jcorenman

And stock bearings? Let us know how that works out for you...

Ian928

I have to pull this thread back up, I was waiting for someone to answer the bearing issue. Will the extra load really be to much for the stock bearings? It seems they are holding up well for the guys running boost?

I also think the idea of keeping the stock crank and rods is well for economic reasons. I would like to see a 5.77l kit consisting of block and pistons only.

The Fixer

I was hoping Carl would answer the question of revs and rev limit with such large displacement, I have never know of larger displacement motors to rev up quicker than smaller displacement motors.
Mid 80s Corvettes never wanted to race me in my basically stock '86 Carrera.
My smaller 911 motor revs much quicker and the car is quicker.

Carl Fausett

It doesn't appear that will be a problem. My own 6.5L motor is about to enter its 4th season, and has been tuned to 750 CHP for road racing and 930 CHP for Bonneville. And we are still going. Credit the very good Porsche oil pump (it flows more than you need - its an excellent pump) and the after-market crank with improved oiling routing.

I can't give you "mileage" as a race engine is aged in "engine hours", not mileage. But I can tell you that this engine has 21.7 hours on it and I inspected the rod bearings last June and they were so good I did not replace them.

So no, based on my experience, I don't see a problem where bearings for the 7.0L motor are concerned.

123quattro

Mid 80s corvettes were pretty damn slow. That wasn't because of engine displacement, but more that they were strangled by emissions equipment and were low compression.

Carl Fausett

Several posts touched on the bore to stroke ratio, let me add something new to that topic.

The Bore:stroke ratio is one of those great barstool arguments (those that have no answer and can be argued untill the beer runs out). You can just Google "Bore to Strioke ratio" and read all you might like on that topic. Its even on Wiki.

As it relates here, with our Moldex crank at 3.75" stroke, this engine is already oversquare. But as we have grown from 100mm bore to 4.0" bore, then 4.125" bore, we have seen the engine become more oversquare, and the power band flatten out and become longer as a result.

I expect this to continue, that the 7.0L motor will have even a longer power band than the 6.5L motor.

As a general statement, engines with the same stroke gain a flatter, longer power band the more oversquare they become. They are less "peaky" in their performance. I don't see this benefit mentioned in articles about bore:stroke ratio, but just wanted to drop into the conversation.

BTW: Bore:Stroke ratios for the following motors
Stock 1988 928 M28/44 = 1.27:1
Stroker crank with stock bores (3.94" bore x 3.75 stroke) = 1.05:1
928MS 6.5 L motor (4.125" bore x 3.75" stroke) = 1.1:1
928MS 7.0L motor (4.250" bore x 3.750" stroke) = 1.13:1

Note the bore:stroke ratio of the stock motor, and you can see part of the reason Porsche got that nice long power band for us. (Camshafts, intake runner lengths, gear ratios, and other factors being the other reasons)

Note this topic is not the same as "where does the engine make peak power". That's a whole different conversation.

Strosek Ultra

When Carl is using an after-market crank (Moldex) with improved oil drilling he must be running Clevite H-series high performance con rod bearings. I like to know what dimension 48mm, 2.000", 2.100" or any other?

Ake

The Fixer

Porsche had the same emission and fuel regs/laws to comply with as GM.

An '86 Vette wasn't slow, 0-60 in under 6 seconds was fast back in the day. It was as quick to 60 as a Ferrari Testa Rossa with 400 BHP.
Vettes made around 230 HP / 330 lb ft from a 350 cu in motor, they weighed about 200 lbs more than a Carrera. The Vette made gobs more torque than the '86 911 which was basically the same as a 1984 911.

My 911 at 193 cu. in. made SAE net HP 200 and about 200 lb ft of torque

Based on the torque comparisons alone you would think the Vette would smoke the 911 but they were pretty much about the same from 0-60

These big bore 928 motors make so much power you don't need to rev them out, but i am curious to know more about the character of them.

With almost all motors i can think of, like the 911 motor, as displacement increased the rpm limit decreased and power was made lower down in the rpm range.

I like a motor that makes it's power down low. That's the appeal of the 928 for me.

ptuomov

Holding the valve sizes constant and the engine redline constant, I don't understand why the bore to stroke ratio has any significant implications on the powerband or the torque curve shape. What specifically are you talking about?

In a clean sheet engine design and holding the displacement constant, larger bore to stroke will allow for larger valves to be fitted and a higher redline because of lower piston speeds and lower stresses on the rotating assembly. But one can't fit really large valves to the 928 4V head without moving the valves (not practical) and I don't see anything obvious here that would allow for a higher engine speed. I don't understand any benefits here beyond larger displacement and availability of cheap piston ring packs.

Carl Fausett

It is believed that increasing the bore to gain displacement has fewer internal losses due to friction as opposed to gaining displacement by increasing the stroke. The longer piston stroke increases the internal losses.

Some say that (where common high-tension piston rings are used) that the piston rings are some 60% of the internal losses due to friction. That's why increasing the stroke increases the frictional losses.

Not really the reasons we did this.... we stopped at 3.750" stroke because thats the most the 928 lower block cradle will allows without wild modifications (which would weaken the cradle, and we didn't want to do that). So the stroke had become "maxed out".

So we turn to the bore - and are discovering how large we can go there....

I mention this because we are not starting with a blank sheet of white paper on this engine - instead we are focused on what we can do with what we have. That makes the theoretical stuff interesting, but in some ways not very applicable.

Carl Fausett

Meaning no disrespect, I'd like to not answer this question. Some of our knowledge has been hard fought for and painfully won. I share a great deal of what we have learned, but there are some solutions and improvements we like to keep to ourselves when they provide us with a competitive edge.

I hope you understand.

inactiveuser1

Smaller journal within reason allows for more stroke.A rod journal smaller
allows the od of a custom rod to be smaller plus it allows more clearance for added stroke.
On upper end of a rod I used small block Chevy sized wrist pin size on a motor.
Went from wrist pin of .984 on the stock motor to .927 Small block Chevy size.
That allowed for the piston pin area to be higher in the piston for more added stroke.
Moldex made a crank for me years ago they will make any size you want.
Back then I always ran Clevite Tri-Metal 77 engine bearings .
Sticking with Small block Chevy sizes makes it easier to get rods and rod bearings also.
Piston speed velocity changes when stroke or rod lengths are changed.
Close to oversquare high torque low rpm motor like Carl is talking example is 455 Pontiac motor.
Overall longest rod with shorest piston makes a good light weight assembly and best rod ratio.
Lower rod ratio causes more sidewall pressure on cylinder walls which creates more heat.
Rod ratio is length of the rod divided by stroke.An area from 1.48 to 1.90 is what most motors fall into.
If you want a longer rod yet you can get an even shorter piston going to a small block ford wrist pin size which is .912" diameter.
Even with Fords on Carillo web site most rods use the .927 small block Chevy wrist pin size.
Better explanation on piston speed weight of rotating mass effects even on crank journal bearings etc.
http://www.wallaceracing.com/enginetheory.htm

123quattro

I'm going to stand by my statement that early C4 Vettes are slow.

Standard engine power in the C4 Corvettes ranges from 205 horsepower in 1984 up to 230 horsepower in 1985 and then variations up to 250 horsepower by 1992.

Carl Fausett

We split a 1987-88 32v head right down the cylinder wall line so we could learn where the cooling passageways in the head are. Now we can modify the head gasket and head with a small steam vent/coolant passageway to promote coolant flow near where the cylinder walls are closest. This will prevent any gas pocket from forming in that area.

inactiveuser1

Darton seems like they put the sleeves together to improve strength under high horsepower and
also coatings to protect from corrosion and rust.This technology for wet sleeves looks great with the cooling.
They have the nested sleeve patent design.Also ported waterflow for cooling.If the sleeves workout maybe get with them
to make something more for production?
http://www.dartonsleeves.com/midinfo.html