Do I need to change port timing?

[merc1250bayliner]

I have a project block I'm working on building. It's a 1992 175 block. I'm currently working on the exhaust chest. I cut it at 30 degrees very similar to the way Forbesauto did his in the 2.5 chest thread. I would like to turn this block into a 200. Would I just need to raise the exhaust port to 1.600? With both of these mods done will I need to touch any of the other ports? Or can I just add stuffing and intake spacer, bigger jets, etc? This will be going on a 1700lb deep vee. I want to retain torque and avoid any melted pistons. I'm new to this but am enjoying learning.

[FORBESAUTO]

I’m always using a degree wheel, but from I recall a 200 exhaust opens at 92 degrees and intake and transfers at 120. Just remember messing with port timing can kill low end if not carefull. You want to keep your blowdown for what your doing in that 27-28 degree range for what ive found. I normally set my transfers for the rpm range I’m wanting first and then set exhaust by blow down from the transfers I used to match that same operating range. For keeping good low torque I would keep the transfers around 120 degrees ATDC, which equals 120 degrees duration and exhaust around 92.5 to 93 to put blowdown in the 27-27.5 degree range. Also found to keep your transfer roof angled from exhaust port side to boost port side with it being shorter (open later and close sooner) on exhaust side like to 175 should already be and no angle on transfer roofs makes better low end- mid range power. You can widen the exhaust port a little to help power and I would leave the boost and transfer widths alone to keep up velocity. Also don’t get crazy with stuffing without opening the transfers: and boost ports up, I would use the intake stuffers only or shave front whichever you choose and fill in oil pump cavity, if your deleting oil injection.

[merc1250bayliner]

My exhaust chest looks like yours from the 2.5 exhaust chest thread. It is roughly 30 degrees angled. I might have to get a degree wheel once I have enough parts for my rotating assembly. Is it just a regular automotive degree wheel for a camshaft? What would happen if I left all ports alone except raise exhaust to 1.6"? Would I be lacking on the intake side? I will be using wh22 carbs, reed stuffers, maybe spacer plate.

[FORBESAUTO]

Yeah, I use a Moroso pro wheel so I can split hairs. What max rpm you wanting to run? I would definitely use the spacer, they significantly help low end and
mid range.

[FORBESAUTO]

Blowdown is a major factor in what rpm power is made. I’m not sure where 1.6 would put you on the 175, I have a note pad at the shop that I record info on for reference, I’ll check and see if I have the 175 info recorded. Raising the exhaust only will increase blowdown and will make more power, but on a heavier rig I just wouldn’t raise it more than 28 degrees of blowdown. Leaving your transfers alone and setting the exhaust to 27-28 blowdown would work really well, wherever that will be as a distance from deck. If ya get your blowdown and transfer durations mismatched, you could still make more power but in a shorter window resulting in a peaky power curve instead of nice flat one, which is more beneficial especially on heavier set ups. Kinda like mismatching intake and heads with camshaft, there may an rpm window in there where they all cross each other and make good power numbers but would do much better if they all came in the same time, if this help convey what I’m saying lol. The shapes, angles also play a major role that you can play with also to change how the port reacts that can effect it as much as port timing also, but that’s a whole ‘nother can of worms

[merc1250bayliner]

Sounds like I need to find out how much 28 degrees is. Thank you for your help.

[FORBESAUTO]

I think so, or at least that’s what I would do, actually I’d be targeting closer to 27 depending on hull,don’t mean it’s right lol. Unfortunately I haven’t recorded a stock 175 in my book of life, but did find deck measurements for 200, 1.600 on exhaust and 2.100 on gasket motor.

[FUJIMO]

...a 1992 175 is about 200 crankshaft h.p. from the factory...

[merc1250bayliner]

Does an intake spacer increase crankcase volume?

[FORBESAUTO]

No, it goes between the reeds and intake, reeds position in front half doesn’t change. The spacer helps straighten out the path into reeds on a carb or laser style injection system. You won’t regret using one on these style induction systems, not as much of an improvement on a crankcase style injection system due to path already a straight shot, other than maybe some low end improvement as a result of a longer runner.

[merc1250bayliner]

I am wondering too if I should cut my exhaust in my 2.0 150. But not as much as this one. Just cut a 30 degree angle down but leave the webbing or whatever you call it. Kinda like a Promax chest. The 2.0 is on the same heavy deep vee. Or would this even be too much for the little 2.0?

[FORBESAUTO]

I ain’t cut on no 2.0’s, so have no hands on experience of what works and what doesn’t . But if I can recall some of Rex’s, your thinking right. I think he just angled the lowers some, probably around the 30 degree mark if I had to guess and would mill down in between the ports just a little with a flat mill and leave some of the ditch. If ya wanted to play with it, I would make small changes and see how it effects and go from there, only way to learn. Sometimes it’s painful, but you’ll also find things may react differently than you expected and was told. So many things I’ve been told works turn out to suck and some that wasn’t supposed to work, turned out to be the ticket. I think this is mainly because of so many variables than change the way things react more than just pure misinformation. To me, the experimenting is what makes this fun, just take it in baby steps and not make too many or too large of a change at a time and record the results. Don’t be scared and have fun and learn. Brush off the negativity and find out what works for you, it only makes you better in the long run.

[Chaz]

Forby .. brudda you much wize beyond yo yeers ... every word, good levelheaded advice ..

[FORBESAUTO]

Thanks for the kind words Chaz! If they where only true lol! I know I do stuff that makes some say what the heck, but it compliments my other what the heck’s and the what the heck’s come together and work for me and what I’m trying to complish.