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Old 06-02-2013, 11:36 AM
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Timing Roots Blown SBC

Hi everyone,
I'm new here, i'm hoping to gain some insight and help on my latest project!

The Build: 388ci sbc, 9.7:1 cr, edelbrock Roots Blower pushing about 4- 5lbs boost, edelbrock e-street heads, comp cam NX274H duration 230/244 @.05 113 Lobe.

Ok, the plan is to make this a wild street car on 93 pump gas. The car will be lucky to be driven once a week.

Now I would like your opionions on..... wait for it.... Timing!! I have a msd digital-6 plus ignition box & msd billet distributor #85551. I would like to run an initial timing of 22 & be all in at 28 @ 3000rpm. However the springs & bushings are preventing me from doing this. The other option was to just lock-out the timing at 28 and take advantage of the start retard the digital-6 box offers.

Your opionions please......

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Old 06-02-2013, 11:50 AM
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fbird88 likes this type of engine combination,hold tight.
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Old 06-02-2013, 12:09 PM
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You need the boost timing master box. So you can retard the timing as when boost comes in.
For pump gas.


26 at idle 36 at max advance ( a 10deg mech advance curve)

then take timing out as boost comes up, as required to get 28 to 32deg @ max boost WOT Using the BTM.

Water methanol injection will allow driving the blower harder (pulley swap) for more boost and power.

But you will need a fail safe system.
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Old 06-02-2013, 12:15 PM
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google search 4secondsflat for a 10deg advance curve stop bushing for the MSD distributor.
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Old 06-02-2013, 12:30 PM
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Yeah I was trying to avoid installing a BTM, is it absolutly essential?

Is locked-out timing at 28 no good for my application?

I'v currently got my engine out, I had cracked the ring-lands on 4 of the pistons running too much boost (7lbs), too much timing (40) & a small cam (214/214 @ .05). We Live & We Learn!!

Is the BTM the best way of avoiding Detonation without compramising HP?
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Old 06-02-2013, 01:21 PM
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Has anyone had any experience driving a street car with locked-out timing with a simmilar setup to mine; sbc 388ci Roots mini Blower 9.7:1cr??
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Old 06-02-2013, 01:45 PM
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I run an 8.4:1, 8-71 383 on the street with a BTM and injection system. The one time I sent my BTM to El Paso for repair, bought a 6AL for the weekend test and tune, before the inj system, with fixed timing at 34*, I melted 7 pistons, an expensive lesson, I couldn't hear it detonating, but sure felt it lay down on the third run at 1,000 feet.
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Old 06-02-2013, 08:51 PM
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Ok, I guess I'll get the msd boost timing master, I do not want to strip the engine down a second time to replace pistons!

So the accepted timing is 26 initial with 36 total (10 of mech advance) & have a boost retard of approx -2 per lb boost (I will be running about 4lbs boost retarding the timing from 36 back to 28).

At what rpm do I want to be all in? 36 advance @ 3000?

Oh and one other thing.... Recomended Spark plugs for these heads are champion RC12YC are these appropriate for a blown engine @9.7:1 scr running 4lbs boost?
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Old 06-02-2013, 09:12 PM
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You probably know a roots blower makes power right off idle, I'd have timing all in by 2500. Four pounds of boost is a cake walk but keep it low due to your high SCR, 9.7.
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Old 06-02-2013, 09:52 PM
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Thanks for the input 68Nova

Any thoughts on use of advance springs to get 2500 all in? Going off the charts & using a 10 advance stop bushing with initial set at 26 im guessing the stock heavy silver springs should fitt the bill.

Any ides on thoes spark plugs?
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Old 06-02-2013, 10:15 PM
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Ok, I guess I'll get the msd boost timing master, I do not want to strip the engine down a second time to replace pistons!

So the accepted timing is 26 initial with 36 total (10 of mech advance) & have a boost retard of approx -2 per lb boost (I will be running about 4lbs boost retarding the timing from 36 back to 28).

At what rpm do I want to be all in? 36 advance @ 3000?

Oh and one other thing.... Recomended Spark plugs for these heads are champion RC12YC are these appropriate for a blown engine @9.7:1 scr running 4lbs boost?
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Old 06-03-2013, 06:14 AM
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Build the engine with a LOW COMPRESSION RATIO

8.5:1 or LESS low compression ratio is better.

9.7:1 is way too high.

you make more power on pump gas with LOW COMPRESSION and more boost.

Then add .001" to the piston to wall clearance and open up the piston ring end gap 40% bigger.

It is a lot easier to build a 355 based engine with low compression, using easy off the shelf parts, than a 383.

The easier cheaper 355 engine that has low compression and high blower boost will amek more power than a 383 ever will. 7.5:1cr allows 10-12psi boost on pump gas. Now you are making supercharged power.
A 383 is a mistake.

Change the combo to low compression or run high octane fuel ///water meth.
Exceeding the laws of physics is expensive.
Do it right this time. 4psi is a waste of time. Use a RC9YC

That cam has way too much exhaust duration (244) to be used with a 2200 stall converter. You won't like it.
The 3.08's aren't helping either.

Different cam or different 3000++10" converter and gears. Pick one.

This a bad combo.
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Old 06-03-2013, 06:22 AM
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You need custom -34 to -36 D dish pistons to build this 388 for a blower. (low compression using 64cc heads)
The laws of physics don't care if you don't like that.

The E Street heads need porting for power.


LOW COMPRESSION is BETTER for non intercooled supercharged power on pump gas.
You need to re-think this whole combo.

Last edited by F-BIRD'88; 06-03-2013 at 06:35 AM.
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Old 06-03-2013, 07:45 AM
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I ran a simmilar combo before that rocked;

Same 388ci sbc, same shortblock, but running a 214/214 duration cam @.05, running the stock 3.6inch top pulley (pushing 3lbs boost), SCR was the same @9.7:1 The car hauled ***!!

This setup worked well! but of course I was looking for more....

Switching to a smaller 2.8inch pulley, gave me 7lbs boost, but cracked my pistons.
Running initial timing of 22 & all in at 40 didn't help either!

Anyways "the re-build" my local Comp Cam rep recommended me this cam; NX274H. I'm going to switch to a 3.2inch pulley (a more safe 4 or 5lbs boost) & sort out my Timing!

Now you tell me its all messed up?!?
The 2200 converter & 3.08 rear gears are a little small (but it is being used as a street car 95% of the time).

Where is the happy-medium?!?

Will the cam mess this build up that much?

Last edited by blowme77; 06-03-2013 at 07:56 AM.
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Old 06-03-2013, 08:04 AM
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ya its all messed up. and so is the engine compression ratio.
How many more engines do you want to have to build/rebuild.???

3.6" and 2.8" top pulley diameter....

What is the bottom crank pulley diameter?

YOU want a LOW engine compression ratio. 8.5:1 OR LESS

Fix that This is where you START.

Last edited by F-BIRD'88; 06-03-2013 at 08:15 AM.
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