390 intake upgrade suggestions

So I’m about to make the plunge and rebuild my original 68 x code 390. I’m just looking to increase the street performance reasonably, not make an unreliable race car. Unfortunately given the shock tower situation, I’m stuck with reusing the factory cast iron “log” exhaust manifolds which are very constrictive. I’ve been looking at Edelbrock’s line of aluminum performance intake products. Most likely will be using one of their performer intake/cam combos, and possibly one of their carbs to match. I was wondering if anyone has used their performance heads as well with the factory exhaust manifolds? I’m concerned it would be a waste to spend the money on the heads if the exhaust will choke the flow? My other thought was I might be spending at least close to the cost/hassle getting the cast iron heads redone?

I am not going to be much help. The X code has fantastic performance in stock form. It has a huge 470 CFM two barrel carburetor. It runs great the way it came. It’s just slightly less performance than the S code engine with none of the drawbacks. And less weight because of no smog system.

I’m not going to disagree with Royce’s assessment.

If you’re determined to change the intake and heads, why wouldn’t you just install Cobra Jet exhaust manifolds and 2.25” exhaust? They are designed to fit in the Cougar engine bay.

I think you may be astonished at just how strong that X code is when it is not completely tired. I do get the desire to have a little something extra going on.

You could put CJ sized valves in the original heads and then add CJ cast exhaust manifolds. I like the stock GT cam and that carb you have is bullet proof and runs great. Plus it will look completely stock on the outside. The X code 390 is really a great motor, a high compression 2V is pretty cool.

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I did originally consider the CJ exhaust manifolds, but trying to find a usable set is next to impossible. Unless you know of a pair for sale? Also in doing more research, the FE “guru” Jay Brown didn’t have very positive results when testing CJ manifolds. For what that’s worth. Though I find it hard to believe they wouldn’t be an improvement over the log manifolds.

I am going to look more into redoing the original heads, but trying to find a good auto machine shop any more, in my area at least, is getting very difficult at best. I’m even having a hard time finding a body shop that will even consider a paint job!

Reproductions are a fairly good deal, and brand new!

I managed to find a decent set of originals for less then the ones in the first link. But that doesn’t happen often.

Thats the way I’m leaning towards, Eldelbrock heads and these CJ manifolds. I just want to feel confident that combo will fit in the shock tower compartment before I pull the trigger on them.

It’s a total bolt in. The 428CJ was optional in 1968 and is the same size externally as a 390.

." Unfortunately given the shock tower situation " ? What situation would that be ? And what makes you think your stuck with the factory cast iron exh manifolds ?
Headers fit, it’s tight getting the header bolts in, but doable. And installing the headers, you just jack up each side of the motor and slide the headers in.

Manifolds, the performer or rpm are good manifolds, but I like the factory stuff, the factory Police Interceptors are good choice.

Cams, I don’t know about the edelbrock cam combos, for a better choice, check with Blair Patrick or Brent Lykins for a better cam option.

Heads, just depends on how much money you have to spend, Blair Patrick has some CNC options both in cast iron and aluminum, reasonably priced. I went with a set of his Pro Ports, but they are a bit more than what you would need for a 390.

Carburators, I am not a fan of the Edelbrock AFB’s, I think a better choice would be either the 735. 750, or 780 Holley, pick up a used one and have it gone thru.

Thanks for the input, good options to consider. I wasn’t familiar with Blair Patrick or Brent Lykins, I’ll look into them for sure. After a quick search for Blair it brings me to Bear Block motors, not sure if it’s the same outfit though. It also lists their heads as out of stock, but I have some time, just pulling the engine out this week for R&R.

I don’t see the Edelbrock heads has being the best option, their chamber size (at 72cc and 76cc) is greater than the 390 head (at 68cc to 71.1cc), thus a lower compression ratio. Ideally you need a higher compression ratio.

So say you now decide to raise the compression ratio by replacing the original dish styled piston with a flat top piston. All is good you say…

But by raising the floor of the original dish piston to a flat top, you have placed the centre of the piston closer to the hottest part of the burning fuel mass in the combustion chamber. Now a normal flame temperature for a typical engine at full throttle ( rich) will be about 2400 °C, ( and 3000 °C at lean cruise conditions)

BUT A typical aluminium piston and its alloys will melt in the 600°C to 660°C. Not good.

3 things save a piston from instant destruction-

  1. the distance the piston crown is from the centre of the burning flame, thus the piston face will run at about 300°C.
  2. The fuel mixture, that is, the engine turner will richen up the engine to control pinkin/detonation which in turn cools the burn and thus the piston crown.
  3. Aluminium pistons have a very good heat transfer rate, thus can transfer heat away from a hot spot to the whole of the piston.

Let’s look at the problems with number 3 above.The heat in a hot spot in the piston crown travels so fast compared to heat transfer in the fuel mixture, that the whole of the piston will heat up faster than the fuel mixture above it. The result is, the whole surface area of the piston crown becomes an ignition source for the unburnt fuel/air above it.
Result is detonation, if the engine is allowed to run too long after detonation, the piston will melt/collapse at the point where it entered the hottest center of the burning mixture.

Okay, what am I trying to say here…
I am saying, it is better to run a small chamber head with a large dished piston (dish in the center of piston crown), than it is to run a large chamber head with a flat top or dome piston.

If you was to build two identical engines, both with identical compression ratios, one with a large chamber and flat top piston, the other with small chamber and dished piston. The engine with the large chamber and flat/domed top piston is more likely to encounter pinkin/detonation.

So good is the dished piston/small chamber design at preventing detonation, that the engine builder can now start experimenting with raising the compression ratio.

So, you need to start with a cylinder head with chamber which is smaller than what you have now.

And if you still don’t under stand what I said, then

" If it ain’t broke, don’t fix it"
:slightly_smiling_face: :grin: :slightly_smiling_face:

The reality is that the Edelbrock heads are very similar in terms of chamber size compared to stock 428CJ heads. Both are in the 76 - 78 CC range. The difference is that the Edelbrock heads are brand new and not worn. The comments about aluminum melting point are baseless.

Royce- what you are saying here is to use heads with larger chambers which in turn lowers the compression ratio, and this is better, huh?
That is what I understand you are saying? Correct me if I am wrong.

As for aluminium heads being new, and not rebuilding cast heads, it all comes down to the skill of the head reconditioner. A rebuilt cast head will out last an aluminium head. Think about it, the original cast head is now 56 years old, but it can be rebuilt. You will not get 56 years out of a aluminium head, it will be a pile of corroded aluminium oxide by then.
Second point is that when building an engine, you port the head to suit the camshaft and the criteria that engine is built for. For example, if the engine is built for maximum power only, the flow rates between the inlet port and exhaust port are different to that of an engine built to have good power and good fuel economy. In this case the exhaust port will need more flow. Just buying a head off the shelf where one head fits all is possibly not the best way to go.

I have missed what you are saying here? My only guess is you didn’t understand what I was saying, or I didn’t explain it well?

:slightly_smiling_face: :grin: :slightly_smiling_face:

The Edelbrock heads are very similar in terms of combustion chamber size to stock C80E 6090-N castings. They are about 76 CC out of the box. Same / same - not a big change in chamber size. The melting point of aluminum is totally unimportant, useless information. It doesn’t happen. Maybe you have some inflated vision of yourself. Sharing useless information doesn’t benefit anyone.

No personal attacks folks.

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I agree here,
I am not trying to attack anyone, not even Royce.

I do understand that it can be very difficult to interpret what I say.
It is also difficult for me the convey an idea, whether by written word, by reading or even by speech.

I also think and write a little too logically with a lack of emotion, which in turn offends some people. If this is the case don’t be offended.

Peter :slightly_smiling_face: :grin: :slightly_smiling_face:

Royce, my post is about raising compression ratio above and beyond what is considered the normal, and solving the detonation problem.

I thought if I explain the concept, it would be easier accepted than just stating facts. The melting point of aluminium is only part of the explanation of why detonation occurs.

The 390 “X” is rated about 280 hp @ 4400, but it consumes enough fuel/air mixture at that rpm to make 840 hp @ 4400. All I want to do is tap into some of that potential lost power of 560 hp. We can debate it, or go to our corners and then everyone will lose.

Peter :slightly_smiling_face: :grin: :slightly_smiling_face:

So you are imagining a custom set of pistons and a custom made set of heads? I think you are dreaming. I spent something like 6 hours on the phone last week to order a single set of stock pistons for an FE. With that done, they will take 10 weeks give or take to arrive. The cost is going to be something like $900 because that’s what it costs to order a set of pistons. Making a custom set is going to be something like $2500 to start with - and that’s only pistons.

I am talking about things I have actually done. Things that help.