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Old 03-22-2009, 01:45 PM
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Tuning GMC 270 straight 6

Hi all, can anybody out there suggest how to increase the power and RPM on a 270 in-line 6 cylinder GMC engine, I have herd about fitting a 302 head but then this will require new pistons as the 270 ones are domed what sort of pistons would be needed to give a reasonable compression ratio. For increasing the RPM does anything need to be done to the crank or rods ? Is the 302 head ok for more RPM ?

Any suggestions please.
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Old 03-22-2009, 02:03 PM
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270 Gmc.

You are dealing with some old and dated stuff there. I would go to JalopyJournal.Com, and the HAMB forum, for help with this engine. Most everyone there has experience with old and dated. Might also try Inliners International, and Patrick's Antique Cars and Trucks. Patrick's has a lot of GMC engine parts, and even has the article, "Jimmy Basics" that covers the different heads, pistons, blocks, and swapping parts thereof.
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Old 03-22-2009, 02:26 PM
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Thanks for the reply do you know what the web sites are or any other people.
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Old 03-23-2009, 08:31 AM
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The 270m H head and the 302 head have the same size ports. check out
www.inliners.org
A street 270 or 302 are good for 5000 rpm, any more and you taxing the old thing.
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Old 03-24-2009, 11:20 AM
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Quote:
Originally Posted by 35chev/302gmc
The 270m H head and the 302 head have the same size ports. check out
www.inliners.org
A street 270 or 302 are good for 5000 rpm, any more and you taxing the old thing.
Thanks, you have lost me a bit there what is a 270m H ?
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Old 03-24-2009, 01:36 PM
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Quote:
Originally Posted by junior stocker
You are dealing with some old and dated stuff there. I would go to JalopyJournal.Com, and the HAMB forum, for help with this engine. Most everyone there has experience with old and dated. Might also try Inliners International, and Patrick's Antique Cars and Trucks. Patrick's has a lot of GMC engine parts, and even has the article, "Jimmy Basics" that covers the different heads, pistons, blocks, and swapping parts thereof.
I have just phoned and spoken to Patrick's Antique Cars and Trucks, they were very helpful and are going to send me a catalogue all the way to the UK listing bits for the GMC engines, so things are getting there.
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Old 03-24-2009, 04:09 PM
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I have met and talked with Patrick. He's very knowledgable and helpful and about as cool as old guys like me get.
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Old 12-28-2010, 09:39 PM
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Thanks for the help
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Old 05-09-2012, 01:34 PM
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I have decided to stay with the standard engine layout but have made a set of banana tubular exhaust manifolds to help with breathing, also a tubular inlet manifold to fit an Edelbrock 500 4 barrel carb.
The problem I am having is that although it is jetted for a 4.6 Ltr engine and fitted to a GMC 270 which is roughly 4.4 Ltrs. It starts really well and once warm idles very well with no stuttering but when the revs are steadily increased just above idle it starts to stutttter and die, if the same is done but with full choke on it will rev a little bit or rev it hard from idle to 2500 - 3000 rpm with no problem !!!
Can anyone suggest what to change first, suspect weak mixture it has standard rods and springs. Also mechanical fuel pump, which I am going to check the fuel pressure at the weekend should be about 5 to 6 psi ?

Thanks
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Old 05-09-2012, 02:35 PM
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There may be a problem (possibly lean, possibly rich) in transitioning from the idle to the main circuit.

First set the idle mixture screws using a vacuum gauge. But before tuning the carb further, set up the ignition curve to give you a decent baseline to work from. Are you using a vacuum advance? If yes, manifold or ported vacuum, and how much advance is it adding? What is the initial timing? How much timing is the mechanical adding, and how much total timing does it have?

If the vacuum advance is dropping a lot of advance out or the mechanical advance not adding any timing, the drop in the timing can cause a drop in vacuum. Besides the drop in vacuum causing metering problems w/the carb through the transition and main circuits, if the vacuum drops enough it can allow the power valve to add fuel because the carb thinks it's under a load when it isn't, and the air/fuel ratio will go uber rich, causing a stumble or hesitation.

Once the timing is baselined, if the carb still needs tuning, go from there. The plugs can give you an indication of the mixture being lean or rich, as well as the exhaust (black smoke = rich).
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