Relieved Block
 

Hi All.. Just wondering if a relieved block has anything to gain over the standard type block. Do you think it might be prone to burning exhaust valves because the outside edge is more exposed during combustion?
Regards Kevin.


I've just acquired a relieved motor. was told it was from a Brengun carrier. It's not the type of thing I would do to an engine myself.

This is the second one to have passed through my hands.
The first one was recovered from a boat in the 1980's and the sump was full of sand. I noticed that the heads were different in the area over the piston tops. They had much more meat covering 'more' than half of the piston. I assumed this was to compensate for the loss of material from the block so as not reduce compression.
I think I still have these heads in my stash.

Could I bolt them onto a standard unrelieved block to increase compression?

Re: Relieved Block
 

Good thread here > https://www.jalopyjournal.com/forum/...how-to.596271/
https://www.rlengines.com/images/fla...flathead24.jpg

Re: Relieved Block
 

Heck, that looks like my work! God only knows I've relieved a few flatheads . . . of all shapes and sizes. LOL

Most of us flathead horsepower chasers believe that relieving a block helps at higher RPMS, with a well ported engine, with a cam that needs it, bigger valves, etc.. If you're creating a typical street flathead and are not really going all out for higher RPMs (over 4500 or so), and don't do the REST of the necessary valvetrain work, then you'll probably gain nothing from a relief.

Actually you'll probably hurt lower RPM performance as you're reducing the compression ratio. This is a real consideration for most street engines . . . who don't have the ports, the valve sizes, the cam, the RPMs or the lower-ends to use a relief.

So, there yah have it . . . I relieve all my engines, but I don't build the type that most build.

Also, as I wrote in my article - there are as many opinions as bung-holes on the subject . . . pick the ones you like, listen to whatever 'guru' floats your boat! LOL.

Boy, I'm RELIEVED I got this off my ole' chest . . .
B&S

Re: Relieved Block
 

I relieved one of mine. Run a 3/4 Iskederian cam . 11:1 Offenhauser heads , stalite valves and tri power carb set up. Been on the road for over 30 years . Still running great.
Vic

Re: Relieved Block
 

Could you please share how you are able to get 11:1 compression on a relieved engine? Do you have a large displacement engine? Can you tell us how may ci?

Re: Relieved Block
 

Did not say he got 11:1 compression but has "11:1 Offenhauser heads".
Everything else is just 'puffery'.

Re: Relieved Block
 

I'd really like to see what the O/P has to say.

Re: Relieved Block
 

I don't believe in relieving the block anymore. I did back in my early days , but after due consideration in airflow decided that relieving a block was a waste of time and just lowered compression. So for the last 50 years I skipped this mod. Besides, it makes it hard to install rings in the bore.
Gramps

Re: Relieved Block
 

Came to the same conclusion more than 60 years ago after spending many hours relieving my first (one and only) block.

Re: Relieved Block
 

Wonder if unrelieving is possible? Jack E/NJ

Re: Relieved Block
 

As mentioned I am running 11:1 Offenhauser heads . With the relieved block I figure that I was in the neighborhood of 9:1 compression ratio. I was not wanting to build anything radical , but just wanted a old school looking engine. I am also using baffles in the center exhaust port , Fenton cast iron headers and 12 VDC electrical system.
It runs on regular pump gas and reliable to drive.
Vic

Re: Relieved Block
 

Offy heads come with valve lift clearance specifications, not compression. Pretty much the only Offy heads around now are .400 and .425. Per the attached flathead compression chart, you can not get to 11:1 unless you are running a very big inch engine and .350 or .325 Offy heads.

Per the chart, my 284" factory relieved engine with .425 Offy heads has a compression ratio of 8.5, which is pretty close to optimum for a street engine. Runs strong with no pinging on regular gas and timing mark advanced 3 degrees.

Re: Relieved Block
 

A relieved block can take advantage of the crow foot chamber shape in many aftermarket heads. Where lower compression won't hurt is when running a blower or turbocharger set up. A lot of truck blocks were relieved at the factory so a person is sort of stuck with it if they have one of those. Make the best of it if that's what you have.

The decks aren't all that thick without a relief and relieving tends to make them more prone to heat cracking if the engine ever gets overheated.

Re: Relieved Block
 

I have been fortunate to have had 2 new 59L blocks with the factory relief. Both were bored to 3 5/16th with 4 inch Merc cranks and the Offy 400 heads. I always figured the compression ratio at close to 9to1. Both seemed to operate alright on regular pump gas. I'm not smart enough to know if the relief helped or hindered their performance?

Re: Relieved Block
 

THEOffy 400 heads won't clear the lift of the 400jr cam. In fact neither will the 425 head, I made a fixture to clearance the head at a 4 deg angle so i wouldn't remove much material. Most of the engined I buikd back in the 90's were 276, blocks and Merc cranks were cheap and the 400 jr wa a nice cam. i went to the L-100 soon after. It was allot cheaper and didn't need much spring pressure, but the offy heads still had to be clearanced for cam lift
Most of the aftermarket HC heads had the area over the relief area lowered ito increas CR and a relief job just lowered it
Massive waste of time. I started cutting my own chambers.

Re: Relieved Block
 

Both my 59L engines used a reground cam I got from Baxter Ford Parts that was similar to the Isky 88 cam. It's been a while but I don't remember having any clearance problems on the heads. But I could be wrong?

Re: Relieved Block
 

I've just acquired a relieved motor. was told it was from a Brengun carrier. It's not the type of thing I would do to an engine myself.

This is the second one to have passed through my hands.
The first one was recovered from a boat in the 1980's and the sump was full of sand. I noticed that the heads were different in the area over the piston tops. They had much more meat covering 'more' than half of the piston. I assumed this was to compensate for the loss of material from the block so as not reduce compression.
I think I still have these heads in my stash.

Could I bolt them onto a standard unrelieved block to increase compression?

Re: Relieved Block
 

Post some pics so we know what you have.

Re: Relieved Block
 

I'll try to locate the heads and upload a pic. If they arn't here at my place then they may be at my sisters. We are currently locked down here in NZ so it may be a while. Also unsure how to load a picture here.
Regards Kevin

Re: Relieved Block
 

Fab link. Great info tutorial etc.. create your flow pattern maybe.. Regards Kevin

Re: Relieved Block
 

I have a relieved 99A block stored at my sisters place under a tarp. I filled all the bores with marine grade grease before I left it there 17 years ago when the marriage went down the shitter. I'm looking at dragging it out next year for a project when I retire. I have always wondered it was a Bren gun carrier motor. It was supposed pulled from a 42 woody that was seen a round Wellington city yonks ago but it is not the original engine to that of course.

GB

Re: Relieved Block
 

The Iaky 88 doesn't have much lift

Re: Relieved Block
 

Good reading,
B&S knows his stuff!

I think relieving for stock rebuild or mild street is waste of time.
High rev engine will be different game.

One thing to remember is that bare block will not work, there is a head top of it.
Together it makes combustion chamber.