1725 over bore

[mototron]

Hello all
what is the max I can bore out a 1725 block
that will have 12:1 compression ?
I have a block that is 030 over but needs to go to 050
to clean up ,did anyone ever sonic test the cylinders to see the thickness ?
this will be strictly a race motor .
Does any one have a std bore 1725 block in the NY area ?

Thanks
Rocco

 

[jumpinjan]

If you want to run 12:1 compression, you might try to locate (I doubt if you could) an old set of 12.5:1 Jahns pistons. I purchased a set in 1971 for my Alpine and ran the engine on the street for about 3 years. I still have my series I Alpine (please don't ask if I would pull the motor and sell them)
Jan

 

[sunbeam74]

Jan,

I don't think they'd be right for the engine Rocco would be building.

Given he is planning boring it to the max. and probably will do the chevy rods with a stroked crank. Add to that a Vizzard modification... and you really would have to have a custom pop-up calculated based on all these changes to end up with 12:1.

BTW< JIm, Jarrid, or Jan, what is the ususual end combustion chamber volume of the Vizzard modification? Based on all the other changes he might be able to use flattops.

Steve

PS: One alternative to pop-ups might be to machine the snot out of the head and relocate the bolt holes for the tappet cover, etc. The Joe Mondello head that I have was done that way. If I remember correctly it was about 31-32cc total - or there abouts - it worked out to be just about 12:1 on a 1592cc motor, .040" over. (Of course, the Mondello head doesn't have the vizzard mod. either)

 

[Bill Blue]

Why not shave the block to achieve the desired compression ratio? I know that is supposedly more expensive/difficult, but it seems to me you reach a point where it comes the best option. Maybe it is my convoluted way of looking at thing, but it seems logical to optimize the head and piston for power and reliability reasons, then set the comp ratio by machining the block. Is the deck to thin to remove this relatively small amount of material?

Bill

 

[sunbeam74]

Technically, you will want the top of the piston to be a few thousandths from the top of the deck. It won't affect your ratio too much since the piston should almost be at the top already.

Steve

 

[atallamcs]

race specs

This worked well for my car.
Sunbeam Alpine 4 Cylinder 1725cc
Venolia Pop-top Pistons
Eagle Connecting Rods (Ford Pinto 2300cc)
Crane Cam - custom grind
Datsun 1600cc Valves
BMW 1600 Schrick double valve springs
Sumbeam Alpine 1600 oil pump
Custom deep sump oil pan with baffle
Mocal oil cooler
3 qt Accusump
Gilmer Belt drive water pump
Weber 45 DCOE Carb
Custom intake manifold (Lynx)
Custom high efficiency raditor
Mallory dual point distributor
Custom header/exhaust
Bill

 

[RootesRacer]

Your really dont want to do this, one the pushrod length is based on proper valve geometry when the engine deck is at the correct height, and the head is at standard (unmilled). Two, getting custom forged pistons that at TDC are right at the deck is not a problem, or terribly more expensive than stock cast pistons. In fact on several motors, I have gone beyond the deck and into the gasket area for even more compression, with excellent benefits to chamber "squish". I pulled the head on my motor a few months ago, on an engine that was twin DCOE carbed rather rich, and the chambers were carbon fouled (as would be expected), but the pistons and the face of the head were nice and clean due to the turbulence from the "squish".

Why not shave the block to achieve the desired compression ratio? I know that is supposedly more expensive/difficult, but it seems to me you reach a point where it comes the best option. Maybe it is my convoluted way of looking at thing, but it seems logical to optimize the head and piston for power and reliability reasons, then set the comp ratio by machining the block. Is the deck to thin to remove this relatively small amount of material?

Bill

 

[jumpinjan]

Jan,I don't think they'd be right for the engine Rocco would be building....you really would have to have a custom pop-up calculated based on all these changes to end up with 12:1.

Steve,
The Jahns pistons do have pop-ups. Haven't you ever seen them before? I'm surprised at you that you haven't...

 

[sunbeam74]

Yes, I am aware of the Jahn pop-ups, the black race car has them.

I am just saying you would want the pop-up to occupy a certain volume to achieve the 12:1 ratio based on the CC oc the combustion chamber. I don't think off the shelf would be the best approach.

The quality of the Jahns don't really compare to newer products available. They do the job but they were probably cast in the 60's.


Steve

 

[jumpinjan]

I have a block that is 030 over but needs to go to 050....

Rocco,
I would like to advise you NOT to go wild on the boring, I would be conservative on the bore size. You are going to have many head gasket problems as it is, and a big bore will make it worse. And I wouldn't think that a solid copper gasket will solve problems, I been there, but with enough money to develop alternative measures, anything could be possible.
Jan

 

[alpine_64]

A few questions:

1. What class are you running? How large can you go with the motor?
2. What size motor are you aiming for?
3. Regarding HG's, if you are going big overbore maybe look at modifing the water passages in the head, do a custom HG and that should help prevent the HG failures.
4. Are you looking for a motor with max HP (i assume you are given the compression you are looking for) is it for circuit or hillclimbs.. maybe a motor with lower comp but good flow and lots of mid range punch might be a better option.

justa few questions and thoughts

 

[Jim E]

Actually push rod length is no big deal custom push rods are cheap.

V chambers are usually in the 41.5 to 43 CC area depends on the head.

Remember that the larger the bore the higher the compression with no other changes.

 

[sunbeam74]

Jim,

Your so right about the compression ratio increasing with the increase in cylinder volume. I am pretty sure that is what killed the Sports Car Forum Alpine in the 60's. (or complications from the compression being too high)

The head I found with the car would have provided 12:1 compression on a 1592cc motor. Unfortunately, it was a 1725 so it was probably closer to 13.2:1 (or something like that). Technically, anything over 12:1 would have been illegal for SCCA but I suspect Bill Swegler was doing what ever to keep it on the track by 68'.

Anyway, the head was off and that is the way it sat for 40 years. May have been a blessing that Bill Swegler had problems... now that I look back on it. At least it sat dormant in the garage.

Steve