PROS and CONS of a high compression head

[boomerusmc]

This a good thread...I've been contemplating the HCH as well. Thanks for all of the info. I think I'll make the final decision after riding with a few of my club members who've made the switch.

[Mitch//pa]

Putting a HC head on a worn out or weak bottom end will accelerate the destruction of it. It is the best bang for the buck and careful operation of the advance is essential

[Purdy Swoft]

I agree with John LaVoy, I run 5.9 on most of mine, it is like the difference in day and night. Creeping and lugging the engine in higher gears will do more damage than anything. Makeing corner turns at low speed and making it chick ah lunk around the corner in high gear will beat the babbit out. Changing to a higher gear ratio adds to the risk of lugging. You've got to know how to keep the momentum up and avoid lugging. If it begins to lug, ping and buck, if you are going too fast to down shift, retard the spark just enough to cushion out the ping and buck.I set my timing the same on all of mine, stock or higher compression. When the timing pin drops in place I lock the points cam down with the trailing edge of the rotor tip pointing at the number one contact in the dist. cap with NO clockwise back lash . In other words, if you want the timing right, all of the back lash must be in the counter clockwise direction. I set my points at .022 and feel that this is important. I run 3.78 gears and have no problems with lugging. I crank the engine with the spark retarded and pull the lever down quickly when the engine cranks. I drive at speed with the spark fully advanced . I've run my model A's this way for over 50 years with no problems. If its not lugging I've never had a problem running at full advance. I don't retard the spark going down hill, I use to do that as a kid to make it pop while decellerating. After cranking the engine I don't mess with the spark much unless I'm driving at very low speed as on trails. Creeping isn't really being easy on the engine. Straining the engine at low speed in high gear will ruin any engine.

[Terry,NJ]

I read this thread this AM. Then I went for my usual ride (about 5-6 miles) To the flea market, in my 1930 a coupe. I thought about this thread while driving, especially uphill. My engine was bored .125 and the head was cut down about .090. I haven't calculated the compression ratio yet. It's a older restoration/modification so I doubt many HC heads were available. I wouldn't want to climb some of the hills of eastern Pa with out that extra boost. My head is probably not equal to a Snyder's (Lion, Thomas, etc) But it's a heck of an improvement. I also have a somewhat lightened Flywheel, with about 10# (53#) off of it. I don't want to start a "Discussion" of the advantages of lightening FWs, but I gained, not lost, something when it comes to climbing hills. Perhaps it's because there's only 10 # off if it. It still retains most of it's rotating mass.
Now I can almost keep up with the rest of the cars on hills with a 55 speed limit.
Terry

[PC/SR]

Retarding spark descending a hill is not necessary for a high compression head, its just a habit I got into. The full expansion effect of the combustion explosion happens with the piston further descended into the cylinder and imparts less force on the piston when it fires, than it does at optimum advance. I would feel a slight bit more engine braking effect since less power is sent to the crankshaft. Never really noticed the difference on the stock head, but could feel a bit of difference with the HC head.

[gwhite]

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Originally Posted by PC/SR

gwhite: Weiand 7:1 head, carbs, cam, more speed stuff, no pressure oil except center main, babbitt bearings no problems. Personal redline is 4000 RPM. The limitation of the stock Model A babbitt engine is not compression but RPM.

I agree completely, just wanted to see where tbirdtbird came up with 7:1 as being the upper limit. IIRC, diesel engines used babbitt for many years with much higher compression ratios...

[marc silva]

Originally Posted by John LaVoy

We have had high compression heads (5.9, 5.5, 6.1) on various engines with no problem. The timing suggestions are important. When you say you drive it like a wuss, I'm hoping that is saying you lug the engine. Lugging will damage the bearings more than anything else you can do. You need to keep the crank turning and downshift when necessary to do that.

Thanks John. When I say wuss I mean I don't push it hard. I don't want to break it.It is old and tired with varying compression across all 4. If I break it then I can't drive it..and I am having too much fun Driving it with my club the Sonoma A's. That is why I am building an engine up.

[James G.]

With a stock head and a 60 thousands over bore, I had to pull crank main shims at about 28,000 miles. Then at about 35,000 I had to do the rods. So, had it been a high compression head giving another,say 10 HP, how much sooner would this work have to be done? I'm now up to just over 46,000 and running strong.

[28ACoupe]

As I recall, there is a very "Stock" looking HC head out there. Does anyone happen to know the make? Also, what would be wrong with simply shaving the stock head an appropriate amount to get to the CR you are looking for?

[CarlG]

The Snyder's 5.5 head, is as close to factory look as you can get. That is what I have and am very pleased with it.

[Jim Brierley]

your gas mileage will go up and the engine will likely run cooler as more engine heat is now going to power the car instead out the radiator and exhaust.

[Charlie Stephens]

Originally Posted by 28ACoupe

As I recall, there is a very "Stock" looking HC head out there. Does anyone happen to know the make? Also, what would be wrong with simply shaving the stock head an appropriate amount to get to the CR you are looking for?

There is not enough metal to shave the head enough to make a difference. The overall head is thick but when you look closely at some of the head to water jacket thicknesses you will see what I mean. Plus if your pistons extend slightly about the surface of the block you will have an interference problem between the head and piston.

Charlie Stephens

[28ACoupe]

Got it! Thanks. I rather suspected this was the case.

Originally Posted by Charlie Stephens

There is not enough metal to shave the head enough to make a difference. The overall head is thick but when you look closely at some of the head to water jacket thicknesses you will see what I mean. Plus if your pistons extend slightly about the surface of the block you will have an interference problem between the head and piston.

Charlie Stephens

[Ron/IA]

You cannot shave a stock head enough to raise the compression any appreciable amount. The HC head builders do so by reconfiguring the combustion chamber.

Note - I apologize, Charlie beat me to it.

[Mikeinnj]

Originally Posted by PC/SR

gwhite: Weiand 7:1 head, carbs, cam, more speed stuff, no pressure oil except center main, babbitt bearings no problems. Personal redline is 4000 RPM. The limitation of the stock Model A babbitt engine is not compression but RPM.
Put on the HC head and enjoy. You will learn to use the timing lever as never before, and become a better driver because of it. Set initial timing in the normal manner. Play with timing to find your sweet spot. Advance and retard as you drive and learn how the engine responds. The engine is more responsive to timing adjustments. Half advance at full throttle for starters, more at cruise, full retard descending hills, for example. As you get used to using the timing you will start to keep your fingers on the lever and adjust it as you drive. It becomes kind of fun and part of the Model A experience.

I think the fellow meant to say: Retard the spark when you ascend |əˈsend|: verb1 [ trans. ] go up or climb a hill

[P.S.]

What good timing (pun). After a discussion at the club meeting a couple nights ago, I spoke to Joe at Turlock Machine this morning about this very same topic.

Joe says that even with a Snyder 5.5 head, if your Model A motor is pinging with full advance, then you have the timing set wrong. Joe says that even after setting the timing the Henry way, it's prudent to check using a timing light with a timing mark on the motor to be sure that full advance does not exceed 26 degrees. Beyond 26 degrees, pre-detonation can occur and it will damage the lower end of the motor.

Joe went on to explain that even at (let's say) 25 degrees of advance, you will not harm the motor in any way as it was designed to run that way and even aircraft motors run full advance all the time after starting. That guy knows his stuff, and I believe his information to be gospel.

Joe is installing a 5.5 head on the long block he is building for me. He says it will run much better with the 5.5 head over a stock head or a 6.0 head.

[Terry,NJ]

As I said before, My engine was bored to 4.000 and the head was cut down .090 . The engine feels strong! If someone better at the math than I am, would calculate the new compression ratio, I would be curious what it is. Using the old percentage method and rule of thumb, it's about 41.6 HP with the boring alone. How much the .090 off the head adds to that. I can't figure out. However I seem to recall a table that someone once put up to show tho increase in CR by .010 increments.
Terry

[colin1928]

Originally Posted by gwhite

I agree completely, just wanted to see where tbirdtbird came up with 7:1 as being the upper limit. IIRC, diesel engines used babbitt for many years with much higher compression ratios...

Diesel Babbitt is a different mix to A Model Babbitt
it can and has been use in cars
may be 1 of the Babbitt guys can explain the differnce

[V4F]

high compression heads are all different . I used a snyders 6-1 for years with 0 problems . I now have a 7-1 lion 3 head . much , much different from stock type heads (snyders included) . it requires much more fuel to keep up with the increase in air flow . less advance for safety (detonation) . I love the newly acquired torque !!!!!!!!!! and yes it gets out its own way quite nicely .........
just be sure you understand the heads needs . .....................

[Brobrian]

I have a 28 Coupe with a Snyder 5.? (Can't really recall) which I've been running for about 5 years and 3,000 miles now. I'd driven the car 25,000 miles with the stock head but wanted more power when I changed the ratio in the rear end. I also have a 31 PU, with stock motor and stock rear end. I enjoy both. The Coupe requires more fiddling with the spark advance, but has a bit more power. It sounds differently and runs a bit rougher at low rpm. On the open road it's a dream. I'd head out for Colorado in it tomorrow with confidence. In other words, I don't feel the head placed significantly more strain on the motor. It simply change some of its characteristics.

I also enjoy the pickup with it's stock head. I'm doing an engine swap in that one this summer, but won't be putting an HC head on the new motor. It's easier to drive in town with all the stops and starts. And I just jump in, start up, yank the spark lever down and go. The pickup is geared low anyway, and I just don't need more power.