383 rebuild - kinda whimpy on the dyno?

[JAndrea]

Long post… and my first engine build (aside from a motorcycle engine), startup, and dyno.

After getting it hooked up to the dyno, the motor started right up after one second and it sounded amazing... that was a relief and a great feeling. After about 4 seconds, we saw that my 750 DP HO carb was overflowing. I had forgotten to set the float levels. No big wup. He (Ron) shut it down immediately and bolted his 750 DP on. Again, it started right up. breakin was about 20 minutes. All vitals looked excellent... no issues, except for.... yes... omg yes... and I just knew it was going to happen.... the dreaded rear main seal leak.

Ron hooked up a PCV hose to slow the leak during the pulls and it made a big difference in the leak flow. So, we then proceeded to the pulls. Various pulls and timing changes found 34 degrees advance to be the optimum for the setup. We also added a 1” carb spacer, which the engine definitely liked. Air fuel ratio seemed right on as well. Plugs look great upon inspection.

After doing about 12 pulls, including setting my float levels on my AED 750 HO HP carb andmounting that (which it liked)... I have to say… I was only going for old school horsepower, but I’m pretty disappointed. Never even came close to 400TQ and 338HP was the best it could make. Granted, I have no problem with those numbers in general, but I didn’t pay the extra $, and take so much meticulous time, for stock 383 figures. I was thinking it would come in around 400hp, with a chance of a little more. Even with the 906 heads, I thought I had pretty realistic expectations.

Specs and work:

block
1968 383 magnafluxed and bored to 4.28”
Zero decked
Align honed
Machined block parallel to mains
Trq plated
Int Balanced crank assembly

Block, heads, crank, pistons all mated and matched during machining process by engine builder.

Crank
Forged, ground, reconditioned crank.

Heads
#906 iron heads completely redone (NOT ported)
Int: 2.14
Exh: 1.81
Milled to 79cc
.039” head gask
Hardened seats
Springs - Not sure on these, but I’ll try to find out
Stamped, stock rockers
I don’t know for sure, but i think I’m around 9.6:1 CR

Ignition
Electronic Distributor - MOP440-431 Mopar Perf w/MSD 6 AL (dyno-owned)
34 degrees was found to be optimum

Pistons
Flat top w/valve reliefs cut by builder
Forged aluminum, Speed Pro (L-2315NF 30) 4.28”
Moly rings

Cam
Hyd FT
COMP Cams Xtreme Energy Camshafts 21-224-4
274/286, Lift .488/.491
Degree’d by me

Lifters
COMP Cams High Energy Hydraulic Lifters 822-16

Intake
Performer RPM

Carb
Holley AED 750 DP HO HP
Jets
P: 74
S: 82
Powervalve: 6.5

Dyno Headers
1 ¾” to match what I plan to use

I did the tolerance checks and assembly on the entire motor, except the heads. After being “done” the heads came back to me without too many details, which I should have asked for right away.

Anyway, Ron found no strange sounds, etc.. during the dyno session and said everything looked great, it must just be a “mild” motor. Ron has dyno’d over 10K motors in 30 years. We also pulled the valve covers to make sure the cam and valves were behaving.

I did screw up and mix 3 gallons of 98 octane race gas and 1 gallon non-oxy unleaded… as opposed to the other way around… 1 gall race gas and 3 gall unleaded). I don’t think running race gas quality through a mild compression ratio motor would have set me back too much though, but not sure.

When I got home, I drained and checked the oil, let it sit for a while... and it looked just fine, no metal, etc. Super clean actually. Then I cut open the oil filter and checked the paper. There was definitely the black moly lube residuals and some tiny flakes in the oil. As far as the paper… once I cut that out and squeezed it in my vice, there were definitely some flakes, but not excessive. I brought it to the builder, who spec’d my components, and he said the filter “looks pretty typical”. He thought I should have hit 390hp or so, but didn’t give any ideas on why I didn’t hit that mark.

I then measured the pushrod height (dial indicator) on #1 and #7, and found .327” and .325” for int/exh… when multiplying by 1.5 rocker ratio.. lift was exactly where it was supposed to be according to cam card.

I’d appreciate any ideas, comments, advice, etc… if this is all I’ll get for hp and tq, that’s cool. It’s just a hobby car, not a racer… but if anybody can think of some things to check… let me know.

I’ll soon be doing a leakage test on all cyls, and going after that rear main seal leak.

 

[Garrett Ellison]

Sounds about right to me, given the combination. Maybe it's just me, but I remember most stock 383s magnums came with around 10.25:1 compression. Those larger valves don't do diddly in your 906s without some port and bowl work, at the least they need to be modded out to the D.C./M.P. porting template set. And your probably getting close to the limit of what those stamped stock rockers are going to be able to stand before the dreaded "push rod push through" happens. First step, do a compression test and see if it matches the cranking pressure on the Dynamic Compression Calculator. Wallace Racing: Dynamic Compression Ratio Calculator

 

[fishmens67]

The head killed it.

 

[Jerry Hall]

A/F 14 @ 3500 is fine, but it seems to be leaning out. It needs a little more fuel throughout the range 14.5 but less than 15 at your max rpm. I agree with the heads are killing it. A little port work is in order.

 

[Murray]

Just dyno'ed mine. Stock rebuild '68 383, reduce compression to 9 to 1, hydraulic roller cam, stock heads no porting (906's), original AFB, original intake manifold, original distributor with points. Factory rated at 280HP. Got 390HP at 5400. I am happy.

 

[Darter6]

Timing ?

 

[moper]

I would be looking at heads and valvetrain. Which spring and lifter was used? What was the preload? Labor on heads could be suspect. Big valves won’t hurt or kill anything, because the rest of the port just won’t flow anyway. But as cast 906s should move enough air to make 400 hp with that camshaft. So I’d be looking there.

 

[JAndrea]

Timing ?

34 degrees was optimum. He tried several.

 

[JAndrea]

I would be looking at heads and valvetrain. Which spring and lifter was used? What was the preload? Labor on heads could be suspect. Big valves won’t hurt or kill anything, because the rest of the port just won’t flow anyway. But as cast 906s should move enough air to make 400 hp with that camshaft. So I’d be looking there.

I hear ya. I used COMP Cams High Energy Hydraulic Lifters 822-16. I don’t know about the valve spring pressure, but I’ll try to find out. I forgot to post pics of the invoice details machining and head work, so I’ll add those.

 

[dukeboy440]

My guess is the heads.

 

[Garrett Ellison]

Either bad sealing (hence a compression and a leak down test) or bad valve angles/sunk valves (should never be an issue when installing oversized valves). You had stated that you weren’t completely sure of your compression. You will need to get your actual chamber, head gasket, and piston (+dish-dome) volume and double check. Often times, even with zero deck, compression doesn’t meet expectation because ChryCo head volumes tend to be larger than stated.

 

[JAndrea]

Either bad sealing (hence a compression and a leak down test) or bad valve angles/sunk valves (should never be an issue when installing oversized valves). You had stated that you weren’t completely sure of your compression. You will need to get your actual chamber, head gasket, and piston (+dish-dome) volume and double check. Often times, even with zero deck, compression doesn’t meet expectation because ChryCo head volumes tend to be larger than stated.

Thanks for all the tips and ideas so far, everyone.

 

[Mattax]

My .02
Since these pulls were done right after break in, the engine may still pickup some compression.
Overall sounded good. I'm assuming the acceleration was at 600 rpm/sec but you should ask and put in your notes.
Why were the pulls ended while the power was still climbing?
I'd expect that cam to peak a little higher and that's what it looks like it wants to do. Somewhere between 5400 and 5700 would be my guess.

Don't see any experimentation with the fueling. There could be more power there.
As was posted earlier, the curve looks relatively flat from 3900 up although leaning a bit.

I'd try 76 primary jets and see what happens. (typically expect best WOT power to be in the 12.5 to 13.2 range that's why I'd be testing 2 jets richer first)
Then, depending on the results I might experiment a little more with the jets, or plug the middle e-holes on both blocks. Of course I'm guessing its got more than 2 emulsion-holes, but that's pretty much what all the new school carbs come with. Two around .027" diameter is about right, or 3 much smaller (maybe .021?) would have to experiment. The goal here would be to correct the start up (2500 - 3500 rpm at WOT) AFR and observe if it effects the upper rpm. Then I'd measure the air bleeds and mostly likely leave them alone and go back to experimenting with jetting.

Timing. The rpm you choose for "all in" on a Chrysler distributor is important to note, and be consistant with. In general they continue to slowly add some advance past 3000 rpm. This is fine. We just need to be aware that measuring 'all in' at 2500 can be different than 'all in' measured at 4000 rpm.

 

[duster360]

I agree with Mattax. That combo should build power to 5800+ rpm. Is there more to the dyno sheet after the 5300 rpm limit posted in the picture above?

 

[RustyRatRod]

It's all in the heads. You upgraded everything else from stock and put a stock head on it.

 

[ir3333]

i think those numbers look good but how it performs driving is what matters.That cam may have taken away the lo rpm grunt.
A long time racer in my area never fails to mention that he raced a lot high HP cars that ET'd poorly.
Peak hp can be very misleading.The important number is lo rpm torque for a fast fun car on the street.

 

[RustyRatRod]

i think those numbers look good but how it performs driving is what matters.That cam may have taken away the lo rpm grunt.
A long time racer in my area never fails to mention that he raced a lot high HP cars that ET'd poorly.
Peak hp can be very misleading.The important number is lo rpm torque for a fast fun car on the street.

Exactly. We all know the best dyno is the drag strip.

 

[YY1]

Looks like someone else caught it but I was going to say it looks like your cam is still building power way late in the RPM range you pulled.

I'd look at a cam that comes in sooner or maybe pull to 6K or 6500 if you think it's safe.

Cheap way may be to advance the cam a degree or two.

 

[Garrett Ellison]

Just wondering what it’s going into. If it were mine, my gut instinct would tell me not to worry about any more dynamometer (well, more my wallet would, unless dyno time were free), and to compression test it. If all goes well, get it in the car and finish tuning it and see how it carries on out at rpm.

 

[RustyRatRod]

All of this discussion and you have to think about the accuracy of the dyno. Put it on another and it might measure 100 HP more. On yet another, less. So there's "THAT".

 

[YY1]

but the fact remains that the peak HP number was the final (and highest) RPM logged.

 

[RustyRatRod]

but the fact remains that the peak HP number was the final (and highest) RPM logged.

I don't disagree. But "how much" would a different camshaft add down low? Now if he changed the heads and cam......but then better heads would need the cam he HAS. See how that works? lol

 

[YY1]

my point is- I don't think that's the true peak, since it wan't tested any further.

What does the RPM range of the cam card say per the MFG?

If the OP is not satisfied with the power, he can either adjust the "peak" downward or extend the RPM range upward to see if there's any more on the table.

Potentially without replacing any parts.

see how that works?

 

[fishmens67]

as it is now, the op's engine is done by 5300

 

[RustyRatRod]

my point is- I don't think that's the true peak, since it wan't tested any further.

What does the RPM range of the cam card say per the MFG?

If the OP is not satisfied with the power, he can either adjust the "peak" downward or extend the RPM range upward to see if there's any more on the table.

Potentially without replacing any parts.

see how that works?

I do. Nonetheless, it's an "underheaded" engine. That cannot be debated.