Maximum Streetable Compression Ratio?

I'm used to hotrodding modern fuel injected automobiles. (Gen III 5.7 Chryler Hemi) W/the proper tuning compression ratios of 10.5:1 are perfectly happy W/93 octane unleaded. This is W/full ignition advance. As long as A/F ratio is kept in the 12.8:1 range, full power is available W/O pulling ignition timing.

Compression is a wonderful thing. (within reason of course) It gives more torque throughout the power band & increases fuel economy.

When I put 10.5:1 pistons in my '67 A65 BSA, it really woke things up.

I would like to build an 850 Commando in the 10.0:1 CR range but is that practical? I can get 110 octane locally, but I want to ride in the Adirondac Mtns & don't want to be tied into using 110 octane fuel.

What do guys do W/10.1 CR Combat engines?

Can the right cam grind make higher CR streetable W/O pulling ignition timing while still maitaining good street manners?

There is bound to be a lot of opinion on this. And some of them may even have some experience and theory to back them up. I would be talking to people like Kenny Cummings, Jim Comstock and Jim Schmidt. Depending upon your budget I would be looking at Fullauto heads. The report is that they give an 850 a ton of mid-range grunt without a lot of other modification. If you haven't visited the web sites for NYC Norton: http://nycnorton.com/ or JS Motorsport: http://www.jsmotorsport.com/default.asp then you should have a look at their offerings. As well, a call to Colorado Norton Works for a chat with Matt is usually enough to get the salivary glands flowing. Another good resource for this sort of thing is Mick Hemmings in England. All of these people have experience with modifying Commando engines and have some idea of what the trade-offs are going to be.

A lot of us like to think we know what we are doing but the truth is, riding a well sorted Commando is like having sex with a willing partner. You usually aren't very objective about it until you find something that works better!

I am currently riding a Combat in stock configuration regarding everything except the ignition. I run 92 octane fuel in it and the bike runs fine. I do not have any ethanol in the fuel where I live.

Russ

As a PS, Jim Comstock aka Comnoz here on the forum was talking A:F ratios in a recent thread that I remember reading but I didn't find it when I went to look for it.

rvich said:

There is bound to be a lot of opinion on this. And some of them may even have some experience and theory to back them up. I would be talking to people like Kenny Cummings, Jim Comstock and Jim Schmidt. Depending upon your budget I would be looking at Fullauto heads. The report is that they give an 850 a ton of mid-range grunt without a lot of other modification. If you haven't visited the web sites for NYC Norton: http://nycnorton.com/ or JS Motorsport: http://www.jsmotorsport.com/default.asp then you should have a look at their offerings. As well, a call to Colorado Norton Works for a chat with Matt is usually enough to get the salivary glands flowing. Another good resource for this sort of thing is Mick Hemmings in England. All of these people have experience with modifying Commando engines and have some idea of what the trade-offs are going to be.

A lot of us like to think we know what we are doing but the truth is, riding a well sorted Commando is like having sex with a willing partner. You usually aren't very objective about it until you find something that works better!

I am currently riding a Combat in stock configuration regarding everything except the ignition. I run 92 octane fuel in it and the bike runs fine. I do not have any ethanol in the fuel where I live.

Russ

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While ethanol has its disadvantages, my experience W/it is that it doesn't promote detonation. In fact, although I know it's not comparing "apples to apples", a lot of the Gen III Hemi guys are going to E-85 as it allows CR in the 14:1 range.

My car ran just as well, maybe better when I would run @ tracks in NJ where 10% ethanol was all that was available. At that time E-10 was not sold up here on the Canadian border (it is the norm now)

My car was set up W/10.4:1 static CR W/a cam that produces 235# of cranking pressure W/a custom ground cam. Full advance did not create any detonation problems running E-10. Here is a short clip of my 1st track session W/that cam. Now 12 flat might not seem like a great accomplishment but the all up weight on those passes was in excess of 4200#.

http://www.youtube.com/watch?v=T-Sbz2Etl_A

I realize that this is not a direct comparison to an air cooled Norton but it does demonstrate that E-10 can make good power. The same car on 110 octane race fuel was a dog in the 60' BTW. That engine was set up W/.030" deck reduction on stock profile chambers & had a quench height (squish) of .035" With the stock pre ’09 5.7 rotating assembly.

I wonder if .035" quench is feasible in an 850 Norton W/O getting above 10.0:1 CR?

Boy oh boy is this an issue I'm still studying up on but can say pretty confidently that Nortons are pretty darn detonation resistant and I've crept up on using 87 no lead no ethanol in my factory 10CR 2S Combat w/o a cylinder base gasket or plate and a thin Flamering headgasket in hi heat summer time Mt. Climbs keeping up with 150 hp squad of elite moderns, and actually had to out do their accelerations as I was last in line of 6 > so the short passing zone was about used up by the time I realized my Trixie could pull it off in time > so did a number of times when the cars didn't pull over for the rush of bikes to go by. They would only hit 120 in the opens which Trixie could too but I only pulled to 115-ish in the short opens to protect the Combat crank flinging wear and they'd slow up so much for turns I could close the 100-200 yd I let them gain in the opens.
Of course they could of pulled to higher speeds faster than my Combat but non of them were crazy just out for sane grins
not racing but fun zooming in paradise like conditions. This is routine occurrence to me, pulling into rest stops with flashy plastic balloon riders telling me how cool the ole Commando was and then recoil with cocked head and sneers when asked which way they were going or if I could tag along... These fun and games require WOT to about red line in each gear - so do not think ya have to worry if racing around on 91-93 on even 10.5+ CR Norton>>> IF IF IF ya have aggressive enough cam profile installed for it.

Of course above is on pure points factory Combat, If I was on Ms Peel Combat with std small port head and 2-1 megaphone 21T+ ratio, Triple Iso Linked and rather lighter than factory issued, I'd of left them corner cripples in the dust pulling to 135 so even if they'd risk 140+ in the short 1/4-1/2 mile opens - would not of been enough to pass before they had to slow for turns Ms Peel didn't. The 900's and above did over take Peel after 90 mph when we take off from rests before much turns encountered so my Next Ms Peel will fix that short coming.

I did survey on list a few yrs ago to find only 3 situations a Norton pinged or detonated, The decades ago attempts to de-tune Combats by only putting in the std cam w/o lowering CR, some racers pressing their luck on sustained WOT heating heats and some choked up or bad spark timed road goers. We have been shown heat images of Commando head to see the hottest area is front and outside area of Exht. port, not the center of head, so racers have added an extra fin to the stack at front. Some thoughtful fin drilling can help heat flow away too.

Detonation stifler's besides HI Octane gasoline or other special fuels like Alcohol which moon shiner's got more power with than the NASCAR back when.
1. Aluminum head
2. Squish ~.035" or even bath tub filled in head flats
3. Ceramic insulating coatings
4. Late closing intake valve cam timing to dog it down low but pack it in up Hi.
5. Dual plugs with hot ignition voltage
6. Powerarc triple spark programmable ignition -with cam end stabilizer adapters or crank triggered pick up.
7. Black Body Emissions coating all over and nothing polished on heat radiation surfaces but piston & chambers

Ms Peel's blown 920 cc 10.5 CR Norris D+ [drag only] cam could see 17:1 dynamic CR so also gets water/methanol injection and exterior water spray cooling and retaining small std valve sizes [6mm K/W Black Diamonds] for the heat flow conduction to stave off pre-ignition of glow plug diesel combustion up to maybe 8000 rpm.

I run 10-1 in my Commando. No problem with premium pump fuel.

It will ping on regular unleaded at 5000 ft. or lower although you might get away with regular at higher altitudes.

I am running a fairly stiff cam. Jim

Although mine is a 750 and I believe the compression is very close to 10:1 I run the JS stage 1 cam which is suppose to be a copy of the Axtel#3.

I have ALWAYS run a 50/50 mix of 93 and 110 leaded, always.

For the first time I stopped by a Sunoco station and filled up with 93. Although it did not smell as good, it seemed to run just fine. This bike is butter smooth going 80mph at 4500. 70 is 4000 and 90 is 5000, on the noze (or very damn close). 20t sprocket with RGM belt drive. Cam comes on at 3750.

So I would say, particularly for an 850, 10:1 should be as far as you want to go and still ride the range while kicking the punks around town.

I just recently posted the research references of detecting ping and detonation in noisy engines whose normal sounds are of similar freq/tones as ping, to realize by the time we can hear it past a big IS tank sound cover and wind blast - its been ruining constantly in some light detonation anyway. So Jim or anyone, unless some new principle of combustion dynamics revealed or you put on a headphones to listen in > I believe you and me and most everyone else pressing some fun in our Norton are often running in some detonation but the dang ole Nortons take it in stride. The very best Master Engine power contesters run in constant detonation just short of blowing up. So if ya really tune and run throttle/gears to never detonate even some on pump gasoline then you ain't having much Norton fun for sure, which is a wise thing just not worth mentioning or remembering as ya age away.

comnoz said:

I run 10-1 in my Commando. No problem with premium pump fuel.

It will ping on regular unleaded at 5000 ft. or lower although you might get away with regular at higher altitudes.

I am running a fairly stiff cam. Jim

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I'm looking @ either a JS Stage 1:

.402" gross valve lift, 276° duration @ .040", 104° LSA

Or, the JS stage 2:

.427" gross valve lift, 278° durartion @ .040", 104 LSA

JS high compression lightweight pistons/Carillo rods (10.5:1 CR)

How do those grinds compare W/yours.

Riding will be in Northern New York state near the Canadian border. Rural riding in the St. Lawrence Valley, Adirondack Mtns & small towns. Seldom over 85°F, usually cooler. No bumper to bumper city riding. 93 octane 10% ethanol fuel.

Might as well get some conversation going about "quench" area on the fullauto or standard head with high compression JS pistons. Please?!!!

From JS:

Our High Compression 850 pistons have approx 10.5:1 C.R. with the deck height raised .110" higher than the original 850 piston. These High Compression pistons will drop right into your stock street motor if your head has not been milled, leaving a squish band clearance of approximately .050". Milling your head .040" with our High Compression pistons will bump the compression ratio up to approx 11.5:1, however this will require machining an outer ring relief in the top of the piston to give you the necessary squish band clearance - or instead of milling your head you can use our .003" (.1mm) thin head gasket ring. All our Racing/High Compression pistons have deep valve pockets to accommodate big-valve heads using up to 5mm oversize intake and 3mm oversize exhaust valve, as well as stock diameter valves. You can adjust compression with our various copper base and head gaskets

rvich said:

Might as well get some conversation going about "quench" area on the fullauto or standard head with high compression JS pistons. Please?!!!

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I had a conversation W/Jim @ J&S about that very thing just a short while ago. (today)

My question was brought up due to the use of either thicker head gaskets or lower deck height on the pistons to adjust CR lower.

Either would increase the already marginal .050" "quench distance. (squish) Jim opines that because the Norton head really didn't have a significant "quench area", increasing the quench height would have little effect as far as increasing the tendency to detonate. The stock Norton head combustion chamber is not a "bathtub" shape like that of the 3rd Generation Chrysler Hemis that I am familiar with.

In a 5.7 Gen III Hemi, I successfully ran .035" quench @ 10.4:1 static CR W/a custom ground cam that had moderate duration W/a very aggressive opening ramp profile & .554” lift @ .050”. That engine made 235# cylinder cranking pressure hot or cold.

http://lxforums.com/board/f71/does-235- ... gh-160689/

http://www.lxforums.com/board/f71/golly ... ff-162867/

Everyone thought I would have to pull ignition timing to prevent detonation, but I was able to run a full 36° total advance W/O any knock retard running N/A & even W/a 175HP shot of N2O, it only pulled 4° spark after shifting to 4th gear (3.23 FDR)

Unfortunately, my funds are limited & going exotic W/head/pistons to gain quench is out of the question.

The thought was to reduce CR to 9.5:1 W/medium compression (shorter deck) pistons W/the .003” copper head gasket & run the Stage 1 cam.

Jim opined that if the 10.5:1 CR did cause pinging, (perhaps less likely given the environmental conditions where I live) I could easily reduce CR via a thicker head /cylinder base gasket.

After considering the conversation I thought that the “stiffer Stage 2 cam might be a better option/the higher CR.

The higher CR will help the lower range torque curve of the Stage 2 cam while the stiffer cam would simultaneously help to alleviate some detonation tendencies while making more power up top.

So I’m thinking either 10.0:1 CR W/the Commando stage 1 J&S cam or 10.5:1 CR W/the Commando stage 2 J&S cam.

My cam has an intake closing time close to the JS stage 1.

I use a quench clearance of about .035 with steel rods. You will be looking for trouble if you go less than about .050 with aluminum rods and iron barrels. Jim

comnoz said:

My cam has an intake closing time close to the JS stage 1.

I use a quench clearance of about .035 with steel rods. You will be looking for trouble if you go less than about .050 with aluminum rods and iron barrels. Jim

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Aren't the J&S Carrillo rods steel? Not looking to go under .050" quench anyway. CR would be nearing 11:1 W/an 850.

From my experience having a standard cam in a combat engine, the cranking compression is a more important factor for pinging than compression ratio. At 182psi cranking compression on my 750 engine, it had a real problem with fuel available at the pumps. Compression ratio is a calculation of how much gas volume is compressed into the combusion chamber area. It does not account for effects of camshaft valve overlap that can reduce cranking compression.

Aren't the J&S Carrillo rods steel? Not looking to go under .050" quench anyway. CR would be nearing 11:1 W/an 850.[/quote]

Yes they are. But with his short pistons you can't go under about .050 due to piston rocking. Jim

illf8ed said:

From my experience having a standard cam in a combat engine, the cranking compression is a more important factor for pinging than compression ratio. At 182psi cranking compression on my 750 engine, it had a real problem with fuel available at the pumps. Compression ratio is a calculation of how much gas volume is compressed into the combusion chamber area. It does not account for effects of camshaft valve overlap that can reduce cranking compression.

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That's the difference between "static CR" & "dynamic CR"

Static CR is just a geometric calculation while dynamic CR takes the cam spec & even rod legth into account.

The KB Pistons website has a dynamic CR calculator.

https://www.uempistons.com/index.php?ma ... 5e1b136f18

The variables are cam timing, ignition timing, mixture, and compression ratio and exhaust system. How many of these do you intend to change at the one time? My approach is to start with standard compression ratio and ignition timing, fit a good well-known cam with sensible timings to suit the exhaust system, then jet up to that combination.

acotrel said:

The variables are cam timing, ignition timing, mixture, and compression ratio and exhaust system. How many of these do you intend to change at the one time? My approach is to start with standard compression ratio and ignition timing, fit a good well-known cam with sensible timings to suit the exhaust system, then jet up to that combination.

Click to expand...

So then you tear the engine down & re-assemble it W/the higher compression pistons & retune?

Not wanting to sound like a smart A$$ but that sytem only applies to a stock engine when the cam is swapped.

I used the calculator. https://www.uempistons.com/index.php?ma ... 5e1b136f18

It won't be 100% accurate as it require .050" lift duration values & the cam specs are @ .040" lift so accurate dynamic compression would be a bit higher. How much would an additional .010" lift spec skew thye results?

Assuming the cams will be installed "straight up", I took the duration, subtracted LSA/2 & added 15° as instructed. Is that the correct way to calculate intake valve closing W/that calculator?

I came out with:

8.6:1 DCR for the 10.5:1 SCR/Stage 2 cam
8.3:1 DCR for the 10.0:1 SCR/Stage 1 cam

Would the "professional" engine builders please chime in & tell me if I'm doing this right & whether either of those combos would be streetable?

My point is don't do everything at once. Progressive development is better because you can observe the effects of the changes. It doesn't matter how you change the internals, you still must carburete for your setup. My feeling has always been that race cams and exhausts give the motor an easier time as far as combustion temperatures are concerned, however you have to tolerate the more well defined power band. I think the Jim Schmidt rods and pistons are a really good idea. The light piston weight will give a performance boost, and add reliability. The longer rods give a better rock-over time with little loss of angularity.
As far as pulling the motor apart is concerned, I don't enjoy it however it is a fact of life when you race a bike. I never speculate when making changes - if the bike loses performance when it has been jetted correctly, I take a step backwards, and reverse the major change. It sometimes happens that you correct an error with an error.