Topic: Compression Ratio for 836cc Race Engine

[ealanm]

I'm just starting an overhaul on a CB750 race engine.  This is an engine that I acquired when I bought my car (yes, it's being used in a race car), and I know very little about it.  What I have learned so far is that it was set up to run on methanol, with a Hilborn injection system, and the compression ratio appears to be about 14.5:1.

I need to convert this engine to run on some kind of gasoline.  (My local sanctioning body doesn't allow methanol.)  I would strongly prefer to be able to use pump gas, such as Chevron 94.  But I would consider using a higher-octane racing gas if it were important enough.  So I have two questions at this point.

1.  What would be the highest, safe compression ratio I could run on gasoline?  This is for vintage racing, so my emphasis is much more on durability than on power.

2.  What's my best option for reducing the compression ratio?  It appears that I might be able to get down to the level I need by using thicker base and cylinder gaskets, although I have not yet looked at how this will affect the pistons at BDC.

Some other details...

The engine appears to have lots of whizzy stuff on it.  The camshaft and valvetrain are completely aftermarket, including  rocker-tower braces.  The valves look to be somewhat bigger than stock, and the camshaft looks quite aggressive.  It has concentric valve springs.  It was clearly set up to run at higher-than-stock RPMs.  I haven't yet looked at the bottom end, but I was told by the former owner that it has Carillo rods.  (I've learned to take everything he said with a grain of salt, though.  Still, it would be consistent with everything else I've found about the engine.)

It looks as though it had some kind of electronic ignition on it, but many pieces are missing so I haven't been able to identify what kind.  I'm not sure what I will run for ignition.  I'd like something good, but also something period-authentic, for vintage racing.  I may try to get the Hilborn injection working, or I may just slap some Webers on it.

[Big Jay]

Is this motor in a TQ (three quarter midget)? Does it have the transmission on it?

[turboguzzi]

12:1 would be pushing it with 94 octane off the pump me thinks

those 14.5:1 pistons are bound to be pretty pointy, so just jacking up the block (assuming the timing chain will let you) will result in a pretty nasty CC shape with zero squish effect. not good IMHO.

Would look for proper 12:1 pistons if i were you,

[ealanm]

Is this motor in a TQ (three quarter midget)? Does it have the transmission on it?

It's not a TQ, but it's similar.  It's a Formula Four, which is a road-racing class based on 750 cc bike engines (or 850 cc for two-valve engines).  But, yes, the transmission is still attached.

[ealanm]

turboguzzi:

Thanks, that sounds like sage advice.

After I made my post above I discovered that the rules do allow me to run M85 (85% methanol, 15% gasoline).  So I'm going to investigate that.  It might be simpler than changing the engine.

[Big Jay]

If you can run alky, keep the injectors and dial them in. If gas, get rid if them.

[bear]

Our old girl runs hi-comp and methanol. But I would be surprised if your motor would be running anything over 13.5:1.
From our experience the hilborn units work well on Speedway and  Dirt Track, but you will be a lot better of with the Webbers for road racing.
Stick with the methanol blend, if nothing else it will help with any overheating issues you are likely to suffer.

Cheers,
Brian

[ealanm]

Thanks, Brian.  It's good to hear from someone who's doing something so similar to what I'm trying to do.

I actually measured nearly 15:1 and rounded it down to 14.5:1.  But my method was a little bit inaccurate in that it didn't allow for the volume inside the spark plug, so you could be right.  The purpose of the exercise was to determine whether or not I could use gasoline and the answer would seem to be "no," anyway.

If I can find a local source of methanol then I will use M85, as the rules here allow.  I'm also going to see if I can get them to write E85 into the rules.  Can't see why they wouldn't if they allow M85.  And I'm going to give the Hilborn a go, since I already own it.  The car was raced that way on road courses for years, so presumably it works at least reasonably well.