Inliners International Forums General Discussion Hi Performance An interesting McGurk split grind 3/4 cam

In this article there is a interesting cam grind.

http://www.selectric.org/55chevy/soup.html

If their is anybody out there who could shed some light on the cam shaft that was mentioned in the article (in test #5) the McGurk number 18, 3/4 grind, I would appreciate it. I had talked to a cam grinder about it's spec's and about it being a split grind with more duration on the exhaust than the intake and he said his info indicated that those spec's would not work. ?? I have complete trust in the work from Frank McGurk in the article, but would like to hear from others on their opinion. I sure would like to try that grind.

As far as I know, Tom Langdon`s "Bulldog" cam has a similar asymetrical grind, he might have the answer you want.

If you grind a cam like the #18 3/4 please let us know how it runs.

Frank

Hi Ed . . .

There's has to be something to this type of grind. Isky offered a simliar regrind in the D.P. #2 (250 intake/280 exhaust) basically a 3/4 spec on the intake lobe and approaching 'rumpy' on the exhaust:

regards,
Stock49

Greetings . . .

Another thing you may want to check with your grinder on is what was meant by "that won't work." Did he mean that the engine won't run right or that he couldn't grind a core that way?

I had the guys at Delta Cams regrind a cam for me. The Isky DP #2 specs were what I asked them to shoot for. But there are limits to what can be done given the core they are working from. In my case it was an NOS TRW cam for the early 235 Powerglide cars.

They were unable to duplicate the exact 'advertised' DP #2 durations, but they were able to provide more lift:
----------Timing-----Lift----Adv Duration
Intake----20.1-47.6--0.40878---247.7
Exhaust--64.4-20.1--0.39862---264.5
And this added lift leads me to believe that the .050 specs of this regrind are perhaps better:
----------Timing-----Lift---.050 Duration
Intake----1.7-23.1---0.40878---204.8
Exhaust--51.2-1.7---0.39862---232.9
then what Isky offered on the DP #2.

But since I haven't had a chance too run it yet - it's all 'on the drawing board' at this point

best regards,
stock49

Hi Stock49
Thanks for your reply. Sounds encouraging for me. How far are you to getting your project running?
I am doing a ground-up build of a Ford 1930 truck streetrod with a 261 and I am a good year or so from getting it on the road. I sure could use your input on how your cam reacts. What other mod's do you have on the drawing board for your engine?
To answer your question the cam grinder said the motor would not run worth a crap with the spec's i gave him on the McGurk #18 cam.

Ed

Hi Ed . . .

I am atleast a year from being on the road as well. It's taken alot of patience to research, locate and acquire the parts I wanted.

I am doing an early 50's hop up on an outwardly stock '49 Chevy Fastback. I am following the guidelines set forth by Roger Huntington and Bill Fisher to modify the 216 that's in the car. An antique hot rod built from antique speed manuals. I am doing most of what's in those books - except converting to full pressure oiling.

I think that this type of grind has lots of promise for a street engine. I have studied the specs on alot of I6 cams. The problem with 'symetrical' long duration cams - like what California Bill describes as a Super Grind:
---------Timing----Adv Duration
Intake----35-73--------288
Exhaust---31-74--------285
is that you get killed at low RPM by early/late valve events and extreme overlap:

1) Opening the exhaust valve so early during the power stroke is a killer on low RPM lugging abilility. One needs to provide alot of additonal compression ratio, displacement or both to over come this low RPM loss of power.

2) Opening the intake valve so early causes exhaust to backup into the intake manifold. This kills vacuum, which impacts low end throttle response, and dilutes the intake charge effecting voumetric efficiency.

3) Keeping the intake valve open so long after BDC causes the intake charge to backup. This reversion further kills intake plane vacuum - so throttle response suffers further still.

These 'late closing dual pattern' grinds we're talking about try to address these three low RPM issues. They do this by only lengthening the exhaust duration to one extreme. This is accomplished by shifting/retarding the lobe center. Which means that the exhaust valve stays closed during more of the power stroke (not opening so early - addressing issue 1). But stays open longer providing exhaust scavenging during the overlap (which helps volumetric efficiency). While the comparitively shorter intake duration and valve events provide a good idle (Issues 2 & 3).

It's a 3/4 race on the intake - and a rumpy full race on the exhaust . . . that's what sold me.

You may want to give the guys at Delta Cams a call. They did right by me Delta Cams Posting on Inliners BB

regards,
stock49

Sounds like the old "Blower" cams to me...In fact I think they still do it that way somewhat.You can "blow it in there anyway,but keep it open late to get it all out"...I'm with IGOR..work on your exhaust port/valve and some good headers and keep a stock cam and with a 216 I would bet you would have more overall power....SCRAP

Dear Ed;

It's the old "titer-totter" theory. Whatever you add to the 'top end' you subtract from the lower.

I don't know of anyone that builds a 'Hot Rod' engine that is concerned about the idle when runing a standard transmission. Once your above 1500 RPM It's all academic anyway.

Personally; I want all I can get in the higher RPM ranges and that means a lousy idle.

Remember, engines are not made to idle, except at stop signs.

Good luck.

Thanks,guys.
Stock49, that was a very informative reply, makes it pretty easy to understand.
Thanks for your reply!
What are you going to use for intake and carb/carbs?
I am going the pretty much follow the McGurk 261 buildup from the 1955 article. (antique style also)
I am going to play around with a 3x1 intake with Strombergs, but I also have a 2x1 intake with strombergs as a backup if I run into trouble for a street setup. Being a open front streetrod the motor will be exposed for the world to see and I can't think of anything better to look at than a 3x1 setup.
Ed

Hi Ed . . .

I am going with a fairly rare piece; the 2nd design Edmunds 2 deuce for the Chevy Standard. It is designed to fit the Stromberg 97/Holley 94:
These carbs are alot easier to come by then the one barrel BXOVs.

Its a very well made manifold with a split plenum to provide good balance to the center port. Most log manifolds (like the Offys) are rich to the center port and lean on the ends - this makes tuning a bit of a balancing act.

This 2nd design over came the problems with the 1st design - which was a bit primitive looking and had brass tubes feeding a water heated 'hot spot' that would corrode and leak. The newer design is much more attactive and has a cast in water jacket that extends under both carbs with NPT threaded inlets.

I have heard that these can be modified to fit on the later 235 engines (by reaming the three exit ports to a larger diameter). They are rare but around. In fact there is one on ebay right now Edmunds Two Deuce

As for fitting a trip - the old speed manuals show that it is only effective for the very top end of the RPM range - and that the biggest bang for the buck comes from a 'two jugger'. But indeed a triple looks awesome! If you go this path you'll want to go for the smallest venturi you can get your hands on - because at most of the operating range you'll be over carbed. Zenith's would be a good choice in this regard - but I don't know where to find them.

regards,
stock49

True on the 3x1, I have heard the same thing that not much gain going from the 2x1 to the 3x1.
That is why I also have the 2x1 backup.

But I really like the 3x1 looks !!

I do have 3 Stromberg (BXOV's) with the 1 3/32" ventri and that will help somewhat. As well as a few with 1 5/32" venturi for the 2x1.
One advantage of the 3x1 would be the fuel distribution would be better than the 2x1. And I am thinking maybe I will try the 216 Offy 3x1 intake and leave the runners alone and not inlarge them (that may help with the vacuum??).
Offy still offers the 2x and 3x for both the 216 and later 235's.
I have a about any size jets to play with so I kind of think I could get a 3x1 pretty well up and running. (I did say THINK didn't I)??
I gave up on finding Zeinth's and stock piled a bunch of the old Strombergs BXOV's just for this project.

Ed

If you have your heart set on a triple and you have lots of Stromberg BX parts, then you may want to look into the older BXV carb (which was standard equipment on the '39 and '40 Stude's). The 'O' in BXOV is stands for oversize throttle barrel - by an 1/8". The smaller throat might help in keeping velocities up on a triple. I think some of the parts are interchangeable between the BXV and BXOV.

Are you sure that manifold will fit a 49s firewall?

There's not to much room there.

Not without a little tweaking. It's 5 1/2" deep compared to the 3" of the stock unit. I am going to dimple to get the additonal 1/2" clearance instead of cutting. It's such a nice manifold it seems worth the effort.

It is/will be.

A Clifford 4bbl should fit okay then.

My 49 Fleetline was delivered the other day. Was it you that said they weigh 3100# I can't remember.

Yeah. In Deluxe trim the minimum weight is 3065 and the convertible (with frame reinforcements and hydraulics) weighs in at 3485. These specs are from the Carnut Website .

When you get a chance post some pics of your car.

There are pics of mine at my Home Page

That's what I bought a while back, only the standard model.

Looks nice (yours), talk more later.