Dyno run

[big_G]

Trying to figure out the low hp peak and why at 5,300 rpm. The torque drops like a rock at the same time. A little info: 383 sbc, Inter-cooled centrifugal s/c'er making about 10 lbs. boost at 6,000 rpm's. Edelbrock Vic. Jr. heads, Vic. Jr. intake, 8.3:1 scr. Cam is 242/248 @ .050 on 110* lsa. Installed straight up. Holley 750 dp'er, A1000 fuel pump, boost referenced. Timing is 34* all in at 2,800. No retard. A/F ratio was consistent 11.8 to 12.1. My gut tells me to look at ignition. Running MSD dist. and 6AL box. Any ideas? Thanks, Gary

[84Dave]

I'm not an expert on blowers, by any means, but seems to me you'd need 'boost retard' on the ignition system as the boost gradually increased. The dyno graph makes it appear the engine is 'hurt' @ 6000? Dave

[rskrause]

Is the cam a HR? I have seen that happen when the valves are too tight. Try backing off the valve adjustment. Also might not be enough valve srping. Also, 34 degrees is a lot for a SC SB. I would take out some timing, though I am not suggesting that is why your power is dropping off. Blowing out the spark is a possibility, but the MSD 6 should be up to the job unless it is defective. What is your plug gap? Try tighteneing it up to 0.035" unless it's there already.

Richard

[big_G]

Originally Posted by rskrause

Is the cam a HR? I have seen that happen when the valves are too tight. Try backing off the valve adjustment. Also might not be enough valve srping. Also, 34 degrees is a lot for a SC SB. I would take out some timing, though I am not suggesting that is why your power is dropping off. Blowing out the spark is a possibility, but the MSD 6 should be up to the job unless it is defective. What is your plug gap? Try tighteneing it up to 0.035" unless it's there already.

Richard

Cam is a solid roller, springs are set right at 200 lbs. closed. No timing retard, as being inter-cooled blow thru, the fuel charge should be cool enough to keep timing. This week-end I found erratic timing caused by an un-shielded MSD distributor pick-up harness. that may have some merit for a solution? Gap is .035.

[Lasse]

34 degrees of timing is extremely too high for 10 psi boost. You will need boost timing retard. Make sure retard is relative to boost, on-off style retard will not work.

[big_G]

Originally Posted by Lasse

34 degrees of timing is extremely too high for 10 psi boost. You will need boost timing retard. Make sure retard is relative to boost, on-off style retard will not work.

How did you determine that? From the dyno graph?

[BlackCat13]

Weak valve springs.

When boost pressures finally get to the limit the springs can stand, the boost pressure pushes the intake valve open...... screwing up the induction process.

Increase only the intake valve springs over-the-nose pressure by 50 pounds and see a difference.

[big_G]

Originally Posted by BlackCat13

Weak valve springs.

When boost pressures finally get to the limit the springs can stand, the boost pressure pushes the intake valve open...... screwing up the induction process.

Increase only the intake valve springs over-the-nose pressure by 50 pounds and see a difference.

I have them at 200 lbs. on the seat now...Quite high enough, IMO.

[MadBill]

I've yet to hear an explanation of why one would want the intakes shut if the port pressure was higher than the cylinder pressure. Wouldn't 'blowing them open' just let more mixture into the cylinder?

I'd be looking at spark output; maybe try closing the gaps 0.010 -0.015" as a test...

[BlackCat13]

Originally Posted by big_G

I have them at 200 lbs. on the seat now...Quite high enough, IMO.

spring pressure is in the ball park according to some cam manufacturers, recommending 350+ over the nose
and recommending 114* LSA for blown engines

maybe a coil breaking down. ran across that recently.

How does it run without boost?