Pop-up Pistons In A Flathead. Wasted Energy?

OK, I looked up my source for the 4:1 comment - Jerry Hatfield's "American Racing Motorcycles.

I was wrong, but not by a lot. To wit:

(Hatfield) "Additional horsepower was also a goal, and to achieve this, the 1968 KRTT's were equipped with twin Tillotson diaphram carburettors. Intake valve timing was unchanged from the 1966 engine, with the intake opening at 66º BTDC and closing at 66º ABDC. The exhaust valve timing was changed somewhat, and provided five more degrees of duration to the opening. The exhaust valve opened at 65º BBDC, and closed at 40º ATDC. Because of the new twin carburettor setup, it was necessary to reshape the combustion chambers, after which the design compression ratio reached an all time low of 5:1."

This change added a "definite" improvement over '67's 52 hp, stated Dick O'Brien. The Daytona qualifier showed 8 mph improvement.

As an aside, my own KR was absurdly easy to kick (in comparison to, say, an XLCR, or stock KHRM), had little bottom end power, but revved like there was no tomorrow, and would easily keep up with a late Evo 80 incher.

True, a motorcycle engine is not a car engine, but I maintain that compression is not the be-all and end-all for flatheads. Flow is your friend.

Cosmo

 

I think we need to differentiate between the radical pop ups used with matching Baron and Kong heads and normal domed pop ups seen on lots of aftermarket flathead pistons. The giant pop ups do allow a much improved transfer port. I am not sure that any of the current really fast Bonneville flatmotors are still using Kong heads. You might try to get ahold of Kenny Kloth or Ron Main or Jimmy Stevens to get real advice. If they are using it, it works.

 

Funny you say that Rich.

I spoke with both Ron Main, and Jimmy Stevens yesterday. I didn't bring this up specifically, as I was picking their brains about cam selections, valve train,stroke, rpms, and any other words of wisdom they were willing to impart.

Both guys are very helpful, and obviously very intelligent and talented!

 

Jimmy has Kong heads on his. I don't know what the combustion chamber looks like. But if your looking for a model to copy for your heads those guys are the fast guys.

 

What's tractor gas?

 

Rich, when I spoke with Ron, this is on of the 'wisdoms' he shared:

"Quit thinking like your dad and grandad did when they built a flathead. You don't want it to run like their flatheads did- they didn't know what they were doing! Forget it's a flathead- you need to think of it as an upside down Hemi, and build accordingly."

 

Seems to me force is equal to pressure * area. Since a pop up piston has more surface area the force generated during combustion is greater with a pop up piston.

Jim

 

This theory really applies to a change in the shape of the pop-up. That is the reason that the Navarro High-Dome heads have a large (smaller radius) dome - to increase the surface area of the dome itself. The High-Dome pistons have a really high dome that is much rounder (More like an early 392 pop-up piston).

In the case of a flat top, doesn't make any difference if it is popped up or not. On the Harley KR, they switched over to a dome shape in the last years -- but many of the guys that built a lot of them (like my buddy) said that they didn't run any better than the flat tops. I'm sure there are other things going on in a flathead - like the quench area and how the quench area works . . .

On the FlatCad, we decided to go with flat tops and mimic the KR head that I've shown pictures of.

 

Btw.: Have you seen KLOTH flathead combustion chamber shape? Is it future design or not,and this whith popup piston with Makuch racing shaped pistons?? He told when Bill ask;Engine product whitout nitro about 400hp! So open transfer area may make big differences?

 

The Kloth chamber is very interesting and unique - I may have to make a custom head design someday to try it in a BLOWN application. Kenny's engine was naturally aspirated - which tends to be a completely different beast than a supercharged/boosted engine . . . but it would be fun to try it!

 

So, which engine ( of these 2 choices of FH ) would you use for a pickup 252 in 3.75 stroke or 274 in. 4.00 in. Stroke. The 252 is a 59 block that is factory relieved and the 274 is an 8ba with eab heads.

 

Kind of hard to answer in that all you've given me is a cubic inch difference. With just that, I'd always take the larger engine - everything else being equal. What other parts are in them? Can, heads, intake, etc? Did you build them - and know the details, or trying to determine what to buy from whom??? You can PM me if you have more details.

 

Any pictures of Kenny Kloth's Combustion Chamber out there to share ??

 

Think like this