TR2/3/3A TR3 Compression Ratio with larger bore liners?

[PatGalvin]

I just purchased the 87.5mm liners for my TR3 rebuild and I am installing stock head. Because swept cylinder volume is increased by larger bore, this wil result in increased compression ratio. Does anyone have any experience with this and would this increse in CR give me any problems? Better to know this while my parts are still at the machine shop.

The engine will be basically a stock motor. I am adding a BPNW reground cam (street performance grind). Most all else is stock at this point. If I win the lotto, I'll buy a tubular exhaust but for now, my wife tells me I've hit my spending limit on engine mods.

Thoughts?

Pat

 

[MGTF1250Dave]

Aloha Pat,

I would think that although the volume (engine displacement) has increased, the compression ratio will remain unchanged. Even as you draw in a great volume of the air/fuel mixture, the travel of the piston will be compressing it the same amount with in the larger cylinder. Compression ratio would change if piston travel changes with different length connecting rod, crankshaft is changed with different throw dimensions or the cylinder head is shaved.

I'm sure a more positive answer will be forthcoming from one of the professional mechanics or racer.

 

[TR3driver]

Changing the bore does affect the compression ratio, even with the same stroke and combustion chamber, etc.

However, the original CR was low enough that a step up is usually not a problem. Within the limits of available fuel, higher CR actually improves engine efficiency, one of the very few modifications that improves both fuel consumption and power.

(Geek alert) Compression ratio is defined as (swept volume + unswept volume) divided by (unswept volume). This can be re-written as (swept / unswept) + 1. If we assume the deck height to be zero (which is very close to correct for a stock TR motor), then the bore affects only the swept volume. If the stroke remains the same, then the swept volume changes with the square of the bore.

So, assuming the original specifications (8.5 CR @ 83mm bore) are correct, the swept volume goes up by (87.5/83)^2, or about 11%. Original swept/unswept was 7.5, so now it is 8.3, giving a new CR of 9.3.

Worst case, you may need to run premium fuel, or retard the spark just slightly. But in my experience, 87mm liners with a stock head run just fine on regular. (I always set the timing by the "road test" method anyway, as the optimum setting varies with the fuel from the pump, which varies with the season.)

Disclaimer : I am neither a professional mechanic, nor much of a racer (yet).

 

[martx-5]

The compression ratio will definitely increase. You can use this calculator to see the effect. Put the numbers in as millimeters. Use 83 for bore and 92 for stroke. for head gasket thickness, I used 2mm. for the bore of the head gasket, I kept it simple and used the bore of the cylinder. I used 60cc for the chamber volume and zeros for the other two factors. I got a compression ratio of 8.0. The I substituted 87.5mm bore and also changed the head gasket figure to 87.5mm. Compression ratio went up to 8.7.

I have 87mm pistons and liners with a head milled 0.085", and don't have a problem. I have to use high test gas though.

 

[PatGalvin]

Thanks Art and Randall.
This is exactly what I needed. Happy to buy premium gas. I don't think gasoline will be my largest expense on this car.

Kind regards,

Pat

 

[MGTF1250Dave]

Mahalo Randall & Art,

Thanks guys for correcting my misconception. Randall, even if you are not a professional mechanic or racer as noted in your disclaimer, you are a font of knowledge and a respected source of information.