Darinm

Good morning all
I have a question on my quest to learn more. I make no claim to be an engine tuning expert so my best bet is to gather all opinions, learn then try and see what works best.

This has to do with timing on my 383 stroker

Here are my engine specs
4 bolt main 350 bored .30 over and clearance
Eagle Stroker crank
Eagle H beam connecting rods
Speed Pro forged flat top pistons with heads came to 10.5:1 compression ratio
Edelbrock Performer RPM 64cc heads Flow ported by engine builder
Lunati 1.5 ratio full roller rocker arms
Comp Cams 314HR retro fit full roller cam w/Comp hydraulic roller lifters
Cam specs:
600 lift
252 duration@50
110 degree lobe separation
DUI HEI distributor curved by DUI to provide 20 degrees mechanical advance by 3000 rpm
Initial timing set at 14 bdc.

Here is my question on my quest for more knowledge.
The initial timing was set by the engine builder on this engine dyno which came out with 460hp, 460lb ft of torque.
This was achieved using his custom build carb which he uses to see if he can get a performance upgrade over the customer provided carb. In my case, it did. My original carb was a Holley Street demon 750 cfm. Using his custom-built Holley double pumper 750 cfm, the engine picked up 40hp and 48lb ft of torque. So, once the dyno was done, using my dyno output I went to the carb builder and had one built for me.
After getting my custom-built carb, the carb builder recommended I change the springs on my HEI distributor to have my timing all in by 2000-2500 RPM vs. the 3000 it’s currently set at. I have a spring kit to do this, but before working with it, I was looking for any advice for or against doing this ?
The car is a 1980 4 speed with stock rear end ratio. It’s only used for cruising, parades and car shows. No racing.

If i'm missing any critical information for advice, please let me know.

Thanks in advance

REELAV8R

Given the long duration on that cam and the fact that you do not race it I would think you would want the timing set up for best drivability.

An initial of 14* is pretty low for a 252@.050 cam. Does it idle ok with not lots of stinky exhaust?
Just balparking it I would think more like 20*ish of initial and 14 to 16 degrees of mechanical. This would require limiting your machanical advance from the stock movement.
More initial would give it better off idle response and make starting out easiser vs having to rev it up a bit to get going.

As far as all in by 2000..that's probably a bit early, 2500 sounds ok. Each engine is going to be a bit different. If timing comes in too early you are working against the rising piston on the power stroke creating more work for the engine and loosing some power as well as stressing the bottom end.
Ideally peak cylinder pressure is achieved around 15* ATDC. Too much advance and you achieve that peak pressure too early and maybe even before TDC.

OMF

For bombing around the street, I think you'd be alot happier by taking 20 degrees duration off that cam than worrying about your timing....

Darinm

Exhaust was very rich in the beginning, but after adjusting the secondary idle screw and getting the proper size power valve the smell is gone. I had read that too rich a mixture also has a louder exhaust and sure enough it did. Now quieter and no smell.

For initial timing, i can certainly try 20, but would need to replace my vacuum advance to an adjustable to limit mechanical since mine was fixed by DUI.

Very good information by the way. Thanks !

Darinm

I'm sure you're absolutely right and in the future, that may be where i'll need to go. I'd love to utilize what i have for now and exhaust all my options as it sits before tearing the top end down again. When and if it comes down to that, it would give me a good reason for a new intake manifold :-)... and if it's taller than what i have, a good excuse for a different hood....

gkull

quite the mismatch on the cam and the other parts. Fast all in below 3000 rpm is not the way to go with a modern aluminum heads, 10.5 C/R and 252 duration.

You best timing choice is to limit the mechanical to about 14 degrees and that would give you 20 initial.

I would not even use a Bubba dyno operator who just wants to show you big numbers. When I was helping with the dyno, the engine was as it will be in the car with it's own intake & exhaust system to prepare it for the real world. Not some gimic carb with a super flow bell velocity stack on to and some shop unmuffled dyno headers.

Adjustable vacuum advance is for engines that ping at cruising speed on level ground like 70 mph lite throttle

Darinm

Interesting opinion and thank you. How would you limit the mechanical if not with an adjustable canister ? Is there another way ? Then if using that method to limit mechanical to 14 degrees, would you leave the curve as is to be all in by 3000?

REELAV8R

Just to clarify, you have three types of ignition advance.
Initial
Mechanical
Vaccum

At idle initial and vacuum advance are in play. With that cam you likely don't have much vacuum. So you need to measure what that vacuum is and get an vacuum advance can that will apply it's advance down to the vacuum you have available.
Ie if you have 10" of vacuum at idle then the vacuum can needs to be all in by 10". Most vacuum cans are more in the range of 14" or more so yours may require a change, if you want vacuum advance at idle.

Having vacuum advance at idle is a lot of help to an engine with low cylinder pressure at idle, like your's is likely. Fuel burns slowly under lower pressures.
With a vacuum advance of say 16* and an initial of 20* now you have an advance of 36* at idle. This is a good thing for off idle response and cleaner burning.

The vacuum advance drops out at full throttle and you are left with only the mechanical advance and the initial advance. IE 20* initial plus your 16* of mechanical. Once again giving you a total of 36* of advance.

At cruise all three are in play. 20* of initial plus 16* of mechanical plus 16* of vacuum. This gives you a total of 52* of advance. This is in the range of a desirable cruise advance of 50* -52*.

To get less cruise advance the likely candidate to limit is the vacuum advance movement.

It's a complicated sort of dance to get them to play together well. Limiting the movement of both the mechanical and the vacuum advances is the best way I have found to get all relms of operation to function well.
I highly reccommend a digital timing light to get this sorted out if you plan on diving into it. Using and old style timing light with a balancer tape will leave you pulling your hair out.

This is the one I have and it is an excellent tool.

Darinm

Correct. Idle vacuum 5-6". Currently running a 2.5 power valve.

On the cylinder pressure, how does that correlate to compression ratio? I would think the higher the compression ratio the higher the cylinder pressure ?

On the timing light, strangely enough i JUST purchased that one last week and haven't even hooked it up yet ! I was just using an old school version before.

So this brings up a question about my HEI distributor. Is this worth keeping ? or should i be looking at replacing it with something more adjustable ?

gkull

You got your answer. vacuum and mechanical are two separate items.

Some dizzy mechanical advance plates have adjustable mechanical advance stops. I use the MSD E-tec and adjust things with dial switches.

I have my all in set to 3000 now. I can change a dial and set it to 3200 rpm and I have experimented up and down. I have 14 degrees mechanical giving me 20 initial. Then when I plug my vacuum in I have it set to 8 additional and I use the partial vacuum port. so I do not get additional advance without some throttle input like cruising down the freeway.

I honestly believe that people that say all in at 2500 or even 2800 have never worked on a modern performance motor or anything hooked to a dyno.

I've rebuilt motors that Bubba the ignition man set the timing and super lite spring sets to bring the advance in. The rod bearings are just toast. Firing way early before the piston reaches TDC creates excessive cylinder pressure. It could have mild detonation rattle that the driver never notices because he has his foot on the floor.

REELAV8R

Yes cylinder pressure is related to CR as well. It is also related to the duration on the cam. The higher the duration on your cam then the later the intake valve closes to start compressing the intake charge.

On a cam with a 252@.050 the piston is farther up the cylinder before the valve closes vs a lower duratation cam. So there is less remaning travel up the cylinder to TDC for the compression to take place reducing the cylinder pressure at least at low RPMS and idle, things change farther up the RPM range.
This is lower cylinder pressure vs a cam that closes the intake valve sooner snd thereby the piston farther down the cylinder on the compression stroke with the same CR.
Long duration cams are good for producing power at higher RPMs. Just curious was your max HP reached in the 6500ish RPM range, or maybe higher?

I can't really speak to using something other than an HEI as I haven't used anything else myself.
There are people on here who do use the MSD box and maybe can school you on it.

Good choice on the timing light. It'll save you lots of time over the fixed type timing light.

I also know squat about the holley and it's associated settings. Although the 2.5 power valve does ring a bell for low vacuum engines.
I still use a Q-jet carb. Modified for my engines needs.

Darinm

Thank you and great explanation of duration and cylinder pressure. That makes alot more sense now. I dont have the dyno sheet with me, but it seems peak horsepower was in the mid 4000 range. I'll check that when i get home tonight.

REELAV8R

Anytime, just keep asking questions, it's easier than heading off in the wrong direction. Probably peak torque was in that range.

Darinm

RELLAV8R, you were absolutely correct. Sorry for the delay on my response. Kids and homework took priority over finding dyno sheets.

Peak Torque was at 4500rpm. Peak Horsepower was at 5700 RPM

Darinm

GKULL, re-reading your response. When you say "partial vacuum port" is this the vacuum port on the passenger side of the carb's metering block ? I know the manifold and bottom of carb plate i think are called full vacuum.
Thanks

USAFVeteran

Commonly referred to as ported vacuum.

Gunfighter13

With the set up you have I would do away with the vacuum advance. Correct distributor to 26 mechanical all in by 3000 to 3500 with 10 to 12 initial. Your cam power curve does not start until 3500 rpm and you have a stock rear gear ratio. That cam needs at least a 3.73 to 4.10 rear gear. Also consider what fuel you can get in your area. Don't need any detonation with the new engine.

v2racing

I would have to agree this is a mismatch of parts, at least the cam anyway. It's not hard to make 500 horse out of a 383 with 20 degrees less duration. That cam is probably costing you more than it's helping, especially on the bottom and mid range.

Mike

Darinm

Thanks Mike. If this were yours and replacing the cam was the route you chose to go, what would you choose ? Also, given my setup and replacing the intake were also an option, would you replace it ? If so, which one to fit under the hood ? Cam obviously being my biggest concern.

Gunfighter13

This is the Cam I would have used:

Xtreme Energy XR294HR Hydraulic Roller Camshaft
Lift: .540''/.562''
Duration: 294°/300°
Lobe Separation Angle: 110°
RPM Range: 2800-6100