new blasters compression psi???

[Budget_Blaster]

Ok i put the gf's motor together torqueing it all with a freshly calibrated snap on torque wrench and heat cycled it 3 times and then did a compression test and i got 137psi.

I dont remember what it was before i pulled this topend but with a no frills build a wiseco .050 fmf gnarly/dg silencer jetted carb and boysens is 137psi ok?i know 120 is what the manual calls for but with my mods should i have more?

I used cometic gaskets from hell bound racing and as i said i did the heat cycles and then a compression test before a short ride up and down the road never getting past 3rd gear.

So what you guys think 137psi good and where i should be considering the build or should i be concerned?its starting easy and idles no problems so far but the 137 seems low to me.

 

[Awk08]

did you re-torque the head bolts after the heat cycles, whats a leakdown test reveal, and without a modified head, i believe thats a normal psi.
(not positive on a bbk though)

also, a compression test should be done with the motor cold, holding the throttle wide open, kick it till the needle stops on the tester

 

[Budget_Blaster]

did you re-torque the head bolts after the heat cycles, whats a leakdown test reveal, and without a modified head, i believe thats a normal psi.
(not positive on a bbk though)

also, a compression test should be done with the motor cold, holding the throttle wide open, kick it till the needle stops on the tester

No bro this aint no bbk its a 94 stock jug and head punched out to .050

Yea i retorqued every time i heat cycled

And yea it was cooled down but not stone cold held open the throttle pulled tether switch and kicked til the needle stopped and it stopped at 137psi.

 

[Budget_Blaster]

And no leak down test done dont have the means to do so.

 

[245stroker]

Once the motor's broke in it should come up another 5-10 psi. Where you're at right now seems fine, imo

 

[Budget_Blaster]

Once the motor's broke in it should come up another 5-10 psi. Where you're at right now seems fine, imo

The topend in question is off my blaster before the 240bbk and had roughly 4 hrs on it id think it was broke in.outside of the compression it runs awesome starts right up idles like a champ.

Where are the rest of you guys at compression wise with similar rebuilds?

 

[Awk08]

Where are the rest of you guys at compression wise with similar rebuilds?

i was right around that same psi, with kens modded dome, no milling head, which is what i'm running now,
my 162 psi oconnor modded/milled head will be going back on over the winter

 

[Budget_Blaster]

Well good i wont worry about it then i just thought that was a bit low but obviously i was wrong.Good to know.

While on the subject is my 167psi on my 72.5mm bbk about right above average or what?

Thanks.

 

[Awk08]

Well good i wont worry about it then i just thought that was a bit low but obviously i was wrong.Good to know.

While on the subject is my 167psi on my 72.5mm bbk about right above average or what?

Thanks.

i'm not positive just a bbk cylinder will give you more compression,
i thought a milled head was what determined that ???
but 160+ sounds about right for running pump gas, any more may require higher octane race fuel, or a mix of it and pump gas, to raise the octane level to avoid detonation

both your compression readings sound good to me
now, do you need to purchase or borrow a leakdown tester ???

 

[super noober]

137lbs is decent for a stock jug, im running 180lbs+ with my BBK

 

[yfz450bence]

stock head anything over 120 will be just fine. youve got good compression, after 2-3 heat cycles the compression stops going up.

i get them real hot then ride hard. i dont do heat cycles. make sure on the tester the treads are as long as the plug or it will read much lower if its not the same length.

 

[Blaaster]

after 2-3 heat cycles the compression stops going up.

Uhh, please explain.:-/

 

[bobrown]

I do know that on my yz, i put a brand new prox top end in it, and after heat cycles the comp. went up good, after break in it went up and made its best power after 2 hours of hard riding....

 

[Blaaster]

I do know that on my yz, i put a brand new prox top end in it, and after heat cycles the comp. went up good, after break in it went up and made its best power after 2 hours of hard riding....

Now I am more confused than ever, you said you never do heat cycles, then you said after heat cycles the comp went up good.

The reason for the compression going up after use is because the rings have settled in and are now bedding in to the cylinder walls stopping blow by.

The reason for heat cycling is to allow the copper head gasket to be compressed sealing any imperfections in the mating surfaces between the head and jug. IF then the head is not re tightened it stands a fair chance of leaking.

 

[bobrown]

I didnt say i dont do heat cycles... that was him up there ^^ lol.

 

[Blaaster]

Oops, smack me on the wrist, I should pay more attention.

 

[bobrown]

Im about to start rebuilding a 1959 evinrude 5.5hp boat motor, i got it running a couple years ago but it needed help.. I think it would be perfect to try modding on.... i saw the intake ports could be opened up to match the carby, wouldnt tht be cool...

 

[Blaaster]

On the older model outboards Evinrude used exactly the same motor for the 5hp and the 7.5 hp, exactly the same specs.

The difference was a restriction in the intake stopping air flow in the 5hp model.

If you match the porting you may well up the power to 7.5hp.

 

[bobrown]

Man, i didnt know that, i read that they used the same pistons and crank and all that but didnt know what made the difference, im going to start a new thread on it and i will post some pics so maybe you could give advice on my lil outboard... rep given..

 

[Blaaster]

Look foward to it.