Compression ratio

Hello,
§I have a little problem i have two high compression Dunstall piston and a head steady Combat. If i fit them what is the real compression ratio? It 's for my racing bike 750, crankshaft balancing 65% %, camshaft PW3, head steady stock Combat.

For me i think the compression ratio are too high and i think fit a thick copper head steady gasket but what sthikness?

Can you help me?

Gilles

if everything clears to run then higher CR helps add another few percent detectable-pilot felt power and better lower rpm response so then its just a matter if enough octane not to detonate on best torque ignition timing. Online calculators can ease solving for static CR and then other calculators with cam profile data to get effective/dynamic CR for octane consideration and ignition timing tolerance.

The conventional ring piston Dunstall supplied for the 750 was a nominal 10:1 compression ratio, and the racing piston with the Dykes top ring was 10.5:1. Trying to use either one with a RH3 Combat head (.042" skimmed off the standard head) will raise the CR even higher, certainly too much for street use with pump gas. You might also find that you don't have enough squish clearance between piston and head with this combination. You need at least .040" squish clearance. A quick calculation shows that using the Combat head in place of a standard head will increase a 10:1 compression ratio to 11:1. The actual CR will vary, depending on whether you use a base gasket, which head gasket you use, and whether the head or cylinder have been skimmed any additional amounts to true them up. In any case, I don't think 11:1 static CR is going to work on pump gas, even with the PW3 cam to help reduce the dynamic compression ratio. With the standard flame ring head gasket and no base gasket, you'd have to use a base gasket around .050" just to get the CR back down to 10:1. That's .042" to make up for the Combat head skimming and .008" to make up for leaving out the base gasket. Or some combination of head and base gasket (like .040" head gasket and .040" base gasket) to accomplish the same effect. To get below 10:1 would require even thicker head/base gasket combinations. Final choice depends on the grade of gasoline you will have available where you are riding.

Ken

Hi Gillou, let assume you have 10/1 Dunstall pistons, I think you mean you a Combat cylinder head (forget the head steady, I think you had mismatch the translation!!), so you should have sometning like 11/1 , just with my rule of thumbs ( 1mm equals one point of ratio) , I am french too so maybe my english is not good enough ....so add a one mm thicker gasket, take care too with your valves kissing the piston heads .........Pierre

hi , sorry Ken , i do not see your post ........

marinatlas said:

hi , sorry Ken , i do not see your post ........

Click to expand...

No problem, Pierre. Your rule of thumb gives pretty much the same answer as my calculation.

Ken

Yes, thanks Pierre i make an error in the word... Cylinder head not head steady !

I have'nt enough time to prepar my standard cylinder head then i think with your answers and counsel, i'am going to do this :

* To reduce the CR i put this, under the cylinder base :

http://www.rgmnorton.co.uk/buy/barrel-base-de-compression-plates-750-850-030-75mm_147.htm

it is 0.75mm thick (0.030") and a copper head gasket :

http://www.rgmnorton.co.uk/buy/750-head-gasket-copper_115.htm

It's 1mm thick (0.040")

Whit that the CR was 10.25:1 and i must check the squish clearance to have .040" and use an additive gasoline to have the good grade.

Is it righ ??

Thanks.

Gilles

For any race bike where you are bringing in new pistons, cams, who knows what to the cylinder head in 40 years you should do a dummy assemble and check all clearances valves, pistons, squish, valve coil bind, valve spring pressure lbs over the cam lobe and static etc. You can actually measure the real CR when you have it dummied up. It probably won't be the same each side.

Standard for a race bike.

If you dont do this then your shooting in the dark.

Racing means doing it properly.

FWIW, raising the compression results in much less power increase than is commonly believed. For example, a one point increase in compression - say from 9:1 to 10:1 is worth about 2 (TWO) HP on an otherwise stock Commando motor...

Obviously, for racing, every last HP counts but many times folks increase compression thinking they are going to get some major difference in power when, in reality, there is rather little...

We ran comps as high as 13.5:1 on some competition motors and there are certainly benefits but, again, they are not as much as is often believed.

There is quickly decreasing power benefit as CR goes up so mainly hi CR helps an agressive over lapped cam to idle and response to throttle down low w/o just bogging out. Likely 2 identical Nortons, one with 1 pt more CR might get a bike length ahead in a straight to enter a turn with 1st choice of line through it so pretty meager advantage for sane folks : )

compression from a lower to a higher level.

Original CR top vs New along vertical for expected increase of % power
CR 8:1 9:1 10:1 11:1 12:1 13:1 14:1 15:1
9:1 3.5
10:1 6.5 2.9
11:1 9.2 5.5 2.5
12:1 11.5 7.7 4.7 2.1
13:1 13.6 9.7 6.6 4.0 1.9
14:1 15.4 11.5 8.3 5.7 3.5 1.6
15:1 17.0 13.0 9.8 7.1 4.9 3.0 1.4
16:1 18.6 14.5 11.3 8.6 6.4 4.4 2.8 1.4

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Hi Gillou , PM sent........

gillou said:

Yes, thanks Pierre i make an error in the word... Cylinder head not head steady !

I have'nt enough time to prepar my standard cylinder head then i think with your answers and counsel, i'am going to do this :

* To reduce the CR i put this, under the cylinder base :

http://www.rgmnorton.co.uk/buy/barrel-base-de-compression-plates-750-850-030-75mm_147.htm

it is 0.75mm thick (0.030") and a copper head gasket :

http://www.rgmnorton.co.uk/buy/750-head-gasket-copper_115.htm

It's 1mm thick (0.040")

Whit that the CR was 10.25:1 and i must check the squish clearance to have .040" and use an additive gasoline to have the good grade.

Is it righ ??

Thanks.

Gilles

Click to expand...

That sounds about right, but remember that the 10.25:1 is just an estimate. As johnm pointed out, the only way to be sure is to measure everything and calculate the real CR. I missed your comment that this was a race bike, not a street bike, so you can ignore anything I said about pump gas. As long as you use a decent grade of race gas, you can run as high a CR as you wish. Even with the spacer, you should still check for squish clearance, valve-to-valve clearance, and valve-to-piston clearance. Since you are running a race bike, I assume you are already familiar with all that. If the Combat head still has standard size valves, you shouldn't have a valve-to-valve clearance problem, and the Dunstall pistons have valve notches meant for race cams, so you should be OK on valve-to-piston clearance, but you really need to check to be sure. Good luck at the track.

Ken

Hi, check too the pushrods lenght when fitting the alloy base gasket , specially if they had been shortened previously for the combat head.........just tell me if you had received the Pm , I am not sure to have sent it properly (getting old......!)

lcrken said:

That sounds about right, but remember that the 10.25:1 is just an estimate. As johnm pointed out, the only way to be sure is to measure everything and calculate the real CR. I missed your comment that this was a race bike, not a street bike, so you can ignore anything I said about pump gas. As long as you use a decent grade of race gas, you can run as high a CR as you wish. Even with the spacer, you should still check for squish clearance, valve-to-valve clearance, and valve-to-piston clearance. Since you are running a race bike, I assume you are already familiar with all that. If the Combat head still has standard size valves, you shouldn't have a valve-to-valve clearance problem, and the Dunstall pistons have valve notches meant for race cams, so you should be OK on valve-to-piston clearance, but you really need to check to be sure. Good luck at the track.

Ken

Click to expand...

Hello Ken,

Yes it's a standard Combat head with standard size valves. I'm going to fit my engin with all those parts and check all to be sure.

You say : "Since you are running a race bike, I assume you are already familiar with all that" it's the second year i running with race Commando then i ask questions and i learn !!!

Gilles