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HP gains from additional compression

> I know that some of the more seasoned and/or engineering types out there
> know the answer to this question. My machinist claims that an increase in
> compression from 10.5 to 1 to 12.5 to 1 will produce serious HP gains,
something like 40 to 50%. Is that true?

From Jack Wheeler:

He is right that there is a lot to be gained with compression, there are also some negative tradeoffs, like higher maintenance and lower engine life. Based on my experience, I don't believe the 40-50% figure. In addition, compression won't do it alone. To increase the horsepower of your engine, you have to look at cylinder head porting and combustion chamber profiling, valve grind geometry, and flow around the valve stems and guides. Also important are carburetor and manifold porting, and matching of manifold to head and carburetor body, camshaft grind, valve springs, and rocker geometry. All of these items help get more air/fuel into the combustion chamber. Without improving these, compression by itself will help, but not nearly as significantly as your machinist suggests.

You also need to be able to get the exhaust out. This was always the restriction on the TR-4 heads, due to the poor shape of the exhaust port. Again, cam, porting, matching of ports, and headers are all important.

When I first started racing, I was always looking for the one 'magic secret' which was going to give me that amazing horsepower that everyone else seemed to have. A lot of well meaning people were willing to give me advice as to the 'magic secret'. Unfortunately, I eventually learned that there is no magic secret. It isn't magic which makes horsepower. As Kas said in his book, it's attention to all the little things. If you do all of the things I mentioned above, you will find 100 other 'little things' that you will also have to fix or improve along the way. For instance, rocker pedestal studs will break as you go to higher lift cams, so better designed pedestals and stronger studs need to be used. More stress requires better quality and lighter weight valve spring retainers. Higher RPM's are also required to generate more horsepower. This means Carillo (or other aftermarket) rods, billet cranks, etc. On the TR4, the stock rear seal leaks pretty badly at 7,000 Rpm's, thus a new seal design is required.

I could go on and on. I'm sure most of the people on the FOT list could as well. Don't be fooled into thinking that more compression will give you lots of horsepower without fully developing the rest of the engine, and beefing it up to withstand the additional stresses.

Good luck, Jack Wheeler


FROM Kas Kastner:

Boy I wish was that easy to get horsepower. No way on that percentage of increase. BUT, the one thing about compression increases they are effective right from the first rpm all the way to the end and thus the increase will be felt all through the range. Because of this you can then run a bigger camshaft (more duration) and still have a reasonable bottom end range. Depending on the cylinder head milling to get that ratio might be touchy and THIN deal to say the least. Obviously with too much milling to get the ratio you then have a problem sealing the head gasket as the floor of the head does not have enough material to support the pressure of the fixing bolt torque. This is especially true with the wet sleeve engine. Too much milling on the wet sleeve engine will generally give you a compression leak to the water around the two center cylinders on the exhaust valve side. More available on all this stuff .. Kas


Also from Kas Kastner:

The issue of popup pistons has come to the fore several times over the years and the information never seems to be saved or passed. To wit: I made a series of engines for exp. purposes in the early 60's to view the effects of compression ratio increases.

In a nut shell here is what I found on an honest engine dyno with an operator that didn't lie to himself (me). The 13:1 popup piston that entered the combustion chamber confused the flame pattern in the combustion chamber so much that the measured BHP was the same as an engine with flat top pistons and 10.25 ratio.

The slightly domed pistons were better than the popup by a considerable measure but NONE were as good as the flat top. The combustion chamber in the wet sleeve engine series is so ancient that there is not room for a decent flame front to progress.

Look at the tops of your pistons if you have the popup or domed type. See if the carbon is a reddish or off color brown instead of black and that there are swirl type patterns on the piston top as opposed to a flat covering of carbon. Generally you will find exactly that as I have described. The flat top piston allows the flame front to progress evenly to the back of the combustion chamber and reach there before the end gas has time to heat up and explode on its own (detonation). On the flat top pistons you should see that the carbon is thinnest or maybe even nonexistence on the squish area side of the piston and closest to the exhaust valve because this is where the detonation is going to occur.

I ran cast pistons for years and years in these engines both the stock 83 mm,86 mm and the Hepolite 87 mm. The Hepolite in my opinion was a great deal better as it was a lot lighter than the stock castings. When we ran forged pistons they indeed had the longevity but did not produce the power as the bore clearance was twice that of the cast pistons and they were quite a bit heavier. The thing here being that if you control the detonation you can run a cast piston. Of course I am speaking from efforts 30 years ago but gasoline still is supposed to burn not explode so maybe some of this is still true.

The best and most powerful engine I made were done with 87 mm cast pistons, chrome compression ring and .003" bore clearance. To eliminate the run in time and additional wear on the engine I lapped all the rings into the individual mating sleeve and had the highest effective compression immediately and it showed on the racetrack. With all my work in these areas I have a small caveat that must be remembered, and this is that I was only interested in something to last the weekend not the year and there is the "wolf in the bush" Kas.


FROM Jack Wheeler

Kas, I don't think my experience is completely contradictory to yours. First, my pistons were custom made by me with a die grinder to get a good fit into the combustion chamber. As a result, the popup part of the piston was more like a dome which projected into the combustion chamber. I don't remember the exact height, but I think they were about .140" above the normal height of a flat top piston.

Our (I have to give credit for the idea to Hardy, who suggested I try popups in about 1983) objective was not to create a better flame pattern, or more efficient combustion. At least in my case, the sole objective was to get the higher compression without having to mill the heads any further. With flat tops, I was milling almost .200", and most head castings wouldn't last long with this much milled off them. After investing all the money to port and polish a head, I needed for it to last a few years. By using the popups, I was able to get the 13:1 compression and only had to mill the head .150". The heads lasted forever.

I also found a considerable improvement in power with the higher compression. With everything the same except the cylinder head and pistons, I went from 155 HP with flat tops to 167 HP with the pop ups. The new head may have been better, and the dynos were different, but the order of magnitude increase we saw on the dyno related directly to improved times on the track. My conclusion is that the increase in compression more than offset and 'confusion' caused by the popups.

This analysis does not follow the scientific method completely, but it made sense to me at the time and, as we said, "the proof of the pudding is in the lap times". Does this make sense?



From Kas Kastner:

Sounds like you have hit pay dirt Jack. This only proves again that "One valid test is worth 10,000 expert opinions". What I was referring to primarily was that the flat tops gave a more efficient combustion therefore you gained more power with a lower ratio and less chance of detonation. But I never argue with a guy who has found a way to be fast.


From Harvy Printice:

<< The issue of pop-up pistons has come to the fore several times over the years and the information never seems to be saved or passed. >>


Once the good heads were gone, it became clear to me that pop-up pistons were the only option that I could think of for a motor to win the ARRC.

It took a large investment of time and money to make it work in the TR3 motor. I doubt that it was easy for Jack Wheeler, even after I let him know that it could work.

Kas is very correct about the flame front getting upset! The likes of Lester Lichty would be appalled.

The increased octane of available, legal, racing fuel; the more exact timing of beyond distributor ignition systems, and legal alternative con rods are the major changes to this situation since the 60s and early 70s.

I did win many earlier races with the fine Hepolite 87mm pistons.

I also figured out how to make the factory rear crank seal work at high RPM, and always check the flywheel runout.

I had a long call from Steve Froines today (TR4 Natl. Champ and the last West Coast JRT Comp Director) He could not help me with my wondering about 67 TR3 Natl. Champ Lee Midgley

The Hardy HP

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