The Jensen Owners' Club

Hi everyone.

Im in the depths of the engine rebuild now.

The motor is in lots of bits, gearbox is out for rebuild and parts from the US have been ordered for the motor.
It would seem that my engine has seen some interesting times with what looks like a catastrophic failure (rod cap/bolt) of #8 at some point in its past, and the crank has had some serious work/machining done to resurrect it but unfortunately is past its best so a new 383 crank needed (luckily I have 2 spare!).
All the rods and rod end-caps are in the wrong order, with some stamped with numbers, some not but none of the marked ones in the right spot. It could be that the rods are from another engine/donor after the failure, but they will all be re-checked for ovality and marked to make 100% sure its as good as it can be on a budget.
Block will be chemi-dipped, re-bore to +040 (its +030 currently), new pistons, rings, bearings, cam, followers, chain, oil pump and a host of other things are coming soon so Im hopeful this will see the engine smoother and with a few of its lost horses returned.
Im not going for higher CP or fancy pistons or rings. I`ll leave the block deck as it is with just a clean up skim if needed.

Because everyone loves a picture or two:

Bare block - cam bearings still need to be knocked out and replaced Spare Crank (luckily I picked up a good one that has already been ground and polished to +010 locally). My question for today is port matching. Is it worth it, both in power/torque AND likelihood of over-doing it.

This is what I am faced with (inlet). A small amount of material removed at the corners will see the gasket match the head. If its worth doing, I`ll also be doing the same on the inlet manifold as well as some work on the exhaust side to make sure the exhaust manifold matches with the head.
Its not a large amount of 'meat' to remove, but nevertheless an opportunity to mess it up if Im not careful.

So whats the consensus on these engines? worth doing or lots of work for little to no gain?

Im not after a big HP engine, but if a few free horses can be gained for a bit of work then Im keen to hear about any tips/tricks.

Any suggestions?

Smoothing out those ports will not provide you any noticeable increase in horsepower, or a noticeable decrease in fuel consumption*.

All of these minor, incremental, changes/updates/improvements build upon each other to provide a more efficient engine, each providing a very small improvement.

The only reason to do it on your build (IMHO) is so that you know it’s done.

Doing so will give you some points in future bench racing contests.

Nevertheless if you don’t do it now, you will never do it. What’s another 6 to 8 hours into your engine at this point?

I vote YES!

Rocky

* That you can feel, because you can't do a "with / without" dyno test, to address Ray's point, below.

Agree with Rocky, WILL be better and personally I think you would quite easily find the difference on the dyno.

However I do think 6 to 8 hours is a bit optimistic!!

The one item to be careful with is to understand exactly where the inlet and exhaust manifolds are when bolted down, the gasket is not a good enough reference. There are plenty of good youtube videos on how to match/mark the parts. In my case the exhaust alignment was fair but the intake (Edelbrock Performer) was awful and the attachment holes had to be modified and then carefully assembled with guide marks to get good port match.

As Rocky says - this is your one opportunity.

Pocket porting is well and truly worth the effort, but port matching commonly yields little to no improvement.

Agree the pocket porting is at another level, a seat of the pants benefit when I had mine done.

I think the matching benefit would depend on how bad it is, there must be a point where mismatches will reduce flow capacity which of course depends on the application. I did a dyno exercise some years ago using special adapters creating different types and size of mismatch on a VW engine, the conclusion was that zero mismatch was the best solution but only by a small amount.

An extreme example of how port mismatches may not make much of a difference relates to the Big Block Chevrolet (BBC). A square port inlet manifold has been proven to work very well on an oval port head. See the image to get an idea of the degree of port mismatch.

You used to be able too buy some templates for the ideal port shape. Some one I know bought a set of heads that had already been ported and he reckoned the engine was a lot more free revving.

If you are after a little more efficiency from your engine getting the compression close to 10 to 1 is a good idea.

Steve

Steve Payne wrote:You used to be able too buy some templates for the ideal port shape. Some one I know bought a set of heads that had already been ported and he reckoned the engine was a lot more free revving.

If you are after a little more efficiency from your engine getting the compression close to 10 to 1 is a good idea.

Steve

Here are the porting templates.
https://www.bigblockmopar.nl/wp-content ... Magnum.pdf" onclick="window.open(this.href);return false;

If you are going to skim the block to square it up, then you might as well ensure that the quench height is optimised.

- It will increase the compression ratio which will help with energy efficiency and power as well as giving you more options when it comes to camshaft selection
- It will improve quench and hence detonation resistance.

The right quench height can be achieved by balancing the deck height of the block, compression height of the pistons and the gasket thickness.

What do you think an optimum quench height is? I’m nearly ready to press the go button on my 383 rebuild - 438 stroker with the original 906 heads. The 440Source pistons have a compression height of 1.320”. My block has not been decked as far as we can tell (stock pistons are within 0.005” of the deck at TDC which is very close to the factory 0.005-0.006” proud deck clearance for a 1969 383). With a rod length of 6.760” and 438 stroker stroke of 3.750”, and a 0.019” steel shim gasket (to a achieve a static CR of 9.6:1) the quench height should be:

9.980” deck height less 6.760” less 3.750”/2 less 1.320” = 0.025” deck clearance plus 0.019” = 0.044” quench height

Thoughts?

I'd say 0.040" to 0.045" is a good range for quench height.

I hadn’t come across the term “quench” so looked it up and thought others in my position may find it useful. Interesting that they make reference to the 1970’s 440’s indicating how bad they are based on current thinking.

https://www.hotrod.com/articles/ideal-quench-height/" onclick="window.open(this.href);return false;

Hopefully this link work.

If you were to use Edelbrock heads they have a squish of 38thou or more precisely the thickness of the head gasket they recommend you use if your pistons go to the top of the bores.

Steve

Probably the most common gasket compressed thickness would be 0.040". So it is straight forward to go with zero deck height (pistons flush with deck) and 0.040" head gaskets, which gives a good quench height of 0.040". You could use the same approach with any gasket between 0.039 to 0.045" and get a good result.

And here is one we prepared Earlier just for this thread .. Edelbrock heads with "0" Deck Height with 10.5:1 compression,.
The block and deck faces have been squared with the bores, this insures that all compression ratio's are the same from back to front, this is something that alot of Mopars when checked have intolerance's