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Author Topic:   MOTOR PROBLEMS!!!! NEED HELP QUICK
DOUBLE BARREL
Dirt Roller

Total posts: 15
posted August 01, 2005 12:14 AM  
Here is the problem...The last three motors my dad has run have washed out the block and head between #5-7 cylinders...He sets the total timing by light at about 28-30 degrees.....I told him to set it at 36 and leave it there...These are machine shop built motors and we have had good luck until this...He runs a 350 with S/R torquer heads 275 dome with Holley 750 vac. secondary (76 Jets) (89 octane)...290/550 lift solid cam turing around 6800 rpms...Temp runs about 230 during feature..The last motor had double hump heads but every thing else was the same...The carb could be a little lean I guess but the exhaust never get cherry red as I have seen some other racers... Is it possible that the timing is too slow?..Any help or suggestions would be appreciated..I am getting tired of donating heads to this cause...Thanks for any suggestions..


waltonjr1
Dirt Forum Champ
Total posts: 516
posted August 01, 2005 04:56 AM  
89 octane could be a problem. Being your running domes you must have some decent compression unless your running open chamber heads.


fast_crew
Dirt Forum Racer

Total posts: 99
posted August 01, 2005 02:59 PM  
those S/R torquers should be 67cc and with domes i would bet some where close to 12.5- 13.5 comp. way to much for 89 pump gas. and with racing fuel you could run 36*timing. Also make sure you timing marks on the balancer are correct.
Josh K.

[This message has been edited by fast_crew (edited August 01, 2005).]

way2fast72
Dirt Freak

Total posts: 303
posted August 01, 2005 04:22 PM  
to much compression,not enough octane.gotta run good fuel.

DOUBLE BARREL
Dirt Roller

Total posts: 15
posted August 01, 2005 04:50 PM  
I believe the heads are 76 cc from the factory...They have been milled just enough to clean them up...Compression is about 180 per cylinder..Is it possible that the timing could be too slow?....He was running aviation fuel but in the last 3 motors he has ran 89 octane....The timing and fuel is the only thing I can think of that may be different...

Raz_900
Dirt Forum Champ
Total posts: 635
posted August 01, 2005 07:44 PM  
The timing 'being slow' is the only thing keeping it from blowing to pieces the first time it's under load. With those heads and pistons, the CR is about 10.7:1 You need at least 100 octane to keep it alive. Keeping running 89 and it'll never stay together.

Less timing than the motor wants will kill some power, make it run hotter and rev slower but won't make it burn stuff up. In my supercharged 3.8 in my T-bird SC I run about 20-22 max on pump gas and 12 lbs of boost. On 104 it can handle 26-28 and 15 lbs without pinging like mad. So timing is relative to what the motor wants and, within reason, having less than optimum won't make a motor detonate. You'd have to be at 15 or less before it would do much damage.

DOUBLE BARREL
Dirt Roller

Total posts: 15
posted August 02, 2005 07:52 PM  
I don't want to sound to stupid but when you run that low of octane gas how does it actually contribute to tearing up the motor ?...

kylejs2005
Dirt Roller

Total posts: 5
posted August 02, 2005 09:33 PM  
Yea them boys are right, you can get by running octane like that if you don't run domes but with all that compression you have no octane to back it up

Raz_900
Dirt Forum Champ
Total posts: 635
posted August 03, 2005 08:00 PM  
quote:
Originally posted by DOUBLE BARREL:
I don't want to sound to stupid but when you run that low of octane gas how does it actually contribute to tearing up the motor ?...

Octane is actually a measure of the detonation resistance and burning characteristics. A low octane fuel will burn faster and less controlled than a high octane fuel. So, 89 burns quickly and relatively uncontrolled. 110 leaded race fuel will burn more slowly and controlled which actually requires a higher CR for optimum results. Running 110 race fuel in a 9:1 motor is throwing money down the drain since 93 pump fuel would actually make more power since it would burn more completely. Just like running 87 or 89 in a 10 or 11:1 motor would be disasterous since it would ignite too quickly (detonate) and burn something up (head gasket, valve, piston, block, head casting etc) It's actually a tuning item to match the octane rating of the fuel to the compression ratio, cam specs, cylinder head design and timing. Cranking compression gives an idea of octane required. Generally, over 180 PSI cranking compression requires something more than pump gas.

Vortec heads can get by with a little more CR without needing high octane fuel because of the fast burn style chamber. The World SR Torquers are better than all stock heads except the Vortecs in chamber desing and 76cc smog heads are horrible in that respect and would need more octane everything else being the same.

Quench has a lot to do with it too. Much over .070"-.080" quench and you have to run about 1 point lower CR. Example, a 10:1 CR motor with .040" quench distance would probably get by on 93-94 pump fuel. Drop the piston down the hole for .080"-.090" quench distance and it would detonate under load on the same fuel.

So... to sum it up... running 89 in a 10.7 motor with domed pistons, that actually make the chamber shape worse, will detonate severly under load. How it doesn't sound like someone's beating on the block with a sledgehammer under load is beyond me.

Good luck
Steve

[This message has been edited by Raz_900 (edited August 03, 2005).]

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