The true compression of stock 426 street hemi !

[427dart]

Found some info from a person involved with the 426 street hemis at Chrysler regarding the actual compression of a stock 426 hemi as delivered.
It calculated out to 8.5 to 1 even though the factory rated it at 10.25!
This was due to the large amount of material left on the block deck height and on the cylinder heads. Chrylser always made note of how much to cut from block and heads when blueprinting these engines.

Now when you installed the factory Race Hemi 12.5 pistons without any mill work done you really had 10.5 to 1 compression.
And to think back in the day,for us it was 1970, it was only Sunoco 260 going in the tank of our then stock 1970 Hemicuda!

Imagine how much stronger the average stock Street Hemi may have been with the true 10.25 compression!!!
I wonder how far off the mark other makes were during the 60's -70's?

[mag2555]

I will bet ya lunch that you could also factor in another 2 CCs total for the top and second ring back land clearance!

[Geoff2]

Doubt very much that Street Hemis were ever 8.5: 1 CR.
I have an article written by Keith Black, who you might say was the Hemi Godfather on blueprinting the Street Hemi. He says the stock pistons were 10.25:1 ratio & no mention is made of decking the block or heads to raise the CR.

[bigfoot584]

An old machinist once told me Ford and Chrysler had some of the worst
factory machining he'd ever seen, and that would've been about 1970
when I'd heard that from him, he was surprised they didn't have worst
problems then they did back then.

[Geoff2]

Well the Chryslers I owned & worked on must have come from a different Chrysler company...

[Geoff2]

And just to add: at the very first race that the 426 Hemis debuted, not only did they finish the race , but Hemis won the first 4 places [ 1964 Daytona 500 ]!!! You don't win races with poor machining...

[MotionMachine]

I recently machined an original hemi. The owner bought it without any main caps so I had to fit aftermarkets. This was by far the best Chrysler block I've ever seen as far as bore locations and decks being 45 degrees from cam centerline.

[427dart]

This discovery came about after my brother did a CC check on the block and heads of his original 1970 426 Hemi that came in his 1970 Hemicuda.
Block date is 1969.
He did it by the book several times and with the factory "Race" 12.5 piston the correct compression came out to 10.5.
Yes this was a bit confusing to us since we figured maybe a real 11.5 0r 12 to 1.

However he found some info from within the Ramcharger group that brought up this same situation.
We will do some more checking and see if there is more info on this.

We still have the original 10.25 pistons we could use to do a another check.

So has anyone here actually CC'd a stock 1966-69 vintage Hemi block and cylinder head to back this up?

[pamotorman]

425 426 DP .536ab Dome 88.30 cc
from the NHRA spec sheet for a 1970 hemi.

[novadude]

I'm curious about this one. Most GM engines missed the mark when it comes to advertised vs actual, but not by almost a full 2 points.

[wyrmrider]

the dimensions in the Chrysler Shop Manual and NHRA etc are all the after blueprinted max specs
Out of the box YMMV on head cc and block deck
piston dome not so much
even off the showroom floor it was hard to keep the plugs clean without going through the carbs and tune
did a lot of them and ran a SS 413> 426 Wedge stick converted later to AL front end then TF then Hemi

[bigfoot584]

And just to add: at the very first race that the 426 Hemis debuted, not only did they finish the race , but Hemis won the first 4 places [ 1964 Daytona 500 ]!!! You don't win races with poor machining...

WORST may have been a poor word to use sorry.

Read up on the Hemi block sometime you'll find
how close they came to not making that race.

[427dart]

the dimensions in the Chrysler Shop Manual and NHRA etc are all the after blueprinted max specs
Out of the box YMMV on head cc and block deck
piston dome not so much
even off the showroom floor it was hard to keep the plugs clean without going through the carbs and tune
did a lot of them and ran a SS 413> 426 Wedge stick converted later to AL front end then TF then Hemi

Yes about the plugs! We ran a one of those Mk 10 CD boxes and still have that.

Now do you recall checking the CC's and noting how far off they were from specs?

[pamotorman]

using NHRA blueprint specs for a 1970 454 450 HP chevy you get 12.39 CR not the 11:0 GM stated

[PackardV8]

I'm curious about this one. Most GM engines missed the mark when it comes to advertised vs actual, but not by almost a full 2 points.

Back in the day, Tonawanda was getting complaints their engines were down on power. Random checks of engines coming off the line showed all of them at the minimum compression spec, caused by maximum deck height.

Cause was the engineers gave a target in the middle, but with quite an allowable high/low deck height variance. The foremen on the line soon learned they could run faster and make bonus by removing as little iron as possible.

Solution was to tighten the over allowable variance.