Building a 350: newbie finer points?

You can't just assume that more horsepower automatically lowers low end torque. Go back and look at the mercruiser graph versus Curt's LT1/ Comp Cams graph. The LT1 has superior heads and a superior intake, which shows in the fact that Curt's engine's torque peak is at a lower rpm than the mercruiser engine's, meaning it is superior at BOTH low rpm and high rpm. This is due somewhat to the cam, but mostly due to the superior LT1/vortec heads. The ports are so small at only 170 cc that port velocity is very high even at low rpm, thus the superior low speed torque numbers; yet the intake port is still able to produce extremely good flow numbers, which accounts for the higher horsepower over the mercruiser engine. You don't see much drop in low speed torque production until the intake port volume goes over 200 cc or so.

Also, props do stall in the same sense that a torque converter does. If you were to tie the back of the boat to the dock and floor it, the engine would hit a certain rpm and stay there, which is basically what torque converter stall is. At some point you'd cavitate, of course, which from Curt's and my point of view, would be equivalent to spinning the wheels of the car on launch, and actually both situations require the same remedy - drop the rpm until you get traction and then floor it again.

 

Mercruiser already uses Vortec heads. The only difference indicated in the whole thread is mild porting. This should decrease air speed at low RPM and decrease torque.
If you tie a boat to a dock and floor it, the type of prop and transmission ratio will make a huge difference. For example, a tug with a low turning big surface prop designed for 55% slip will either max the engine's torque or pull the dock away. A prop and transmission designed for speed, with low slip, will cavitate and over-rev the engine. With your setup it would cavitate.

 

So you are saying just use the vortec head stock?

 

I am saying that he is thinking that installing a Vortec head will better the engine's performance. However, the Mercruiser HP and torque curves are for an engine with Vortec heads. There is a shift of torque to the uppper RPM range which leads me to believe the lower end will have less torque. His software doesn't do predictions for low ends because it is for cars. In an automotive application, a high stall torque converter fixes that. In a boat the low end torque is necessary to get up on plane and is probably more important than maximum torque.

 

Pocket porting will not decrease low speed port velocity. That's just wrong. In fact it usually improves low-lift flow numbers. Sharp ridges and poor port to bowl to chamber transitions are quite common on OEM cylinder heads. These poor transitions do not help with anything. Conversely getting rid of them hurts nothing. It's often the low flow numbers that improve most with mild porting as high-end flow numbers are more influenced by port volume. If you have ported heads and worked with a flow bench you would know this. Of course all this assumes proper port design without many of the 'errors' found in head designs from the 60's and 70's. But these are Vortec heads; they have very good port shapes without any tinkering.

Jimbo

 

Dude, you're in denial. There's no shift of torque to the upper rpm band. The LT1's torque peak is actually LOWER in the rpm band than the merc's - meaning that even if they were equal in force, the LT1 would still win. But they're not equal - the LT1 makes 50 ft/lbs more torque than the merc, which is why it also makes more horsepower.

 

Stonebreaker,

I'd like to ask you a few questions via email about a engine application for a boat. Could you contact me at joemaj888@hotmail.com?

Thanks,

SalmonMan

 

If those questions are applicable to a 350 and this thread, it would be nice if they are posted here as we could all maybe learn something. If they are not related to a 350 please disregard this post.
Thanks

 

Where does the LT1 come in? The thread is about a Mercruiser with mildly ported heads. The graphs, when I put them side to side show an almost equal curve but shifted.

 

The LT1 comes from my original (slightly unrealistic) choice for this boat.

Here's what I'm looking at. The stock engine spec'd out like this:

-305
-8.5.1
-Melling 22124 cam
-iron intake
-rear riser exhaust manifolds
-probably 214 heads, 160cc intake, 58cc chambers, peak flow of about 184 cfm
-230 hp

The one that is replacing it looks like this:
-355
-9.0:1
-Melling 22124 cam
-bowtie aluminum intake
-center riser exhaust manifolds
-906 heads, 170cc intake, peak flow of about 245 cfm
-300+ hp

I guess where I'm confused is why are we saying that my numbers will be inadequate for the boat. I'm sure that I didn't shift the torque peak up by more than a couple hundred RPM while adding 50 cubes and more compression. I might make more torque with a roller, but I think my point is that even though the graph doesn't show it I gotta be making considerably more than stock across the entire band.

I think I've decided to go with the flat mercruiser grind. I would love to go roller and I might someday, but for now the two cams I was looking at offer similar peak HP, but the flat gives up 50 lb-ft down low. Since I know I'm making more than stock I'll feel more comfortable dragging skiers out of the water with the Alpha. Same top speed, just an extra second getting there.

And, if I'm not happy with it, I can always change it.

 

Here are some dyno charts that show as close as I can get with the stock 305 versus the 350 I'm building with the flat cam. Tell me how my numbers under 2000 rpm could be too low with that combo. If the stock 305 could do it, my 350 will be a real kick in the pants.

 

I think the confusion was that I though you were comparing a 350 Mercruiser with a 350 with slightly modified heads. Now it makes sense. However, since the graph show nothing below 2000RPM it is anyone's guess what will happen at the lower end. The 350 will have more torque than the 305 for sure.