High Compression Head ?

J

jaguar6165

Senior Member
I am looking at going with a high compression head and see a lot of people using 5.5 to 1 heads. I have a stock 4 banger and was wondering if it is ok to go with something more, 6 to 1 or 7 to 1. i am also looking at a downdraft setup, a header and exhaust to give it a little more go. Will this be ok with a stock 28 motor?
 
How long since the motor was last rebuilt? Does it smoke, knock or anything? The 5.5 should be ok if it's in good condition. 6 or 7 to 1 may be a little too stong for an older engine.
 
I dont really know if it has ever really been fully rebuilt. if it has it was over 30 years ago. it doesnt smoke but i think there is a slight knock in it.
 
the 5.5 head will wake your motor up & not hurt it like a 7-1 will ...... steve
 
jaguar6165 said:
I am looking at going with a high compression head and see a lot of people using 5.5 to 1 heads. I have a stock 4 banger and was wondering if it is ok to go with something more, 6 to 1 or 7 to 1. i am also looking at a downdraft setup, a header and exhaust to give it a little more go. Will this be ok with a stock 28 motor?
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An important addition to the engine is a counterweighted crankshaft. It will improve the life of the babbitt quite a bit, especially for those that like to drive over 40 MPH. Remember the forces quadruple as the speed doubles, so the force of the crankshaft trying to bend and wipe out the center babbitt is 4 times at great at 60 MPH than it is at 30 MPH.
 
I agree with the 5.5:1 suggestion if you see the need for a few extra horses. That's what I have in my '31oadster. But I firmly believe that the higher the compression (thus, higher the horsepower and bearing loads), the more accelerated wear and tear you're putting on an engine designed with babbitted, un-pressurized bearings and putting out 40 HP.

The babbitted bearings in all four of my lower rods failed recently with only 6500 miles on them. An excellent Model A mechanic (Ora Landis) could not absolutely tie it to the higher-comp. head and allowed that maybe the original pour or material was sub-standard. But he absolutely stressed that hi-comp. heads cause increased pounding (impulse) loads on these bearings and definitely won't INCREASE your bearing life!

He emphasized the need for more careful driving habbits with a Hi-Comp head by avoiding low RPM "lugging", unnecessarily fast acceleration, and constant driving with full spark advance.

When I took the car in for the rod bearing problem, I had new rods with insert bearings installed. In doing my homework, I get the sense that a vast majority of folks who run them are happy with them.

We'll see...

Earle
 
what about 6 to1 heads. I see Brumfield has a 6 to 1 head or the say mill the 5.5 to get it to 5.9 to 1. would i be able to ge away with that? I really want to overhaul the entire motor, but unfortunately i have a some what limited budget. a few hundred dollars is manageable but a few thousand is gonna have to wait.
 
Kevin, Yes. But the 5.5:1 head had only been on for about the last 2500 of those 6500 miles. Hence, the liklyhood that the babbit had a problem prior to the hi-comp head being installed. BUT the hi-comp head certainly hurried up the failure process if the babbit was weakened/sub-standard/marginal in the first place.
 
jaguar6165 said:
what about 6 to1 heads. I see Brumfield has a 6 to 1 head or the say mill the 5.5 to get it to 5.9 to 1. would i be able to ge away with that? I really want to overhaul the entire motor, but unfortunately i have a some what limited budget. a few hundred dollars is manageable but a few thousand is gonna have to wait.
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The decision is yours. Given the engine conditions you describe I would say...5.5/1 = moderate risk....6/1 = moderate to high risk...7/1 = high to "don't do it" risk. Also, please consider the "surprises" that you could encounter when changing a head on your current motor. It's not all that uncommon for someone to break studs or find the costly results of some monkey fisted - half butt tree surgeon ;).

Wild guess on my part but I would assume your ultimate goal is to have a fresh engine with the highest ratio head you can get? If so I would recommend you table the head decision save the money for it and tinker on other less costly restoration issues. While doing so you may change your mind and do something different all together.

As Bob from Michigan use to say, "Make haste slowly".
 
Yeah that is the goal unfortunately diving into the motor proved a little more costly then i was hoping for so i was thinking about going with bolt on parts for some more get up until i got around to wanting to spend the money and diving into the internals.
 
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