Racing Commando 750

Champy72 · Jan 23, 2020

Hi all.
The 850 to 750 Commando Mk 2 conversion is nearing completion, and the engine rebuild is just around the corner.
Question 1. Originally, the previous owner (920 engine) had set the Boyer to 35 deg. BTDC. I used the same on the 850. However, Boyer and Norton manual state 31deg BTDC. The unit has PWC (?) crank, P3 cam, thunder rod con rods, Omega pistons and a Maney stage 2 head. Boyer suggest that moving timing more towards 35 may be better for the engine. Any comments on this?
Question 2. I used to race the 850 at 6500rpm, and up to 7000rpm on the odd occasion!!.
What difference will there be with the 750 unit?

Thanks in advance!

Danno · Jan 23, 2020

I've always set my Boyers at 32 deg btdc. That's 4 deg advanced over dual points-and-coils. 35 sounds a bit much, but my 850 motors are nearly stock spec.

850s are more of a flywheel motor and run out of pep before redline, whereas 750s like to rev more. Of course, I'm speaking of stock or nearly stock spec, not a built race motor. I would imagine the two different cranks still display some of the stock engine's characteristics, depending on modifications.

kommando · Jan 23, 2020

Book a dynometer and have race fuel to answer your timing question plus a few jets.

Bernhard · Jan 23, 2020

kommando said:
Book a dynometer and have race fuel to answer your timing question plus a few jets.

I agree with Kommando, your race engine should be set up to its own individual requirements, you may have to adjust setting here or there that the book does not cover

oldmikew · Jan 23, 2020

Champy72 said:
Hi all.
The 850 to 750 Commando Mk 2 conversion is nearing completion, and the engine rebuild is just around the corner.
Question 1. Originally, the previous owner (920 engine) had set the Boyer to 35 deg. BTDC. I used the same on the 850. However, Boyer and Norton manual state 31deg BTDC. The unit has PWC (?) crank, P3 cam, thunder rod con rods, Omega pistons and a Maney stage 2 head. Boyer suggest that moving timing more towards 35 may be better for the engine. Any comments on this?
Question 2. I used to race the 850 at 6500rpm, and up to 7000rpm on the odd occasion!!.
What difference will there be with the 750 unit?

Thanks in advance!

oldmikew · Jan 23, 2020

Am puzzled that Boyers are suggesting so much advance... would expect it to kick back ... Why not ask Steve?

olympus · Jan 23, 2020

Original fuel spec was i believe 98 Ron (4 star in the UK) , regular unleaded is now 95... how much affect is this having with regard to ignition timing advance settings??

jseng1 · Jan 23, 2020

The high advance is for low compression engines. More efficient and smaller combustion chambers with higher compression use less advance around 28 deg.

Champy72 · Jan 23, 2020

No kick start on this monster! Have to start with driven rollers... If you mean talk to Steve Maney, he retired to Thailand about a year ago.

Champy72 · Jan 23, 2020

olympus said:
Original fuel spec was i believe 98 Ron (4 star in the UK) , regular unleaded is now 95... how much affect is this having with regard to ignition timing advance settings??

Race fuel is 102 leaded, or 50/50 Avgas and 98 unleaded.

Champy72 · Jan 23, 2020

jseng1 said:
The high advance is for low compression engines. More efficient and smaller combustion chambers with higher compression use less advance around 28 deg.

Now that is interesting! The 750 certainly has higher compression ratio. Believe I'll take it to a dyno and check it out. Thanks all.

Dances with Shrapnel · Jan 23, 2020

You've apparently sunk some time and monies into this build and if you race it these expenditures will be the tip of the iceberg. Install an RTD crank triggered ignition and set yourself up to get all there is from the motor. Get it to an eddy brake dyno with a good operator who can tune your engine to the current configuration and map the ignition.

Boyer ignitions are fine for most applications and I have raced with them before but in my opinion, the boyer ignition curve is compromise with regards to engine performance and an over glorified starting ignition retard; not much different from an AAU.

oldmikew · Jan 23, 2020

Champy72 said:
No kick start on this monster! Have to start with driven rollers... If you mean talk to Steve Maney, he retired to Thailand about a year ago.

Presumably he has an Email.... I also have a Maney stage 2 head ,but nothing was ever said by Steve about it needing more advance. Personally I would start at about 28 and that might be too much given the cam and the increase in compression . It will do no harm and its easy enough on a Boyer to make incremental changes to your timing.

acotrel · Jan 23, 2020

When you tune your motor, there are five things to consider - compression ratio, jetting ignition advance, exhaust system type and the sort of cam you are using. Cam and iexhaust system set the power band and torque characteristcs. The other three are a balance and are affected by the fuel you intend to use. I usually choose the compression ratio, and ignition advance, then jet to suit in practice sessions. I try to never change the ignition advance once I have jetted the carbs. If the advance changes, you need to check your carburation. Changing the ignition advance or the comp. ratio or the exhaust system affects the jetting.
The actual value I choose for the ignition advance depends on the fuel. For methanol in a low comp motor, I add 4 degrees to the recommended 29 degrees advance used for petrol. If I was using high octane petrol, I would probably add two degrees. But it does not really matter because when you jet carefully, the result is pretty much the same. At higher comp., you use more fuel - that is where the extra power comes from. But you usually use less advance, because of the knock-rating of the fuel.

Champy72 · Jan 23, 2020

oldmikew said:
Presumably he has an Email.... I also have a Maney stage 2 head ,but nothing was ever said by Steve about it needing more advance. Personally I would start at about 28 and that might be too much given the cam and the increase in compression . It will do no harm and its easy enough on a Boyer to make incremental changes to your timing.

I emailed him not long after he set off to Thailand. Never received a reply...

Chris · Jan 23, 2020

Ian
F*** 35'
It's all about the compression.
Mine eat the engine at 31 I have run 28 for 3 years although Steve did say I needed more than avgas to run at the original compression. It's lower now.
Chris

Champy72 · Jan 23, 2020

Chris said:
Ian
F*** 35'
It's all about the compression.
Mine eat the engine at 31 I have run 28 for 3 years although Steve did say I needed more than avgas to run at the original compression. It's lower now.
Chris

Thanks Chris. I will be running 102 leaded race fuel this season, but from all the threads so far, I definitely need to reduce timing...!

acotrel · Jan 23, 2020

It is usually expensive to lower the compression, so retarding the ignition is the way to go. The last thing to do is re-jet the carbs.

acotrel · Jan 23, 2020

My 850 has never run out of power below 7000 RPM. It is very difficult to avoid over-revving as I accelerate up through the gears. If you are using a 2 into 1 exhaust and you lose top end, the tail pipe might be too small in diameter. The cross-sectional area of the tail-pipe needs to be double that of one of the header pipes. However, noise can become a problem.

oldmikew · Jan 23, 2020

Chris said:
Ian
F*** 35'
It's all about the compression.
Mine eat the engine at 31 I have run 28 for 3 years although Steve did say I needed more than avgas to run at the original compression. It's lower now.
Chris

What were you running ? Much more than 10.1 ? Many years ago holed a piston on a Dommy ... it was only a few degrees overadvanced on a magento. My thoughts at the time were that the damage was done not at 6000 or so rpm but at 3,000 when the engine would be proportionately more and fully advanced.

Racing Commando 750

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