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Old 01-26-2005, 10:23 PM
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Ignition timing curve help?

We have built a 383 sbc for our project car 87 camaro that weighs in around 3400 lbs the engine will be bolted up to a WC T5 standard transmission Which will be driving a 9 bolt posi with 3.26 gears.

The engine build up looks like this
650 demon double pumper
on top of air gap manifold
bolted on to SR home ported heads with 190cc intake runners Valves are only 1.94
compression ration on engine is about 11:1
Cam is comp cams XE284H -10
Valve lift is .506 intake and .510 exhaust
Duration is 284 intake and 296 exhaust at .006 lift
Duration is 240 intake and 246 exhaust at .050 lift
exhaust system is headman shorty headers into headman y pipe
Dynomax 2.5" cat back

So what do I run for ignition timing

Base timing ????
Vacuum advance?????
mechanical advance????

I know the cam is a bit long on duration to get a really smooth idle but this is the smallest one that my son and I could agree on.

Your suggestion will be greatly appreciated.

Thanks
Ric

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Old 01-26-2005, 10:37 PM
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Best starting point is 36 degrees advance with those heads, have base about 10-12, so that gives you mechanical of about 24-26 all in by 3000 at the latest.

I shoot to have 8-10 degrees of vac advance and hook to manifold to get the benefit of the vac advance on the idle to smooth things out. You'll have reversion with that duration and therefore a slower/richer fuel requirment.

I'd bet money this will dentonate on pump 93 if it truly pumps at 11:1. I suspect also your next purchase will be a set of 3.70 or higher rear gears to get the motor up into the RPM a bit quicker.
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Old 01-26-2005, 10:55 PM
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WE are hoping that the extra duration in the cam will aleviate some of the detonation. What do you think?

AS for the gears the transmission is a 5 speed and first and second gear is very low. This car used to be my daily driver it had a 2.8 litre v6 then. and belive it or not the thing would get rubber in first gear and a chirp going into second.
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Old 01-27-2005, 01:17 AM
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timing

Detonation may be a fuel problem.
Use premium fuel.
Total advance 38 deg.
If this don't work.
Install one medium spring in the distributor
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Old 01-27-2005, 06:01 AM
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timing

Can you give us some more info..

What pistons, head gaskets, what size are the chambers in those heads. was the block zero decked??? what ignition system??

IMO as a starting point i would not use the vacuum advance. I would set the curve for 18 deg initial with 36 total in by 2500 or so. This would be a starting point.If it really is a 11:1 engine and you want to run it on 93 octane,,, i would limit the total to 32/34 untill you get some tuning time on it to see what works.

Make sure you start with 1 heat range colder plug also untill you get it sorted out.

Keith
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Old 01-27-2005, 07:46 AM
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Quote:
Originally posted by Ratchet
WE are hoping that the extra duration in the cam will aleviate some of the detonation. What do you think?

AS for the gears the transmission is a 5 speed and first and second gear is very low. This car used to be my daily driver it had a 2.8 litre v6 then. and belive it or not the thing would get rubber in first gear and a chirp going into second.
I think you'll be on the ragged edge if not over it.
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Old 01-27-2005, 05:36 PM
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Quote:
Can you give us some more info..
The pistons are Keith Black 111's they have a .100 pop up and decrease the volume of the combustion chanber by 5cc

The heads have 69cc chambers

block is stock deck but with the pistons in I measured the deck height at .040

Head Gaskets are.039

I did the calculations for Dynamic Stroke that were at this link http://members.uia.net/pkelley2/DynamicCR.html I found in a post from macx, thanks macx! and it came out to a dynamic compression ratio of 8.5

Keep those suggestions comming guys

Ric
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Old 01-27-2005, 08:00 PM
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Your going to detonate on pump 93 no doubt about it. at least the 93 octane you get here in the US. 8.5 dynamic is WAY too high.

Your .040 deck height makes no sense though. I assume you mean you zero decked the block but pistons are still .001 in the hole so with .039 gasket you have a .040 quench.
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Old 01-28-2005, 02:57 AM
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compression

Your whole combination is a little drastic for the heads your using. With 1.94 valves I would have stopped at a .477 or .488 cam.

Damage control:
Personally I would invest in a set of 70cc Edelbrock heads. They'll flow better, have 2.02 valves, take off 50 lbs, and get your compression down enough to stop detonation. Also, they will work great with the pistons and cam you have chosen.
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Old 01-28-2005, 06:17 AM
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I am a little lost also.
Those pistons have a comp height of 1.433 So with a 9.025 deck they should be .017 down in the hole.. Did you zero deck the block (9.008 height) and you are using the gasket thickness in the .040 deck calculation???

I am getting that with a 4.030 bore 3.75 stroke 10 cc gasket 69cc chambers and a piston volume of -5CC It's going to be a 11.5:1 static.........

Look on the cam card and let us know where the intake valve closes at in degrees......

Keith
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Old 01-28-2005, 06:40 AM
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.100 pop up piston

Ratchet
Quote:
.100 pop up and decrease the volume of the combustion chanber by 5cc
Kstar
Quote:
I am getting that with a 4.030 bore 3.75 stroke 10 cc gasket 69cc chambers and a piston volume of -5CC
This would reduce the 69cc head to a 64cc head. A flat top piston would have been sufficient.
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Old 01-28-2005, 06:44 AM
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Re: .100 pop up piston

Quote:
Originally posted by bracketeer
Ratchet


Kstar


I believe it's a +5cc.
Yup,, i put a minus sign instead of a plus.... thanks for the catch

Keith
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Old 01-28-2005, 10:08 AM
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The reason we have the pop up pistons is because they were purchased assuming that the 76 cc heads we had would actually measure 76 cc. In actual fact when we ccd them they came out at values that read from 69 to 70 cc's

As for the deck height you are correct .017 is the calculated value you get with a stock block and these pistons. I can't go back and check now but to the best of my memory it measured .040 with my depth mic.

Ric
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Old 01-28-2005, 11:12 AM
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deck

Were the rods re-sized???

keith
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Old 01-28-2005, 01:27 PM
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no the rods where those powdered metal ones from a vette engine.

Ric
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