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1500 7.5:1comp or 9:1comp piston which one?

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recordsj

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I am rebuilding my midget 1500 engine and I am trying to figure out if the pistons are a 7.5:1comp (dish piston) or 9:1comp (flat piston)? Can someone direct me to a website or attach a picture to help me figure out what pistons I have?

If I have a 7.5:1 compression and convert to a 9:1 are there any other modifications I need or should do with the conversion?
 
recordsj said:
I am rebuilding my midget 1500 engine and I am trying to figure out if the pistons are a 7.5:1comp (dish piston) or 9:1comp (flat piston)? Can someone direct me to a website or attach a picture to help me figure out what pistons I have?

If I have a 7.5:1 compression and convert to a 9:1 are there any other modifications I need or should do with the conversion?
Click to expand...

You'll need to look at your pistons in order to identify them. The 7:5 have a dished cut out in the top, the 9:1 are perfectly flat. You'll probably want to buy the 9:1 since no other mods are required to fit them.
 
Lots of combinations can be done to increas compresion ratio, including block/head decking, piston dish volume and deck hieght, you nned to check and see where you are now, that will tell you alot in the equation. Sometime higher compression piston offer for engine could be in more compression height or less dish or flat top in the case of the 1500 high compression pistons. Cahances are you could go to the flat top higher compression piston, but you need to know where you are now. Which intails you recording piston deck heights, head combustion chamber volumes, compresswed head gasket thickness, bore andstroke, with data you can use a online compression caculator to do the math and see where you are. You can do this stuff pretty close with a few simple cheap tools, a dial caliper, a feeding tube syringe, an piece of plexigalls for sealing the combustion chamber, it not that hard with the help of the caculators. Hope this helps.
 
If I went to the 9:1 compression ratio would I need to use a higher octane of fuel (as opposed to regular 87 octane)?
 
recordsj said:
If I went to the 9:1 compression ratio would I need to use a higher octane of fuel (as opposed to regular 87 octane)?
Click to expand...

Thats kinda border line, maybe, maybe not, if you went any higher towards 10.0 to 1, I'd say yes for sure, but at 9.0 to 1, it's coin toss. I like to use unleaded premium in all my non regular use stuff, classic car, 4 wheeler, genorator, etc, premium unleaded with it increased octane has much longer shelf life and it worth more to pay at the pump that clean out gunked up carbs and drain bad fuel. On one of cars it maybe only a couple more dollar on a fill up with premium, well worth it to me.
 
Yep, no use to build a hot rod, if you don't want to run the better gas, with racing gas $6-8 a gallon that we use in those cars, premium unleaded sound cheap to me for the street car :smile:I think in the MGB, it like $2.50 more to do premium as for a tankful, so less in a Spridget, now trust me I'm cheap, I'm just not that cheap :smile:
 
see the attached picture, I assume this is what they mean by a disshed piston?
 

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Ok- I'll be optimistic and say "1%". Maybe more if you keep your foot out of it!!
BillM
 
depends to i think on your carburetor and state of tune.


m
 
recordsj said:
If I do go to the 9:1 comp pistons and use the higher grade fuel any idea of how much fuel efficiency would go up?
Click to expand...

Keep in mind, the flat top pistons alone will not get you to 9:1. You'll need the 76 head, or a skim on the one you've got. Assuming you have a later head and it's never been skimmed, the pistons alone will get you up to about 8.4:1.
 
Is there a dimension I can check to see what head I have and if it has been skimmed or not? If so what dimension should I compare it to?
 
recordsj said:
Is there a dimension I can check to see what head I have and if it has been skimmed or not? If so what dimension should I compare it to?
Click to expand...

You could check the part number on the head, but that will only confirm what you had when it left the factory. Lots could have been done to it since. TKC2748 is the 1500 9:1 USA head. TKC1155 is the head to have, it's 9:1 and is not drilled for smog; was only available in the UK though. TKC1409 is likely what you have. (7.5:1, drilled for smog)

As Hap suggests, you really need to cc the combustion chambers to know where you are.

What is your goal for the engine?
 
where is the p/n on the head located?

goal for engine: street use, since I am rebuilding it because of a bad piston ring and need to replace the pistons I am looking at my options of what I can do....
 
recordsj said:
where is the p/n on the head located?

goal for engine: street use, since I am rebuilding it because of a bad piston ring and need to replace the pistons I am looking at my options of what I can do....
Click to expand...

Top of the head, opposite #3 plug (on the intake/exhaust side). It will be inset/engraved, not cast/raised.
 
This is very helpfull and usefull.
The head is TKC2748. It looks like I have the 9:1 head but from a previous owner rebuild the pistons are dished (see attached photo). I have no smog equipment installed (no CAT or air pump, EGR ports in head are plugged), since a previous owner installed the 1500 engine in a '74 car MG Midget car.
 

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