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1275 Compression, Camshaft, et al

lbcfan

Jedi Trainee
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I could use some advice, or please point me to a previous thread. I haven't been on for a while.

Last year I rebuilt my bugeye, and I bought a rebuilt 1275 engine that for various reasons I've had to tear down after 200 miles. The more I uncover, the more I find that I cannot trust what the builder did, so I'm treating it as a used engine.

I have an unidentified reground camshaft that I'd like to replace. There seems to be a lot of slop in the chain drive so I would replace the set, and the tappets as well. Springs- not sure... I'm trying to do the work myself, but I've never done this before, so I could use guidance.

The pistons are AE 21251, which provide an (original) 8.8 compression ratio. The head and block were supposedly milled, but I haven't obtained measurements on how much. When I ran the engine last year the hot compression readings were around +/-168 PSI.

A decision to make before replacing the camshaft- replace the perfectly good pistons with something in the 9.8-10.0 compression ratio area, or keep the 8.8 pistons. Using high test or a fuel additive is not a concern- this is a weekend car. I just want it accelerate, and I need the power for the 3.9 rear end that I just put in.

I'd appreciate thoughts on whether it is worth going to a higher compression ratio, at the otherwise needless expense of new pistons? I want to get the most out of the engine for street use, but without sacrificing reliability. I'd hate to spend the money on the cam without maximizing the impact, and I'm willing to replace the pistons if it will make a noticeable difference.

The bigger question- depending on the pistons, what cams are best in each case, and whether and when to increase to 1.5 rockers. Obviously I'd like to keep the stock rockers if possible.

The camshafts I'm looking at are the Kent 266 or the Swiftune SW5-07, but I'm not clear on the applicability regarding compression and lift.

Thanks for the help.

Michael
 

BlueMax

Jedi Warrior
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1) The first thing you need to do is measure your deck height, (Top of piston to top of Block)
2) Second you need to CC the dish of the pistons.
3) Third you need to CC you cylinder head. That needs to be done on all the combustion chambers.
4) Used compression calculator so that you know what you have to work with. If you are using a copper head gasket calculator about .030 thickness.

You’ll need precision measuring instruments to do all of these calculations. All of this information must be recorded if you decide to make changes by machining block or head.

For 93 octane fuel you need to stay well below 10.5 to one compression.

Rocker ratios are chosen with the appropriate camshaft. You would get a great education by buying David Vizard book, Tuning A Series Engines. There is so much information in this book, all you’ll need to know is right their.

HTH

 
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lbcfan

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Thanks. I can't make the measurements myself. I am looking for a shop in the Jersey shore area. If anyone has a recommendation...

For the sake of the discussion, it would be reasonable to assume the compression ratio that would result from standard milling of the head and block in a rebuild, and the 8.8 pistons.

Thanks,

Michael
 

BlueMax

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Not to be sarcastic, but there are no reasonable assumptions in building your engine properly. You have already made a reasonable assumption by the purchase of the rebuilt engine. (IE, The out come your dealing with now) I know a very reputable builder in the New England area if you’re interested or send it to Acme Speed shop? :encouragement:
 
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lbcfan

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Not to be sarcastic, but I'm just looking for friendly advice. While I imagine you know a lot, it's not coming through in your replies. Thanks anyway.
 

AN5Sprite

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Michael
I remember Frank Clarici (RIP) posting the name of the machine shop he used in the Jersey shore area, either here or on one of the other forums. Maybe someone else will chime in with that info. I can't find it.

I've got a Hap Waldrop built 1275, it's a VERY strong motor that was built for the street. Pulls like a tractor, revs easily when I need it to.
.040 overbore
APT VP 276 cam
9.75:1 comp
There's some Hap ju-ju in the head porting and I'm leaving out some stuff, but to help answer your query - just under 10:1 and that 276 cam work really well on the street. I'm using the 3.9 rear gear, ribcase trans and the car will lope along happily in 4th gear down to 2K rpm. Seems just as happy at 6K rpm too.
 
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lbcfan

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Thanks Steve.

Like everything else, I want to do the work on the engine myself. I need a shop for certain measurements and necessary machining, but I'm looking forward to assembling the package- maybe with the local club. Trouble is, internet research only gets you so far, and I'm not fanatical enough to study Vizard.

It's helpful knowing the 276 is useful for the street with the higher compression. Do you know if your rocker arms are high lift (1.5/1)?

As for porting, etc., maybe next time. At least I don't have to pull the engine for that...

Regards,

Michael
 

racingenglishcars

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Hi, I don't want to step on anyone's toes, and I've been criticized in the past for providing a bit more specific and/or generalized information so take it or leave it as it is. I apologize in advance.

It's quite true what Bluemax says and I don't want to imply in any way that an engine rebuild can be done better, but in my opinion, some generalizations can be pursued.

The pistons have a dish which could be 'accurately' measured with a calibrated syringe, or one could use the volume of spherical cap formula
https://en.wikipedia.org/wiki/Spherical_cap

The 1275 head was originally 2.75 inches in height. If it's been milled (probably) this can be measured with a caliper. *some heads were slightly different or even milled from the factory.

Generally the combustion chamber of the 12G940 head has a face area of around 29.2 cm². Therefore for each 0.010 inch removed from the head, around 0.74cc is removed from the chamber. The original chamber capacity was around 21.4cc and the removed volume can be subtracted from the original volume.

Or the chamber can be measured with nearly the same method as with the pistons, using a calibrated syringe and with the head held chamber-side up and level. It's necessary to use a piece of glass covering nearly all of the chamber, allowing just enough space to inject antifreeze from the syringe. The glass can be sealed to the head with grease.

A used gasket could be measured for thickness, one could use the assumption from Bluemax or take the information from the manufacturer. The compressed volume is usually listed on their website, also if it's sold by a reputable reseller. If not, ask them.

An important part is the deck height. This should be measured before removing the pistons. A straight-edge over the piston bore, while the piston is at TDC, and use a feeler gauge to measure the distance between the straight-edge and the piston. *Measure both sides at the same time to ensure the piston isn't rocking.

The end measurements you should have are the piston dish volume, chamber volume, gasket volume and deck volume. You should also know the overbore size. The stroke is a known value.

The formula is found in places like here
https://www.muller.net/sonny/crx/compression/

Aside from the above generalizations, I think even more generalizations are made regarding camshaft choice. My personal choice would be the Kent 266 (or equivalent) for a very good street driver or a 276 (or equivalent) for a bit hotter street cam. More than that becomes harder to drive on the street, but does produce more power.

Another generalization regarding rockers is that more is better. However as bluemax says, follow the recommendations of the camshaft. Personally (being thrifty) I got the offset bushing set from APT
https://www.aptfast.com/ShowItem/103066 A Series Offset Rocker Bushings.aspx
which should give 1.3:1.

Regarding compression ratio, Higher is better, if you can get the fuel, and if the pistons can take the pressure / fatigue.

So go ahead and slam me. I'll just stay off the board for a year or two.
 

smaceng

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I don't think that any of the comments have been off subject, inappropriate, or incorrect, it is just that sometimes it is hard to answer a question in just one answer when we don't know person asking. If I could make a recommendation Michael, is to continue to refine your question as you get more information, and you can then make decisions about how you want continue with your rebuild.
Yes, by all means, find a machine shop that can cc your engine.
I had a machine shop build a short block for me, as my 1275 CIMG1540.jpg
block needed to be bored and cleaned up.
I too used the 21251 pistons. I also shaved my cylinder head to came up with a 9.5 to 1 compression ratio. I used the Apt VP266 cam with new springs, lifters, retainers, and push rods. All was balanced Rod and main bearings were the original tri-metal (King bearings are vg also). Standard valves and rockers.
My car is also just a weekend car, and it runs great. It has good low end torque, and will rev like crazy right past red line, but I stop at 6K.
Good luck and don't hesitate to keep asking questions until you get what you need. We need all the Sprites on the road not up on blocks!
Scott in CA
 
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lbcfan

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I cannot have asked for a better response to my question(s). Last year I did my windshield glass myself. I could find no guidance at all, except to take it to a professional. I did it and I posted a detailed description so that there would be something out there. Some people were derisive about it- others were grateful. Your Email will help other shade tree mechanics as it helped me.

Thanks.
 
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lbcfan

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Thanks for the feedback. No-one seems to put in high lift rockers. That was a big question for me, now answered.

I would have thought that after all these years there would be reasonable rules of thumb for an engine with known characterists, and for the changes in the compression ratio given certain changes. You could almost prepare a matrix and avoid the calculations. Good weekend project, but obviously not for me. racingenglishcars' response put things on a level I could use.

Regards.
 

nomad

Yoda
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In my area it is common to not be able to find gasoline with over 87 octane. Most station's carry only 91 any more. IMHO, I would not consider over 9.5/1 compression. Again IMHO. Cam's can influence the compression a bit, I believe, but you would need an expert to guide you on that.

Kurt.
 

racingenglishcars

Darth Vader
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Yes, longer duration, lower compression gauge readings, but higher effective compression at the speeds the camshaft is designed for.
Example: The camshaft closes the inlet valve during the compression cycle. Close later, less "volume" compressed, but at speed the inertia of the inlet air will keep filling the cylinder. It's a trial and error balance designing camshafts!
 

HealeyRick

Yoda
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Thanks for the feedback. No-one seems to put in high lift rockers. That was a big question for me, now answered.

I would have thought that after all these years there would be reasonable rules of thumb for an engine with known characterists, and for the changes in the compression ratio given certain changes. You could almost prepare a matrix and avoid the calculations. Good weekend project, but obviously not for me. racingenglishcars' response put things on a level I could use.

Regards.

This is kind of what BMC did with the "Special Tuning" guides, listing the modifications that were available from the competition department that an owner could follow to produce a proven result. If you don't want to go into Vizard-like depths, the S/T guides might be of some help to you. Go here and look under the "BMC Special Tuning Manuals" section for the 1275: https://www.spritespot.com/referencedesk.htm#Special_Tuning It's not the most recent info and doesn't take into account the recent advances in engine theory, but it might give you some ideas.
 
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lbcfan

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I found a very detailed discussion on camshafts at the link below. It may help someone making choices on a camshaft.

https://www.mlmotorsport.com/forum/index.php?action=printpage;topic=334.0

I decided to go with a swiftune SW5-07, and keep the standard compression pistons and rockers. Looking for more oomph with very good drivability. Apparently with the lower compression, higher lift rockers will not offer any advantage.

Thanks to all for your input.
 
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