed for 30 months.
Personally I would just go ahead and install an HEI

http://skipwhiteperformance.com/detail.aspx?Item=6500-R

You can't beat these for value and if you have the room at the fire wall do it. After that you can set up the mech and vacuum advance with ease.

Either way get your base timing to about 18-20 BTDC add in another 16 with mech advance by 2800 RPM, and then add another 10-12 deg vacuum advance on manifold vacuum. So that when you are at idle you are seeing about 30 degrees advance, adjust your carb to get an idle of 800-900 rpm and see how it likes it.

edit,,,duplicate info, was typing whille fbird posted,,,no offence
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Old 01-13-2011, 05:50 PM
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You are getting plenty of good advice here but I would like to point out that it could indeed be a stroker with that piston. If they used 400 SBC rods (5.56") the stock pin height 350 piston could be used. That's how the whole 383 stroker thing started. I built my first 383 back in '96 from a Speed-O-Motive kit that used a scat cast steel crank, the 400 chevy rod and the same L2256 pistons you have.
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Old 01-13-2011, 07:07 PM
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Quote:
Originally Posted by Custom10
The run on can be a number of things but the best case senario is that by increasing the base timing and adding vacuum advance at idle you will be able to close off the base idle adjustment screw on the carb some more, this may help with the run on.

According to the info you are running a old points type dist that has been retro fitted with the petronix pickup. You need to ensure that the air gap between the rotor and the coil is correct, there is a special feeler gage that comes with the kit, its about 0.030 thick. Also there is a ground wire inside the dist to make contact between the module base and the dist base, check that these connections are tight.

I am not savvy to these kits/modules but they play around with dwell, this is taken from the vendor:



Personally I would just go ahead and install an HEI

http://skipwhiteperformance.com/detail.aspx?Item=6500-R

You can't beat these for value and if you have the room at the fire wall do it. After that you can set up the mech and vacuum advance with ease.

Either way get your base timing to about 18-20 BTDC add in another 16 with mech advance by 2800 RPM, and then add another 10-12 deg vacuum advance on manifold vacuum. So that when you are at idle you are seeing about 30 degrees advance, adjust your carb to get an idle of 800-900 rpm and see how it likes it.

edit,,,duplicate info, was typing whille fbird posted,,,no offence
not to get off track too much but are the skip white distributors any good? I'm finishing my 383 stroker and was looking for a distributor. I planned on using a Taylor unit... sells for $125 on summit... if this skip white job is as good as the Taylor unit I'd GLADLY shell out only $45 for the same thing... thanks.

Josh
"Drive It Like Ya Stole It"
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Old 01-13-2011, 10:25 PM
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Yes 45 bucks and I have ran one for a few thousand miles now on my 383. No issues. So take it for what it is worth,,,via my part it is money well spent.

Simple to setup with adj vac advance, just get the crane limiter plate for a few more bucks and you can cover the 3 amigo's of your timing curve with a 8-32 screw limiter for mech adv, spring set few more $, a 10-12 gage switched 12+ volt wire on the battery side and they light up to 5800 rpm no sweat.

They have been around for a while now and there have been some pretty good reviews/comparisons. Of corce there is going to be a lemon or two and then they will be just more asian junk.

TJ

Last edited by Custom10; 01-13-2011 at 10:32 PM.
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