high compression head modification ?

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Wayfarer
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high compression head modification ?

Post by Wayfarer » Thu Feb 27, 2020 8:01 pm

Since I got my car I have been reading all the high compression head discussions, but there's a question I haven't seen asked...

From what I gather the available heads are at most 6 to 1 or just shy. The Prus heads are advertised at 8 to 1, but are in fact, also shy of 6 to 1

I live at 6000 feet altitude. Our average altitude density offers a 25% loss ( I have seen as low as 18% and as high as 28%) of effective compression.
I have built other engines (even a flathead) raising the static compression ratio to compensate for the effective compression loss, with excellent results.
using a 6 to 1 static here, would be like having a 4 to 1 at sea level.

has anyone modified an existing head / chamber to further increase static compression ? Whether to compensate for altitude or just to get more power ?

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DanTreace
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Re: high compression head modification ?

Post by DanTreace » Thu Feb 27, 2020 8:30 pm

Guess that would depend on what your need of speed and power for the T.

Have the Prus head on my '27 touring, and love its performance in flat Florida. Think just having more umpth over stock Ford iron head with high compression head would make things happy.

Have to say, reaching high altitude didn't really affect driveability of my touring in Montana, easily climbed up the Road to the Sun. :)
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kelly mt
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Re: high compression head modification ?

Post by kelly mt » Thu Feb 27, 2020 10:42 pm

I run a Prus head on two of my cars and have pulled nearly every major pass in Montana with them. They pull long grades and operate real good at high altitude. Pulled Beartooth Pass Montana at 10,947 feet real good. The thing to remember is the lower end.


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Re: high compression head modification ?

Post by Wayfarer » Thu Feb 27, 2020 11:13 pm

I've picked up on a few mentions of broken cranks here and there.

I understand that pulling a hill in any fashion is increasing stress on the crank, as is increasing cylinder pressure. too much, no matter where it comes from, is bad.

for you guys with high compression heads at low altitude - would you go back to stock compression / stock power at low altitude ? Can you / could you still drive the car as you do now if you went back to stock compression ?

how much stress is that 6 to 1 head putting on the crank at sea level ? pushing it at or near its breaking point ?


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Re: high compression head modification ?

Post by Dennis Prince » Thu Feb 27, 2020 11:34 pm

I run a Z head and have driven over The Trail Ridge Road at over 13,000 ft elevation and I didn't notice any difference, I didn't even mess with the carb adjustment, it is set the same as when I am at sea level and runs great at all altitudes.
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Re: high compression head modification ?

Post by modernbeat » Thu Feb 27, 2020 11:50 pm

I've sent a Prus head to Weisco to have it scanned so they can make custom pistons trying to get the compression up. Also getting a lighter piston with a modern ring package.
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Mark Gregush
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Re: high compression head modification ?

Post by Mark Gregush » Fri Feb 28, 2020 1:45 am

Wild idea. If you are having pistons made, have them made with room on the top slot to fit 2 narrow compression rings. I installed two 1/8 rings on the top slots of my standard cast iron pistons in a worn block, I didn't have the stock 1/4" rings at the time to get it running. Chances of the joints getting lined up as they rotate might be low and by the seat of the pants, feel like better compression then my 21 with aluminum, fresh bore, new rings. I got the idea from seeing ads for rings that had tapered ends that over layed to form a continues ring.
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Wayfarer
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Re: high compression head modification ?

Post by Wayfarer » Fri Feb 28, 2020 5:21 pm

modernbeat wrote:
Thu Feb 27, 2020 11:50 pm
I've sent a Prus head to Weisco to have it scanned so they can make custom pistons trying to get the compression up. Also getting a lighter piston with a modern ring package.
makes sense. I'd like to hear more as you progress.

My goal is local drivability. There's a pretty slim chance that I'd ever trailer (or drive) the car somewhere low altitude for a tour, but, ...never say never. I can picture in my head a "high altitude head" and a "low altitude head" on my shelf - one being high compression, and the being even higher compression.


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Re: high compression head modification ?

Post by Scott_Conger » Fri Feb 28, 2020 6:20 pm

Corey

you're losing 20%, not 25% compression due to altitude. At 6-1 with Prus head, you're ending up with a car which at 6000ft has more compression than a T ever came with at any altitude with the addition to a MUCH improved combustion shape.

You're seeking to improve and add complications where none is needed (a second head for different altitude??)

Pay attention to what Dan Treace and Pat Kelly are telling you, as they have extensive touring experience. Additionally, Pat lives in Mountain Country, and let me tell you, Bearthooth Highway is nothing to sneeze at, nor is Going to the Sun Road.

There are lots of folks touring Colorado with whom you can speak, who tour successfully all over. Find out what they're doing...(and you're going to find out they're not swapping heads around).

Also understand that any comression boost will shorten life of rod and main bearings and make the location of your timer much more critical in having an engine last almost forever vs needing significant refreshing after only a few seasons. I'd suggest putting on a Prus head and learning about your car for at least a year, understanding that a Model T is a 20HP car from the factory, and that it is a "system", meaning that simply bolting on one thing (head) comes with upside and downside, and that significant powertorque improvements require a lot of things from cams, to pistons, carburetion, to gearing. My opinion is that you're too new at this to start planning on doing things like having 2 heads, or seriously increasing compression without understanding the significant issues it can create.

Sometimes it's as simple as stripping a head bolt hole during a head swap which can really ruin your day. For some folks, we can take it in stride and fix it in an hour, others, it becomes a significant issue. Which person are you today? Which person will you be a year from now.

Take a breath...slow down...have fun.
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Wayfarer
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Re: high compression head modification ?

Post by Wayfarer » Fri Feb 28, 2020 8:18 pm

Scott - I'm not even far along enough in owning my car to buy a high compression head, much less modify one or have two, or whatever. I've owned my car for a few weeks. The farthest I've driven it so far is 1.2 miles to the gas station, and back home. Before I change the dynamics of the car, I need to learn the dynamics of the car.
Right now I'm focused on making the car as reasonably drivable as possible - tail lights, brake lights, all the proper carb linkages. I just figured out how to get the stuck radiator cap off today. I still haven't adjusted the parking brake to keep the car from rolling backwards in my not very steep driveway.

that being said... I'm the obsessive type. I read and research constantly, whatever I'm focused on at any given time. I'm asking questions trying to gain knowledge, not to put myself in a "hold my beer and watch this" situation. I NEVER make a move until I am darn good an ready, and that includes researching to death.

I am brand new to Model T's, but not brand new to cars in general. Not brand new to vintage vehicles, and not brand new to engine building, designing or modifying. Most performance principles will apply to the T engine just as they would another engine. Airflow, cylinder pressure, metallurgy, etc. I have built stock and performance L head engines before, and have another in progress now. While I have yet to experience the effects on a T engine, I have a good general idea of what to expect. That is exactly why I came here, to those with the experience, to ask very carefully formulated questions.

as far as atmospheric psi loss - two days ago in my classroom the weather station gave me 68 degrees and 44% humidity. 5800 something feet. Density Alt. was a bit over 8000. 26.7% loss. ( I may be a bit off with the temp / humidity numbers, but the 26.7% is accurate, the students ooh'd and aaaah'd for several minutes) Obviously it's not 68 degrees outside this time of year, but it certainly gets warmer than that here. 25% loss is a reasonable expectation here in Colorado Springs.

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