Flatty Compression ?

Why is it that all of the hot street use flatties being built only run 8 to 1 or lower compression?

In my very limited experience w/ BBC's and SBC's we built em' at 10 to 1 w/ good ignitions. These ran just dandy on 93 octane pump gas w/ out any knock.

Is there an issue of diminishing returns w/ upping the compression on flatties or something? Just curious.

 

Well after reading Tex Smith's flathead book. A few guys in there indicate that the Ford design of the valves along with the heads have clearance issues and the flow of intake and exhaust gasses don't allow for higher compressions at SB or BB limits. Some have redesigned the whole block and heads to bring one of a kind flatheads up to higher than normal horsepower limits. You can always increase the compression but the original designs don't give the exhaust and fuel/air anywhere to go.

 

I agree with Pete. I have read that the design of the block itself limits the amount of fuel that can go in and out. If I remember correctly, a guy built a saltflat car and modified the motor so that it would run backwards, huffing the fuel in from the exhaust side and dumping it out the top where the intake was. From what I have read from Tex Smith and also Flathead Jack, the flattys run better for street use at a lower compression ratio and run more efficiently. Just my $.018

 

The flow in and out has to go from the valves over to the cylinder essentially sideways. It is impossible to get very high compression without restricting this flow area too much to allow the engine to breathe its maximum, which isn't very much anyway. The drop off of breathing is pretty rapid, as the space for extra compression normally comes out of the top where flow is probably highest. Also, the real running compression pressure as distinguished from static mathmatical compression ratio depends on breathing--if not much air gets in there, there's nothing to squeeze, and the high ratio won't translate into any real pressure anyhow.

 

Interesting stuff and what I suspected.

Now what about supercharging? Is this a recipe for disaster as you are trying to move a larger amount of air through the same sized hole(s)? Wouldn't this pressurize everything and up your chances of blowing stuff apart?

 

If I remember right, and there's a good chance I don't, the bottom end becomes the weak point with a supercharger because there aren't enough main bearings for proper support. Can anyone verify or deny that?

-Bugman Jeff

 

[ QUOTE ]
.... the bottom end becomes the weak point with a supercharger because there aren't enough main bearings for proper support.
-Bugman Jeff

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Blowers on flatheads DO place additional strain on the main bearings. But with a slightly lower compression (mine was 7:1) and not overdriving the blower (mine was putting out about 5 pounds boost) you can run a blower on a flathead on the street quite safely.

I have a SCOT blower (going in another car) that helps "push" air/fuel into the cylinder to make up for the lower compression and the "winding path" that the "L" design of a flatty presents.

I don't know what the final compression worked out to be, but it did improve performance. Put over 14,000 miles on the other engine. When I removed it, and checked main bearings, there was no signs of unusual wear.

 

Nothing wakes a flatty up like a blower. Eventually you can make enough power to break the bottom end, but there are girdles and whatnot that can be used if you are going high end. A couple of folks make beefier main caps. Offenhauser has made center main supports for ages, but these aren't the strongest option. Some even say the Offy support can split the main cap. I've never seen it happen, though.

There are lots of trains of thought on how to get a flathead to breathe better. It's usually a trade off between breathing and compression ratio. One super quick flathead racer I know says the key is compression ratio. Another super quick flathead race I know says the key is breathing rather than compression ratio. All I can say is that old hot rod V8 still isn't all figured out.

 

To answer the initial question, a flatty gives up about 2 CR points as compared to a good generic OHV design for the flow/restrictions as mentioned by several.
So a 8:1 flatty is running the same octane (roughly) as a 10:1 Chebby. This is one of the reasons I stress CCing any unknown head before using.

Unlike a Chebby, you cant retard the timing more than a hair before you get into overheating problems. I can usually run my 286 (cr around 8.9) in the 53 on 93 Octane with 89 thrown in on some days.
About the same mix on my 10.75cr 396 Chebby with a bit of retard. It really wakes up with proper timing and some go juice when I feel like blowing around $80 on a tank full; doesnt last long tho.

With a blower, running stock cast iron heads is the best way to go for a street budget. Better sealing than generic aluminum types and they flow very well with minimal work.
About 6# boost is considered safe with a simple aftermarket main cap from F$$HJ. My 276 computes around 7.9 cr with EAB heads which is a bit high for a blower. Id go with 8CM heads when I make the switch, that gives 7.0 cr.

Cam choice is different for a blower also, I already planned on one so cammed accordingly.

 

I think old Henry was a prankster at heart and designed his flattie just to tantilize folks who wanted to modify his engine.As mentioned by several here it is definitely a question of compression vs breathing .In the 50's it seems like every issue of Hot Rod mag had a definative article on this question.
I believe the answer is in properly relieving and porting the engine.Again, this question has been thrashed around for 60 or so years.Some guys knew (know?) how to do it and some don't.Both 286merk and AV8 have contributed great knowledge on this here on the HAMB.
If I were build a new flattie today I would go with Navarro'sheads along with his matched intake manifold aand attempemt to duplicate his block relief method.
A small blower with mild boost is probly the easiest way to get big gains for a flathead.
I'm certainly no expert but that is my $.02 worth.
Ted

 

[ QUOTE ]
There are lots of trains of thought on how to get a flathead to breathe better. It's usually a trade off between breathing and compression ratio. One super quick flathead racer I know says the key is compression ratio. Another super quick flathead race I know says the key is breathing rather than compression ratio. All I can say is that old hot rod V8 still isn't all figured out.

[/ QUOTE ]

Ain't it the truth! 50 years old, and we STILL keep trying!!

But ya gotta love em!

 

If yer interested in putting a blower on, ya gotta get Joe Abbin's book, Blown Flathead. They're on eBay or you can check his website (do a search for Roadrunner Engineering) & I can vouch that Joe is a super nice guy & loves to talk flatheads all day long!

As for raising compression w/o hurting flow on a flathead - what about pop-up pistons with relieved heads? Seems like you could keep a reasonable area above the valves & make up a bit with the pop-up...or am I just out in left field again? I know the early K-model sportsters used this trick...

 

[quoteIf I were build a new flattie today I would go with Navarro'sheads along with his matched intake manifold aand attempemt to duplicate his block relief method.

[/ QUOTE ]

When I first started my whole flathead odyssey years ago, I bought some manifolds from Barney. I asked (begged, pleaded) him for some drawing of his porting and releif on paper. This is what he sent me, I think you guys will get a kick out of it:

 

Ernie, I cant say too many good things about Abbins book. It is the only one with documentation to back up the step by step relief process along with easy to follow diagrams.
Ive chewed his ear off, or was it the other way around, several times during the build process.
The best thing was that our dyno results were nearly identical for an unblown 276. There has been a lot of talk about porting and relieving but none of the others back it up with flow data.
BTW, Speedway has the book now also.

Barney has those pop up pistons to be used with his special head; or at least he did in the past.

 

Thanks 286merc. In my post re: Navarro heads he uses pop up pistons.I believe the guys around using his set up are using pistons made by Ross.I might mentin that by the time you get the complete set up you're talking pretty big $$.
Ted







44

 

Digger Dave was kind enough to respond to a PM I sent him and it got me wondering, what do you guys have tied up in your flatties (no blowers)?

As best I can tell it looks something like this:

-$500 to start
-$250 for tanking and testing
-$1000-$1500 machine work (boring, porting, relieving, etc.)
-$2500 parts (pistons, bearings, intake, cam, carbs, heads)
-$500 assembly and run in

That's about $5000! Yikes, that's spendy. Am I in the ball park?

 

[ QUOTE ]
Navarro heads he uses pop up pistons.I believe the guys around using his set up are using pistons made by Ross.I might mentin that by the time you get the complete set up you're talking pretty big $$.

[/ QUOTE ]

Not to take anything away from Navarro, but it would seem a fairly simple machining job to relieve the heads to accept the pop-up pistons (used to do this on two strokes all the time). So, pick your head, pick your piston, & machine your heads. Maybe you could even use off-the-shelf Ford (3.75" stroke) pistons on a merc crank (4" stroke) & they'd stick up 1/2 the stroke, so .125-ish. Certainly do-able on a cast iron head...might keep the cost down some???

Don't mind me - I'm just babbling...

 

As best I can tell it looks something like this:

-$500 to start
-$250 for tanking and testing
-$1000-$1500 machine work (boring, porting, relieving, etc.)
-$2500 parts (pistons, bearings, intake, cam, carbs, heads)
-$500 assembly and run in

That's about $5000! Yikes, that's spendy. Am I in the ball park?

That is roughly twice what it should cost.
I can build a nice 258 or 276 for around $1500 and sell for about twice that. $50-100 buys a good 8RT engine around here, they often have been upgraded with Merc crank and Johnsons which can be resurfaced. Lots of ways to cut costs without sacrificing quality and performance in a streeter.

1. Dont bother with a relief unless you are going high compression and a pretty radical cam and also do all the porting yourself. Both are labor intensive, no rocket science degree needed.
2. Dont waste your money on 97's; use 94's or a small pattern 4bbl...both can be found for $10-20. Or use a 60's 450-500 Holley with an adapter. Do your own rebuilds, they often work better than a high volume shop job
3. Iron heads if aluminum not in the budget. They perform well and can be made to look great.
4. Shop for an intake, often find cherry items at swaps for 1/2 new.
5. Dont cut corners on pistons/rings or cam. Reds & Schneider is all I will use. Try and get most of what you need from one source and one time if mail order. Shipping multiple times really adds up.
6. DO have the block magnafluxed and Sonic if going big bore. But do all the cleaning yourself and then dunk in the molasses elixir to clean the water jackets better than any shop tank.
7. Use a Mopar or GM HEI dizzy, cheap to convert and save big bucks over aftermarket.
8. Stay away from the big name shops, they will take all your money and do no better than a GOOD small operation.

Those are just the basics. Same line of attack for all the bolt ons.

 

286,
Right on.
Good blocks around here a little higher unless you're real lucky.Usually bout $150-200. Irecently scored a good 59A engine in a swap for about $15 scrap metal. complete less carb and dizzy.
With some scroungingaround and doing every thing but machine work yourself I think you can build a damned good engine around here for $2500-3000.(Salem, Or)
Ted