Let's get technical.

[SuperChris]

Lets get technical. I have a 74mm stroke x 72mm bore engine that has just under 300PSI static compression. The sqush band is 12.45mm wide with 38 thousandths squish clearance. Port timings are as follows, exhaust opens at 85degrees, transfers at 115 degrees. I need to know if these port timings are bad and how to cut the head, like how wide should the squish band be and how many CC's. Any help would be awesome.

[RichM1983]

EDIT: Misread the question. Sorry.

[C-LEIGH RACING]

What head gasket you useing, the thin CR250R one.
What cylinder, stock or Pro-X
Those port numbers are upper midrange, but the transfers are a little high, only 30* blowdown.
Those number not bad for mild drag racing but that compression is W A Y to high.
Neil

[SuperChris]

Should I raise the exhaust a bit? Maybee 2 degrees to get 32 total blowdown? This is a 72MM sleeve in the stock jug. I wish the CR250 gasket would work. I have only found the Cometic junk that comes as a whole kit for WAY to much coin. I'm thinking of cutting a little off the squish band and blending it in to about a 2CC increase to see what happens. Is that to much? not enuph?

[deathman53]

why not a stock trx head gasket? They are still available.

[C-LEIGH RACING]

Depends where you want the power.
85 is a little high for a midrange & not high enough for full drag.
Be better to lower the cylinder some more & then cut the exhaust roof. You can cut the head & let the piston run up into it.
If you got 300 psi, you need to get another head & then cut it for that build.
Neil

[SuperChris]

This is a high RPM outlaw kart motor. I don't need much bottom end. The wierd thing is that this motor pulls HARD on most of the other motors in my class as it is. Powerband feels enormous compared to a stocker.

[atvmxr]

if you have high enough octane gas to not detonate, how can you have too much compression??????????

I thought the pro stock drag cars were in the 15 -16:1 CR

[wilkin250r]

Something HAS to be wrong, I just can't see 300psi compression.

I went through the calculations using Boyle's Law, and 300psi comes out to be a little over 20:1 compression ratio. That's like a diesel engine, I just can't imagine that.

[novaracr70]

300 my brother was at 290psi for a day . due to a math errore . it ran with no sighns of problems but i would worry about reliability.

[C-LEIGH RACING]

It just dont add up, I know those kart engines are turned up high in rpms & with that high of a compression it cant last.

Wait a minute,,,,, those kart guys think just like the dirt track car guys & build a engine every race,,,sooo it could be they only want it to last a race or two,, so I guess you could be right.
Those kart guys aint like the bike or atv crowd, its go or blow so who cares about a engine.

Super
You need to be thinking about getting you another head & getting it cut.
You could get a cool head with like a 21 or 23cc dome & probably be ok, just tell them its for a big bore 295.
Neil

[wilkin250r]

Compression aside, let's talk about the rest of the build, because it sounds like you are thinking about cutting the head anyways to customize the squish band and perhaps tweak the compression.

What do you plan on using this engine for? A wide squish band gives better cooling for end gasses, and helps control detonation during peak loads. However, a narrow squish band allows a flatter and broader combustion chamber (for a given size dome), which improves flame travel and gives better high-rpm power.

If you want to get really technical, we need to know the intended application.

[SuperChris]

Intended application is a Kart motor. It's a 310 big bore stroker that has the stock head just taken out for the 72MM bore. I tried on 2 different guages just to be sure. The builder put it together with way to much squish clearance, I fixed it and this is what happened.