2.5 exhaust chest mods....

[texasvector]

some info on port timing between the 260/280/drag

http://www.stvowners.com/topic/1227-...t-280p-part-i/

http://www.stvowners.com/topic/1228-...#comment-10613

[FORBESAUTO]

How’s your motor project coming?
After reading those articles, reminded me of something that could be an interesting topic. I’ve always put a radius on my exhaust ports because always saw it on port maps like the described on the drag. My new block was one T-Rex started on and stopped when he decided to give it up. I finished the porting on it, he had half of the exhaust ports done and noticed no radius. I asked him about it and he said he doesn’t put any radius on it, just enough to not snag the ring. So I thought about it, and came to the conclusion, with the roof raised a tad more to the point where the radius would be, the port would start flowing at the same time but at a higher rate and instead of the initial flow being directed to the floor of the pocket, it’s now directed out towards chest. May loose some bottom end, but would think would gain upper end power. Thoughts?? New mota is this way and so far seems to not be lacking much in the low end, in any it would be right off idle to less than 3k. Once finished break end, I have a really stiff overhub prop I use to test the very bottom end.

[dwilfong]

Just some thing that was shared with me.

[Smalltownbassin]

Answered all my questions! Thanks S&F!

[johnboy 88 vegas]

7 Petal 260 reed blocks are the ticket for these kind of builds....made a hell of a difference on mine

[Smalltownbassin]

Just some thing that was shared with me.

Can anyone explain the measurements in this drawing? Is this represented in degrees of timing or actual measured distance? The pocket cut into the port roof is interesting as well but again, I'm learning here so I don't fully understand the benefits of this compared to raising the entire port roof. The transfers and boost ports are what I was told not to ever touch when tinkering around but that's mostly because I dont know what the heck I'm doing. If anyone has the time and patience to talk about it I'd be grateful to learn from your experience.

[Stoker boy]

The way I check my port timing is with the crank and pistons in the block. I go off the piston at its highest point with a dial indicator. I don’t go off the top of the block because I don’t know if it’s been decked in the past if I didn’t purchase it brand new. Generally you widen and raise the ports. I think the timing you’re referring to is the distance from the highest point of the piston till it sees the crack of the ports. I actually made some templates when I’m porting a motor so it speeds up the process for me. That’s the way I’ve been doing it for the last 25 years it works for me.

[LakeFever]

7 Petal 260 reed blocks are the ticket for these kind of builds....made a hell of a difference on mine

what reed config were you running before?

[Baker343]

Asking Forbesauto and others who have first hand knowledge and experience of porting and running their own motas to educate us less experienced engine assemblers on how to properly setup and degree a mota for grinding. Sharing secrets is a bonus on 2.5 fishing motas. It's a long winter and
pictures would be a big help. Thanks

[johnboy 88 vegas]

what reed config were you running before?

Stock 5 petal fishing motor parts

[johnboy 88 vegas]

With the stock heads its almost the same as a bone stock motor with 150 lbs compression. But when you introduce a 7 petal front to 160 lbs compression on the same motor it is a whole different animal even with factory port timing.

[FORBESAUTO]

I have to respectfully disagree. Depending on what you are after and the set up you using. If you are after pure top speed and fuel injected turning high rpm, then yes 7 is better. If carbed turning less than 8 and want all around performance, then no, a 4 petal will give ya much better mid range and acceleration and will run within a fraction or even faster depending on rpm operating range your in. On a carbed intake and laser style injection, approx 75% of a 7 petal intake at reed area is a dead zone due to the configuration of intake hurting velocity and causing tumble. Velocity is most important on these things on intake side, if you kill it, you’ve killed the power, this is reason of the mention of being told don’t mess with booster and transfer ports. If you don’t know what your doing and make em big and lazy, they wont perform.

[Smalltownbassin]

I have to respectfully disagree. Depending on what you are after and the set up you using. If you are after pure top speed and fuel injected turning high rpm, then yes 7 is better. If carbed turning less than 8 and want all around performance, then no, a 4 petal will give ya much better mid range and acceleration and will run within a fraction or even faster depending on rpm operating range your in. On a carbed intake and laser style injection, approx 75% of a 7 petal intake at reed area is a dead zone due to the configuration of intake hurting velocity and causing tumble. Velocity is most important on these things on intake side, if you kill it, you’ve killed the power, this is reason of the mention of being told don’t mess with booster and transfer ports. If you don’t know what your doing and make em big and lazy, they wont perform.

What method is used to determine transfer and boost port size in relation to exhaust? Port timing? If i should have 30ish degrees of blowdown, at what point should my transfers open and for how long? If ive raised my exhaust port roof, cut rod slots and opened chest, would stock transfers be a loss? Im full of questions so im sorry if im getting carried away. Just trying to connect some dots.

[FORBESAUTO]

Kinda a loaded question . As with most everything, there’s a lot of variables and this effects that and so on. But in a nutshell, it’s takes a certain amount of duration to support a certain amount of rpm, just as more blowdown is required for more higher rpm power. But at the same time as you increase both/either lower rpm power will suffer. If your running 30 degrees blowdown then you are after high rpm power(8 and above), I’m gonna step out on the ledge and say this will require at least 124 degrees duration (open at 118atdc) under most situations on boost/ transfers. Just keep in mind the more you open ‘em, then more doggy the bottom end will be. You can fudge duration by playing with angles and crankcase pressure to a degree. I’m messing with mostly bass boats so I have to be cautious with losing too much bottom end, they’ll let ya know quick when you’ve messed up, so I’m usually not running quite that high on blowdown, I pretty much stay under 28 degrees on b/d and 124.5 on boost. When I run the higher boost timing I generally run my crankcases intake the tight side to increase the velocity on the larger ports to help bottom end. There is no much more involved than this, but I think this should get ya in the ballpark and hopefully give enough info to get tha wheels turnin. There’s is no correct answer and one size fits all with all the many variables that effect each other.

[Smalltownbassin]

Great info! And you're right, it's definitely got my wheels turning. Sounds like crankcase stuffing can help if one goes overboard with a die grinder haha. Thanks for explaining this in detail to me. Anyone can cut what they're told to cut. Understanding why is the key. Thanks again!