An Engineer I work with bought a Mustang with a 302. It had issues so the PO put 351 heads on it. It runs, but runs badly. He suspects timing, but being an Engineer, he has to calculate compression etc.
His calculation show that a 1cc larger combustion chamber lowers the compression from 8 : 1 to 6.5 : 1, I'm having a hard time getting that one.
Any thoughts? Will a 351 head work on a 302? With positive results?
Dan
Needs more boost and it will be fine (once the turbo's spool)
after 77 they're almost identical - but head bolts are different size - 351 would need a bushing to fit. They flow about the same as a 302 head - so no real gain. Earlier 351 heads flow better, but have more fitting issues with water passages.
In reply to 914Driver:
Try 289 heads for a higher comp ratio. IIRC when I was a much younger Eric, a friends father created a monster with the 302 block and 289 heads (massaged gently) for a 37 hot rod. Well a monster for 1988 anyway.
Run a thourogh compression test and leakdown, get out of the notebooks and calculators and get hands dirty... Check timing, vacuum, etc and convince yourself that something is wonky, then hit the books!
Eric G
Near as I know, there is no gain going to 351W heads on a 302. There's not much loss, either. I'd be curious to know what numbers your engineer friend is using to determine such a tremendous drop in CR.
If you or your friend is looking for cheap replacement head, look to the last generation of 5.0 Explorers (circa '98). They used the GT40 heads from the 5.0 Cobras, and you should be able to pull a set from the junkyard for fairly cheap and see ~ 20-40hp gain over stock Mustang heads.
Isn't the Boss motor 351 Cleveland heads on a 302 Windsor?
Shawn
I doubt you're going to see that much of a compression drop by losing 1cc.
The 351 heads have bigger valves, but also have a slightly larger chamber?
I agree with Dexter that the GT-40 heads are the way to go.
The question was more about the compression drop; the heads are already on the car. Along with an aluminum hi rise and a new Holly. It's a 1973 Mustang Grande, heads of unknown vintage.
It's a DD/Project for his son.
I suggested he start from scratch with TDC, check cam/crank alignment, distributor location.... the basics. It already has new goodies, just gotta make it happen.
Dan
Trans_Maro wrote:Isn't the Boss motor 351 Cleveland heads on a 302 Windsor?
Shawn
Boss and Cleveland heads are similar, but not quite the same. I think the swap you're thinking of is a "Clevor" engine, when you put Cleveland heads on a 351W.
For the most part, a Windsor head is a Windsor head. That is to say it doesn't really matter if it came off of a 302 or a 351. OTOH, the newer castings are much better.
You might be able to find a good used set on NLOC, some of the Gen1 Lightning guys take of their GT-40's (351) for AFR's.
My dad had the opposite issue with his late-model van when the 351 dropped a valve. Stranded in the middle of nowhere 351 heads were impossible to find, but the local machine shop had several sets of 302 heads and was willing to ream the bolt holes out for cheap. The engine had such high compression afterwords he had to run premium with octane boost, but it got him home.
Somewhere in there there were some head volume differences 302 vs. 351, but you'd need details on exactly what he has to know for sure.
914Driver wrote:His calculation show that a 1cc larger combustion chamber lowers the compression from 8 : 1 to 6.5 : 1, I'm having a hard time getting that one.
Dan
You're right. I think he confused cubic inches and cubic centimeters. With a 4" bore and a 3" stroke, you get about 37.7 in^3 of swept volume, so an 8:1 compression ratio gives you about a 4.7 in^3 combustion chamber. A 5.7 in^3 combustion chamber would give you a 6.6:1 compression ratio. 1cm^3 is about .06 in^3 and would only drop the compression ratio to about 7.9:1.
Someone check my calcs please. It's been a long day.
Bob
914Driver wrote:The question was more about the compression drop; the heads are already on the car. Along with an aluminum hi rise and a new Holly. It's a 1973 Mustang Grande, heads of unknown vintage.
It's a DD/Project for his son.
I suggested he start from scratch with TDC, check cam/crank alignment, distributor location.... the basics. It already has new goodies, just gotta make it happen.
Dan
You have to remove heads to see casting numbers, but one quick identifier of D0OE heads (i.e. the only worthwhile 351w head) is 18mm plugs. Comparable flow data to the GT40 casting series. 58cc.Cylinder head flow data
In reply to 914Driver:
He should invest the time he's using to incorrectly computer compression ratio changes (that he can't easily perform or correct...) and get a cylinder compression tester and a leak down tester and start figuring out if there are issues with this little project he's picked up. Testing the ignition circuit, making sure the carb and fuel system is up to snuff, those sorts of things.
Another idea would be to start looking for a good engine to start building up to replace the meatball under the hood now. Fuel injected 302 with actually decent heads; the smog monster under the hood of that Grande probably has dished pistons and the slightly larger 5.8 cylinder heads aren't going to help anything, but at least there's a single-plane intake to remove that annoying torque...
In reply to 914Driver:
What year 302 and what year 351 heads? Early 302's ran much smaller combustion chambers than any 351 head, so what your friend calculated may be true. I run 64cc chamber 351w heads on a 302 and have 10.25 compression, which I acheive with ..2" domed pistons. So yes, you can do it and it will work, but generally speaking (depends on year of block and heads), just bolting down 351 heads on an early 289/302 WILL decrease the CR. Later 302/5.0's have larger combustion chambers with flat top pistons. My '87 heads have 64cc chambers also, so swapping 351 heads on to that short block would maintain compression. The 351 heads flow much better than any 289/302 head, which is why people used to use them for increased performance (need to use special head bolts, BTW), but nowadays if you have the money, heads from AFR/TrickFlow, etc... are much better than any factory head.
In reply to bravenrace:
^ I agree. Of course we are making generalizations, but for the O.P. question about another person's engine, this information is more than accurate.
As far as 351w heads flowing much better than any 302 head this is true, up to the "H.O." E7TE castings. After that 302 head, it's not worth the trouble for stock heads.
GRM's own Fox mustang picked up a decent power boost when they moved to GT-40P heads. There were other swaps as well though.
That said, the cheapest set of aluminum Edelbrock, AFT or TrickFlow heads currently on the market would make much more sense than going after a set of factory heads for this thing. For a short block that they actually build, not whatever is in the car now; I don't think I'd trust all the question marks that would bring with it.
