Just can't my 340 to run well on 91 octane.

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340_833 dart

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When I built my engine I measured everything except the cylinder volume and the combustion chamber volume. I don't have the stuff needed to measure liquid volume.
But, here are the specs that I was able to measure;
Bore: 4.08"
Pistons are 0.020" above the deck. (The block required decking of 0.002" in order to get it square and level.)
I used a Fel Pro head gasket with a crushed thickness of 0.039".
The heads are iron "O" heads with 2.02/1.60 valves.
I installed KB hypereutectic pistons. Using their compression chart I figured my engine is, at the highest, about 9.8:1.
I'm running 16* initial advance with a total of 34*.
Given that I figured I would easily be able to run pump 91 octane gas.
Wrong.
The engine runs hot, rattles and just plain lousy with 91 octane.
A gas station about 10 miles from me has Sunoco 104. And, their mid-grade gas is 94 octane. So, I tried a 50/50 of those two grades and the car immediately responded.
The engine runs cooler, is quiet and so much more responsive with the high octane.
Unfortunately, the 104 is currently $6.99 per gallon and the 94 is $4.89. Compare that to $3.53 for 91. My car gets about 8 mpg. Fill ups are kind of expensive. Thank goodness it's not my daily driver.
I know if I go to aluminum heads the engine would probably run just fine on 91.
But, I constantly read about iron headed motors being able to tolerate up to 10.0:1 on pump 91.
What am I missing?
 
You should be right there with that cam and combo. Did you degree the cam? if it to far advanced it can cause an issue. Ask any engine builder, and we will tell you, a cam set can be off as much as 7*, at least thats the worst I have seen.

Another thing to look at is the timing marks. Are you sure your timing marks on the damper and tab are correct?
 
In my experience 91 octane is pushing it with nearly 10 to 1 compression and open chambered heads. Especially with a smallish cam like yours. I'd expect it to take at least 93 octane to be happy. My old 360 with the next size up Lunati cam and X heads and KB pistons (9.85 to 1 measured compression) would spark knock with anything short of 93 octane in it. That was with 34 degrees total timing and the cam was degreed in and balancer checked to make sure things were where they were supposed to be. Maybe the guys you've read about that got away with less octane had closed chambered heads and good quench?? Have you tried yours on straight 94 octane?
 
Are you running vacume advance or full mec have you tryed recurving your distibutar are you running full vacume to your distributare are you running full vacume at idle or not. A lot of varables tell us what you have right know, you should be able to run what you got with now problems.
 
fishy68 said:
In my experience 91 octane is pushing it with nearly 10 to 1 compression and open chambered heads. Especially with a smallish cam like yours. I'd expect it to take at least 93 octane to be happy. My old 360 with the next size up Lunati cam and X heads and KB pistons (9.85 to 1 measured compression) would spark knock with anything short of 93 octane in it. That was with 34 degrees total timing and the cam was degreed in and balancer checked to make sure things were where they were supposed to be. Maybe the guys you've read about that got away with less octane had closed chambered heads and good quench?? Have you tried yours on straight 94 octane?
Click to expand...

I agree completely! You have already proved it yourself, at the pump. That's why Chrysler put 9:1 pistons in 340's in 1972 and 73. Gas is even worse today. That's why my favorite 273 is having the domes milled off the pistons...
 
Another possible issue is quality of fuel. It may state 91 at the pimp, but, it may be down a point or two. In addition to the above......


I myself would not have run that cam in that compression ratio. Next cam up or minus some compression. About .5 of a point, min.
 
Have you tried cooling the air coming into the carb ?
Maybe adding a spacer under the carb and blocking off the exhaust cross over ?
Cool Can ?

or last resort : Methly/water injection
 
That head gasket seems a little thin for a piston out of the deck, i have 340 that .025 out and i run a .066 head gasket.
I also have a 360 that is out of the deck 11.5 comp and runs on 91
 
I think you have a lot higher compression ratio than you think you do. Combined with an unknown cam degree, plus it's a small cam in general. Don't use the chart KB has on their site, instead use their calculators. I calculated that you would have to have a head chamber in the low 70cc's to have a static c/r of 9.8:1. That head gasket is way too thin for having the pistons .020 above deck. Pull the heads, either pay a shop to measure and have them order you the correct gasket or you could take a thick piece of plexiglass, cut a small hole and use a syringe or pipette and fill the chamber with water to get a rough estimate. And degree that cam, my cam was almost 4* retarded if installed straight up. Thats a lot!
 
MRL Performance said:
You should be right there with that cam and combo. Did you degree the cam? if it to far advanced it can cause an issue. Ask any engine builder, and we will tell you, a cam set can be off as much as 7*, at least thats the worst I have seen.

Another thing to look at is the timing marks. Are you sure your timing marks on the damper and tab are correct?
Click to expand...

yep, and it may be the cam, the gears, cranks keyway or all of the above.

Ive had to set a cam 8* retarded to get an 2* advance.

Cant say forsure...but I want to agree with others about the 52* of overlap being less than desirable for the octane/comp being run...but Id have to fiddle with it to find out.
And 8 miles to the gallon with that cam sound like you have a good amount of advance into the cam.jmo
 
Lots of great questions and responses here. I appreciate the help and will try to answer as best I can.
I am not running vacuum advance. The advance plate is locked in place with two screws. The distributor is a MP unit with easily adjustable mechanical advance.
I had the harmonic damper rebuilt and checked the timing mark against the tab on the chain cover and it came out perfect.
I tried degreeing the cam and feel like I got it set like Lunati said to. The engine idles down to 700 with a nice lope and it pulls hard to the rev limiter at 6250.
The heat crossover on the Performer RPM manifold is blocked and I'm running a one inch spacer under the carb.
The carb is a Holley 750 with vacuum secondaries. I have spent a lot of time tuning the carb and it is dialed in pretty well.
As far as running the straight 94, I haven't tried that yet. I'm thinking it will do OK. But I do admit to liking the smell of the 104 out the tailpipes.
Now y'all have me thinking about a cam change.
Again, thanks for the help. If you have anymore ideas please post them here.
 
What do y'all think about this cam? It's the next step up Lunati Voodoo.
My car's got a 2800 PTC converter and 3.91 rear gears. I have no desire to mess with nitrous.

Hydraulic. Hot Street cam, likes 2800 converter, Hi-Rise type dual plane intake with 750cfm carb, headers, 10:1 compression and 3.73 gears. Likes up to 200HP nitrous.

Advertised Duration (Int/Exh): 276/284
Duration @ .050 (Int/Exh): 234/242
Gross Valve Lift (Int/Exh): .513/.533
LSA/ICL: 110/106
Valve Lash (Int/Exh): Hyd/Hyd
RPM Range: 2200-6400
 
The whole voodoo line is good. You will see the mid/upper power pick up with that cam.
 
Did you try just backing off the timing a little ? The marks might be right but not all timing lights read correct. Turn the timing back--might be all that is needed. Make sure the spark plugs aren't too hot also.
 
That would be my smallest cam to try with the C-ratio you have. It should work. Timing the distributor and carb jetting will be of the most important issue's to tackle next.
 
on my 750 vac sec liked to have 80 jjets all the wat around go with stiff spring the grey one and 35 to 38 squirter..and take choke off and it should fix problem..or u can get a dp like i di alot alot easier...i have a 408 tho...good luck
 
moper said:
What's the tune on the 750 VS like? What power valve? what jet?
Click to expand...

IIRC, I've got a 6.0 pv. The engine has 11" of vacuum at idle in gear.
I'm pretty sure I have 72 jets up front. It's been a while since I have been into the carburetor.
It's got a metering block on the secondary side.
The choke plate and shaft have been removed. It's got the tan accelerator pump cam and a 32 squirter.
Plus, it's got the white vacuum secondary spring.
The spark plugs are Autolite 66s and are very clean.
Judging by the gas smell out the tailpipes I have always wondered if the set up is a bit too rich.
 
Clean is lean, not rich. I think you're overly lean. My changes... The power valve is way low. You should be closer to an 8.5 or 9.5. The problem with your setup, as I see it, is you ahve squirtor that ends early, and a power valve that opens "late". The main jets are fine, and I think the secondaries are fine if that's the factoy plate... That equals 72s. But the white spring? You should be able to run the plain steel, or the purple with no issues with the convertor and gearing. The problem is, ping is self perpetuating. Once it starts, it wont stop because the chamber temps spike. So stop it from starting, and it should calm down for you. Least that's my take. That's where I'd start from in terms of tuning.

Edit - the squirtor should go back to 27 or 28. I forget which is "stock" on the 3310.
 
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