Air cooled motor cylinder pressure for pump gas?

[atvmxr]

The motor in question is a blaster, but I figure the air cooled pre 85 atc and etc should be on same principle.
What is the cylinder cranking pressure you need to run race fuel? I know on the water cooled motors you can run in the 165 -180 psi on pump gas. The blaster motor I have had since 1991 from a former national mx racer. Motor ran strong, no issues with denotation or burning up pistons. Very reliable. Fast forward twenty some year and the big crank rod bearing lets go and smashes the head up good. I sent the head out for repair and the shop said the head was shaved down way too much and the pressure would be in the 165 psi range and totally un reliable for an air cooled motor and should be running 125 psi. If my water cooled trx was running less than 150psi, I would think the motor needs rebuilt.

Is there really that much difference between the air cooled and water cooled motor operating pressure???

[barnett468]

Hello atvmxr




The motor in question is a blaster, but I figure the air cooled pre 85 atc and etc should be on same principle.
What is the cylinder cranking pressure you need to run race fuel? I know on the water cooled motors you can run in the 165 -180 psi on pump gas. The blaster motor I have had since 1991 from a former national mx racer.

He was running race gas if he was a pro guaranteed. He could not be competitive without it.

Honda says 170 for 93-100 octane on the 81 and 82 air cooled atc 250r’s. See orig ad below.

http://www.3wheelerworld.com/content.php/180-ATC250r-Technical-Data

My experience is 170 on an air cooled and 180 on a properly cooled water cooled engine and Honda is stretching the limit with 170 cylinder cranking pressure on 93 octane in around 80 degree weather on their air cooled 250r's. Others experiences will vary.








Motor ran strong, no issues with denotation or burning up pistons.

It was likely detonating if you ran regular gas you just didn't hear it. Detonation is NOT always audible.







Very reliable. Fast forward twenty some year and the big crank rod bearing lets go and smashes the head up good.

Some of the causes can be, cumulative detonation, big bore kits, bad rod rebuild, rod replaced with non original type rod and/or poor quality oil.

What oil were you running exactly?

Is the rod orig type, what does it say on both sides?







I sent the head out for repair and the shop said the head was shaved down way too much and the pressure would be in the 165 psi range

Its impossible and irresponsible for them to make that statement without knowing exactly what the cyl bore, stroke and exhaust port height is unless they are psychic.







and totally un reliable for an air cooled motor and should be running 125 psi.

Not according to my experience as head of Kawi R and D and being a professional racer.







If my water cooled trx was running less than 150psi, I would think the motor needs rebuilt.

LOL.







Is there really that much difference between the air cooled and water cooled motor operating pressure???

No, maybe 10 psi as mentioned above. Others experiences will vary.

[atvmxr]

Thanks for the detailed reply. I was disappointed in the builders response as he is supposed to be very reputable but I thought he didn't know what he was talking about. So I checked here. He had the cylinder too as I was getting it bored also.

As for the blaster motor, at some point the rod bearings are going to fail regardless. I'm more than pleased with a twenty year service life. We run belray mc-1 at 50:1 with 93 octane, if we were running 32:1 would start adding race fuel to the mix.

[barnett468]

Hello atvmxr






Thanks for the detailed reply. I was disappointed in the builders response as he is supposed to be very reputable but I thought he didn't know what he was talking about. So I checked here. He had the cylinder too as I was getting it bored also.

Well, since he had the cylinder it is possible for him to calculate your cranking compression if he’s smart enough so you need to ask how he arrived at the 165 number to see if it’s correct. As far as recommending 135 cranking pressure you can see he’s incorrect.







As for the blaster motor, at some point the rod bearings are going to fail regardless. I'm more than pleased with a twenty year service life. We run belray mc-1 at 50:1 with 93 octane, if we were running 32:1 would start adding race fuel to the mix.

Bel Ray was developed in the early 70’s and has not been changed since. There have been many improvements in 2 stroke oil since then, many that would extend the life of your rod. I was sponsored by them in 74 thru 76 and have nothing against them but simply think there are better oils out there these days. If you run hard all the time with 0 trail riding then Castrol XR77 is the best and Castrol A747 is the next best. Both are used in 16000 rpm moto gp race bikes, lol. If you trail ride mostly Maxima 927 is probably the best.




SHELL ADVANCE RACING M designed for alky and cart racing at 9000 rpm. $14.00 a qt.

http://www.opieoils.co.uk/p-69340-shell-advance-racing-m-2t-sae-30-kart-oil.aspx



Castrol A747 used by the 16000 rpm factory GP bikes, lol. This should work but you can’t run under 3000 for long or your plugs will load up. Available on ebay. Should call to Check compatibility with methanol vs Castrol M $23.00 a qt

http://www.castrolmoto.com/en/our_oils_new/a747.php



Castrol M for use with methanol $25.00 a qt

http://www.bikebiz.com.au/products/Castrol-M-Racing-Oil-5-Litre.html



CASTROL XR77 The latest and greatest, check for methanol compatibility ONLY $43.00 A QT!

http://www.easyparts.nl/documentation/3387.pdf


MAXIMA 927 $20.00 QT.

http://www.maximausa.com/shopping/index.php?main_page=product_info&products_id=13


KLOTZ SUPER TECHNIPLATE $8.00 QT I WOULDN'T USE THIS

http://www.powersportsplace.com/parts/klz-kl-103?seid=pspse1&gclid=CNSo5bKDmbgCFeRZ7Aod8E4AOw

[atvmxr]

Yes, at the time of our conversation when he said I should be running 125psi he made it clear that he hadn't checked the porting. All he was going off of how much the head was shaved.


Oh well, at least I know not to use them in the future.