To much back pressure cause lean condition.

[pfi572]

Have a fellow that i know that has a 860 and running Dyno Port Y Pipe and Pipe with HPS can. He is running the high elevation pipe . I was told to not run the high elevation pipe and HPS as it was to much restriction with a big bore.
His is not a RKT but one that was built to the same specks out of Edmonton. The engine is torn down and i looked at the pistons and they are lean and just about seized at one time in the 350miles.
Pistons have carbon build up on the under side and have been hot. Rings flaking and piston has been rocking due to marks on the piston that you can feel.

The jetting is 480 with 20 thou shim and this is what i had ran with the RKT and 14/1 head but with low elevation pipe and different can that isn't so restricted. (No Problems with running lean and ran great)

I think the problem is to much restriction for the engine. Jet up or knock the restricter out of pipe .
He also runs strait 110 with the 14/1 head and ecm reprogramed by RKT. The guy is a fuss nut also and the machine is show quality but doesn't thing the restriction is the problem.

People in the know let me know your thoughts as i would like to know also.

 

[maxwell]

i wouldnt be running any pipe on that sled. especially if its anything like kelseys he will tell you to leave it all stock. including the can.

i will say this. hps cans lean out sleds period. thats why mine runs so good as a stocker because it leaned it out just enough rather than channging the stock jets.

 

[pfi572]

Thanks Maxwell and you are correct as this is what i found also.

 

[climbin]

If you are going to run the dynoport Y-pipe and pipe you should mate it with the dynoport can (the big one) not the smaller barker model which has too much restriction. Most of the aftermarket cans restrict too much for a bb and some too much for a stock bore. I even opened the outlet up in my can a couple mil for the 880 as per a reputable tuner. I tested the dynoport pipe against a non-modified stock pipe on the 880 and found the dynoport to pull a little better (marginal) in spite of what some builders say. Could have just been my build (could have been in my head lol).

 

[LRD]

This is what I have learned from various sources over the years including my 2-stroke software for porting and pipe design.

A tight restricted pipe builds more pressure and this pushes more heat back into the piston dome. So when you big bore, ported and the engine pumps a lot more air/exhaust through the same pipe it raises the pressure in the pipe. That pressure is also dependent on altitude. From sea level to about 9 to 10,000 ft we lose near 30% hp, so 30% less heat building pressure in the pipe.

Turbo Al at the SledShed builds the Thing silencer to work with this phenomena and increase backpressure as altitude increases. Most cans don't make as much HP as stock silencers I suspect because they restrict the pipe less. But when you increase HP you can run into the opposite problem, hot pistons, oil burned black on backside etc. Al said on a forum number of years ago optimal pressure was around 3 psi if I remember correctly.

Good Luck

 

[sledshedrevy]

over the years i have tested pretty much every brand and engine size made . in relatiionship to pipe pressure most off all of the manufactures achieve 3psi at wide open throttle and under full load on my guage.when i build systems to run on any pipe on any motor i drill a hole in the middle sectiion of the pipe and weld a piece steel brake line to it . this i attache a pressure guage .
on internal stinger pipes the need to take the pressure at the middle shows best results .
if i find the system is running at 4psi the throttle responce is noticable crisper and you will have running issues on the top end due to the over fuelling , not under fuelling ,
reason is simple if you also plumb a pressure nipple into the intake side , you will see that the vaccum signal is higher .
reason you see the burn on the piston look like it does is due to the excess fuel burning in the pipe / with egt's you will see a higher temp

once you take the sled over the measured elevation you take your readings at you will see the pressure drop and the sled run normal '
building for efi sleds in the can department is a little more difficult but once you match the numbers correct they work fine .

to loose a can runs poor at low end .and never gets any better with elevation