383 rebuild - kinda whimpy on the dyno?

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383's are kind of a different breed of cat to build up. A 1/4" bigger bore than a 350 Chevy but a .100 shorter stroke. That doesn't necessarily make them a high rpm engine. The 350's have a 5.7" rod where the 383's have a long 6.36 rod. Kind of a strange combination. Not quite a horsepower engine but more a torque engine. I don't remember too many that were over the top screamers. They ran good stock back in the day though.
 
fishmens67 said:
as it is now, the op's engine is done by 5300
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I don't think you can say that as it wasn't tested past that RPM.

...unless there are other factors.

I've had relatively mild 318', 360's and a moderate 383 that were good to at least 5500 if not 5800. All with stock heads.
 
If you pop off the heads goback with a steel shim head gasket to help comp. I've seen stock heads make over 400 hp with not much more cam in a 383 and peek at 6000 rpm. I read an article once that showed big valves without porting actually hurt flow on the 906 heads. That should be a 360 to 375 hp build imo.
 
I really appreciate everyone’s knowledge and constructive input. Lots of good reading and information here.

I was thinking at the time that the revs could have gone higher, and that I would have liked to experiment more with the jets that I brought to the session... but then my instinct of “hey, you’re the expert here” took over and I let him run the show. If there is a next time, I won’t be so passive.

I think the plan is just to get it back in the car, get it running, then work on some of these tweaks.

As far as a/f ratio... I’d definitely like to play with the jetting. Here is a pic of a plug...

FC70AE66-ABBE-4EF8-B842-97309330184E.jpeg
 
toolmanmike said:
383's are kind of a different breed of cat to build up. A 1/4" bigger bore than a 350 Chevy but a .100 shorter stroke. That doesn't necessarily make them a high rpm engine. The 350's have a 5.7" rod where the 383's have a long 6.36 rod. Kind of a strange combination. Not quite a horsepower engine but more a torque engine. I don't remember too many that were over the top screamers. They ran good stock back in the day though.
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It was the whole combo that was legendary. Especially a 68 Road Runner 383, 4 speed, 4.10 gears.
 
JAndrea said:
I really appreciate everyone’s knowledge and constructive input. Lots of good reading and information here.

I was thinking at the time that the revs could have gone higher, and that I would have liked to experiment more with the jets that I brought to the session... but then my instinct of “hey, you’re the expert here” took over and I let him run the show. If there is a next time, I won’t be so passive.

I think the plan is just to get it back in the car, get it running, then work on some of these tweaks.

As far as a/f ratio... I’d definitely like to play with the jetting. Here is a pic of a plug...

View attachment 1715359342
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That seems pretty lean.
 
JAndrea said:
but then my instinct of “hey, you’re the expert here” took over and I let him run the show. If there is a next time, I won’t be so passive.
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That's fine. There's always something to learn and IMO it always is better to be respectful of a person's experience. If you go back, you can then say 'hey, I'd like to run it a bit higher this time.'
 
Definitely get the jetting and timing up to at least what Mattax suggested. If it's running that lean on the dyno, it's not going to be a very happy combination when you get it in the car. Always safer to start out a little rich and work the lean out of it.
 
Not while cruising, per say, but upon acceleration, where the engine demand is around 12.5:1, definitely so.
 
Carb choice: good. Intake choice: good. 9.6 heads: Even a bowl blend would help more than you would suspect with 79cc heads. She wont be done chirping till past 5500 with that cam and springs. What RPM your springs throw in the towel is a guess until the builder gives you a part number.
 
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Here's an example on a chassis dyno.
This engine didn't like richer than 12.5 on the dyno, and it also started to lose hp when above 13:1.
upload_2019-7-5_21-42-9.png

Had to show it in mph as the rpm wasn't fully captured.
Numbers next to the AFR are primary main jets, secondary main jets. vac secondary with plain spring.
Best drag strip run (previous to dyno) was jetted 74, 78 so prob 12.7 AFR
Although its not important for the discussion 060 over 340 with Comp cams "High energy 280" SD intake, 340 manifolds.
Car would run 14.0 typically with a best of 13.77, 99 mph. 3.23 gears on 24" tires, 3000 rpm stall.
 
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four things, I normally top out at 3, but there are so many loose ends here,,, why not add one more

1) I am always suspect of any engine build where the actual static compression is not so solid it is written in stone
2) the OP says he degreed the cam, OK, at what?
3) I would think a 383 would make power beyond 5300 RPM
4) the comment about the dyno ( post #20) is extremely valid, different dynos, different results. Especially considering the operator did not spin it past 5300, for some reason.
 
Post #42 didn’t just hit the nail on the head, but drove it and the hammer all the way through both boards, with the first swing...
 
I'm always a skeptic on dynos - I don't really take them as bible truth. Ever. But they are useful tools - and in this case the tool is saying it's way down on power to where I'd expect it to be. Maybe not 400, but certainly 40-50hp (15% +) more than it is showing.

If the cam was degreed at all, chances are it's close. Or maybe the better way to say it is it's not "15%" worth of out. I've never had to move a Comp product more than 4* when the base machining was "good" and the parts were mid-price point or better.

Tuning the A/F may give a few hp here or there but it's not going to "make" this combo. It's close already.

Rings should be seated, but might not be, so it may get a little better as things seal.

This is where I'm at: I think the valvetrain is the problem. Lifters may be floating keeping the upper rpm power lower and limiting the ceiling. It should not be done under 5500. It should pull cleanly (and make power) past 6K with that cam and as-cast 906 heads. It's not.the camshaft has a "recommended" lifter and spring. IMO That is what should have been used. I wouldn't use the stock lifters and weaker springs on anything pas the XE268. That upgrade would have meant spring seat and guide work, plus adjustable rockers. Those would have added $1K or more to the build and the builder may have thought it not worth the price. It's a good engine, just not quite where the "sum of it's parts" should put it.
 
moper said:
I'm always a skeptic on dynos - I don't really take them as bible truth. Ever. But they are useful tools - and in this case the tool is saying it's way down on power to where I'd expect it to be. Maybe not 400, but certainly 40-50hp (15% +) more than it is showing.

If the cam was degreed at all, chances are it's close. Or maybe the better way to say it is it's not "15%" worth of out. I've never had to move a Comp product more than 4* when the base machining was "good" and the parts were mid-price point or better.

Tuning the A/F may give a few hp here or there but it's not going to "make" this combo. It's close already.

Rings should be seated, but might not be, so it may get a little better as things seal.

This is where I'm at: I think the valvetrain is the problem. Lifters may be floating keeping the upper rpm power lower and limiting the ceiling. It should not be done under 5500. It should pull cleanly (and make power) past 6K with that cam and as-cast 906 heads. It's not.the camshaft has a "recommended" lifter and spring. IMO That is what should have been used. I wouldn't use the stock lifters and weaker springs on anything pas the XE268. That upgrade would have meant spring seat and guide work, plus adjustable rockers. Those would have added $1K or more to the build and the builder may have thought it not worth the price. It's a good engine, just not quite where the "sum of it's parts" should put it.
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It boils down to the old adage, "speed costs money, how fast do you want to go?"
 
Exactly. I don't think the builder was doing anything more than weighing cost vs reward.
I've watched the stamped steel rockers flex and with any real spring and an aggressive lobe. You can see it turning it with a socket, nevermind at 5K rpm. I see valvetrain issues on that trace/result.
 
moper said:
Exactly. I don't think the builder was doing anything more than weighing cost vs reward.
I've watched the stamped steel rockers flex and with any real spring and an aggressive lobe. You can see it turning it with a socket, nevermind at 5K rpm. I see valvetrain issues on that trace/result.
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I’ve seen them more than flex!
Comp Xe275hl

7AB8131F-80DB-4ECF-9002-706083112B75.jpeg
 
are you sure your builder cut your heads from a stock 93 or so cc's to 79?
Would that much require cutting your intake and maybe shimming your rocker shafts?
i'm wondering about your compression...are you going to check it?
 
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