Hemi advance timing?

[AR67GTX]

Probably most people have read the story about the early dyno testing of the 426 Hemi and how initial testing was disappointing and didn’t show any advantage over the Max Wedge motors. Then, based on some race development of the Gen1 Hemi I think I remember, they decided to crank a bunch of timing into it (40 degrees +) and see what would happen. And that was when it came alive and showed its true potential.

OK, so another often stated tuning standard seems to be that street Hemis generally don’t like more than around 32 to 33 degrees total timing. I have the impression that even street Hemis with a bump in compression and cam timing find this lower timing to work best - or is that a mistaken impression on my part? I keep my stock engine set around 33.

These two observations would seem to be contradictory and I have a hard time reconciling them. The only thing I can guess is that maximum power in a Hemi takes a lot of timing and a lot of octane to support (maybe 115 octane + ?) - something that isn’t practical on the street. But with the lower compression of the street Hemi that doesn’t seem to make much sense.

So, what’s the real story here?

 

[Cheapsunglasses]

A story I heard is. Don Garlits loved the 392, and when Chrysler gave him the new 426 he didn’t want anything to do with it, the 392s were perfect. So he wanted to show them their new engine was junk, and advanced the timing to where he thought it would blow up. Instead he ended up setting a bunch of time and speed records, and planting the seed of the Hemis dominating legacy.

 

[Darter6]

^^^^^^^^^^ Yes, I watched a interview with Big Daddy and he told that story. He went from 38,40,44,46,and up to 50 degrees.
Just went faster every time.

 

[AR67GTX]

I think I read a version of that story too but had forgotten about it.

Maybe hemispherical chamber has some exaggerated relationship between cam timing and dynamic compression?

 

[INTMD8]

Did that car run nitro? Or what fuel?

Just as another data point, mine is 12.4-1 with a 104.2cc dome. E85 so not octane limited.

Still, made the best power at 33 degrees total timing.

 

[AR67GTX]

Did that car run nitro? Or what fuel?

Just as another data point, mine is 12.4-1 with a 104.2cc dome. E85 so not octane limited.

Still, made the best power at 33 degrees total timing.

View attachment 1313179

Thanks - reinforces my curiosity. I wouldn’t think Chrysler engineers were doing development dyno pulls with nitro. Guess I’ll look for that old HotRod Mag story and re-read it.

 

[Fran Blacker]

Back in the day 60's and 103 octane 36 degrees was considered max timing with gas. This is to the best of my recollection.
Read that Garlits article that Cheapsunglass' posted. If you ran to much timing on a 392 on nitro you'd blow the bottom end.

 

[diesel_lv]

Hi octane requires more timing as high octane burns slower. Very efficient heads, flow, squish, etc, require less timing as the flame burns faster and thus also a lower octane. The early Hemi design was not super efficient, they also like high RPM's for power along w the additional timing.

A lot of 2 valve heads like anywhere from 32-36° timing. I've got a 4 valve head that makes no more power after 24°. 3 valve same engine 25° and 2 valve same engine 26°. The 4 valve redlines at 8k.

The more efficient the head design, the less timing it will take.

Now, can you make more power by adding timing, yes, "if" you increased compression cams etc which will also require higher octane to slow the burn and prevent detonation.

 

[INTMD8]

If we are talking pump gasoline, high vs low octane has a nearly identical burn rate.

That will be more determined by combustion chamber design.

Different fuels of course (especially racing) can have faster/slower burn rates but it's all very specific and no direct correlation to octane.

 

[Geoff 2]

Never heard of the 426 hemi disappointing anyone. It won the first 4 places in it's inaugral race at Daytona. What other engine can claim that?

The Street Hemi had a 48* curve in the dist. While the hemi chamber unshrouds the valves for good airflow, it has a large surface-to-volume ratio which is bad. It requires a lot of ignition advance to completely burn the mixture & that is bad for power. It also loses a lot of combustion heat to the surrounding chamber metal, also bad for power.

 

[AR67GTX]

Yes, but then why do the street Hemis perform best with less timing In the 32 - 33 range? Only difference would be a bit smaller chamber surface to volume area ratio.

 

[Runcharger]

Since the 70's I always ran 34 total but my 71 Hemi Charger ran best with 40 total. Up here I believe the 94 octane fuel formulation had something to do with that.

 

[diesel_lv]

Hemi's love timing.

 

[Nxcoupe]

A story I heard is. Don Garlits loved the 392, and when Chrysler gave him the new 426 he didn’t want anything to do with it, the 392s were perfect. So he wanted to show them their new engine was junk, and advanced the timing to where he thought it would blow up. Instead he ended up setting a bunch of time and speed records, and planting the seed of the Hemis dominating legacy.

He spoke at the National Hemi Owners meet way back when and I was lucky enough to be there that day. He is a very cool car guy and it was just unreal to meet him.
He told that exact story in great detail, so I can attest to what you are saying as gospel.

 

[Charger 770]

572 Hemi here. 11.2:1, 272/282 @.050, .750/.680" lift, single Dominator, 2 1/4" x 3.5" headers.

From 30-34 it did'nt make much difference, but best HP/TQ was at 33 deg....850HP/770TQ on pump gas.

In the car in summer, I run around 30 deg on premium pump gas here.

Works for me.

 

[6PKRTSE]

Same here. My 528" on motor made the most horsepower and torque numbers on dyno at 33 degrees of timing as well.

 

[Dragon Slayer]

The Street Hemi had a 48* curve in the dist. While the hemi chamber unshrouds the valves for good airflow, it has a large surface-to-volume ratio which is bad. It requires a lot of ignition advance to completely burn the mixture & that is bad for power. It also loses a lot of combustion heat to the surrounding chamber metal, also bad for power.

What specifically do you mean by 48 curve? Including Vacuum? The 66-67 Non emission hemi put in 17 deg mechanical at the crank. Vacuum could add 18-19 deg at 15". The cap carbs and 68 up were putting 30 deg mechanical but used 0 deg initial timing. Same vacuum adv. So they were running total around 30-31 deg. Cruising might be 48 deg with the vacuum.

 

[Geoff 2]

According to my 1966 Motos Manual, page 735, the 1964 Hemi Charger had a dist that had full advance of 24* [ dist degrees ] @ 880 rpm....which is 48 crank degrees @ 1760 rpm. There was NO vac adv.

 

[Geoff 2]

Dist#2444814 for above.

 

[Dragon Slayer]

There was no 64 charger in us, but that is the super stock distributor for the 64-65 super stock cars. I have that distributor. I think Moto Manual gave you the crank timing.
Think of the slot length required to give 24 at the distributor. The largest I have seen is 17 degree slot on mopar emission cars. Mopar used fix mechanical, there was no adjustable mech deg like some of the aftermarket.

From the Autolite/prestolite data, that cam gave 10deg mechanical in at 440rpm which is distributor timing on a machine. At the crank was 20deg in at 880rpm. Pick your initial.

Early street hemi was about 12.5 if I remember right, but starting in 68 it was Zero initial for emissions with 15 deg cam in the distributor.