I have just rebuilt a 1970 340 engine to stock specifications. the engine is basically a 10:5-1 compression ratio. Challenger automatic with a 355 rear end.Is this engine fine to run on unleaded hi-test?

Does it actually have 10.5 compression? Chrysler overrated ACTUAL compression ratios.

If you noted your deck height/head gasket thickness and a guestimate of your head CC's from what they're listed at in the charts you can figure your SCR then with your cam choice an estimate of the DCR & from that your octane needs. The period correct heads would not be unleaded compatible but depending on how hard your run it you can get by with unleaded for awhile but for how long I'm not sure. The open chambers/no quench is going to hurt (raise) the octane requirement EDIT If your close you can slow the timing somewhat to compensate but it will drastically hurt your performance

Yup it is, my 340 is a true 10.5-1 and it runs fine on 91 octane. It runs even better with a race gas mix but most of the time I'm burning 91.
Last night i filled it with some gas that was probably around 90 octane and I drove it today and it was fine. Not as good as last night before the fill up but it runs fine on 90. Now I do have aluminum eddie heads and KB 243 pistons.

I did run it on 91 octane when it had factory hi compression pistons and iron heads also for many years.

..i thought a stock 340 was actually about 9.4.
the 10.5 rating allowed racers to build to 10.5 and be legal

If it is iron headed with a stock(ish) cam and has true 10.5:1 I would expect some difficulty. 9.5:1 would be a typical target for pump gas/iron head.

I guess it will like 100 octane. Tricks to help are cold air intake, slow timing curve and limit total mechanical advance. That may do it on pump gas.

If it is not "quench" now a thick head gasket can get compression down and help.

The motor is in a challenger TA, 340 six pack. I wanted the engine put back to stock. The rebuilder told me that "stock" was pretty close to 10:5-1. Just wanted to make sure that this motor would be fine on pump gas and not be harmful to the engine.

what would the difficulties be? Performance? Damage to the engine?

The main difficulty would be detonation/pinging which can seriously damage the engine over time. Bearings, main caps, head gasket ect.

Retarding ignition timing to compensate for detonation reduces performance. You are better off with an "appropriate" compression ratio and normal timing curve.

If you have the choice, I'd shoot for 9.5:1.

I would be scared of any engine builder that uses the words "pretty close"

When I built my last 340 in 2007 I did CC the edlebrock open camber heads made for the earlier 340s. I don't remember the exact CCs but do remember the pistons were .029" above the deck and I'm sure I used the recomended felpro head gasket and I remember it being 10.3-1 to 10.4-1.

That sound right??

Stock bore, hand honed by me with a sunnen hone. Took quite a few weeks to get stock bore pistons. I've got over 12K miles on it now and did a compression test on it last fall and had a even 165psi on all 8 holes! It has never burned a drop of oil, I have never added oil between changes and I drove it 120 mph for miles at a time a few weeks ago at bonneville for the 2nd time, it's a real test on everything.


Come take a ride with me.

^^^^^^^^
Those pictures are on the way out to California, we stopped on the way back too and I beat the crap out of the car on the roads and on the salt. I didn't want to trash the car on the salt on the way out.
Just a tease as I couldn't hold my chessy camera and drive too.

Not every block needs to be bored, exspecially old rarer motors like a 340 if it's not absolutely necesary, I got lucky.

And most people who think they are using 92-3 or even 94 pump octane are really using something closer to 91 octane.

If you are running ethanol blend you will need to jet up from stock.

My question is, got hardened seats?

Be a shame to dial-down a healthy 340...

Tickle it with some CAM2...

Your engine rebuilder should have brought up the gas requirements before the engine rebuild began. Or and least told you afterwards.

Ask him what the part number of the pistons he put in it were. Should be on your receipt.

From the sounds of things, it's a 99% chance the rebuilder used the thick Fel-Pro blue head gaskets that come in the gasket kit which will lower you "stock" compression anyway.

I say run pump 91 gas. If you hear it ping a little time it back a couple of degrees.