I am rebuilding a pre 1972 4 bolt hp.61 nitro engine. I am wanting to run 10% nitro with it. What I am looking for is a chart or a Idea on what nitro content should be used at what compression. I am smart enough to know higher nitro changes timing and so does compression. What should the compression be for this engine if I turned the crank taking the reading at a moderate rpm of 200 rpms using a cordless drill. Thanks for all your help

Compression readings using a compression guage are useless other than as a comparison on the same engine using the same guage over a period of time for an indication that it's wearing out. Compression guages have a dead volume which effectively adds to the volume of the combustion chamber so gives a false indication of actual compression ratio. In a car engine this dead volume is of no consequence compared to the size of their combustion chambers but a 60 engine will have a total volume (including squish volume) of close to 1cc.

AFAIK there are no charts showing how much nitro can be used at a given compression. The only guide I could give is that I know (from experimenting) that 13.5:1 is ideal for zero nitro and that most engines using nitro have a compression of around 9:1 where they seem to be optimum with about 15% nitro. Some 4 strokes are (I believe) down around 6.5:1 and they run best on 30% nitro. Going by these figures I'd suggest that for 10% nitro a measured (calculated) compression of around 10-11:1 would be close to ideal. These calculated compressions are done by using the swept volume of the engine, not from where the exhaust port closes.

And to add the trouble of heavy (to my observation) dependance of the ignition point mostly on the Temperature and
as well the glow-plug "state-of-health" etc ... too many factor which seems led to the complicated carbutators of the
real gas engines and consequently to injection systems. Yes, would be nice to have had the lux to spend th money to
get to try the OS engine with the injection and associated electronics....

Well, I measured and tried to many things but it seems Nitro % depends on the Engine Gas-Diagram and associated
Exhaust system (resonance-pipe or just open exhaust or .....) and varies so much than I have to keep very diferent
fuels for different engines and also weather and also glow-plugs and also .... too much troubles out of a hobby.

So I settled with 12% Nitro when I have Nitro at low cost ... otherwise there is nothing wrong with FAI fuel (0%).
Nick

It's a glow engine, not a nitro engine. That off my chest , there is no chart other than rough numbers as provided already. Timing is influenced enough by prop load, plug choice, atmospheric conditions to make blanket statements next to impossible. Quantitative readings or charts are useless except to get you in the ballpark, but the fact that it is a commercial sport engine means you are much closer to the ballpark already than you would get with someone else's numbers. You can only compare compression readings with the same engine that way, and then that assumes you have good control over the minute variables. With modern tapered bore engines - ABC/ABN/MTV, whatever - forget it, ground based compression readings tell you nothing and likely do more than cause unneeded wear.

5-10% will be fine with that engine, based on the era and the fact that it is a sport engine, and I would not recommend running 100% synthetic oil either. Don't overprop it to start, I'd say bench run it on an 11-7 or so and find out how it behaves, and make changes to fuel or compression ratio if a need is apparent.

MJD

The engine came to me with 4 head shims and 2 of them are fairly new by looking at them and the other two look like the originals and with a short glow plug, Assuming god help me with that someone was using higher nitro with less compression. I am trying to get a good ball park so I dont have to keep removing the head and adding or taking out a shim. Also I am replacing the bearings in it and purchased the hybrid ceramic, the front one did not have a seal on it is this normal for that engine of the time and should I leave the seal on the new one.

You can leave the seal on the front bearing and hope the internal sealed lubricant will last as long as you need it. Generally, the nose or front bearing does not have a seal, it may have a "shield." Shields are metal and allow lubricant to get into the bearing. Usually the front shield is retained 9to keep debris out of the bearing) and the rear shield is removed (to make access by the lubricant easirer). Seals are just that, seals, they do not allow for lubricant to get into the bearing and they rely on the seal keeping the bearing manufacturers lubricant to remain in place inside the bearing sealed area.

That is why main bearings are either open or shielded to allow for lubricant to get to the front bearing. Open or shielded bearings allow for flow though of lubricant. There have been cases where the main bearing was sealed and the front bearing was not, the front bearing did not last very long. And there is nothing wrong with using a sealed front bearing knowing the limitations involved. I would not use a sealed bearing for the main bearing.

It is best to replace bearings with like bearings. It is your engine and you can do what you want to do.

Cheers,

Chip

That is the first time I have heard of replace bearings with like bearings is there a reason for it? I choose those because they were just a little lighter in weight. Granted its not much but I have always tryed to keep weight down in all my planes. And sorry you are correct on that I should have not used the term seal rather shield both the bearings were open type.


Randy