Fox 35s - hemi heads and stuffer backplates

I've been flying stock Fox 35s since the early 1950s, but before GMA passed away, I had him to do the basic GMA rework on some of my NIB Fox 35s, plus Lew Woolard did his magic basic services on a few of these NIB Fox 35s. None of these Fox 35s have yet been ran.

A few years ago I purchased from Fox NIP hemi heads and stuffer backplates that have never been installed on these engines. What is the advantage in installing these Fox hemi heads and stuffer backplates on these NIB reworked Fox 35 eninges please?

What does the hemi heads and stuffer backplates do for the Fox 35 engines, give them more power, allow you to use a larger prop or what please?

I've heard and read pro & con on the Fox hemi heads, should these not be used as they come from Fox please?

I basically enjoy flying OTS, if that helps to answer my question any.

Thanks to all who may assist me on this matter.

Ken Wilson
"A ukie lifer"
Evansville, IN

Ken,

Can't help with the Fox's but saw you are in Evansville. When I was real young (early 50's) I lived in Wadesville for a couple of years. Had a uncle that owned Lesher's garage on the highway in Wadesville. Just brought back some good memories, great country.

Thanks for the e-mail Bob, I well recall the garage and still travel thru Wadesville often. Do you recall ABC Hobbycraft in Evansville that opened in July 1946? ABC is still here, under the second owner now, Ken Ballard who started ABC is still living and enjoys fairly good health. GREAT to hear from you. Ken

The Fox stuffer back plate was designed SOLELY with the intent of reducing the wear on the crank pin, caused by the rod moving forward and back with each revolution. It also reduced some vibration caused by the same movement. It was NOT designed to improve "breathing" or to correct the burping of profile mounted Fox 35s.(although on some engines it seemed to help abate that phenominum a bit.
The Hemi-Head that Fox mfg sells was intended to improve the power in the "hard four cycle) setting without runaway when the engine breaks into a two cycle. It does that but not as well as the way it was supposed to do as Fox mfg produced it before all development was completed. The "as produced" head button can be modified to do a better job of developing more torque quite easily.
I must stress!!! Neither the stuffer backplate NOR the Fox hemi-Head was intended for racing conditions. Although I use the backplate for practice racing Foxberg and Foxy Hazle racing (to stop the wear on the crank pin).
Bigiron ( I don't know what the Larry Foster nor California Blue Magic heads were designed to do.)

The Fox stuffer back plate was designed SOLELY with the intent of reducing the wear on the crank pin, caused by the rod moving forward and back with each revolution.

Marvin,

This is very funny. As you no doubt know, by the time your stuffer back door got on the market, all the self-appointed Engine Dudes just "knew" what it was intended to do.

For years, I think it even happened a couple times last year, people would ask about my Fox 35s used in OTS and Classic, including if I was using a stuffer. When confirming use of said device, there was always a comment, or at least a leading question, as to more power, elimination of the burp, blah-blah-blah...

I would always shrug and tell them that I didn't know, had not tested for any of that, was only using stuffers to keep rod from walking off the crankpin. Such was not always well received, incidentally. For reasons I have yet to fully understand.

My sincere appreciation for taking time to answer my hemi head/stuffer question.

Ken

Thank you very much for taking time to answer my question.

Ken

I work with gas compressors in my job. A two cycle engine uses the piston to power the crankshaft AND to pump or compress the fuel/air mixture through the engine.
In compressor terminology, "clearance" is the amount of volume remaining in the crankcase after the piston has reached bottom dead center. By using a stuffer backplate, the clearance is reduced and more fuel/air is pumped to the cylinder.
So even though the stuffer backplate was designed only to hold the connecting rod on the crankpin, it serves to increase the throughput of the fuel/air mixture through the engine.
Happy Landings

Hey John;

MANY thanks for taking time for your input on this subject matter, I sincerly appreciate your thoughts.

Ken

If the Fox stuffer back plate was designed SOLELY with the intent of reducing the wear on the crank pin, caused by the rod moving forward and back with each revolution, then consider this.

I have a new Larry Foster mod (L&J) Fox .35 with a stuffer backplate installed. When I grasp the crankshaft with no prop installed and push in and out, I have approx. .020" of play. This is more end play than some of my stock Fox .35's!

When the L&J Fox is running there is theoretically a minimum of .020" of space between the rod end and the backplate. The crank assembly should be pulled to the front (away from the backplate) by the prop thrust. This would allow the rod to walk back and forth .020" on the crank pin.

I have measured the difference in length from the gasket surface to the end facing the rod/crank on the stock and Stuffer backplates. Stock is .365" and the stuffer is .380". Not much there. I can make this up by using something thinner than the .032" thick cork stock Fox gasket or facing the gasket surface of the backplate. Or eliminate the gasket and use RTV gasket material.

Also if this distance is critical, then those folks who advocate making backplate gaskets from anything handy (playing cards, old sneaker bottoms,etc.)need to rethink this practice.

I have only measured one stuffer from Larry so I don't know if this is a typical size or not. Has anyone else measured a stuffer and if so, how does it compare?

I think there is some marginal increase in crankcase pressure change rates due to the reduced volume in the crankcase but the reduction in volume is so small that I don't think there is any real world benefit.

Thank you very much Dick, this really gives one somehting to think about.

Cordially, Ken

I work with gas compressors in my job. A two cycle engine uses the piston to power the crankshaft AND to pump or compress the fuel/air mixture through the engine.
In compressor terminology, "clearance" is the amount of volume remaining in the crankcase after the piston has reached bottom dead center. By using a stuffer backplate, the clearance is reduced and more fuel/air is pumped to the cylinder.
So even though the stuffer backplate was designed only to hold the connecting rod on the crankpin, it serves to increase the throughput of the fuel/air mixture through the engine.
Happy Landings
John O. Wright

John,

While I am thoroughly familiar with the theory you put forth, lots and lots of people have tried to make this case for the Fox 35 in various ways, including use of the stuffer rear cover.

Sorry, it just doesn't work as you might think. I have never heard a credible source claim increased output, which is of course what we would get if the engines with stuffers were pumping more fuel/air mixture.

Don't mess with BigIron when it comes to the Fox 35. Some do--even Fox Manufacturing has dropped the ball when it comes to input from Marvin--but they generally regret it down the road.

As I recall (vaguely), Duke made an adjustable backplate for a stunt 35 such that he could increase or decrease the crankcase volume. He reported that he got maximum RPM's with crankcase volume considerably larger than stock. Does anyone else remember this or is it self-delusional?

>As I recall (vaguely), Duke made an adjustable backplate for
>a stunt 35 such that he could increase or decrease the
>crankcase volume. He reported that he got maximum RPM's
>with crankcase volume considerably larger than stock. Does
>anyone else remember this or is it self-delusional?
>
>Jim

Ron B.
F4Fguy

He was probably making the classic error of changing more than one variable at a time.he was,in the same tests trying smaller and smaller props.By the time he got to Vc=Vd he was running a 5 1/2 -3.

Seriously,there's an optimum crankcase CR in every engine which is almost never the highest possible.This occurs at the point where flow restriction in the case exceeds the gains in theoretical volumetric efficiency.In actual fact stuffers(designed solely to pack the case),can reduce total throughput of mixture.Leaving restrictions out of the equation,it's obvious that the more you can push out of the case on the down stroke,the more you can pull in on the up stroke.Like everything else,you can't have your cake and eat it too.

The claim that blocking part of the bypass "increases velocity"and thereby improves volumetric is of some mild interest to me .It's much more likely that it simply gives a higher crankcase CR without "plugging up" the bottom end.It doesn't hurt power because the porting,and the cross flow system are the limiting factors.Blocking the bypass brings the flow to a volume more closely matched to the useable requirement.As to velocity effects,the lower the better,up to the point where fuel will no longer remain entrained.This is,usually ca.250FPS and is in the same ballpark in any engine because this is where flow losses are lowest.

wright: What I recall was when Fox did its experiment with wildly different crankcase volumes (Duke made a clever, "trombone" sliding adjustable backplate that stuck way out in max volume form), it hardly made any difference at all. Currell

I would expect that the faster you run a two stroke the more detrimental a large crankcase volume would be. This would still back up Dukes findings. The large volume would be soft and the added resistance of the higher transport speed would make pumping more difficult. So why the increased performance?
What happens in these engines is quite difficult to calculate. Air is compressible. What happens to transfer when both intake and exhaust ports are open is anyone’s guess. Also there probably is some fuel air transfer after the piston is BDC. A little more volume could help that situation.
I will go along with Duke, Bigiron, and Dirty Dan. I have seen what Bigiron can do and I know it works.

LAST EDITED ON Apr-15-04 AT 12:55 PM (CDT)
 
>wright: What I recall was when Fox did its experiment with
>wildly different crankcase volumes (Duke made a clever,
>"trombone" sliding adjustable backplate that stuck way out
>in max volume form), it hardly made any difference at all.
>Currell

Ron B.
F4Fguy

"Hardly any" ain't the same as "no".Every engine has an optimum CCV,even Foxes.The fact that you can plug 50+%of the bypass and not negatively effect performance indicates that factors other than the backplate depth dictate in this case.

"Duke"was,by appearances,a TLAR engineer who,on occasion,got it right.If the CCV was too much initially,increasing it wouldn't make much difference.To point out the obvious,the crancase volume comprises ALL the volume enclosed after the piston closes the intake port,not just the area behind the crank.

>The claim that blocking part of the bypass "increases
>velocity"and thereby improves volumetric is of some mild
>interest to me .It's much more likely that it simply gives a
>higher crankcase CR without "plugging up" the bottom end.It
>doesn't hurt power because the porting,and the cross flow
>system are the limiting factors.Blocking the bypass brings
>the flow to a volume more closely matched to the useable
>requirement.As to velocity effects,the lower the better,up
>to the point where fuel will no longer remain entrained.This
>is,usually ca.250FPS and is in the same ballpark in any
>engine because this is where flow losses are lowest.

Of course, the reason for the bypass blocking *is not* claimed to improve the "volumetric" - in fact, it appears to have next no effect whatsoever on the volumetric efficiency. In fact, I would have at best expected it to do no harm (as it does) but I was surprised to find this. I expected that the higher bypass velocity would reduce the power for obvious reasons - i.e. higher velocity = more frictional losses = less pumping efficiency. But it doesn't.

You desription doesn't explain the observed effect - i.e. the outside turn burp, in fact, it doesn't seem to have anything to do with the crankcase volume. But the bypass narrowing clearly addresses this problem, and with a plausible mechanism.

You don't care about flow loss in the bypass if that's not the limiting factor in the first place. Even knocking it down to 1/3 the cross-section has negligible effect on the overall pumping efficiency - with objective dyno information to prove it.

You certainly can't argue that the velocity is not higher as a result of the bypass being 1/3 the cross-section. Same mass flow, roughly the same volumetric flow (both necessary consequences of same power output at the same setting). Divide the volumetric flow rate by the bypass cross-section, and you get the velocity - and its inversely proportional to the cross-section.

Why does it help? Reynolds number is higher, but that's probably not it, since it's not obvious that turbulence all by itself makes any difference to the engine's response to acceleration. But the deflection of the stream as a result of side-to-side accleration is clearly reduced with the velocity being 2x or 3x as fast, and sure enough, cutting the cross-section in half or less fixes an effect of acceleration quite definitively.

Any explanation involving the alteration of crank volume would have to address all of these issues, and explain the observed results. I don't see the mechanism.

LAST EDITED ON Apr-15-04 AT 01:04 PM (CDT)
 
I replied to the above thread last evening, but was having computer problems and I guess the responce is out there somewhere in cyberspace.. I will try again!!

Thanks for the kudos Dan.
I have run many many tests on the effects of the stuffer backplates. They DO what I designed them to do. Reduce the wear on the crank pin. An additional benefit was some reduction in vibration caused by the rod floating back and forth on the pin. On some engines it seemed to reduce or eliminate the dreaded "Fox BURP". Those two added benefits were NOT anticipated nor were they frowned upon when they did help. There has been some (people) who think that the purpose was to get more RPM! Not so! There are some creditable sources who claim an increase in RPM (power)of which one is Currell Pattie. There are others who claim that there is NO change (The late George Aldrich was one) There are some who claim a loss. I ran many many tests with different size props, different nitro and oil contents. There were increases in some combinations and losses in others. That tells me that at different RPMs there is a need for either larger or smaller crankcase volume to get peak effency at THAT RPM. Since that Duke Fox DID run the 35 with a backplate made such that the case volume could be varied, (that backplate setup is in either the AMA musieum or at the factory show case), It can be assumed that he set the volume where he anticipated that most would run the engine, or that he assumed that when he got the best performance with the prop/fuel he was running the test on, that any other variables would not make any change. I suscribe to the first.

Since the original reply didn't take, I went down to the shop and measured seven stock backplates and six stuffers. The difference in the depth that the plates go into the case is: 0.040 inch. The gap between the front of the STOCK backplate and the crank pin was 0.045 to 0.060 (closest to the greatest on six engines checked. I have in other previous tests seen as much as 0.070 gap. That is with the prop, thrust washer and front washer on and the nut tight and the shaft shoved as far rearward as it would go. also with the Backplate gasket REMOVED.I measure that way as some like to use silly-cone as gasket material and others use other things. Even Factory gaskets come in a widely array of thicknesses.

OH! The measurement from the gasket sholder to the face of the Backplates I measured came out as follows STOCK 0.365, STUFFER 0.405 That was the average of six each that I measures this morning and is the part that goes inside the case. That was with NO gasket installed.

By-the-way, Larry Foster mills the back side of the crankcase to true it up on engines that he reworks so the backplates may set a little further in than a stock case.

Bigiron

The response to my hemi head / stuffer backplate question has been more than I ever imagine, my deepest ansd sincere thanks to all who have taken time to respond.

Most Cordially and Sincerely,

Ken Wilson
Evansville, IN

Well Ken, You going to try one?? (G)
Bigiron