270psi cranking pressure!!!

[ib2strokin]

ive got a fresh rebuild that i just put a new piston in and this is the cranking psi i got!

here is setup:
67.5mm bore
86 piston
2mm stroker
85atc bottem end
ftz head cut for stroke but not sure bout dome cc, squish, etc.
im runnin turbo blue 115 race fuel because i was told it needed race fuel
ftz pipe...thinking about switchin to a trx5 though
motor was originally built for TT racing, C&C Racing shop owner's daughter used it at nationals and placed 2nd....

im guessing that would mean that the port work is for mid-top???

my question is... why is my cranking psi so high?
can i change something to run pump gas?
is this a good setup for TT?

motor gurus...if you have any inputs on this motor setup please chyme in, i just wanna know everything i can about this motor!

[deathman53]

I'm suprised you can kick over 270psi. Mine are around 230 and its tough the kick. Your guage working right????

[Aceman]

Originally posted by deathman53
I'm suprised you can kick over 270psi. Mine are around 230 and its tough the kick. Your guage working right????

No doubt! When I rebuilt my 310 it went from 180 to 225 psi. I thought a ring was snagging a port the first time I tried to roll it over!

Kick back at 270 psi would be a real shin breaker...:eek:

[ib2strokin]

trust me it hurts like a Mo Fo when it slips!!
My gauge is new and im pretty sure its right. tell me if im doin it right....

holdin throttle wot, kicking 15 to 20 times, each kick the needle goes up and finally stops at 270 psi.

ive got a pic of it on my phone....ill post it...almost 270 sorry:D

[400EX QUADER]

looks like exactly 260 to me...

[my88r]

Originally posted by 400EX QUADER
looks like exactly 260 to me...

:macho

[ib2strokin]

yea i looked at the guage and seen it was close to 270, my fault :chinese: but still 260 is still really high isn't it?

[ib2strokin]

edited thread title to 260 psi.....

[smr2]

I have the same problem (but about 20 psi less) with about the same set up bore and stroke.

I want to here the answer too!!!

[SuperChris]

I have ran 2 motors with 300+, I say that because they pegged the gages at 300. I mean gages also, I tried three separate ones the first time I saw it.

[ib2strokin]

wow, i dont know if i would even try to kick that over!!

what were the setups?

[C-LEIGH RACING]

Heres the deal, unless your not going to rev it more the 3 or 4000 rpm, best to try & get the compression down.

With the stroker crank & travel of the piston from the exhaust port roof to TDC, your traping to much & no where for it to go.

Could just be a matter of machining out the dome bowl a little more or even cutting the squish & bowl to lower the compression.

You can do a squish test by inserting a piece of L shaped soldier down through the plug hole & then rotate the crank backwards. Just place the soldier in line with the wrist pin where the piston is strongest.
Best way to do the test is pull the head & cut a piece of soldier to fit from one side of the cylinder bore to the other on top of the piston just like the wrist pin. Any closer than 0.040 could be to close.
240 to 245 compression the engine can survive long as you dont twist the crap out of it. Any higher than that, rebuild time come sooner than expected.
I bet thats a grunt engine for sure with 260 psi :eek: but fun to ride or race.
Neil

[ib2strokin]

ok, ill try to do a squish test on it cause i really am curious to find out every spec of the way its setup. its not too bad to kick over, but it pulls very hard!! :D when it gets on the pipe, the front end comes up if i got traction! i would kinda like to lower the copmression some to extend the life of it but i dont wanna lose much power. is there a point where more compression doesnt equall more power?

[Buttermilk]

There is a point where more compression doesn't equal more power in a two stroke.

My motor was setup at one time with 260 psi, then later 250 psi. It ran good, but didn't want to rev out much at all.

I later changed the setup, which resulted in about 209 psi. This motor rev's a lot better, pulling a lot more RPM's on top. Plus, additional changes I've made to the motor also gave me more bottom to mid power as well.

It's all in the whole setup, not just a single item such as compression. Compression is just one piece of the puzzle.

C-Leigh is steering you in the right direction, and he knows a whole lot more about it than I do. Drop the compression, get the squish tolerance set, the squish band width and angle set, the dome cc'd and get a competent builder to make sure your port timings are where they need to be and your motor will come alive.

If you are gonna switch to the ESR5 pipe, I'm quite certain that pipe likes to be in the low 190s (192-194) for exhaust, and around 128-130's for transfer timing (I've ran as high as 198/134 on my motor, but that's a bit too high for broad powerband...).

Regards,

[C-LEIGH RACING]

Thats right, high compression & high rpm fight each other. One counteracts the other.

Whats realy nice to see, is when a rider masters a high compression engine with their shifting & make the compression work for them to out run a screaming engine right beside them. Little left foot working will get the job done & wont work the rider has hard either.
Neil

[All250R]

Originally posted by Buttermilk
There is a point where more compression doesn't equal more power in a two stroke.

My motor was setup at one time with 260 psi, then later 250 psi. It ran good, but didn't want to rev out much at all.

I later changed the setup, which resulted in about 209 psi. This motor rev's a lot better, pulling a lot more RPM's on top. Plus, additional changes I've made to the motor also gave me more bottom to mid power as well.

It's all in the whole setup, not just a single item such as compression. Compression is just one piece of the puzzle.

C-Leigh is steering you in the right direction, and he knows a whole lot more about it than I do. Drop the compression, get the squish tolerance set, the squish band width and angle set, the dome cc'd and get a competent builder to make sure your port timings are where they need to be and your motor will come alive.

If you are gonna switch to the ESR5 pipe, I'm quite certain that pipe likes to be in the low 190s (192-194) for exhaust, and around 128-130's for transfer timing (I've ran as high as 198/134 on my motor, but that's a bit too high for broad powerband...).

Regards,

I know stock TRX transfers are pretty tame. How peaky are you with that pipe at 130? Any dyno curves by chance?

The engine (88 cyl) I've been sitting on lately is at 187/122 with a wide exhaust. In glamis sand I'm coming on kinda hard around 5.5k on the trx5. I dyno'd this engine once. It pulled 45hp at the end of the season. Had a stuck lower ring tho, and slightly sagging crown so I need to try it again.

Neil, I think I've seen you give more conservative advice over the years. What do you think of those numbers, if you don't mind sharing?

[Buttermilk]

Yeah, I've got dyno numbers. Check the dyno section here under my screen name. I've posted several. My last dyno session, I pulled 56.4 HP / 34.56 TQ and that was with axle bearings that were shot (discovered while on the dyno....). I also had about a 1mm mis-match between the carb and intake boot. Had those two things been fixed, I might have pulled another horse or so....


I do know that a lot of people go more conservative on port numbers than what I've posted, but They are missing some hp/tq by doing so. However, If I were building a 250R for woods racing and wasn't going to see a lot of high RPM situations, I would probably go more conservative myself. My quad was built for TT/flattrack racing, so I wanted a bit more high RPM power.

I've attached a dyno chart that shows my motor with two different setups.

Run008:
190E / 133T timing
250 psi compression (~17.9:1 UCCR)
Sparks TT pipe
38 PWK A/S carb
TT111 race fuel

Run071:
194.5E / 128.5T timing
209 psi compression (~15.2:1 UCCR)
ESR TRX11 centermount pipe
41.5 PWK taperbored carb
TT111 race fuel

[All250R]

Nice numbers. Out in so cal we see a lot of desert riding which begs for plenty of open throttle. I have a bunch of oem cylinders. I may have to do some experimenting inspired by your numbers and see how it works out here. Are you doing anything fun with the boost port angle or transfer roofs (those secondaries are a little steep)?
Thx for sharing!

[Buttermilk]

My cylinder is an '87 oem. As you may know, the transfer ports on an '87 cylinder are quite small, interms of volume, compared to the '86, '88, and '89 yrs.

So I've got a lot (a lot!) of time with a porting tool on my cylinder. I did alot of "tunnel porting" on the transfers, as well as re-shaped the port roofs on all of the transfers.

My transfer roofs are flatter than factory, and I've worked over the port entry angles into the cylinder for better directional flow. My exhaust port is quite wide (I'd have to measure my port map to know exactly how wide; don't have it handy at the moment).

This motor as it currently sets has the broadest power of any setup I've ran so far, as well as the highest peak power too.

Regards,

[ZSNOW]

if 270's high i better go check mine again. last time i checked it it was 290...in a paul turner nationals 265. im gonna go do that again and get a pic...:eek2: :eek2:

[ib2strokin]

Originally posted by C-LEIGH RACING

You can do a squish test by inserting a piece of L shaped soldier down through the plug hole & then rotate the crank backwards. Just place the soldier in line with the wrist pin where the piston is strongest.
Best way to do the test is pull the head & cut a piece of soldier to fit from one side of the cylinder bore to the other on top of the piston just like the wrist pin. Any closer than 0.040 could be to close.
240 to 245 compression the engine can survive long as you dont twist the crap out of it. Any higher than that, rebuild time come sooner than expected.
I bet thats a grunt engine for sure with 260 psi :eek: but fun to ride or race.
Neil

Neil, i did a squish test thru the plug hole and averaged right around 0.040". I really expected it to be tighter than than that but anyways. The gasket i was runnin was 85atc gasket (0.045"). I have a 0.058" head gasket that im goin to put on and do an accurate squish test with solder on the piston like you stated. Im just tryin to get my compression down a lil bit to maybe extend the life of my topend and lower the risk of detonation! Do you think this other gasket will help that? ill post my findings in the next couple hours.

[C-LEIGH RACING]

Wouldnt hurt to install the thick gasket, that would releave some of the compression.
Most all 250R engines I have built & after a good discussion with the rider as to what they are looking for from the engine, thats how I set up the compression according to that.
240 maybe 250 at the very highest will I go on compression & even then its because I know the riders going to respect the rpm limit.

Let us know what you get with the thicker gasket.
Neil

[ib2strokin]

well it didnt change the compression much. now im at 250 psi and a squish of 0.051. my squish is about 0.275 wide. any ideas why so little change?

[C-LEIGH RACING]

Now you are holding the throttle W/O when checking, correct.
Start that engine, get it warm ride it around some & then check it.
While your running it, be sure you run it hard enough the clear the bottom end out.
Sometimes fuel puddling in the bottom end will cause wrong compression readings.
Neil

[ib2strokin]

.
i know its been a while but i was about to go rride it any clear it out but now ive got a coolant leak at the head gasket!!:mad:
so, im gna get me a new gasket soon, but i also want to put a stock head to get the compression down so i can run pump gas! c-leigh, since i have a 2mm stroker, do i have to cut a stock head to fit?

[C-LEIGH RACING]

I dont remember, but if you piston is comming out of the cylinder at TDC, yes the head will need cutting.
If its comming up flat with the deck, no, just use the stock thick head gasket.
Neil

[ib2strokin]

ok, thanks! im gonna be pullin this head off to check! also, will going to a stock head automatically let me run pump gas? right now on ebay there is a pair of stock heads for sale, one is stock and the other is rechambered for 26cc. would these benefit me?

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=110281245340&ssPageName=STRK:MEBI:IT&ih=001