3500 vs 3900

[Fastmax32168]

I am all but finished with my 3500 powered Chevy Monza and am already looking ahead to my next build which will be a 60 degree V6 powered Lotus 7 Replica.
I have been shopping for an engine and have found the 3500 to be more plentiful and considerably less expensive than the 3900.
This motor will need to be a revver and calls for ported heads, street/strip cam, long tube headers, and free breathing intake and exhaust.
So hypothetically if I were to build a 3500 and a 3900 both along these guidelines what would the HP and torque differences be between the two?
I am trying to determine if the additional cost of purchasing the 3900 and then having to retrofit it back to non VVT technology is worth it. What type of additional power do I get out of the better heads and additional CC for my money, time, and trouble?
I am hoping to end up near the 300 HP mark with whichever one I choose.
Thanks
Roy

[bob442]

300hp is achivable with a 3500, but a 3.6DOHC makes 306hp out of the box

[Superdave]

More displacement = more power potential. The tradeoff is the amount of money to be invested.


The 3900's heads flow amazing stock, with a good cam and headers it'll make 300+ easy. A 3500 will do that as well but you have to run a bit more cam and ported stuff to get there.

[Fastmax32168]

I went ahead and built both these motors in Dynosim using headflow data garnered from the internet on ported 3500 and 3900 heads and the WOT strip camshaft. Looks like more difference in torque than ultimate HP. The 3500 Hangs on for an extra 500 RPM. At this level there is actually less diff than I thought there would be based on all the talk about how great the 3900 heads are. This is what I call barely making your goal!

This chart is RPM, 3500HP, 3900HP, 3500TQ, 3900TQ


2000 089 101 235 265
3000 147 164 258 287
4000 218 237 286 312
5000 272 288 285 302
5500 287 300 275 286
6000 293 300 257 262
6500 293 283 229 236

[Fastmax32168]

You guys are going to love this. I would imagine a lot of people dont know what a Lotus 7 is so I figured I would post a pic. The completed car will weigh about 1400 lbs, when the 300 HP 3900 is installed into it the theoretical best ET is 9.96!!!!

[Superdave]

I have the perfect engine for that car, you should drop it off this week and don't forget to sign over the title.


FWIW, I'm making almost 350 HP with my 3500... and if i didn't have to haul around kids i'd actually DD it during the warm months.

[Fastmax32168]

Well now this is better. I used the new information Ben posted regarding how much better the 3900 non variable intake was to improve the flow on the 3900 intake on my simulation. It made a big difference. It looks like that intake was a big choke point on the 3900 heads, the 3900 now stomps the 3500 by nearly 40 HP and 40 ft lbs of torque!

This chart is RPM, 3500HP, 3900HP, 3500TQ, 3900TQ


2000 089 098 235 258
3000 147 165 258 288
4000 218 247 286 325
5000 272 307 285 322
5500 287 329 275 314
6000 293 332 257 290
6500 293 325 229 262

[pocket-rocket]

That is one sweet little car

[Fieroghini]

I have a 3900 with all those things and unfortunately your numbers are greatly exagerated. I did install thinner head gaskets but didn't get far enough with the tune to test it out before I parked it for the year. My last time at the track was a 13.35 which puts me about 245hp at the wheels.

[Fastmax32168]

Fieroghini,
Tell me exactly what you have done to your motor. I never expect Dynosim to be completely accurate, but in the past I have found it to be pretty darn close once you have all the parameters right. Thats a pretty big discrepancy so if it is that far off, either you have an unknown issue that is holding you back, or I have something programmed in wrong. I am curious as to what the differences are.
Thanks
Roy

[Fieroghini]

Northstar throttle body, long tube headers, non variable intake, comp cam with .610 lift( I'm trying to attach a photo of the cam card but haven't done it here before) AEM 4'' air filter. 4t65e-hd trans with 3.73 gears. I'm told the lower gears throws off the dyno readings but track times don't lie and they were pretty consistant. I'm not sure if I had the magnaflow muffler with the last track run. I was running dual corsa's but it was long, hot and heavy. It seems to run better with a single 3'' into magnaflow anyway. I also raised compression almost 1 point with the head gasket change but this was after the track run but it definately feels stronger despite the incomplete tune. As for why the low numbers, I have a couple guesses but am no expert. Maybe the cam is too big, along with stock compression ratio. Rear tires are big and heavy. tune needs work. Oil pressure was low until I took off the oil cooler which may have affected the lifters. I know the oil pressure wasn't high enough to use the LS spring upgrade. Again, it's fine now but I'll have to wait until next year to find out any results. I'm open to suggestions of course, 350hp would be a nice improvement.

[Fastmax32168]

Are the heads stock or are they massaged? Also do you know what size your header tubes are?
I will virtual build yours and compare it to the one I built.

Thanks
Roy

[Fieroghini]

The heads are stock. The headers are 1 5/8'. 32'' long but one tube came out short because I ran out of bends. The cam card is for 1.5 rockers so that needs to be adjusted. I know Sappy said it should be 310hp conservatively.

[Fastmax32168]

Ok so using stock head flow numbers I scrounged off of the internet your motor makes 279 Crankshaft HP at 5500 RPM. Given the drivetrain loss that sounds pretty darn close to me.
But that is a pretty big difference between yours and my virtual motor for what basically amounted to a set of ported heads.
So I compared the stock head flow numbers to the ported head flow numbers I had previously scrounged up.
The intake flow was not that much difference, however the exhaust flow difference was substantial.
Either my ported exhaust numbers I previously found are skewed, or porting that exhaust really opens it up. Either way something big is happening with the exhaust.
So I figured I would make only one change to your motor. Still using the stock flow numbers I decided to experiment with that dual pattern camshaft.
I think that cam is kind of odd having more intake duration than exhaust. All the dual pattern cams I have used have always been the other way around. Perhaps that cam was designed around older style heads where the intake did not have such great flow in order to help the intake side out a bit.
I changed the exhaust duration in your simulation from 212 to 230 creating a more traditional dual pattern cam where the exhaust is given the greater benefit. Your power jumped hugely to 321 at 5500 and that number hung on until 6000.
What I learn from this is that I think the 3900 heads can flow substantially more in than they can get out, so I am going to look at a different camshaft choice.
Roy

[Fieroghini]

Sounds about right. Not much I can do about it now though. Thanks for going through the trouble.