fuel requirements for rebuilt 10:5- compression 340

Posted By: sailboat

fuel requirements for rebuilt 10:5- compression 340 - 07/13/12 07:15 PM

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?

Posted By: JohnRR

Re: fuel requirements for rebuilt 10:5- compression 340 - 07/13/12 07:27 PM

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

Posted By: RapidRobert

Re: fuel requirements for rebuilt 10:5- compression 340 - 07/13/12 08:21 PM

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

Posted By: Challenger 1

Re: fuel requirements for rebuilt 10:5- compression 340 - 07/13/12 09:04 PM

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.

Posted By: can.al

Re: fuel requirements for rebuilt 10:5- compression 340 - 07/13/12 09:30 PM

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

Posted By: ahy

Re: fuel requirements for rebuilt 10:5- compression 340 - 07/13/12 11:45 PM

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.

Posted By: sailboat

Re: fuel requirements for rebuilt 10:5- compression 340 - 07/14/12 02:11 AM

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.

Posted By: sailboat

Re: fuel requirements for rebuilt 10:5- compression 340 - 07/14/12 02:14 AM

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

Posted By: LaRoy Engines

Re: fuel requirements for rebuilt 10:5- compression 340 - 07/14/12 02:27 AM

Quote:

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.

Pretty close to huh? How close did he get it? You might be worrying for no reason.

So, what compression was the engine really built to and how was the compression ratio determined. Have him explain how he got 10.5:1. Deck height, piston type (part #, valve relief cc's) cylinder head cc volume, gasket thickness, bore and stroke?

Posted By: ahy

Re: fuel requirements for rebuilt 10:5- compression 340 - 07/14/12 02:32 AM

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.

Posted By: DaytonaTurbo

Re: fuel requirements for rebuilt 10:5- compression 340 - 07/14/12 04:38 AM

Quote:

Quote:

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.

Pretty close to huh? How close did he get it? You might be worrying for no reason.

So, what compression was the engine really built to and how was the compression ratio determined. Have him explain how he got 10.5:1. Deck height, piston type (part #, valve relief cc's) cylinder head cc volume, gasket thickness, bore and stroke?

Without any actual specs on the engine build, any guesses of ours are worthless. Best I can suggest is start with pump 93, get it dialed in and see if you can go down from there without a ton of retuning.

Posted By: AdamR

Re: fuel requirements for rebuilt 10:5- compression 340 - 07/15/12 02:51 PM

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

Posted By: Challenger 1

Re: fuel requirements for rebuilt 10:5- compression 340 - 07/15/12 04:00 PM

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.

Posted By: Challenger 1

Re: fuel requirements for rebuilt 10:5- compression 340 - 07/15/12 04:26 PM

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

Posted By: Rug_Trucker

Re: fuel requirements for rebuilt 10:5- compression 340 - 07/16/12 01:02 AM

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

My question is, got hardened seats?

Posted By: RSNOMO

Re: fuel requirements for rebuilt 10:5- compression 340 - 07/16/12 01:34 AM

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

Tickle it with some CAM2...

Posted By: autoxcuda

Re: fuel requirements for rebuilt 10:5- compression 340 - 07/16/12 01:50 AM

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.