gt 40 upgrade

[jeffatjones]

iv got a 1990 mariah with a pcm 351 and want to go 2 gt-40 heads. iv found a like new set off a ford 302. will they work on my 351? if anybody has any experience with this, your thoughts would be greatly appreciated!

[sybrmike]

From back in my old Mustang days, heads should be interchangeable between the 302 and 351 Windsor small block (what's in the old Supras). The 351 Cleveland was based on the big block & used different heads.

[jeffatjones]

thanks alot mike. my first thread. that was really quick.
thanks from south GA!

[sybrmike]

Sorry, didn't catch the virgin posting - welcome aboard! Better post some pics and background quick before others start harassing you.

[cadunkle]

The bolt holes on the 302 heads will need to be drilled out as the 351w uses larger head bolts (7/16" vs 1/2"). Easy enough to do if you have a drill press. just disassemble the head and clean it up good before reassembling. Good time to do some port work while you're at it, even if it's just some smoothing out of the exhaust.

[jeffatjones]

thanx yall! what im really after is about 4 more mph top end. im only getting about 38mph top end. its fine 4 wakeboarding and slalomn but not enough 4 barefooting. we fat boys need 42. i hope i can get 42mph with the gt-40s???

[cadunkle]

I'm not all that well versed on small block heads and port work, as I do mostly big block Ford builds... But what heads are on the engine currently and what is the factory HP? What RPM are you at wide open doing 38 MPH?

I'm curious of the flow difference from the factory heads to the as cast GT40 heads. Some port cleanup will help. Depending on what RPM your turning a cam and possibly carb upgrade might be the ticket to be sure you can hit at least 42 MPH. Does anyone know the specs on the factory cam in these engines? Also the tune can make a couple MPH difference, timing, jetting, etc.

[Hagman]

What model of Supra are you running. I would think that a good running stock set up should get you up to 42mph. I had a bunch of motor work dun a couple of years ago including gt40 heads I’m not sure that they were worth the money. And I took out the cam upgrade and went back to stock. The balancing & CCing was worth ever penny.

[sybrmike]

It's all about getting the right combination of engine, prop, etc. To get more speed using the same prop, you just gotta spin it faster. So the GT40 heads might help with valve float & allow more rpm, but gotta make sure the rest of the engine is up to it as well.

Alternately, you could switch to a higher pitch prop (13x14?) which would get you more speed at the same engine rpm - but hole shot will suffer (could be a problem for deepwater footin' starts).

Or do it all in the quest for speed - tweak the engine and select the prop that best matches the new performance characteristics. Oh, and don't forget to give the bottom of the hull a good polish & wax - could be good for another 1-2 mph.

[csuggs]

I purchased a new Acme 3-blade prop and was told that I will notice gains in my hole shot and top end. I was running a 13x13 4-blade OJ getting about 41mph and was told the new 13x11.5 Acme 3-blade will get me to 44. I think on the 13x13 I was just running out of HP to get the rpm's up.
I would definitely talk to someone that knows props before spending the $ on changing heads - unless of course there's another good reason to change heads besides needing a few more mph.

[jeffatjones]

im not sure of heads specs. i know its a stock pcm351w. i assume its 240hp. im gettn about 38mph @4200rpms. im turning an acme cc 4 blade 12.5 -15.5 recomended by skidim. very smooth, good hole shot, but struggling with top end. My biggest hangup with switching heads is my boat runs perfect now with the exception of 38mph max. I weigh 230!!!

[cadunkle]

4 MPH might be possible with your current setup with some tuning changes.

What is your initial timing? What is your total timing and at what RPM?

What carb are you running? If vacuum secondary are the secondaries fully opening? How do the plugs look after a WOT run? Jetting front/rear?

If timing and carb aren't quite right you might be able to get a couple MPH by adding some more timing and jetting richer on the secondary side.

If that won't get you a few more MPH and some more RPM a little less pitch on the prop may help. I've also heard 4 blades have a slightly less top end than 3 blade. 4200 RPM is low though, I'd expect more like 4400-4600 RPM WOT. You'd probably be fine spinning it to 5000 or more but I'm not sure what rods PCM used and what they will take for sustained RPM.

Problem is, it's not a great idea to be turning over 4000 RPM continuously, for longevity of your engine. You need more power at a lower RPM. To push a bigger prop without lugging. There's a reason barefoot boats were usually big blocks, but that's not to say it's not possible with a small block. It just may require more extensive engine work, and accepting that to get that kind of performance out of a small block it's gonna be a screamer.

[csuggs]

I think the older big block boats did not use a 1:1 trans and so they could get more top end and still have have a good hole shot because of the added ponies - 330 of them instead of 240 on the 351. I had a good friend time and tune my motor and although I don't know the details, I do know that he drag races a 900+hp carbureted (and blown) Ford 302 so I'm pretty sure he knows how to tune my motor. Anyway, the reason I say that is because I'm pretty sure I'm just plain out of hp at 4100rpm on my boat. That may be your case also. If you use a prop with less pitch, you can spin it faster with less hp and bring your rpm's up where you need them.
Cadunkle is right about the prop. A 4-blade is not as efficient as a 3-blade and so you will sacrifice top-end with the 4-blade, but increase hole-shot with the 4-blade. The new Acme 3-blade CNC is supposed to be the best of both worlds - yet to proven in my case. I just know a GT40 conversion is expensive and there may be another way of achieving the results you need. Plenty of guys on here know more about this than I do though.

[cadunkle]

I've never heard of an overdrive trans is a ski boat, but then again I don't know too much about different models of inboard transmissions.

[jeffatjones]

thanx guys! sounds like i need to do somemore homework. i dont have a clue about timing and carb settings. i do have a holley 600 single pumper and i think its factory. i just found a set of gt-40 heads that are like new and i thought it may b as simple as taking off the stock heads and bolting on the gt40s. 200.00 bucks 4 the set of gt40s. was thinking it may b as simple as that to get 40 or so more hp! i need a good tune up buddy it sounds like!
oh yea if anybody runs across that od tranny yall holler @ me!

[cadunkle]

Csuggs, I think what you mean about the trans gears are gear reduction transmissions. So far as I can find these weren't used behind big blocks and I don't see any reason why you would want to. They were used behind small blocks to turn a bigger prop and have more power off idle. This increased engine RPM but also kept the engine more into it's power band. Higher RPM for the same MPH. No reason to run that behind a 454 that has the torque to turn a bigger prop anyhow.

[csuggs]

Csuggs, I think what you mean about the trans gears are gear reduction transmissions. So far as I can find these weren't used behind big blocks and I don't see any reason why you would want to. They were used behind small blocks to turn a bigger prop and have more power off idle. This increased engine RPM but also kept the engine more into it's power band. Higher RPM for the same MPH. No reason to run that behind a 454 that has the torque to turn a bigger prop anyhow.

OK - you're probably right. For example a 1.23:1 trans would mean 1.23 turns of the motor for 1 turn of the output shaft - right? So that is a gear reduction. I was thinking the other way around. My bad.

[szarik5]

A few years ago I built a new engine for my ts6m comp from a stock block from a f150, gt40p heads, e303 cam and fuel injection setup from FAST. Expensive but fun project. I am running 320 to 330 hp and top speed only increased to about 46.5 to 47 mph. Was hoping for a higher top end but the hull is limiting the top end. You could add a few more hp into that engine but I already hear a sqeeking sound when I open my wallet at the gas station.
Having fuel injection is a sure improvement over the old holly carb.

If you have any questions on your project send me message and maybe I can be of some help.
Mark

[Hagman]

I’ve been hoping someone would come to this forum that has installed a fast efi system. Witch system did you get ? T body or port . And has it improved fuel economy . How did you install the O2 sensor . Send Pictures.