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How high to raise the compression ratio

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70herald

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Since I have to take the head off again /bcforum/images/%%GRAEMLIN_URL%%/wall.gif and probably have it slightly shaved. I think I want to raise the compression ratio a bit.
First of all, I am taking the head off because it turns out the head gasket is junk. Serves me right for using it, I wasn't real happy with it in the first place but the spare head gasket turned out to be for a 1200 engine and I have a 1300. Anyway, the bad head gasket explains allot of the bad behavior of my engine since I rebuilt it all of 15 km ago.
Now I doubt that the head is actually warped, but I don't exactly want to find out that it is after I reassemble it with a new gasket so off to the machine shop for a quicky skim.

That raises the question if I shouldn't raise the cr ratio altogether. The engine was originally a low compression engine (7.5:1) for the low octane fuel available here at the time. I rebuilt it with flat pistons instead of the original dished pistons so that will raise the CR ratio a tiny bit, but it is still way under the 8.5:1 of the standard engine.

How high can a safely raise the CR? fuel here is European 95 octane (similar to regular in the US). I could easily shave as much as 2 mm off of the head, which would put the cr up to around 1:9, according to Teglerizer's web site, and there would still be enough adjustment available in the valve adjusters to set the gap correctly.

How high should I go? I want a reliable street engine which will run on regular gas in the end.
 
I would suggest you do it like a machine shop would. Get a piece of plexiglas and a syringe for large animals, Use some ATF as fluid to measure exactly the volumn of the chamber.Factor in the headgasket thickness and calculate the total displacement of the piston in one cylinder.Then you can figure out how much to plane the head to get the desired chamber volumn......I suggest that 9/1 is a practical limit for the fuel you are getting. Dont forget that the 1500 head has biger valves/hardened seats. if the name of the game is HP plane a 1500 head to your needed ratio.
MD(mad dog)
 
Paul is right on about CC'ing the head. It is the only way to be certain of what the CR is and how much to remove to get where you want it.

Rubbing alcohol works well for the fluid because it can be cleaned off with an air hose between tests. Just be aware it can rust after because it will clean the oils from the metal.
 
I was planning on cc'ing the head. Considering its age, and uncertain pedigree I am not sure how much has been shaved off in the past but considering that it started at 1:7.5 there is still more than enough for me to take off.

Mostly I wanted to know how much I can safely up the CR ratio without running into problems. I don't want to get into a situation where I need premium or cause drive ability problems.
From the responses, it seems that I should aim for somewhere slightly over 1:8.5 That is right about what the standard version of this engine came with so there clearly wont be any major risks.
 
Paul and I run TR6's but it should still be pretty safe to compare. I am running 9.8:1 and am very pleased with the way it runs. I did not want to go over 10:1 because several told me that over 10:1 would be starting to push it.

That said, it does not leave much for future head work if needed. That is fine for me but you may not be comfortable with that.
 
tomshobby said:
Paul and I run TR6's but it should still be pretty safe to compare. I am running 9.8:1 and am very pleased with the way it runs. I did not want to go over 10:1 because several told me that over 10:1 would be starting to push it.

That said, it does not leave much for future head work if needed. That is fine for me but you may not be comfortable with that.
Click to expand...

The basic architecture of the engines are rather similar. TR6 has a longer stroke vs. the 1300 and of course 2 extra cylinders. Since I still have the original stock cam, you certainly have a more aggressive cam also which would further raise the dynamic compression ratio more than what I would get even for the same static ratio.

Tom are you and Paul running on regular gas? If so then I will feel fairly safe shaving to about 1:9+ which would still give me a bit of room for a future skim.
 
I am not sure what Paul is running for gas. Except for one fill up when it was not available, I have run premium but probably would not have to. The thing is it runs great, runs cool, and has plenty of power with premium so I will stay with it. The cost difference for the miles I drove this summer would only be about $30 anyway.
 
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