I was told by my local shop that my compression is 205 PSI cold and 220 hot. They said that it is really high. I was wondering if this is true and what would be considered normal. I have the Stage II big bore kit with the SE 255 cams. Thanks in advance.

 

Hmm! With a Stage II big bore kit and cams,I wonder why they would say that?? Any problems cranking over when hot? If not,don't worry about it.

 

I am no expert, but I'll repeat what I have been told.

If they were talking about CCP (cold ranking pressure), they are technically correct. I believe 195-200 is considered slightly high, over 200 is definitely high. Obviously it will be higher when starting a hot engine.

The SE255, along with many EPA cams, have a very limited overlap for emissions. This contributes to high CCP numbers.

I have a '12 103 with the SE255s and don't have a problem... but I do have the auto compression releases. I forgot the year, but the 103s started coming from the factory with auto compression releases.

If your starter is labored during hot or cold starts, you may want to add compression releases

... or ....

Go to an aftermarket cam with more overlap to bring down the CCP.

FYI: Cam overlap
Stock HD injected cams 07-13 have -8.0°
SE255 cams have 13.0°
Woods 222 (similar low end torque to SE255) cams have 28.0°

The Woods 555 or 777, and a lot of high performance cams, have over 40.0° of overlap.

If I remember all this wrong, I'm sure someone will correct me...

If your bike starts fine.... just enjoy it!

 

Thanks for the responses. It was a poorly worded question. What I should have asked was how much higher than normal is 205 psi. At any rate, I'm having the same issue with hot starts that others have reported. Having compression releases installed and was just wondering what the numbers actually meant

 

What was the compression ratio of your big bore kit pistons ? If it runs good with no pinging just install some compression releases .

 

180 psi is a good target for a street tuned motor. I built my motor to 195 psi cold, and it was a little tricky to tune. Higher compression can lead to detonation and hot start problems. My ideal motor for a touring bike would be large displacement (113 or larger), and relatively low compression.

 

no worries, my tuner told me mine were running 215-220. already had cr, he also told me if I wanted to add 0.01 to my HG which i did,he included another tune in the flash drive but told me 0.01 with map on the bike was fine and he would leave it alone. just make shure your battery,starter and CR are in top condition and put some fans on it imo

 

"180 psi is a good target for a street tuned motor. I built my motor to 195 psi cold, and it was a little tricky to tune"'


+2


High cranking pressure also indicates high cylinder pressure when the engine is in service (dynamic compression). This raises octane requirements, all else being equal.


The overlap of the cams used is certainly a "player" here, but really doesn't factor into the deal for comparison purposes, unless you are comparing all of them at the same specifications. Cams manufacturers typically provide .050" specs, and "advertised" specs as well. The lobe positions play a role here as well, and the placement of the cam in the engine, or ICL. As cams get bigger in duration, they may pull the lobes together some, or tighten up the LSA. They may also grind them to end up with advanced ICL.


The whole deal can be a "bundle of snakes" when it comes to increasing CID and compression. Having over 220psi cranking pressure certainly tells me that the engine is pretty high for static compression ratio, and/or the cams are pretty small as well.


For the most part, to keep cranking compression in check we need to add about 10 degrees duration for every full point of compression ratio increase, assuming the same LSA was used. Most very well thought out combo's designed to manage pump fuel only will end up around 170-180psi cranking pressure, but those numbers are just representative, they can be lower or slightly higher and the engine combo still be a complete success on pump gas only. Tuning comes into play here, and the ideal engine combo will make optimum power and still able to use a "normal" tune in it. If it requires more fuel added and timing pulled out to be happy, then it's simply not got enough cam in it for the compression, and giving up some power that could have been had with better parts choices.


The bottom line here is that the end user is happy with it, and you don't "buck" the starter on hot restarts, and it doesn't run hot/overheat/detonate on pump fuel, and for absolutely sure you will REQUIRE compression releases for that kind of cranking pressure!.......Cliff

 

205 is not unusual with 255s. Manufacturing/assembly differences in piston height and/or chamber cc along with the early intake close of 255s can add up to pretty high ccp. One of the many reasons I think 255s are a poor choice in 103 up motors. This is why proper measuring, part selection and assembly is important in engine building. 205 really isn't a problem with a proper tune and compression releases (I'm running 207) but 220 is getting up there. I recommend swapping to a proper cam if you are unhappy with what ya got.

 

=========================

For comparison stock compression is roughly 120-140psi depending on motor etc so yes even approaching 200 psi cranking comp is high let alone 220psi hot especially high for a motorcycle motor.

Your motors high comp is caused by too short of cam timing/duration/overlap which was due to improper choice of cam vs compression & pistons in the motor cam was going into.

If your having hot start issues due to high comp you have some choices ,you can install compression releases or if you need to remove heads to do that another way to approach it is to install a much heavier duty batter then stock 385cc with one that got a more healthy 500cca or 600cca along with higher capacity battery cable at the same time to ensure the additional cca from more powerfull battery can be fully utilized by starter.

That should help some but if it doesnt fully get ride of hot start issue then its time for a hvy dty HI TRQ starter ,there are plenty of them at decent prices way cheaper the hd starters on Ebay /amazon etc.

The getting a new hvy dty battery,hi capacity battery cables,hi trq starter may seem like a lot but compaires to removing heads those items are much easier to install yourself vs maybe having to take bike to shop to have heads removed ,drilled & tapped for compression releases.

And you dont want guys tearing appart a good running non leaking motor if you dont have to so if i were in your shoes and did have hot start issues due to very hi compression like your motor has i'd go this route vs tearing into motor for comp releases.

If motor was coming appart for a build thats a diff story and i have the comp releases installed at that time but that isnt the case with your motor.

Lastly is you could either install new cams with considerably more duration to reduce cyl / reduce compression, you could install lower comp pistons ,could install heads with larger combustions chambers to reduce compression along with thicker or head gaskets designed to be stacked to redcuce compression if they mfg them for HD's like they do for perf car motors.

Another issues with your motors very high 200-220psi cold/hot cranking compression brings up another issue thats being overlooked.

With 200-220psi compression i would not be surprised if that thing is detonating on 93 pump fuel (may not hear it with loud pipes) which can cause damage or running hotter then normal in which case if you dont have an oil cooler " with elec cooling fans " like an ultracool oil cooler does you should get one along with Wards cooling fans too cool heads and upper cyls too.

An oil cooler are without thermostatically controlled elec cooling fans are just about useless,thats because an oil cooler without elec cooling fans that provide the additional airflow required thru oil cooler to better cool oil & motor in traffic and or at slow road speed where there is little to no airflow going thur the oil cooler without elec cooling fans that greatly reduces the oil coolers efficiency just when the additional cooling is needed most.

But working Collectively together as a cooling team the Wards elec cooling fans and an Ultracool oil cooler with elec cooling fans reduced oil temp on hot days on my stock 09 motor by 50+ deg which resulted in multiple positive improvements right off the bat that any bike inc yours would benefit from.

Oil running 50+ deg cooler on hot days on my bike resulted in significantly increased oil pressure @ idle & cruise , motor stopped detonating under load, valvetrain was much quieter esp when hot, motor didnt loose as much power on hot days retaining more pwr/trq & crisper throttle response, little less heat coming off motor onto rider to bit more comfy on hot days, was able to go back to the less expensive 91 octane fuel HD recs for my motor vs the more expensive 93 fuel i had to run just to reduce but not stop detonation on hots days under load,etc.

And since your motor has very high 200/220psi cold/hot cranking compression even if those 2 cooling mods didnt cool the oil 50+ deg like in my stock motors case with less compression and cooled your motors oil by 35-40 deg (instead of 50+ deg like in my case) that would still be well worth it IMHO.

Anything you can do to significantly reduce oil temp also reducing motor temp at same time on a lrg disp HD motor on a hot day is a good thing.

Thats because motor oil is a major contributor too cooling an air cooled motorcycle motor and when the oil is overheated (due to very hi compression) enough to sig reduce oil pressure that shows its viscosity is being reduced which degrades the oils ability to do its job in protecting/cooling/lubing the motor.

Good luck.

Scott